THE EFFECT OF OXIDATION ON THE VERWEY TRANSITION IN MAGNETITE

At the Verwey transition (T(V) almost-equal-to 110-120 K), magnetite t ransforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change marke dly at T(V) in the case of coarse grains but only slightly or inapprec iably in the case of fine (<1 mum) grains. We find on the contrary tha t remanence changes at T(V) by 50-80% in both large and small crystals , if they are stoichiometric. However, minor surface oxidation suppres ses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 mum whi ch were initially non-stoichiometric (oxidation parameter z from 0.2-0 .7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the ox idized surface of each crystal was reduced to stoichiometric magnetite , the SIRM decreased sharply during warming by 50-80% around 110 K. Th e change in SIRM for the 0.22 gm grains was almost identical to that m easured for a 1.5 mm natural magnetite crystal. Thus a 10(12) change i n particle volume does not materially affect the remanence transition at T(V) but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on T(V) provides a sensitive tes t for maghemitization in soils, sediments and rocks.