Role of heterostructures and multiple magnetic phases in the low-field magnetization of Fe–Cr giant magnetoresistive multilayers

Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations along with ac magnetization versus temperature and M‐H loop measurements are reported for two series of ion beam sputtered Fe–Cr giant magnetoresistive (GMR) (maximum of 33%) multilayers where the interface roughness is different. The ZFC and FC magnetization data follow different curves below an irreversibility temperature (Tirr). The FC data show a T3∕2 thermal demagnetization behavior at lower temperatures with a very small spin-wave stiffness constant but they go as 1∕T at higher temperatures (above Tirr). The ZFC data show maxima at Tm(H), from which we find the glass transition temperature Tg using the de Almeida and Thouless plot. ac magnetization versus T also shows broad peaks roughly near the same Tg. This behavior is interpreted in terms of the coexistence of spin-glass∕superparamagnetic, FM and AF phases. Most of the above exotic low-field features are absent in molecular beam epitaxy grown Fe–Cr multilayers (GMR >50%) with very smoo...