Thermomagnetic hysteresis and electrical transport in amorphous Ni74Mn24Pt2 films

We report resistivity and magnetization measurements on an amorphous Ni74Mn24Pt2 thin film in the temperature range of 3-300K. Two significant features are apparent in both the magnetic susceptibility and electrical resistivity. A low-temperature (low-T) anomaly is observed at about 40K, where a cusp appears in the resistivity, while a concomitant step-like increase in zero-field-cooled (ZFC) magnetization (M) appears with increasing temperature. The low-T anomaly is attributed to a crossover from a pure re-entrant spin-glass within individual domains to a mixed ferro-spin-glass regime at lower temperatures. By contrast, the high-temperature (high-T) anomaly, signaled by the appearance of hysteresis below 250K, corresponds to the freezing of transverse spins in individual domains acting independently. Between the low-T and high-T anomalies a small but discernable magnetic hysteresis is observed for warming vs. cooling in the field-cooled (FC) case. This behavior clearly indicates the presence of domain structure in the sample, while the disappearance of this hysteresis at lower temperatures indicates the complete freezing of the spin orientation of these domains. According to these results, we have divided the magnetic state of this sample into three regions: at temperatures above 250K, the sample behaves like a soft ferromagnet, exhibiting M vs. H loops with very small hysteresis (less than 5Oe). As the temperature is lowered into the intermediate region (the range 40-250K), spins become frozen randomly and progressively within the individual domains. These domains behave independently, rather than as a cooperative behavior of the sample. Weak irreversibility sets in, indicating the onset of transverse spin freezing within the domains. At temperatures below 40K, the M vs. H loops exhibit larger hysteresis, for both the ZFC and FC cases, as in a pure spin-glass. We have also demonstrated giant noise in the resistivity at temperatures just below 250K. Such noise can originate from fluctuations of the domains near the film surface because of competing effective bulk and surface anisotropy fields. The large observed amplitude may be explained by means of a large ferromagnetic anisotropy in the resistivity due to the large spin-orbit effect seen in NiMn systems. Finally, the low-T peak in the resistivity has been analyzed using Fisher and Langer's expression based on the Friedel Model proposed for critical transitions in transition metals (s-d systems). The fitted results are in satisfactory agreement with the predictions of this model