Anomalous exchange bias in Gd/Cr bilayer and Cr/Gd/Cr trilayers

Exchange bias has been observed in Gd/Cr bilayers and Cr/Gd/Cr trilayers systems in which the Curie temperature (TC) of ferromagnetic Gd is less than the Néel temperature (TN) of antiferromagnetic Cr. The exchange bias field HE and coercivity HC of the samples exhibit anomalous temperature dependence behavior in comparison with the conventional ferromagnet/antiferromagnet bilayers with TC > TN. As temperature increases, HE increases monotonically until TC, whereas HC drops first and then goes up; finally, it disappears at TC, which is attributed to the noncollinear interfacial exchange coupling between Gd and Cr. The results show that the exchange bias may exist in the AFM/FM bilayers regardless of the condition of TC > TN.