Electrical impedance measurements in reentrant Au-Fe alloys

We report on impedance experiments carried out in the reentrant alloys Au-Fe 18 at.% and Au-Fe 21 at.%. A spin-glass system Au-Fe 13.5 at.% was also studied for comparison. Measurements cover an extended temperature interval including the paramagnetic state, the intermediate ferromagnetic-like phase and the low temperature reentrant regime. Several frequencies in the range 0.1-1.8 MHz were applied. Magneto-impedance measurements were also performed in low magnetic fields applied parallel to the current orientation. From the impedance versus temperature data, the real and imaginary components of the circumferential permeability were extracted. A Hopkinson maximum is observed in both permeability components of the reentrant magnets. Also observed in these systems is a downwards inflexion in the permeability versus temperature curves at the canting temperature TK where the reentrant regime sets in. The anisotropy field HK was obtained from the magneto-impedance measurements in several temperatures spanning the ferromagnetic and reentrant states. A marked increase of the HK magnitude occurs in temperatures below TK, indicating the stabilization of the reentrant phase by the anisotropic Dzyaloshisnskii-Moriya coupling. These results are in agreement with predictions of the mean-field theory that explains canting and the reentrant state with basis on the freezing of the spin components which are transverse to the field orientation.