NanoStnrchnedM ataisls, Vol. 5. No. 3. pp. 281~287,1995 copyright (B 1995 Fkviez Science Ltd Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Printed hrthe USA. All rights -ed 0965-9773/95 $9.50 + .cm 09659773(95)00248-O A STM STUDY OF THE MICROSTRUCTURE OF AMORPHOUS AND NANOCRYSTALLINE Fe-Zr-B-Cu RIBBONS J. Nogu&, K-V. Rao#, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF A. Inoue*, and K. Suzuki*+ # Department of Condensed Matter Physics Royal Institute of Technology, 10044 Stockholm, Sweden * Institute for Materials Research, Tohoku University, Sendai 980, Japan t Alps Electrical Co., Ltd., Nagaoka 940, Japan zyxwvutsrqponmlkjihgfedcbaZ (Accepted February 1995) Abstract Fes&r# &4 The surface microstructures of amorphous and nanocry stallized melt- spun ribbons have been studied by means of scanning tunneling microscopy . The On theotherhand, thenanocty stallized a~rp~~sample~sas~oth,al~stfeaturelesss~ace. sample consists of a homogeneous distribution of ellipsoidal clusters (= 300 nmxl50 along the casting direction and composed of randomly oriented nanocry stallites nm) aligned ofabout 10 nm in diameter consistent with transmission electron microscopic observations. The nanocry stallites themselves have a smooth morphology , rather than a grainy surface. The smaller size of the nanocry stallites in comparison with the ferromagnetic exchange length (about 50 nm) gives rise to the improved soft magnetic properties in these nanocry stallized ribbons. INTRODUCTION In this communication we present a study of the surface microstructure of amorphous and magnetically soft Fes&@BeCu melt-spun ribbons. The role of the local microstructure on the improvement of the magnetic properties of the nanocrystallized ribbons has been investigated. Fe-rich amorphous alloys exhibit soft magnetic properties which are at least an order of magnitude better than what is observed for their crystalline analogue, mainly due to the absence of crystalline anisotropy. However, in FeBSi-base amorphous alloys, incorporation of finely dispersed submicrometer precipitates of magnetic phases improves their soft magnetic properties even further (1). Such alloys are therefore considered as possible new materials for application in future distribution power transformers. On controlled high temperature annealing, the precipitates of nanocrystallites of metastable bee-phases in an otherwise amorphous Fe-Si-B-NbCu matrix drastically improves the soft magnetic properties. In addition, there is an increase in the saturation magnetization (2-5) over that of the amorphous precursor. These new class of nanocrystalline materials are thus of considerable scientific and technological interest. zyxwvutsrqponmlkj nanocry stiline 281 282 J NOGULS,K’/ ho, A INOUE AND K SUZUKI Pursuit of other possible nanocrystalline materials with compositions consisting of fewer elements has recently produced a new amorphous FeggZr&Cu precursor, which upon controlled annealing below the crystallization temperature gives an amorphous matrix in which nanometersized metastable phases of bee-Fe based particles are densely embedded (45). This process results in a nanocrystalline soft magnetic material with significantly improved properties, such as increased effective permeability increasing by one order of magnitude, especially at high frequencies, and doubling of the saturation magnetization (5). These materials have thus come to be of considerable interest and are being investigated extensively both magnetically (B,, l.@, HJ and structurally (X-rayandTBM) (5). In order to understand the intrinsic mechanisms responsible for these enhanced properties, it is important to study the microstructure on a nanometer scale. In this paper we present such a study using Scanning Tunneling Microscopy (STM). STM, a non-destructive image technique that provides three-dimensional surface topographies over a length scale ranging from subnanometer to micrometer, has proved to be one of the most versatile techniques to study various surface properties of metallic as well as semiconducting materials directly in real space with atomic resolution even at ambient conditions. Thus, surface probe techniques like STM have been used to study materials with or without limited long range order (6-8). STM has already been used to study the atomic scale morphology of both melt-spun andsplat-quenchedamotphousmetallicalloys(8-18). Amorphoussamplesoftenappear(8,11,13,16) flat zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA