Positive exchange biasing in GdFe∕NiCoO bilayers with antiferromagnetic coupling

For GdFe∕NiCoO bilayers after field cooling, hysteresis loops at low temperatures are shifted along both the horizontal and vertical axes. The exchange field H_E changes from negative values to positive values with increasing cooling field H_CF and the coercivity H_C acquires a maximum near the crossover of H_E. At 5 K, H_E and H_C at H_CF=3 T and the peak height ΔH_C are proportional to the inverse GdFe layer thickness. At H_CF=3 T, H_E is always positive in the temperature region from 5 to 350 K. At low H_CF, however, H_E is negative at low temperatures and becomes positive at high temperatures. These results can be ascribed to antiferromagnetic coupling between GdFe and NiCoO layers.