JP7515241B2 - Ferrite powder for bonded magnets and its manufacturing method - Google Patents

Description

The present invention relates to ferrite powder for bonded magnets and a manufacturing method thereof, and in particular to ferrite powder for bonded magnets that contains coarse and fine ferrite particles and a manufacturing method thereof.

Conventionally, ferrite-based sintered magnets have been used as high-magnetic magnets, such as those used in small motors for AV equipment, office automation equipment, and automotive electrical components, and in magnet rolls for copying machines. However, ferrite-based sintered magnets have problems such as poor productivity due to the risk of chipping and the need for polishing, as well as the difficulty of machining them into complex shapes.

For this reason, in recent years, rare earth bonded magnets have been used as high magnetic force magnets for small motors used in AV equipment, office automation equipment, automotive electrical components, etc. However, rare earth magnets cost about 20 times as much as ferrite sintered magnets, and have the problem of being prone to rust, so it is desirable to use ferrite bonded magnets instead of ferrite sintered magnets.

The ferrite powder for such bonded magnets has a composition of (Sr _1-x A _x )O.n[(Fe _1-y-z Co _y Zn _z ) ₂ O ₃ ] (where A is La, La-Nd, La-Pr or La-Nd-Pr, n=5.80 to 6.10, x=0.1 to 0.5, y=0.0083 to 0.042, 0≦z<0.0168) and a saturation magnetization value σs of 73 Am ² A magnetoplumbite-type strontium ferrite particle powder for bonded magnets has been proposed, which is a magnetoplumbite-type strontium ferrite particle powder having an average particle size of 1.0 to 3.0 μm and a molecular weight of 73 emu/g or more, and which contains plate-like particles at a ratio of 60% or more by number in the magnetoplumbite-type strontium ferrite particle powder (see, for example, Patent Document 1).

JP 2002-175907 A (paragraph number 0025)

However, the strontium ferrite particles for bonded magnets in Patent Document 1 contain a large number of plate-shaped particles, and when attempting to align the particles in the magnetic field direction by magnetic field orientation, the plate-shaped particles inhibit each other's orientation, making it difficult to produce a bonded magnet with high orientation.

Therefore, in consideration of these conventional problems, the present invention aims to provide a ferrite powder for bonded magnets and a manufacturing method thereof that can produce bonded magnets with high remanent magnetization Br through magnetic field orientation.

As a result of intensive research into solving the above problems, the inventors discovered that by mixing a powder of a composite oxide of iron, strontium, lanthanum, and cobalt with iron oxide, granulating the mixture, and then sintering it, it is possible to obtain a bonded magnet with high remanent magnetization Br through magnetic field orientation, which led to the completion of the present invention.

In other words, the method for producing ferrite powder for bonded magnets according to the present invention is characterized by mixing a powder of a composite oxide of iron, strontium, lanthanum, and cobalt with iron oxide, granulating the mixture, and then sintering it.

In this method for producing ferrite powder for bonded magnets, it is preferable to coarsely crush the sintered product obtained by sintering, crush the resulting coarsely crushed powder, and then anneal it. It is also preferable that the composite oxide powder is obtained by mixing strontium carbonate, lanthanum oxide, iron oxide, and cobalt oxide, granulating the mixture, and then sintering the mixture at 1000 to 1250°C, and crushing the resulting sintered product. It is also preferable that the composite oxide powder and iron oxide are mixed, granulated, and then sintered at 1100 to 1400°C. It is also preferable that when mixing the composite oxide powder and iron oxide, the composite oxide powder and iron oxide are mixed so that the molar ratio of Fe in the iron oxide to the total of Sr and La, Fe/(Sr+La), is 4.5 to 11.7.

The ferrite powder for bonded magnets according to the present invention is characterized in that it has a composition of (Sr1 _{-xLax )} ·( _{Fe1- yCoy} ) _{nO19 -z} (where 0<x≦0.5, 0<y≦0.04, 10.0≦n≦12.5, -1.0≦z≦3.5) and has an average particle size of 1.3 to 2.5 μm.

The ferrite powder for bonded magnets preferably has a specific surface area of 1.0 to 2.1 m ² /g, and the average ratio of the major axis length to the minor axis length (major axis length/minor axis length) of particles with a major axis length of 1.0 μm or more is preferably 1.55 or less. In addition, 90.0 parts by mass of ferrite powder for bonded magnets, 0.8 parts by mass of silane coupling agent, 0.8 parts by mass of lubricant, and 8.4 parts by mass of powdered polyamide resin are filled into a mixer and mixed to obtain a mixture, which is kneaded at 230 ° C. to produce a kneaded pellet with an average diameter of 2 mm, and this kneaded pellet is injection molded in a magnetic field of 9.7 kOe at a temperature of 300 ° C. and a molding pressure of 8.5 N / mm ² to produce a cylindrical bonded magnet with a diameter of 15 mm and a height of 8 mm (the orientation direction of the magnetic field is along the central axis of the cylinder), and when the residual magnetization Br of this bonded magnet is measured in a measurement magnetic field of 10 kOe, it is preferable that the residual magnetization Br is 2950 G or more. Furthermore, when the maximum energy product BH _max of the above bonded magnet is measured in a measurement magnetic field of 10 kOe, it is preferable that the maximum energy product BH _max is 2.15 MGOe or more.

The bonded magnet according to the present invention is characterized by comprising the above-mentioned ferrite powder for bonded magnets and a binder.

According to the present invention, it is possible to produce ferrite powder for bonded magnets, which can produce bonded magnets with high remanent magnetization Br through magnetic field orientation.

