Negative rotatable anisotropy in IrMn/Cr/Co thin films

This work presents modifications of the magnetic properties of polycrystalline IrMn/Cr(t(Cr))/Co films where the Cr spacer thickness t(Cr) was varied between 0.25 and 3 nm. High-resolution transmission electron microscopy, x-ray diffractometry and reflectivity, as well as x-ray absorption near-edge structure were used for the structural characterization of the films; static magnetization curves as well as ferromagnetic resonance measurements were employed for the magnetic characterization. Decrease of the exchange-bias field and considerable and nonmonotonous enhancement of the coercivity with t(Cr) were observed. The films' anisotropy parameters were extracted from the experimental angular variations of the resonance field and from the hard-axis magnetization curves via numerical simulations. It was obtained that the two phenomena studied, namely, the exchange bias and rotatable anisotropy, have different origins. The rotatable anisotropy was ascribed to Cr interface coupled antiferromagnetically with the Co atoms. On the other hand, the occurrence of exchange bias, even for the film with the thickest Cr layer, was attributed to uncompensated spins at the topmost IrMn interface. Together with the significant increase with t(Cr) of the rotatable anisotropy field, near saturation it is antiparallel to the external magnetic field.