or smooth zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA withoccasional hillyorwavystructures (8,10,14-16,18), zyxwvutsrqponmlkjihgfedcbaZY usuallyalong thedirection of the casting of the ribbon ( 11). These features are most likely due to periodic surface waves frozen during rapid quenching (16). When some ordered regions are observed in these amorphous alloys (8-10,12,14), they are tentatively ascribed to the possible existence of nanocrystals in an otherwise amorphous as-quenched sample (18). In investigations on crystallization of amorphous metals by STM (14,15,17) thepresenceof clear grain-like structuresof several tens of ananometer are found after annealing (14,17). Clearly, STM should be an ideal probe to investigate the particle size, the homogeneity of their distributions and relationship of the interparticle distances in order to gain a nanoscopic insight into the causes of the observed excellent soft magnetic properties of these new tailored materials. This is the objective of the work presented in this paper. EXPERIMENTAL The alloy ingot was prepared by arc melting in an argon atmosphere. The amorphous ribbon was produced by a single-roller melt-spinning method again in an argon atmosphere. The ribbon obtained was confiied to be amorphous from X-ray, SEM, and TEM observations and had a cross-section of approximatelyO.02 mm x 12.5 mm. The as-quenched sample was annealed at 873 K (600°C) for 3600 s (1 h) in a vacuum-sealed quartz tube, followed by water quenching. The structures of both the as-quenched and annealed samples characterized by X-ray diffraction and TBM have already been reported (4,5). Our STM has a digitally controlled tubular piezoelcctric scanner. The STM images, consisting of 400 line scans per frame with 400 pixels per line scan, are taken with a PtIr tip at a scan rate of 1 line/s, on the free surface of the ribbon, that is the surface not in contact with the wheel during casting. The tunneling currents were typically in the range 0.1 to 1.0 nA, while the tunnelling voltages were usually between 500 and 1500 mV for the amorphous sample, and between 200 and 800 mV for the annealed sample. The data were taken in the height mode, which AMORPHOUS ANDNANOCRYSTALUNE Fe-Zr-B-Cu RMONS 283 Figure 1. Top-view STM image of the as-quenched, amorphous Feg&r&Cu, obtained at It = 0.50 nA and Vt = 800 mV. shows the motion of the piezoelectric tube scanner required to maintain a constant tunneling current. Samples consisted of small pieces of ribbon, each about 10 mm x 10 mm area attached to copper substrates clipped to the STM stage. RESULTS AND DISCUSSION In our STM studies the morphology of the asquenched samples is found to be very smooth and similar to what is well known for metallic glasses (8,11,13,16), with occasional hilly or wavy structures of various length scales, as seen in Figure 1, usually directed along the casting direction. Such smooth features have been tentatively ascribed to the surface waves frozen during casting (16). The topography of the nanocrystallized sample, obtained by annealing, is remarkably homogeneous (Figure 2). We obtain similar STM topography on various locations of three specimens from the same batch. The first striking feature is the uniformity of the structure. Figure 2 shows that the sample consists of roughly elliptical bumps of about 300 nm x 150 284 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J N oc uEs, K V RAO, A I N OU E AN D K SU Z U K I Figure 2. Top-view STM image of the annealed, nanocrystalline FessZnB&u, obtained at 1, = 0.65 nA andVt = 200 mV. nm uniformly distributed along the casting direction. The origin of this orientation is still unclear. Presumably this is related to the release of surface stress and to surface tension at the annealing temperatures. Similar bumpy structures, but smaller, have been observed on laser irradiated splatquenchedNb&&o alloy and remains unexplained (18). On focussing into the bumps (Figure 3), they are found to be clusters of small nanocrystallites of about 10 nm. These crystallites have a smooth profile, contrary to what is found after crystallization of other normal metallic glasses (14), so the grain boundaries probably have an amorphous layer, which acts as a barrier against the growth of the bee-phase (5) (20 - 30% of amorphous phase is thought to remain in the annealed samples (4)). Three factors which could limit the growth of the bee-phase (2,4,5) have been suggested: (i) moderate annealing times and