Perpendicular Exchange Bias in (Co/Pt)n Multilayers

For solid-state magnetic memories and other potential devices based on giant magnetoresistance and tunneling magnetoresistance effects, it is invariably required that one ferromagnetic layer remains with its magnetization pinned in a given direction. The other ferromagnetic layer is then free to rotate creating the magnetoresistive effect. The fixing or pinning of one of the ferromagnetic layers can be achieved by using a material with a high coercivity or significantly higher anisotropy than the other layer. In conventional devices with in-plane anisotropy, this is achieved by the use of the exchange bias phenomenon where the pinned layer is coupled to an antiferromagnetic layer typically consisting of IrMn. In this paper, we consider strategies to achieve the exchange bias phenomena in materials with strong perpendicular orientation to the magnetization and the potential for achieving the required exchange bias. We report on successfully achieving this objective in a (Co/Pt)(n) system. We have achieved values of loop shift approaching 700 Oe at room temperature and blocking temperatures in excess of 500 K.