GB655666A - Improvements in or relating to magnetic materials - Google Patents

Abstract

A magnetic material suitable for cores used in communication apparatus contains a cubic ferrite consisting of a substantially homogeneous cubic mixed crystal phase having a Curie-point between 40 DEG and 250 DEG C., a tan d /m value of less than 0.0001 at a frequency of 100 kc./sec., and having a ratio of manganese, zinc and iron compounds expressed in terms of the relative oxides of 15-35 mol. per cent, MnO; 15-30 mol. per cent ZnO; 45-60 mol. per cent Fe2O3. In one method of making the ferrite, a mixture of zinc oxide, manganese dioxide and iron oxide are ground for twelve hours in a ball mill, moulded into a ring with water as a plastifying agent and binder at 4000 kg./cm.2, sintered for two hours at 1300 DEG C. in oxygen and cooled in oxygen to 200 DEG C. in three hours. The proportions of starting oxides may vary, the iron oxide being preferably at least 52 per cent of the total. The oxides may be precipitated from a solution containing the three metals, or carbonates or other compounds forming oxides upon heating may be used, being compressed and sintered until they become ferrite. The sintered product may be ground and re-sintered, and the cooling rate should be about 5 DEG C. per minute. For ferrites produced by various slightly different methods values are given of tan d /m , the Curie-point, the initial permeability, and the hysteresis factor Rh/Z. Ferrites with a reluctance having a negative temperature coefficient of 1/m may be obtained, and when combined with a ferrite having a positive coefficient, a material with a substantially zero coefficient may be obtained. Specifications 626,946 and 631,886 are referred to.