Direct observation of frozen moments in the NiFe/FeMn exchange bias system

Abstract

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the elementspecific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin-orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energy. 5 Current address: Outokumpu Stainless AB,

Probing the Transfer of the Exchange Bias Effect by Polarized Neutron Reflectometry

“…This is true for all of the FeMn thicknesses measured. This model of interfacial layers agrees with previous reports of uncompensated spins at the FM/AF interfaces by PNR measurements 42–45 , and by x-ray techniques 46,47 in a variety of systems, such as Co/FeF 2 42 , Py/CoO 46 , NiFe/FeMn 47 . From our subsequent analysis we see that these uncompensated moments are not frozen and are able to rotate with applied field.…”

Nonswitchable magnetic moments in polycrystalline and (111)-epitaxial permalloy/CoO exchange-biased bilayers

“…Brück et al showed that in a MnPd/Fe exchange-bias bilayer, the NS Mn moments extend into MnPd about 13Å [33]. More recently, Mohanty et al observed that in a NiFe/FeMn bilayer, the NS moments exist in both FM and AFM layers [34]. In the case of our polycrystalline Py/CoO system, the NS moment per unit area is estimated to be +4.9 × 10 −6 emu/cm 2 , which has the same order of magnitude as the estimate from the hysteresis loop measurement.…”

Evidence for asymmetric rotation of spins in antiferromagnetic exchange-spring

“…Although experimental techniques, such as nuclear magnetic resonance and neutron diffraction, have been adopted to identify the antiferromagnetism and magnetic structures of AFM [9,10], they are constrained by the sample thickness, which should be thick enough to generate visible signals. Besides the earlier theories of exchange bias that focused on the interface of FM/AFM [11], x-ray magnetic linear dichroism (XMLD) and x-ray magnetic circular dichroism (XMCD) are preferable to detect the arrangement of interfacial spins simultaneously in FM and AFM, taking advantage of the element specificity and shallow probing depth for the total electron yield (TEY) mode [12][13][14][15]. Nevertheless, previous XMLD experiments were generally focused on the collinear AFM, such as NiO and FeRh [3,13], regardless of the noncollinear AFM, due to their complex spin axes.…”