Positive and Perpendicular Exchange Bias CoO/Co Bilayers

TL;DR

This study investigates the exchange bias effect in CoO/Co bilayers, revealing asymmetries in magnetic reversal processes, interfacial magnetic domains, and temperature-dependent bias fields through comprehensive experimental techniques.

Contribution

It provides detailed experimental insights into the magnetic reversal mechanisms and interfacial phenomena in CoO/Co bilayers, highlighting the effects of Co layer thickness.

Findings

Asymmetry in magnetic hysteresis during reversal

Existence of interfacial magnetic domains

Temperature-dependent positive and negative exchange bias

Abstract

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co(111) textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, SQUID magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature. For very thin Co-films the coexistence of ferromagnetic domains with parallel and perpendicular magnetization directions leads to a peculiar shape of the hysteresis with an extended plateau like region of almost zero magnetization.