FIG. 2 is a diagram showing the measurement results of the ferrite powder for bonded magnets obtained in Example 1 by powder X-ray diffraction (XRD). 1 is a scanning electron microscope (SEM) photograph of a cross section of the bonded magnet obtained in Example 1. 1 is a SEM photograph of a cross section of the bonded magnet obtained in Comparative Example 1.

In an embodiment of the method for manufacturing ferrite powder for bonded magnets according to the present invention, a powder of a composite oxide of iron, strontium, lanthanum and cobalt is mixed with iron oxide (preferably hematite (α-Fe ₂ O ₃ )) (so that the Fe/(Sr+La) ratio in the iron oxide relative to the sum of Sr and La is preferably 4.5 to 11.7, more preferably 9.0 to 11.0), granulated, and then fired (preferably at 1100 to 1400°C, more preferably 1100 to 1300°C, most preferably 1150 to 1250°C), the fired product obtained by this firing is coarsely ground to obtain a coarsely ground powder, which is then annealed (preferably at 950 to 1000°C).

The above-mentioned powder of composite oxide of iron, strontium, lanthanum and cobalt can be obtained by mixing and granulating strontium carbonate, lanthanum oxide, iron oxide and cobalt oxide, and then sintering the mixture at a temperature of preferably 1000 to 1250°C, more preferably 1050 to 1200°C, and most preferably 1050 to 1150°C, and pulverizing the sintered product.

The coarsely pulverized powder is then pulverized (wet pulverization) using a wet attritor or the like (preferably for 20 to 80 minutes), the resulting slurry is filtered, the solid matter obtained is dried, the resulting dried cake is crushed in a mixer, and the crushed matter obtained is crushed using a vibrating ball mill or the like, and then the annealing process is preferably performed.

In this manner, it is possible _to produce ferrite powder for bonded magnets having a composition represented by (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19-z} (where 0<x≦0.5, 0<y≦0.04, 10.0≦n≦12.5, −1.0≦z≦3.5).

Moreover, an embodiment of the ferrite powder for bonded magnets according to the present invention has a composition of (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z} (wherein 0<x≦0.5 (preferably 0.03≦x≦0.5, more preferably 0.1≦x≦0.5), 0<y≦0.04 (preferably 0.004≦y≦0.04), 10.0≦n≦12.5 (preferably 10.0≦n≦12.0), -1.0≦z≦3.5 (preferably -0.5≦z≦3.5)) and an average particle size of 1.3 to 2.5 μm (preferably 1.3 to 2.0 μm).

The specific surface area of this ferrite powder for bonded magnets is preferably 1.0 to 2.1 m ² /g, and more preferably 1.2 to 2.0 m ² /g.

Furthermore, when 10 g of the ferrite powder for bonded magnets is filled into a cylindrical mold having an inner diameter of 2.54 cmφ and then compressed at a pressure of 1 ton/ ^cm2 , and the density of the ferrite powder for bonded magnets is measured as the compressed density (CD) of the ferrite powder for bonded magnets, the compressed density (CD) is preferably 3.0 to 4.0 g/ ^cm3 , and more preferably 3.2 to 3.6 g/ ^cm3 .

In addition, 8 g of ferrite powder for bonded magnets and 0.4 cc of polyester resin are mixed in a mortar, 7 g of the resulting mixture is filled into a metal mold having an inner diameter of 15 mm and compressed at a pressure of 2 tons/ ^cm2 for 60 seconds. The resulting molded product is removed from the metal mold and dried at 150°C for 30 minutes to produce a green compact. The magnetic properties of this green compact are measured using a BH tracer to determine the coercive force iHc and residual magnetization Br of the green compact in a measuring magnetic field of 10 kOe. The coercive force iHc is preferably 2000 to 4000 Oe, and more preferably 2300 to 3500 Oe, and the residual magnetization Br is preferably 1700 to 2000 G, and more preferably 1800 to 1950 G.

In general, it is known that ferrite magnetic materials with a magnetoplumbite crystal structure have a negative temperature coefficient for residual magnetization Br and a positive temperature coefficient for coercivity Hc, with the temperature coefficient of coercivity Hc being approximately 0.2 to 0.3%/°C. In other words, the lower the temperature, the lower the coercivity Hc of ferrite magnetic materials with a magnetoplumbite crystal structure. Therefore, when used in bonded magnets, unless a ferrite magnetic material with a higher coercivity H than necessary is used, there is a problem of irreversible demagnetization (low-temperature demagnetization) occurring due to temperature cycles from low to high temperatures. Such low-temperature demagnetization of ferrite magnetic materials is particularly problematic when the ferrite magnetic material is used as a material for bonded magnets used in outdoor units for air conditioners and motors for automobiles, which are subject to large changes in temperature outdoors. Therefore, 20 mg of ferrite powder for bonded magnets and 10 mg of paraffin are packed into a measurement cell, held at 60°C for 10 minutes, and then cooled to fix the ferrite powder for bonded magnets in the measurement cell. The coercive force of this ferrite powder for bonded magnets is measured at three points, -25°C, 0°C, and 25°C, using a full loop (sweeping speed 200 Oe/sec) in which a magnetic field of up to 5T (10,000 Oe) is applied, and the temperature coefficient of the coercive force Hc of the ferrite powder for bonded magnets calculated from the rate of change in the coercive force Hc is preferably 0.1%/°C or less.

In addition, 90.0 parts by mass of ferrite powder for bonded magnets, 0.8 parts by mass of silane coupling agent, 0.8 parts by mass of lubricant, and 8.4 parts by mass of powdered polyamide resin were charged into a mixer and mixed, and the resulting mixture was kneaded at 230°C to produce kneaded pellets with an average diameter of 2 mm. These kneaded pellets were injection molded in a magnetic field of 9.7 kOe at a temperature of 300°C and a molding pressure of 8.5 N/ ^mm2 to produce cylindrical bonded magnets (the magnetic field was oriented along the central axis of the cylinder) with a diameter of 15 mm and a height of 8 mm. The coercive force iHc, residual magnetization Br, and maximum energy product BH of this bonded magnet were measured. When measured in a measuring magnetic field of 10 kOe, the coercive force iHc is 2200 to 3700 Oe, more preferably 2400 to 3500 Oe, the residual magnetization Br is preferably 2950 G or more, more preferably 2970 G or more, and the _maximum energy product BH _max is preferably 2.15 MGOe or more, more preferably 2.2 to 2.5 MGOe.

