Thermomagnetic hysteresis effects in NiMn and NiMnPd thin films

dc magnetization measurements, for zero-field cooled $(MZFC)$ and field-cooled $(MFC)$ cases, have been carried out for flash-evaporated Pd-doped NiMn thin films. These included reentrant phases $(Ni76−xPdx)Mn24,$ for $0⩽x⩽5,$ and $Ni75Mn23Pd2,$ a pure spinglass phase. The studies were performed over the temperature range 3–300 K. Low-field magnetization measurements show the irreversibility effect $(MZFC$ and $MFC$ diverge) at temperatures below the Curie temperature $Tc.$ In $Ni75Mn23Pd2,$ $MZFC$ falls below $MFC,$ as usually observed. However, in reentrant compositions, $MZFC$ crosses $MFC$ upon warming into the ferromagnetic regime, where it stays above $MFC$ at temperatures below $Tc.$ This unusual behavior is attributed to a model of Imry and Ma in which, in a ferromagnet with antiferromagnetic impurities, the impurities can couple to the host ferromagnetic alignment and force the system to break into domains antiferromagnetically coupled to each other. Field-cooled hysteresis measurements indicate the uniaxial anisotropy in these samples to be small, in contrast with the rigid uniaxial anisotropy reported for the corresponding polycrystalline bulk samples. Since the lattice-orbit coupling is weak in the amorphous phase, this clearly demonstrates that the physical origin of the unidirectional anisotropy is associated with the spin-orbit coupling.