GB859937A - Improvements in ferromagnetic materials and processes for their preparation - Google Patents

Abstract

A ferromagnetic material comprises tetragonal crystals of chromium oxide containing as an integral part a modifier consisting of one or more of the elements Ru, Sn, Sb, Ti, V, Mn, Fe, Co, Ni and F. The material comprises 42.5 to 61.9% by weight of chromium and 0.008 to 25.0% of the modifier. Further modification can arise by the additional inclusion of elements from the group N, P, As, Sb, Bi, V, Ta, Pa, Cd, Li, Na, K, Cs, H, Mg, Zn and Pd. The different modifiers provide a diversity of properties, e.g. coercivities may be above 400 oesteds and Curie temperatures between 80 DEG C. and 170 DEG C. With Ru, Sb and Sn acicular particles 2 m in length and having an axial ratio from 4:1 to 35:1 are obtained and exhibit high coercivity. Low coercivity is obtained with Ni and Co, the particles having irregular shape and an axial ratio from 1:1 to 6:1. Fluorine, which replaces oxygen in the crystal structure provides irregular particles less than a micron across. The material is prepared by heating CrO3 with the modifying elements or compounds thereof at 225 DEG C.-500 DEG C. and pressures of 1-3000 atmospheres. Amongst suitable modifier compounds specified are oxides, acids bases, salts, sulphides, sulphates, halides, carbonates, and nitrates. Fluorine may be introduced as HF or metal fluoride. The reaction which may last from 10 minutes to 3 hours may be carried out in water steam or a molten salt such as lithium nitrate. Oxygen is evolved and the product corresponds to a modified chromium dioxide. The processing equipment is of platinum or Ni-Cr-Fe alloy to be resistant to O2. The product obtained after heating is generally filtered, washed and dried. The materials are suitable for recording media, permanent magnets, transformer cores and memory devices. In one example CrO3, Sb2O3 and water are hermetically sealed in a platinum tube with flexible walls which is subjected to a pressure of helium of 700 atmospheres and heated for 3 hours at 400 DEG C. The antimony modified chromium oxide produced is boiled in water and dried. For a recording tape this material is dispersed in a toluene/butyl alcohol mixture containing acetyldimethylamine, milled, added to a solution of polyvinyl butyral, remilled, filtered, degassed, coated on to a 1 mil polyethylene terephthalate tape either in or in the absence of a magnetic field, and dried to give a magnetic coating of 0.65 mils. Specification 824,323 is referred to. p