Furthermore, the above bonded magnet is cut parallel to the direction of the applied magnetic field, and the particle shape is observed at 2000 times with an electron microscope. The obtained electron microscope photograph is then digitized to determine the particle shape index, and the ratio (long axis length/short axis length) of the long axis length (the maximum distance between two parallel lines when one particle is sandwiched between them (the length of a line segment drawn perpendicular to the two parallel lines)) of particles having a long axis length of 1.0 μm or more to the short axis length (the minimum distance between two parallel lines when one particle is sandwiched between them) is determined (long axis length/short axis length) (aspect ratio) (each particle is assumed to be a plate-like particle, and the volume-average aspect ratio weighted by the volume is calculated as long axis length x long axis length x short axis length). The aspect ratio is preferably 1.55 or less. If the aspect ratio is 1.55 or less, the particles of the ferrite powder for bonded magnets can be easily aligned in the magnetic field direction by magnetic field orientation, making it easier to produce bonded magnets with high particle orientation and high remanence Br and maximum energy product BH _max .

Below, we will explain in detail the examples of the ferrite powder for bonded magnets and the manufacturing method thereof according to the present invention.

[Example 1]
(Production of coarsely ground powder)
Strontium carbonate ( _SrCO3 , specific surface area 5.8 ^m2 /g), lanthanum oxide ( _{La2O3 ,} specific surface area 3.8 ^m2 /g), hematite (as iron oxide) (α _{- Fe2O3} , specific surface area 5.3 ^m2 /g), and _{cobalt oxide (Co3O4 ,} specific surface area 3.3 ^m2 /g) were weighed and mixed to a molar ratio of Sr:La:Fe:Co = 0.70:0.30:0.70:0.30, and this mixture was granulated in a pan pelletizer while adding water. The resulting spherical granules with a diameter of 3 to 10 mm were placed in an internal combustion rotary kiln and fired in the air at 1100°C for 20 minutes (primary firing) to obtain a fired product. The fired product was pulverized in a roller mill to obtain a powder of a composite oxide of iron, strontium, lanthanum, and cobalt. The specific surface area of the composite oxide powder was measured by a BET single point method using a specific surface area measuring device (Monosorb, manufactured by Quantachrome Corporation), and was found to be 3.5 ^m2 /g.

This composite oxide powder and hematite (as iron oxide) (α-Fe ₂ O ₃ , specific surface area 5.3 m ² /g) were weighed and mixed so that the molar ratio in the oxide to the sum of Sr and La (Fe/(Sr+La)) = 10.0. 0.17 mass% boric acid and 2.3 mass% potassium chloride (as additives) were added to this mixture and mixed, after which water was added to granulate. The resulting spherical granules with a diameter of 3 to 10 mm were placed in an internal combustion rotary kiln and fired in air at 1250°C (firing temperature) for 20 minutes (secondary firing). The fired product was then pulverized in a roller mill to obtain a coarsely pulverized powder.

(Manufacturing ferrite powder for bonded magnets)
The obtained coarsely pulverized powder (100 parts by mass) and water (150 parts by mass) were put into a wet attritor and pulverized for 20 minutes to obtain a slurry. The solid matter obtained by filtering the slurry was dried in air at 150°C for 10 hours to obtain a dried cake. The dried cake was pulverized in a mixer, and the pulverized matter obtained was pulverized for 20 minutes at 1800 rpm and 8 mm amplitude using a steel ball having a diameter of 12 mm as a medium in a vibration ball mill (Uras Vibrator KEC-8-YH manufactured by Murakami Seiki Kogyosho Co., Ltd.). The pulverized matter thus obtained was annealed in air at 985°C for 30 minutes in an electric furnace to obtain a ferrite powder for bonded magnets.

A compositional analysis was performed on this ferrite powder for bonded magnets by calculating the amount of each element by the fundamental parameter method (FP method) using an X-ray fluorescence analyzer (ZSX100e manufactured by Rigaku Corporation). In this compositional analysis, the ferrite powder for bonded magnets was packed into a measurement cell, and molded by applying a pressure of 10 tons/ ^cm2 for 20 seconds. The measurement mode was EZ scan mode, the measurement diameter was 30 mm, the sample form was oxide, and the measurement time was standard time. After performing a qualitative analysis in a vacuum atmosphere, a quantitative analysis was performed on the detected constituent elements. As a result, the ferrite powder for bonded magnets contained 0.1 mass% _Cr2O3 , _0.3 mass% MnO, 85.3 mass% _{Fe2O3 ,} 2.4 mass% _Co2O3 , 6.8 mass% SrO, 0.1 mass% BaO, and 4.9 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. Elements such as Cr, _Mn , and _Ba , which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. These trace amounts (1.0 mass% or less in terms of oxides) of elements were regarded as impurities, and the chemical formula of the ferrite powder for bonded magnets was expressed as (Sr1 _{-xLax )} (Fe1 _{-yCoy ) nO19 -z} from the analysis values of the main components Sr, La, Fe, and Co. The values of x, y, n, and z were calculated to be x = 0.32, y = 0.026, n = 11.5, and z = 0.79. Note that z was calculated so that the valence of Sr was +2, the valence of La was +3, the valence of Fe was +3, the valence of Co was +2, and the valence of O was -2, so that the sum of the valences of the chemical formula was 0 (zero).

