Soft x-ray magnetic circular dichroism (XMCD) and element-specific magnetic hysteresis (ESMH) measurements were performed in transmission mode on Mn73Ir27 10nm∕Co70Fe30 2nm bilayers with different chemical orderings of the Mn–Ir layer.The unidirectional anisotropy constant was 0.55erg∕cm2 for the disordered Mn–Ir layer and 1.18erg∕cm2 for the ordered Mn–Ir layer. The XMCD signal of Mn was observed, which means the induced ferromagnetic component of Mn spins through the exchange coupling at the interface. No vertical offset of the Mn ESMH loops was observed for either the disordered or the ordered bilayers, which means that insignificant uncompensated Mn spin was pinned at the interface to induce exchange bias on the Co–Fe layer.

The microscopic origin of the uncompensated antiferromagnetic (AFM) spins was investigated by means of the x-ray magnetic circular dichroism (XMCD) spectroscopy with transmission mode for Mn–Ir/ferromagnetic (FM) bilayers. As the AFM layer thickness increases, resonant absorption magnitude of Mn L edge naturally increases, but the XMCD magnitude does not change so much. When the FM layer material is modified, the XMCD signal of Mn L edge drastically changes not only in its magnitude but also in its sign. The XMCD signal vanishes without the FM layer. These facts clearly mean that the uncompensated Mn components are induced through the exchange interaction between the FM and the AFM layers and are localized at the very interface. Micromagnetic simulation within the framework of the classical Heisenberg model well supported the above conclusion.

Effects of ultra-thin Mn, Ru, Pd, Ta, Gd, and Tb layer insertion at the interface on the exchange bias of Mn 75 Ir 25 /Co 70 Fe 30 bilayers were investigated. Unidirectional anisotropy constant, J K , of the bilayer was significantly enhanced from 0.46 erg/cm 2 to 0.84 erg/cm 2 with the insertion of 0.5-nm-thick Mn layer, while it was reduced with increasing the inserted layer thickness for the other materials. The value of 0.84 erg/cm 2 was not obtained in the variation of J K with respect to the chemical composition of Mn-Ir layer in a range of Ir content of 0 ~ 40 at.%. The cause of the enhancement of J K with ultra-thin Mn insertion might be due to the modification of antiferromagnetic spin structure at the interface.

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