COERCIVE FORCE IN THE SYSTEM OF FERROMAGNETIC GRANULES FOR HALF METAL CrO2 WITH PERCOLATION CONDUCTIVITY

Magnetic and magnetoresistive properties of several samples of compacted powders of ferromagnetic half-metal CrO2, consisting of needle-shaped or spherical nanoparticles coated with thin dielectric shells, were investigated in wide temperature range. The temperature dependence of the coercive force Hc(T) is compared with the temperature dependence of the field of maximum of positive tunneling magnetoresistance Hp(T). The dependence of Hp(T) was nonmonotonic one. It is found that in the low-temperature range (4.2 ÷ 70 K) the ratio Hp ≈ Hc, expected for compacted ferromagnetic powders with particles of submicron sizes, does not fulfilled. It is assumed that the possible reason of the difference between Hp and Hc is the mismatch between the orientation of the global magnetization of the entire sample and the orientations of the magnetic moments in some part of granules that form the optimal conducting channels at low temperatures. Such a mismatch may be due to the multidomain granules are more prone to the formation of optimal conducting chains in the transport channels. That leads to a change in the mechanism of magnetization reversal in these channels and to violation of the ratio Hp ≈ Hc.