How a ferromagnet drives an antiferromagnet in exchange biased CoO/Fe(110) bilayers

Abstract

Antiferromagnet/ferromagnet (AFM/FM) bilayers that display the exchange bias (EB) effect have been subjected to intensive material research, being the key elements of novel spintronics systems. In a commonly accepted picture, the antiferromagnet, considered as a rigid material due to its high anisotropy and magnetic hardness, controls the magnetic properties of the ferromagnet, such as a shift of the hysteresis loop or coercivity. We show that this AFM-FM master-slave hierarchy is not generally valid and that the influence of the ferromagnet on the magnetic anisotropy (MA) of the neighbouring antiferromagnet must be considered. Our computer simulation and experimental studies of EB in an epitaxial CoO/Fe(110) bilayer show that the ferromagnetic layer with strong uniaxial magnetic anisotropy determines the interfacial spin orientations of the neighbouring AFM layer and rotates its easy axis. This effect has a strong feedback on the EB effect experienced by the FM layer. Our results show new physics behind the EB effect, providing a route for grafting a desired anisotropy onto the AFM and for precise tailoring of EB in AFM/FM systems.

Magnetic Anisotropy and Temperature Dependence of Exchange Bias in Epitaxial CoO(111)/Fe(110) Bilayers

“…A close to linear dependence of H EB on temperature is observed and the blocking temperature of CoO(111) on Fe(110) can be estimated as T B ≈ 270 K. It should be noted that in our system exchange bias is present even without field-cool (FC) procedure, however, particular choice of magnetic field orientation during FC is crucial for the orientation of AFM frozen spins and consequently for the magnitude of the shift field H EB . When the sample is cooled at remnant magnetic state, we find that the orientation of AFM frozen spins follows the local (thickness dependent) orientation of Fe magnetization, in agreement with our previous report [13]. In case of FC procedure, when FC magnetic field is parallel to the local easy axis of Fe layer, the corresponding value of H EB is the same like for the sample cooled at its remnant magnetic state.…”

“…We complement conclusions described in Ref. [13] by showing a true bilateral nature * corresponding author of FM-AFM interaction. Specifically we show that depending on the orientation of the frozen CoO spins, that can be controlled either by remanent magnetic state of the Fe or by applied field cool procedure, it is possible to strongly modify the magnetic anisotropy field of the Fe(110) layer.…”

“…Here we report on SRT and exchange bias in Fe(110)/W(110) films coated with a metal-oxide layer (i.e., AFM CoO). For this system, we have recently shown how a ferromagnetic Fe layer can be used to tailor the magnetic anisotropy of the antiferromagnetic CoO overlayer [13]. In the present report we extend this study and present temperature dependence of exchange bias in CoO(111)/Fe(110) bilayers.…”

Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins

“…[8][9][10] This SRT causes the Fe magnetization to switch from the [1−10] to the [001] inplane direction as the Fe thickness increases above a critical value of the order of 150 ± 100 Å, depending on preparation conditions or type of the overlayers used to cover Fe (110). 11,12,20 Importantly, the SRT also occurs in NiO/Fe(110) bilayers and can be imaged using X-ray magnetic circular dichroism photo-emission electron microscopy (XMCD-PEEM), 13 as seen in Fig. 1.…”

Effects of the Annealing Temperature before Cooling under Magnetic Field on Perpendicular Exchange Coupling in (Pt/Co)5/NiO

“…One proof is that the Meiklejohn and Bean model does not explain positive EB. However, it has been experimentally proven that the exchange coupling anisotropy strongly depends on the individual thicknesses of the FM and AFM layers, [11][12][13][14][15] the interface roughness, and the interface morphology. [16,17] Here, we present an experimental study of perpendicular exchange coupling in FM/AFM system.…”