In addition, this ferrite powder for bonded magnets was measured by powder X-ray diffraction (XRD) using a powder X-ray diffractometer (Miniflex 600 manufactured by Rigaku Corporation) with a tube voltage of 40 kV, a tube current of 15 mA, a measurement range of 15° to 60°, a scan speed of 1°/min, and a scan width of 0.02°. The measurement results are shown in FIG. 1. The peak positions of SrFe ₁₂ O ₁₉ , which has a general M-type ferrite structure, are shown at the bottom of FIG. 1. From FIG. 1, all peaks were observed at the same positions as SrFe ₁₂ O ₁₉ , and it was confirmed that the ferrite powder for bonded magnets of this embodiment has an M-type ferrite structure. This result was similar to that of Examples 2 to 8 and Comparative Examples 1 to 3 described below.

The average particle size (APD) of the ferrite powder for bonded magnets was measured by the air permeation method using a specific surface area measuring device (SS-100 manufactured by Shimadzu Corporation) and was found to be 1.72 μm. The specific surface area of this ferrite powder for bonded magnets was measured by the same method as above and was found to be 1.47 ^m2 /g.

In addition, 10 g of the ferrite powder for bonded magnets was filled into a cylindrical mold with an inner diameter of 2.54 cmφ and then compressed at a pressure of 1 ton/ ^cm2 . The density of the ferrite powder for bonded magnets was measured as the compressed density (CD) of the ferrite powder for bonded magnets, and was found to be 3.45 g/ ^cm3 .

In addition, 8 g of ferrite powder for bonded magnets and 0.4 cc of polyester resin (P-resin manufactured by Nihon Chikagaku Co., Ltd.) were mixed in a mortar, and 7 g of the mixture was filled into a mold having an inner diameter of 15 mmφ, compressed for 60 seconds at a pressure of ² tons/cm2, and the molded product obtained was removed from the mold and dried for 30 minutes at 150°C to obtain a green compact. As the magnetic properties of this green compact, the coercive force iHc and residual magnetization Br of the green compact were measured in a measuring magnetic field of 10 kOe using a BH tracer (TRF-5BH manufactured by Toei Kogyo Co., Ltd.), and the coercive force iHc was 3060 Oe and the residual magnetization Br was 1870 G.

20 mg of ferrite powder for bonded magnets and 10 mg of paraffin were packed into a measurement cell, and the ferrite powder for bonded magnets was fixed in the measurement cell by holding it at 60°C for 10 minutes and then cooling it. Using a vibration sample magnetometer (VSM-5HSC manufactured by Toei Kogyo Co., Ltd.), the coercive force of this ferrite powder for bonded magnets was measured at three points, -25°C, 0°C, and 25°C, using a full loop (sweeping speed 200 Oe/sec) in which a magnetic field of up to 5T (10,000 Oe) was applied. The temperature coefficient of the coercive force Hc was calculated from the rate of change in the coercive force Hc. As a result, the temperature coefficient of the coercive force Hc of this ferrite powder for bonded magnets was -0.024%/°C. This temperature coefficient was calculated as the slope of the relational equation between y and x, where y is the coercive force Hc and x is the temperature, and the relational equation between y and x was found using the least squares method.

(Manufacturing of bonded magnets)
The obtained bonded magnet ferrite powder (90.0 parts by mass), silane coupling agent (Z-6094N manufactured by Toray Dow Corning Co., Ltd.) 0.8 parts by mass, lubricant (VPN-212P manufactured by Henkel Co., Ltd.) 0.8 parts by mass, and powdered polyamide resin (P-1011F manufactured by Ube Industries, Ltd.) 8.4 parts by mass were weighed, filled into a mixer and mixed to obtain a mixture, which was kneaded at 230 ° C. to obtain kneaded pellets with an average diameter of 2 mm. The kneaded pellets were loaded into an injection molding machine (manufactured by Sumitomo Heavy Industries, Ltd.) and injection molded at a temperature of 300 ° C. and a molding pressure of 8.5 N / mm ² in a magnetic field of 9.7 kOe to obtain a bonded magnet (F.C. 90.0 mass%, 9.7 kOe) of a cylindrical shape (the magnetic field orientation direction is along the central axis of the cylinder) with a diameter of 15 mm x height of 8 mm.

As the magnetic properties of this bonded magnet, a BH tracer (TRF-5BH manufactured by Toei Kogyo Co., Ltd.) was used to measure the coercive force iHc, residual magnetization Br and maximum energy product BH _max of the bonded magnet in a measuring magnetic field of 10 kOe. The coercive force iHc was 3017 Oe, the residual magnetization Br was 3069 G, and the maximum energy product BH _max was 2.33 MGOe.

The bonded magnet was cut parallel to the applied magnetic field direction, and the particle shapes were observed at 2000x magnification using a scanning electron microscope (SEM). The SEM photographs were then binarized to obtain the particle shape index. The average value (aspect ratio) of the ratio of the long axis length to the short axis length (the minimum distance between two parallel lines when a particle is sandwiched between two parallel lines) (long axis length/short axis length) was calculated for 200 or more particles in the SEM photograph (200 or more particles with a long axis length (the maximum distance between two parallel lines when a particle is sandwiched between two parallel lines) of 1.0 μm or more, the entire outer edge of which is observed within one or more fields of view in the SEM photograph). The average value was 1.43. Note that the aspect ratio was calculated by assuming each particle to be a plate-like particle, and the volume was weighted by the volume as long axis length x long axis length x short axis length.

