Memory of frozen and rotatable antiferromagnetic spins in epitaxial CoO(111)/Fe and NiO(111)/Fe bilayers.

Abstract

We combined XMLD, XMCD and MOKE measurements in order to follow the magnetic properties of epitaxial CoO(111)/Fe(110) [1] and NiO(111)/Fe(110) [2] bilayers. We find that in both studied cases FM sublayer plays a dominant role and determines the magnetic state of the neighboring AFM, however completely different interaction mechanisms are involved.In CoO/Fe bilayers the AFM spins are frozen and their orientation is imprinted by magnetization of Fe layer when the system passes the Neel temperature of CoO. Once the Fe layer grafts the particular magnetic anisotropy (MA) into the CoO overlayer, it later remains frozen and insensitive to external factors like external magnetic field or Fe magnetization direction [3]. Specifically, choice of particular magnetic state of Fe sublayer, when passing Néel temperature of CoO, determines both the axis and direction of interfacial antiferromagnetic spins after the sample is cooled and allows for imprinting their +/- 90° and 0/180° alignment within the sample plane. For example, particular direction of frozen AFM spins determines the corresponding sign of the shift field of exchange biased magnetic hysteresis loop (Fig.1).For NiO/Fe bilayers, due to the weak intrinsic MA of NiO, the AFM spins are rotatable and always follow the reorientation of Fe magnetization that can be controlled by external magnetic field or via the temperature and thickness driven SRT of Fe(110). Recently we showed that in a uniform thickness NiO(111)/Fe(110) system, two magnetic states with orthogonal spin orientations can be stabilized in antiferromagnetic NiO [2]. Field-free, reversible switching between these AFM states was demonstrated. Here we present results of systematic investigations of such field-free, temperature induced switching of AFM NiO spins. Fig. 2 shows temperature dependence of RL2 ratio for various thicknesses of Fe. Critical temperature of SRT in AFM NiO can be tuned in wide temperature window ~ (250 – 380 K) for field-free, reversible switching of AFM spins. XML(C)D-PEEM results obtained on individual self-organized NiO/Fe nanostripes will be also presented.  Fig.1. Memory of frozen AFM spins in CoO(111)/Fe bilayer.  Fig.2. Temperature dependence of XMLD (RL2 ratio) for NiO/Fe [4].