Beating the superparamagnetic limit of IrMn in ferro-∕antiferromagnet∕artificial antiferromagnet

By coupling an artificial antiferromagnet (AAF)$(CoFe∕Ru∕CoFe)$ to a ferro-∕antiferromagnet $(F∕AF)$ system $(NiFe∕IrMn)$ we found a strong increase of the exchange bias blocking temperature, e.g., at 1.5-nm IrMn from 110 to 360 K. By magneto-optical Kerr effect and torquemetry at 10–340 K we determined the exchange bias field $He$⁠, the coupling energy $JF∕AF$ between ferromagnet and antiferromagnet, and the rotational loss for thickness of IrMn from (0.3 to 3.5) nm. By comparison of both systems we concluded that for small thickness of the AF its superparamagnetic state was overcome due to the increased total anisotropy energy in the system with AAF. In this system exchange bias can be realized at room temperature with vanishing crystalline anisotropy of the antiferromagnet and without rotational loss.