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Reduction of coercivity in FePt/FeRh bilayer films by heating

In this study, in order to investigate the effect of antiferromagnetic(AF)-ferromagnetic(FM) transition of FeRh layer on the magnetization reversal of FePt layer, some films of FePt/FeRh/MgO(001) with different thickness are fabricated and the temperature dependence of polar Kerr effect are measured. In addition, details of film structure are investigated by XRD, AFM/MFM and RHEED measurements. Results show that for the FePt(10 nm)/FeRh (7.5 nm) film, the coercive field Hc decreases gradually with increasing temperature, but slightly increase once, and takes a small maximum around the AF-FM transition temperature(Tt). The slight increase of Hc around Tt may be due to a contribution of strain to the magnetic anisotropy of FePt, since the first order AF-FM transition of FeRh gives rise to sharp lattice expansion. On the other hand, for the FePt(10 nm)/FeRh(25 nm) film, the coercive field decreases drastically at temperature above Tt. A shape change in polar Kerr hysteresis loops is observed which suggests the existence of exchange spring effect between FeRh underlayer and FePt upperlayer.