[Example 2]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that the pulverization time using the wet attritor was 40 minutes.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 85.1% by mass of _Fe2O3 , 2.5% by mass of _Co2O3 , _0.1 % by mass of _ZnO , 6.7% by mass of SrO, 0.1% by mass of BaO, and 4.9% by mass _{of La2O3} , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn, Zn, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all of them were only a small amount of 0.4% by mass in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analytical values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.32, y = 0.028, n = 11.6, and z = 0.64.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.62 μm, specific surface area 1.62 ^m2 /g, pressed density (CD) 3.40 g/ ^cm3 , coercive force iHc of the green compact 3130 Oe, and residual magnetization Br 1870 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3052 Oe, the residual magnetization Br was 3038 G, the maximum energy product BH _max was 2.28 MGOe, and the aspect ratio was 1.54.

[Example 3]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that the pulverization time using the wet attritor was 80 minutes.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 85.0% by mass of _Fe2O3 , 2.5% by mass of _Co2O3 , _0.1 % by mass of _ZnO , 6.7% by mass of SrO, 0.1% by mass of BaO, and 5.0% by mass _{of La2O3} , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn, Zn, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all of them were only a trace amount of 0.4% by mass in terms of oxide. Considering these trace amounts (1.0 mass% or less in terms of oxide) of elements as impurities and based on the analytical values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.32, y = 0.028, n = 11.5, and z = 0.79.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.42 μm, specific surface area 1.96 ^m2 /g, pressed density (CD) 3.42 g/ ^cm3 , coercive force iHc of the green compact 3310 Oe, and residual magnetization Br 1870 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3193 Oe, the residual magnetization Br was 3036 G, the maximum energy product BH _max was 2.28 MGOe, and the aspect ratio was 1.50.

[Example 4]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that the secondary firing temperature was 1150°C.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 85.0% by mass of _{Fe2O3 ,} 2.6% by mass of _Co2O3 , 6.8% by mass of _SrO , 0.1% by mass of BaO, and 4.9% by mass of _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn, and Ba _, which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.4% by mass in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The calculations of x, y, n, and z give the following: x = 0.31, y = 0.029, n = 11.5, and z = 0.83.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.53 μm, specific surface area 1.65 ^m2 /g, pressed density (CD) 3.29 g/ ^cm3 , coercive force iHc of the green compact 3410 Oe, and residual magnetization Br 1820 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3407 Oe, the residual magnetization Br was 2985 G, the maximum energy product BH _max was 2.21 MGOe, and the aspect ratio was 1.54.

[Example 5]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that a composite oxide powder similar to that in Example 1 and hematite (as iron oxide) (α-Fe ₂ O ₃ , specific surface area 5.3 m ² /g) were weighed and mixed so that the molar ratio of Fe in the iron oxide to the sum of Sr and La (Fe/(Sr+La)) = 10.4.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained _0.4 mass% MnO, 85.7 mass% _Fe2O3 , 2.4 mass% _Co2O3 , 0.1 mass% _ZnO , 6.5 mass% SrO, 0.1 mass% BaO, and 4.7 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Mn, _Zn , and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.4 mass% in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The calculations of x, y, n, and z give the following: x = 0.31, y = 0.026, n = 12.0, z = -0.04.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.70 μm, specific surface area 1.56 ^m2 /g, pressed density (CD) 3.40 g/ ^cm3 , coercive force iHc of the green compact 2780 Oe, and residual magnetization Br 1890 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2546 Oe, the residual magnetization Br was 3009 G, the maximum energy product BH _max was 2.23 MGOe, and the aspect ratio was 1.53.

[Example 6]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that strontium carbonate (SrCO ₃ , specific surface area 5.8 m ² /g), lanthanum oxide (La ₂ O ₃ , specific surface area 3.8 m ² /g), hematite (α-Fe ₂ O ₃ , specific surface area 5.3 m ² /g), and cobalt oxide (Co ₃ O ₄ , specific surface area 3.3 m ² /g) were weighed and mixed in a molar ratio of Sr:La:Fe:Co = 0.70:0.30:0.85:0.15.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1 mass _{% Cr2O3} , 0.3 mass% MnO, 86.4 mass% _Fe2O3 , 1.2 mass% _Co2O3 , 6.7 mass% SrO, 0.1 mass% _BaO , and 5.0 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, _Mn , and _Ba , which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.32, y = 0.014, n = 11.5, and z = 0.73.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same method as in Example 1. The results were that the average particle size was 1.87 μm, the specific surface area was 1.27 m ² /g, the pressed density (CD) was 3.43 g/cm ³ , the coercive force iHc of the green compact was 2650 Oe, and the residual magnetization Br was 1880 G. The coercive force Hc of the ferrite powder for bonded magnets was also measured using the same method as in Example 1, and the temperature coefficient of the coercive force Hc was calculated to be -0.063%/°C.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2724 Oe, the residual magnetization Br was 3038 G, the maximum energy product BH _max was 2.29 MGOe, and the aspect ratio was 1.52.

[Example 7]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 6, except that the secondary firing temperature was 1300°C.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained _0.1 mass% _Cr2O3 , 0.3 mass% MnO, 86.5 mass% _Fe2O3 , 1.2 mass% _Co2O3 , 6.7 mass% SrO, 0.1 mass% _BaO , and 5.0 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, _Mn , and _Ba , which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. Considering these trace amounts (1.0 mass% or less in terms of oxide) of elements as impurities and based on the analytical values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.32, y = 0.013, n = 11.5, and z = 0.61.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.84 μm, specific surface area 1.36 ^m2 /g, pressed density (CD) 3.49 g/ ^cm3 , coercive force iHc of the green compact 2390 Oe, and residual magnetization Br 1910 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2473 Oe, the residual magnetization Br was 3043 G, the maximum energy product BH _max was 2.29 MGOe, and the aspect ratio was 1.49.

