Magnetic Anisotropy in Ferrimagnetie Crystals

Abstract

Absolute values of K1, the first-order anisotropy constant, have been determined at different temperatures from torque measurements on single crystals of the following compositions: MnxFe3−xO4, where 0.7 ≤ x ≤ 1.0; Cox Mn1−xFe2O4, where 0 ≤ x ≤ 0.25; Co0.04Mnx−0.04Fe3−xO4, where 0.70 ≤ x ≤ 0.91; GaxFe3−xO4, where 0 ≤ x ≤ 0.8; Sm3Fe5O12Gd3Fe5O12. The magnetocrystalline anisotropy of manganese ferrous ferrite was found to vary considerably with the concentration of ferrous ions (Fe2+), some compositions even possessing a positive value of K1 at room temperature, as previously found by Penoyer (1959). In these cases, K1 becomes negative at low temperatures in contrast with the behavior of cobalt-substituted crystals which are found to have rapidly increasing positive values of K1 as the temperature is reduced. The anisotropy contribution of cobalt ions(Co2+) substituted in manganese ferrous ferrite varies with crystals of different compositions. The experimental values of K1 are compared wherever possible with present theories of magnetic anisotropy. In particular, the recent theory proposed by Slonczewski (1958) to explain the anisotropy of cobalt ions substituted in magnetite (Fe3O4) is found to be unsuccessful in the case of Co2+ ions in manganese ferrite (MnFe2O4). Measurements at low temperatures on the gallium-substituted magnetite crystals reveal an additional interesting effect. For certain crystals, the torque curves at 80°K exhibit considerable rotational hysteresis in fields above 10000 oe, when the material should certainly be saturated. This hysteresis is shown to be related to the ordering of the ferrous (Fe2+) and ferric (Fe3+) ionswhich takes place on the octahedral sites at low temperatures.