Experimental investigations of the relation between electrical conductivity and natural magnetic anisotropy in ferrites containing cobalt

Abstract

A deep relation between the crystalline structure, the nature and energy of the interatomic interaction, and the different physicochemical properties is exhibited with special clarity in ferrites. Therefore a comprehensive discussion of the physical properties of ferrites with the peculiarities of magnetic transitions in ferrites taken into account, along with an investigation of the structure, is of great significance. In this connection, we posed the problem of investigating the effect of magnetic anisotropy on the electrical properties of ferrites containing various amounts of cobalt ions, which make, as is well known, an anomalously large contribution to the magnetic anisotropy constant of ferrite-spinels and lead to a change in sign. Investigations of the electrical properties of ferrites have shown that a change in the activation energy of the electrical conductivity [1] and an anomaly in the temperature behavior of the spontaneous Hall field [2] are observed upon the transition of ferritesfrom a ferri- to a paramagnetic state. It is natural to assume that these anomalous changes are related to the disappearance of magnetic order inside a ferromagnetic semiconductor, which indicates a relation of the conductivity electrons to the magnetic structure. However, the magnetic order in crystals of ferro- and ferrimagnets can be altered in other ways besides a transition through the Curie point. The magnetic structure can be altered in a definite way also upon a change in the sign of the constant of natural magnetocrystalline anisotropyň ₁ (i.e., when the direction of weak magnetization is changed in a crystal). Investigations of the effects of these kinds of changes in the magnetic structure on the motion of current carriers in ferrites are of great interest. It is important in this connection to carry out investigations in the temperature region in which the investigated object is in the ferrimagnetic state, i.e., the spontaneousmagnetization is not destroyed, and its direction in the crystal was altered only as a result of a change in the sign ofň ₁. It is completely permissible to assume that there occurs in the ferrite a change in the intracrystalline field, which affects in a definite way the wave functions of the electrons participating in the conductivity. Thus, even these simple qualitative discussions indicate that such investigations can give valuable information on the effect of a magnetic subsystem on kinetic phenomena in magnetic semiconductors — ferrites. Our investigations of the effect of a magnetic subsystem on the transport phenomenon differ from the experiments of a number of authors [3, 4, 5] in that these authors have investigated the temperature dependences of electrical resistance in the temperature range in which the sign of the magnetic anisotropy constant changes, i.e., the direction of the magnetization vector is altered not due to the effect of external magnetic fields, but as a result of a change in the anisotropic properties upon being heated up.