Ferromagnetic resonance study ofNi79Mn21alloy

In this report, we present ferromagnetic-resonance (FMR) measurements on the reentrant ${\mathrm{Ni}}_{79}$ ${\mathrm{Mn}}_{21}$ alloy as a function of temperature (4.2 K<T<300 K), after cooling the sample through the spin-glass–ferromagnetic transition either in zero field (ZFC) or in high field (FC). The FMR spectra were taken with the applied magnetic field both parallel and perpendicular to the surface of the thin disclike sample. The unidirectional anisotropy has been obtained by analyzing the FMR data. The anisotropy effects appear at a much higher temperature than the spin freezing temperature, $T_f$ (∼35 K). In addition to the usual FMR signal, when the applied magnetic field is parallel to the cooling field direction, another resonance signal is observed at lower magnetic field. However, this signal disappears completely when the field is reversed. We interpreted this signal to be due to a new microscopic anisotropy created during the cooling.