Kerr microscopy study of thermal and athermal training effects in a Co/CoO exchange bias system

Abstract

The present work reports the magnetic domain evolution during the magnetization reversal and the training effect in a polycrystalline Co/CoO exchange bias system. Co/CoO bilayers with different cobalt (Co) layer thicknesses are being studied. The measurements are carried out using the Kerr microscopy at different temperatures (≥ 80 K) after the field-cooling across the Néel temperature of the antiferromagnetic (AFM) CoO layer. It is observed that with the increasing ferromagnetic (FM) Co layer thickness, the exchange bias reduces and the training effect becomes weaker. Analysis of the temperature variation of the exchange bias field indicates an increasing disorder at the FM-AFM interface with the decreasing FM thickness. Two different training effects, viz., athermal and thermal mechanisms, are observed depending on the thickness of the FM layer. For the lower FM thickness an athermal training effect is observed, whereas for the thicker FM layer thermal training effect is dominated. The domain structure of FM layers drastically changes during the athermal training effect with the cycling and domain size significantly decreasing during the magnetization reversal process below blocking temperature (TB), whereas in the thermal training mechanism no significant changes are found in the magnetic domain evolution throughout the temperature range with cycling. These results are expected to provide key inputs to various theoretical models that are being used to study the exchange bias phenomena in the recent literature.

Ultradense Arrays of Sub-100 nm Co/CoO Nanodisks for Spintronics Applications

“…Thus, Co/CoO nanodisks with feature size smaller than CoO domains in thin films yield larger exchange energy densities, in agreement with the results in Figure c. The dot diameter, 92 nm, is much smaller than the micrometer size domains observed in Co/CoO bilayers by photoemission electron microscopy and the magneto-optical Kerr effect. − Moreover, intrinsic CoO domains in continuous films can exhibit an averaging effect that is not present in sub-100 nm nanostructures. In this regime, the reversal of FM domains larger than the AFM ones reduces the EB field due to the average of the unidirectional anisotropies experienced by the FM domain. − The Co/CoO films oxidized in the annealing process at 250 °C present an exchange energy density around 0.97 erg/cm 2 at 50 K, which is similar or even higher than other values in the literature measured at a much lower temperature. , As a scalability effect, the exchange energy density of nanodisks increases up to 1.80 erg/cm 2 at 50 K, which is also above the magnitude estimated in other equivalent nanostructures.…”

Angle and rotational direction dependent horizontal loop shift in epitaxial Co/CoO bilayers on MgO(100)

“…The training effect could be separated into an athermal effect dominating for thinner FM layers, significantly changing the FM domain structure, while a thermal mechanism dominated with increasing layer thickness. 12 Differences between the training effects along hard and easy axes in a Co/CoO bilayer system with biaxial anisotropy were correlated to different motion modes of the antiferromagnetic interface spins, triggered by different reversal modes of the FM layer. 13 In an in-situ experiment, the interface roughness of polycrystalline Co/CoO bilayers could be varied with the annealing time of the sample, allowing for studying an EB decrease with increasing interface roughness in just one sample.…”

Local Control of Exchange Bias by Resistive Switching

“…Figure (d) shows the hysteresis loop at HRS and LRS in the second sequence. At HRS (VS), the sample shows a typical exchange bias with the bias field of H E = 1750 Oe, because of the coupling between FM Co and AFM CoO x bilayers . The hysteresis loop changes dramatically when all of the cells are set to LRS.…”