Negative rotatable anisotropy in IrMn/Cr/Co thin films

Abstract

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.

Antiparallel interface coupling evidenced by negative rotatable anisotropy in IrMn/NiFe bilayers

“…Opposite to the previous observations, in IrMn/Cr/Co films H RA is antiparallel to H and the isotropic shift of H res ð/ H Þ is upward, 18 i.e., H RA < 0, which has been attributed to antiferromagnetic Co/Cr exchange coupling. The increase of jH RA j has also led to a significant increase of H C .…”

“…This seems to be the case of IrMn/Cr/Co, 18 resulting in the direct correlation between H C and jH RA j.…”

“…18,31,34,43 Our samples, however, show H Regardless of the type, parallel or antiparallel, of the M/m rot coupling and its strength (J 0 ), H C of an FM/AF bilayer should be enhanced if compared to that of the respective control FM film due to the additional energy necessary to reverse an anisotropic UCS. The same holds for the RA, always positive for J 0 > 0.…”

“…(1); more details can be found elsewhere. 18,30,31 Differently from the previously employed domain-wall formation model, 32 we use uniaxial UCSs as FM's pinning sites instead of interfacial AF domain wall. Note that m set kŝ for very high k set and the model is reduced 33 to the rigid AF moment (RAF) model.…”

The angular dependence of the exchange bias under the planar domain wall model

“…27,28,[42][43][44][45][46] On the basis of the PDW model, Geshev et al have performed a series of researches on the exchange bias effect. [29][30][31][32][33][34][35][36][37] The distribution of the AFM axis, the enhancement of the coercivity, the dependence of the exchange bias on the measurement techniques, temperature, and the rotatable anisotropy have been discussed systematically in their works. These studies suggest that only using the PDW model, the magnetic behaviors of these exchange-biased systems can be well understood.…”