Pinned orbital moments in uncompensated antiferromagnetic Co doped ZnO

Abstract

Low temperature Co K-edge x-ray magnetic circular dichroism spectra at different field cooling conditions were recorded to study the imprinted magnetization in antiferromagnetic (AFM) Co doped ZnO (Co:ZnO) films which manifests itself in a vertical exchange bias effect. Co:ZnO films with 50% and 60% doping concentrations were investigated to provide a high degree of pinned magnetic moments. The measurements reveal a change at the main absorption energy of the spectra, while the signal obtained at the pre-edge stays unaffected by the cooling conditions. Therefore, the pinned uncompensated AFM moments, resulting in an imprinted magnetization, are predominantly of orbital character and are independent of ferromagnetic layers.

Transition from a hysteresis-like to an exchange-bias-like response of an uncompensated antiferromagnet

“…All films have been grown with a nominal thickness of 200 nm. The optimized crystal structure utilizing those parameters was confirmed by x-ray diffraction (XRD) and x-ray linear and circular dichroism (XLD, XMCD) and published in earlier works [25,29].…”

“…Similar to Co:ZnO samples in Refs. [11,29,33], no saturation of the M (H ) curves is visible up to 5 T. From the XMCD(H) of Co:ZnO, it is known that even up to 17 T no saturation is visible for various Co concentrations [25,29,33], which is a clear indication of antiferromagnetic ordering. Furthermore, XLD and XMCD spectra of the studied samples exhibit no sign of metallic Co precipitates; cf.…”

“…Furthermore, the AFM exhibits an increased orbital magnetic moment [23,24]. X-ray magnetic circular dichroism (XMCD) measurements on Zn 1-x Co x O (Co:ZnO) revealed that this increase in orbital moment is independent of a FM layer in proximity [25].…”

“…The size of the hysteresis opening and the vertical shift of the Co:ZnO M (H ) curve depends on the Co-doping concentration. Therefore, for this work samples of different Co-doping concentrations with ample precharacterization [25,29,32] are studied as a function of temperature and cooling field, using superconducting quantum interference device (SQUID) magnetometry.…”

Spin pumping from permalloy into uncompensated antiferromagnetic Co doped zinc oxide

“…Furthermore, Co:ZnO films exhibit a vertical EB in complete absence of a FM layer [34]. This vertical exchange shift is dependent on the Co concentration [32], temperature and cooling field [35] and the field imprinted magnetization predominantly shows orbital character [36]. Note that below the coalesence limit of 20 % the vertical EB vanishes.…”