[Example 8]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 6, except that the secondary firing temperature was 1200°C.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.3 mass% _MnO , 86.2 mass% _Fe2O3 , 1.3 mass% _Co2O3 , 0.2 mass% _ZnO , 6.8 mass% SrO, 0.1 mass% BaO, and 5.0 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Mn, _Zn , and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The calculations of x, y, n, and z give x = 0.32, y = 0.014, n = 11.4, and z = 0.85.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.71 μm, specific surface area 1.49 ^m2 /g, pressed density (CD) 3.40 g/ ^cm3 , coercive force iHc of the green compact 2870 Oe, and residual magnetization Br 1870 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2865 Oe, the residual magnetization Br was 3061 G, the maximum energy product BH _max was 2.32 MGOe, and the aspect ratio was 1.47.

[Example 9]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 6, except that the composite oxide powder and hematite (as iron oxide) were weighed and mixed so that the molar ratio in the oxide to the sum of Sr and La (Fe/(Sr+La)) = 9.8.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 85.7% by mass of _{Fe2O3 ,} 1.3% by mass of _Co2O3 , 6.9% by mass of _SrO , 0.1% by mass of BaO, and _{5.3% by mass of La2O3 ,} and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Mn, Zn, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.33, y = 0.015, n = 11.0, and z = 1.46.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.90 μm, specific surface area 1.32 ^m2 /g, pressed density (CD) 3.41 g/ ^cm3 , coercive force iHc of the green compact 2840 Oe, and residual magnetization Br 1840 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3038 Oe, the residual magnetization Br was 3004 G, the maximum energy product BH _max was 2.23 MGOe, and the aspect ratio was 1.50.

[Example 10]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 6, except that the composite oxide powder and hematite (as iron oxide) were weighed and mixed so that the molar ratio in the oxide to the sum of Sr and La (Fe/(Sr+La)) was 10.4.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained _0.4 mass% MnO, 86.5 mass% _Fe2O3 , 1.2 mass% _Co2O3 , 0.2 mass% _ZnO , 6.7 mass% SrO, 0.1 mass% BaO, and 4.8 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Mn, _Zn , and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.31, y = 0.014, n = 11.7, and z = 0.41.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.97 μm, specific surface area 1.21 ^m2 /g, pressed density (CD) 3.39 g/ ^cm3 , coercive force iHc of the green compact 2540 Oe, and residual magnetization Br 1870 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2564 Oe, the residual magnetization Br was 3013 G, the maximum energy product BH _max was 2.24 MGOe, and the aspect ratio was 1.51.

[Example 11]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that strontium carbonate (SrCO ₃ , specific surface area 5.8 m ² /g), lanthanum oxide (La ₂ O ₃ , specific surface area 3.8 m ² /g), hematite (α-Fe ₂ O ₃ , specific surface area 5.3 m ² /g), and cobalt oxide (Co ₃ O ₄ , specific surface area 3.3 m ² /g) were weighed and mixed in a molar ratio of Sr:La:Fe:Co = 0.80:0.20:0.80:0.20.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 86.5% by mass of _Fe2O3 , 1.6% by mass of _{Co2O3 ,} 7.7% by mass of _SrO , 0.2% by mass of BaO, and 3.5% by mass of _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn _, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} (Fe1 _{-yCoy ) nO19 -z.} The calculations of x, y, n, and z give x = 0.22, y = 0.018, n = 11.5, and z = 0.67.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same method as in Example 1. The results were that the average particle size was 1.98 μm, the specific surface area was 1.20 m ² /g, the pressed density (CD) was 3.40 g/cm ³ , the coercive force iHc of the green compact was 2600 Oe, and the residual magnetization Br was 1870 G. The coercive force Hc of the ferrite powder for bonded magnets was also measured using the same method as in Example 1, and the temperature coefficient of the coercive force Hc was calculated to be 0.020%/°C.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2748 Oe, the residual magnetization Br was 3031 G, the maximum energy product BH _max was 2.26 MGOe, and the aspect ratio was 1.49.

[Example 12]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that strontium carbonate (SrCO ₃ , specific surface area 5.8 m ² /g), lanthanum oxide (La ₂ O ₃ , specific surface area 3.8 m ² /g), hematite (α-Fe ₂ O ₃ , specific surface area 5.3 m ² /g), and cobalt oxide (Co ₃ O ₄ , specific surface area 3.3 m ² /g) were weighed and mixed in a molar ratio of Sr:La:Fe:Co = 0.80:0.20:0.90:0.10.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 87.2% by mass of _Fe2O3 , 0.9% by mass of _{Co2O3 ,} 7.9% by mass of _SrO , 0.2% by mass of BaO, and 3.2% by mass _{of La2O3} , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace amounts (1.0 mass% or less in terms of oxide) of elements as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.20, y = 0.010, n = 11.5, and z = 0.74.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same method as in Example 1. The results were that the average particle size was 2.11 μm, the specific surface area was 1.11 m ² /g, the pressed density (CD) was 3.45 g/cm ³ , the coercive force iHc of the green compact was 2340 Oe, and the residual magnetization Br was 1890 G. The coercive force Hc of the ferrite powder for bonded magnets was also measured using the same method as in Example 1, and the temperature coefficient of the coercive force Hc was calculated to be 0.052%/°C.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2371 Oe, the residual magnetization Br was 3064 G, the maximum energy product BH _max was 2.31 MGOe, and the aspect ratio was 1.43.

