Thermomagnetic hysteresis effects in NiMn and NiMnPd thin films
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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 0x5, 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. 2001 American Institute of Physics.
