Anomalous exchange bias in FeMn/Gd bilayers with T CT N

Exchange coupling has been investigated in FeMn/Gd bilayers where the Curie temperature (T C) of Gd is much lower than the Neel temperature (T N) of FeMn. The coercivity (H C) exhibits unusual temperature dependence. As temperature increases, it decreases at first, then goes up to the maximum and keeps it in medium temperature range, finally drops rapidly and disappears at T N. The exchange bias field (H E) is negative initially and its magnitude decreases monotonically with increasing temperature, then it turns to be positive, finally disappears at high temperatures. The anomalous H C(T) behavior may be ascribed to the non-collinear coupling interaction at the FeMn/Gd interface. The positive H E can be attributed to the antiferromagnetic coupling between the interfacial Gd and FeMn moments. Moreover, the antiferromagnetic ordering adjacent to the FeMn layer may be responsible for inducing the exchange coupling at T C