[Example 13]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that strontium carbonate (SrCO ₃ , specific surface area 5.8 m ² /g), lanthanum oxide (La ₂ O ₃ , specific surface area 3.8 m ² /g), hematite (α-Fe ₂ O ₃ , specific surface area 5.3 m ² /g), and cobalt oxide (Co ₃ O ₄ , specific surface area 3.3 m ² /g) were weighed and mixed in a molar ratio of Sr:La:Fe:Co = 0.90:0.10:0.90:0.10.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 87.6% by mass of _Fe2O3 , 0.9% by mass of _Co2O3 , _8.3 % by mass of _SrO , 0.2% by mass of BaO, and 2.4% by mass _{of La2O3} , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace amounts (1.0 mass% or less in terms of oxide) of elements as impurities and based on the analytical values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.15, y = 0.010, n = 11.6, and z = 0.53.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same method as in Example 1. The results were that the average particle size was 2.01 μm, the specific surface area was 1.09 ^m2 /g, the pressed density (CD) was 3.39 g/ ^cm3 , the coercive force iHc of the green compact was 2290 Oe, and the residual magnetization Br was 1920 G. The coercive force Hc of the ferrite powder for bonded magnets was also measured using the same method as in Example 1, and the temperature coefficient of the coercive force Hc was calculated to be 0.056%/°C.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2414 Oe, the residual magnetization Br was 3038 G, the maximum energy product BH _max was 2.27 MGOe, and the aspect ratio was 1.44.

[Example 14]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 6, except that the temperature of the primary sintering was 1200°C.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 86.2% by mass of _Fe2O3 , 1.3% by mass of _{Co2O3 ,} 6.8% by mass of _SrO , 0.1% by mass of BaO, and 5.0% by mass of _La2O3 , and the main components of the _ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.32, y = 0.014, n = 11.3, and z = 0.90.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.62 μm, specific surface area 1.53 ^m2 /g, pressed density (CD) 3.40 g/ ^cm3 , coercive force iHc of the green compact 3010 Oe, and residual magnetization Br 1570 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3298 Oe, the residual magnetization Br was 2999 G, the maximum energy product BH _max was 2.24 MGOe, and the aspect ratio was 1.49.

[Example 15]
Ferrite powder for bonded magnets was obtained in the same manner as in Example 6, except that the temperature of the primary sintering was 1050°C.

_The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1% by mass of _Cr2O3 , 0.4% by mass of MnO, 86.4% by mass of _Fe2O3 , 1.3% by mass of _Co2O3 , 6.5% by mass of _{SrO ,} 0.1% by mass of BaO, and 5.1% by mass of _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr, Mn _, and Ba, which are thought to be derived from impurities in the raw materials, were also detected, but all were in trace amounts. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.34, y = 0.014, n = 11.7, and z = 0.40.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.88 μm, specific surface area 1.21 ^m2 /g, pressed density (CD) 3.41 g/ ^cm3 , coercive force iHc of the green compact 2950 Oe, and residual magnetization Br 1850 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2880 Oe, the residual magnetization Br was 2998 G, the maximum energy product BH _max was 2.23 MGOe, and the aspect ratio was 1.53.

[Comparative Example 1]
Strontium carbonate ( _SrCO3 , specific _{surface area 5.8 m2/g), lanthanum oxide (La2O3} ^, _specific surface area 3.8 ^m2 /g), _{hematite (α-Fe2O3 ,} specific surface area 5.3 ^m2 /g) and cobalt _{oxide (Co3O4 ,} specific surface area 3.3 ^m2 /g) were weighed and mixed to a molar ratio of Sr:La:Fe:Co = 0.70:0.30:11.70:0.30, and ferrite powder for bonded magnets was obtained in the same manner as in Example 1, except that the primary firing temperature was changed from 1100°C to 1250°C and no secondary firing was performed.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1 mass% _Cr2O3 , _0.3 mass% MnO, 85.3 mass% _Fe2O3 , 2.4 mass% _Co2O3 , 7.0 mass% _SrO , and 4.9 mass% _La2O3 , and _the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr and _Mn , which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The calculations of x, y, n, and z give the following: x = 0.31, y = 0.026, n = 11.3, and z = 1.12.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.26 μm, specific surface area 2.19 ^m2 /g, pressed density (CD) 3.34 g/ ^cm3 , coercive force iHc of the green compact 3590 Oe, and residual magnetization Br 1830 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3332 Oe, the residual magnetization Br was 2882 G, the maximum energy product BH _max was 2.04 MGOe, and the aspect ratio was 1.56.

[Comparative Example 2]
Ferrite powder for bonded magnets was obtained in the same manner as in Comparative Example 1, except that the temperature of the primary sintering was 1200°C.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1 mass% _Cr2O3 , _0.3 mass% MnO, 85.3 mass% _Fe2O3 , 2.4 mass% _Co2O3 , 7.1 mass% _SrO , and 4.7 mass% _La2O3 , and _the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co, were detected. In addition, elements such as Cr and _Mn , which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. Considering these trace amounts (1.0 mass% or less in terms of oxide) of elements as impurities and based on the analysis values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The values of x, y, n, and z are calculated to be x = 0.30, y = 0.026, n = 11.3, and z = 1.09.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.25 μm, specific surface area 2.21 ^m2 /g, pressed density (CD) 3.26 g/ ^cm3 , coercive force iHc of the green compact 3950 Oe, and residual magnetization Br 1790 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 3609 Oe, the residual magnetization Br was 2867 G, the maximum energy product BH _max was 2.02 MGOe, and the aspect ratio was 1.62.

[Comparative Example 3]
Ferrite powder for bonded magnets was obtained in the same manner as in Comparative Example 1, except that the temperature of the primary sintering was 1300°C.

