Exchange bias in Cr/Gd multilayers with TC < TN

We report on the exchange bias phenomenon in Cr/Gd multilayers in which the Curie temperature (TC) of the ferromagnetic Gd is slightly less than the Néel temperature (TN) of the antiferromagnetic Cr. The exchange bias field HE and the coercivity HC of the samples display unusual temperature dependence behaviors in comparison with those of the traditional antiferromagnet (AFM)/ferromagnet (FM) multilayers with TC > TN. It is shown that the temperature dependence of HE and HC relates to the thickness of the Cr interlayer. For [Cr(20.4 nm)/Gd(18 nm)]5 multilayers, the ferromagnetic behavior and exchange bias phenomenon persist to T > TN; Oscillatory HE and non-monotonic HC with the temperature are observed. The anomalous temperature dependence of HE and HC results from the spin-density wave spin structure of the Cr layer and the non-collinear coupling between the Cr and Gd layers at the interfaces.