temperatures which do not favor the transformation of the amorphous phase into a crystalline bee-phase; (ii) presence of the remaining amorphous layer which prevents the already existing nanocrystallites from coalescing; and (iii) compositional fluctuations, due to the presence of immiscible Cu, which inhibit the formation of bee-phase energetically. Of course, it is impossible to distinguish the above factors from STM studies alone. AMORPHOUS AND NAN~CRYSTALLINE Fe-Zr-B-Cu k 20 I 285 RIBBONS zyxwvutsrqponmlkjihgfedcbaZYXW nm *- ~, t Figure 3. Top-view STM image of the annealed, nanocrystalline Fes&?k7&jCU(focusing into an individual cluster), obtained at It = 0.46 nA and Vr = 350 mV. From a more detailed investigation, we find the nanocrystallites to be between 8 nm and12 nm in diameter and distributed uniformly all over the surface of the sample. This size is in very good agreement with the estimates from previous zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK TBM results (5) on the same sample. From these results together with the X-ray studies (5,) which also show a crystallite size of about 10 nm, we can deduce that the crystallites have roughly the same size along the x, y, and z directions (x and y from STM and TEM, and z from X-ray diffraction), and are thus uniformly distributed not only on the surface but in the whole volume of the sample. Such homogeneous distribution is probably due to the immiscibility of Cu in the Fe-based amorphous system, which may ease nucleation, and increase the number of nucleation sites (4). Incidentally, an estimate of the density of Cu atoms is roughly of the same order of magnitude as the density of nanocrystals in the sample. One simple way to understand the origin of the unique soft magnetic properties in nanocrystalline materials is tocompare theaverage separation distancebetween the nanocrystallites with the magnetic exchange length, as discussed by Herzer (19). In the regime where the grain size is smaller than the domain wall width the exchange interaction is expected to impede the magnetization to follow the local magnetic easy axis of the randomly oriented grains. A 286 J Nootis, KV FJAo,A zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK I N OU E AN D K SU Z U K I consequence of this is that the effective magnetic anisotropy averaged over several grains is significantly reduced. This suppression of the local magneto-crystalline anisotropy appears to be the basic mechanism responsible for the soft magnetic behavior of nanocrystalline ferromagnets (19,20). We estimate the exchange length, Lx, to be about 50 nm, assuming that the exchange stiffness constant does not change much from that of crystalline iron (21), using the relationship L,, = (7r2A/2K)lR, where A is the exchange stiffness constant, and K the anisotropy constant. CONCLUSIONS The microstructure of as-quenched and nanocrystallized ribbons Of FegeZt&CU has been studied using STM. The as-cast amorphous ribbon is smooth and flat over large areas. In contrast, in the annealed nanocrystallized samples, roughly elliptical clusters (- 150 nm x 300 nm) oriented along the direction of casting the ribbon are found. These clusters consist of small crystahites ranging in size from 8 to 12 run, consistent with previous TEM studies. The size of these crystallites is probably limited because of the low annealing temperatures and the presence of an amorphous layer at the grain boundaries. The nanocrystallites are homogeneously distributed throughout the volume of the ribbon mainly due to the immiscibility of Cu in the Fe-base alloy; this immiscibility which may ease nucleation and increase the number of nucleation sites. The improvement of the soft magnetic properties can be explained by the fact that the grains are smaller than the exchange length. In the present study the grain size is found to be about = 10 nm and the exchange length is = 50 nm. ACKNOWLEDGEMENTS (JN) thanks the Dr. John Moreland in whose laboratory much of the STM work was carried out, and also the members of NIST for their hospitality during his stay in Boulder. Valuable discussions with John Moreland and V. Madurga are gratefully acknowledged. This research has been supported by the Swedish funding agencies TFR, and NUTEK. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. R. Hasegawa, G.E. Fish and V.R.V. Rarnmanan, Proc 4thint. Conf. on Rapidly QuenchedMetals, eds. T. Masumoto and K. Suzuki, Japan Inst. 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