The composition of this ferrite powder for bonded magnets was analyzed by the same method as in Example 1. As a result, the ferrite powder for bonded magnets contained 0.1 mass% _Cr2O3 , 0.3 mass% MnO, 85.4 mass% _Fe2O3 , 2.4 mass% _Co2O3 , 7.0 mass% _SrO , and 4.7 mass% _La2O3 , and the main components of the ferrite powder for bonded magnets, Sr, La, Fe, and Co _, were detected. In addition, elements such as Cr _{and Mn} , which are thought to be derived from impurities in the raw materials, were also detected, but each was a trace amount of 0.3 mass% in terms of oxide. Considering these trace elements (1.0 mass% or less in terms of oxide) as impurities and based on the analytical values of the main components Sr, La, Fe, and Co, the chemical formula of the ferrite powder for bonded magnets is expressed as (Sr1 _{-xLax )} ·(Fe1 _{-yCoy ) nO19 -z.} The calculations of x, y, n, and z give x = 0.30, y = 0.026, n = 11.4, and z = 0.95.

The average particle size, specific surface area, pressed density (CD), coercive force iHc of the green compact, and residual magnetization Br of this ferrite powder for bonded magnets were measured using the same methods as in Example 1. The results were: average particle size 1.22 μm, specific surface area 2.41 ^m2 /g, pressed density (CD) 3.42 g/ ^cm3 , coercive force iHc of the green compact 3140 Oe, and residual magnetization Br 1800 G.

Furthermore, this ferrite powder for bonded magnets was used to obtain a bonded magnet in the same manner as in Example 1. For this bonded magnet, the coercive force iHc, residual magnetization Br, and maximum energy product BH _max were measured and the aspect ratio was calculated in the same manner as in Example 1. The coercive force iHc was 2885 Oe, the residual magnetization Br was 2930 G, the maximum energy product BH _max was 2.11 MGOe, and the aspect ratio was 1.58.

The results of these examples and comparative examples are shown in Tables 1 to 4. Scanning electron microscope (SEM) photographs of the cross sections of the bonded magnets obtained in Example 1 and Comparative Example 1 are shown in Figures 2 and 3, respectively.

The results of Examples 1 to 15 and Comparative Examples 1 to 3 show that in Examples 1 to 15, it is possible to produce ferrite powder for bonded magnets, which allows the production of bonded magnets with high remanent magnetization Br through magnetic field orientation.

Furthermore, the ferrite powders for bonded magnets of Examples 1, 6, and 11 to 13 have an extremely low temperature coefficient of coercivity Hc of 0.1%/°C or less, and are therefore ferrite powders for bonded magnets that are not easily affected by low-temperature demagnetization. In particular, the ferrite powders for bonded magnets of Examples 1 and 6 have a negative temperature coefficient of coercivity Hc, and are therefore excellent ferrite powders for bonded magnets that are extremely resistant to the effects of low-temperature demagnetization.

Claims (11)

A method for producing ferrite powder for bonded magnets, comprising mixing a powder of a composite oxide of iron, strontium, lanthanum, and cobalt with iron oxide so that the molar ratio of Fe in the iron oxide to the total of Sr and La, Fe/(Sr+La), is 9.8 to 10.4, granulating the mixture, and then sintering the mixture. The method for producing ferrite powder for bonded magnets according to claim 1, characterized in that the sintered product obtained by the sintering is coarsely pulverized to obtain a coarsely pulverized powder, which is then pulverized and annealed. The method for producing ferrite powder for bonded magnets according to claim 1 or 2, characterized in that the composite oxide powder is obtained by mixing and granulating strontium carbonate, lanthanum oxide, iron oxide and cobalt oxide, and then sintering the mixture at 1000 to 1250°C to obtain a sintered product, and grinding the sintered product. A method for producing ferrite powder for bonded magnets according to any one of claims 1 to 3, characterized in that the composite oxide powder and the iron oxide are mixed and granulated, and then sintered at 1100 to 1400°C. A ferrite powder for bonded magnets, characterized in that it has a composition of (Sr1 _{- xLax} )·(Fe1 _{-yCoy ) nO19 -z} (where 0.15≦x≦0.34, 0.010≦y≦0.029, 11.0≦n≦12.0, -0.04≦z≦1.46) and has an average particle size of 1.42 to 2.11 μm. 6. The ferrite powder for bonded magnets according to claim 5 , wherein the specific surface area of the ferrite powder for bonded magnets is 1.09 to 1.96 m ² /g. The ferrite powder for bonded magnets according to claim 5 or 6 , characterized in that the average ratio of the major axis length to the minor axis length (major axis length/minor axis length) of particles of the ferrite powder for bonded magnets having a major axis length of 1.0 μm or more is 1.55 or less. The ferrite powder for bonded magnets according to any one of claims 5 to 7, characterized in that 90.0 parts by mass of the ferrite powder for bonded magnets, 0.8 parts by mass of a silane coupling agent, 0.8 parts by mass of a lubricant, and 8.4 parts by mass of a powdered polyamide resin are filled into a mixer and mixed to obtain a mixture, which is kneaded at 230°C to produce a kneaded pellet with an average diameter of 2 mm, and this kneaded pellet is injection molded in a magnetic field of 9.7 kOe at a temperature of 300°C and a molding pressure of 8.5 N/ ^mm2 to produce a cylindrical bonded magnet (the magnetic field is oriented along the central axis of the cylinder) with a diameter of 15 mm and a height of 8 mm, and the residual magnetization Br of this bonded magnet is measured in a measuring magnetic field of 10 kOe, and the residual magnetization Br is 2950 G or more. The ferrite powder for bonded magnets according to claim 8 , characterized in that the maximum energy product BH _max of the bonded magnet _is 2.15 MGOe or more when measured in a magnetic field of 10 kOe. 10. The ferrite powder for bonded magnets according to claim 5 , wherein the temperature coefficient of coercivity Hc of said ferrite powder for bonded magnets is 0.1%/[deg.] C. or less. A bonded magnet comprising the ferrite powder for bonded magnets according to any one of claims 5 to 10 and a binder.

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