Baric dependencies P≤7 GPa, magnetic field H≤5kOe and dependencies of specific resistivity ρ, Hall coefficient R_H dependencies, dependencies of cross magnetic resistance Δρ_xx/ρ and dependencies of relative susceptibility χ/χ₀ in high temperature ferromagnetic semiconductor Cd_0.82Mn₁₈GeAs₂ have been studied. There were taken several methods for measuring: baric dependencies were measured at rise and fall of hydrostatic pressure using method [1,2], magnetic susceptibility  using frequency method [3], magnetic field dependence using standard method. Structural reversible phase transitions semiconductor–metal at P =5.2 GPa were found on baric dependence ρ(P) and R_H(P). One may outline four regions on curve R_H(P): P≤0.6 GPa – region of impurity conductivity, P= 0.6 – 1.9 GPa – region of exhaustion, P=1.9 – 4.5 GPa – region of fall of R_H (reasons are still in discuss) and P=4.5 – 6.5 – region of phase transition. In the region of phase transition P > 5 GPa, curve comes out for saturation, where electric conductivity σ = 3000 Ω^-1 cm^-1, and concentration of charge carriers 5×10²⁰cm^-3 testifies that there takes place semiconductor–metal transition. Magnetic phase transition ferromagnetic–antiferromagnetic was found for the first time on baric dependence of relative magnetic susceptibility both at rise and fall of pressure. Temperature dependence of phase transition ferromagnetic–antiferromagnetic is measured from temperature, phase transition ferromagnetic–antuferromagnetic with increase of temperature T=310–334 K moves to low pressures, amplitude of phase transition falls.

Cross magnetic resistance was measured in magnetic field up to H≤5 kOe. Magnetic resistance is observed in magnetic field under pressure in Cd_0.7Mn_0.3GeAs₂. The increase of pressure and magnetic field leads to growth of positive magnetic resistance, and the amplitude of positive magnetic resistance reaches maximum in field H = 5 kOe and P≤1 GPa. Further increase of pressure leads to suppression of positive magnetic resistance. In the region of magnetic phase transition ferromagnetic–antiferromagnetic at P ≅2 GPa, magnetic resistance changes positive sign for negative. Negative magnetic resistance is ≅ 3% at pressure P >4.5 GPa and field H = 5 kOe. To our opinion, at pressures P < 1GPa a considerable contribution in magnetic resistance gives the scattering of  carriers on fluctuations of magnetizing–magnetic resistance is positive. With growth of pressure in magnetic field occur regulating of spins of manganese ions, scattering decreases, and it leads to negative magnetic resistance. Hysteresis of magnetic resistance is found on dependencies Δρ_xx/ρ(P) at pressure fall. Hysteresis of magnetic resistance, apparently, is a characteristic of anomalous scattering of charge carriers, occurring at transition in magnetic regulated state at forming of ferromagnetic clusters of nano-measuring (ferrons).

The works has been carried out under financial support of program of Presidium of the Russian Academy of Science “Heat physics and mechanics of external energy influences and physics of heavily condensed matter”.

Reference

[1] L.G. Khvostantsev, L.P. Vereshagin, A.P. Novikov. High Temp.- High Pressure, 9, 6, 637, (1977).

[2] A.Yu. Mollaev, R.K. Arslanov, L.A. Saypulaeva, S.F. Marenkin. Inorganic Materials, 37, 4, 405, (2001).

[3] V.I. Chechernikov. Magnetic Measurements, MGU, Moscow, 1963.

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