Simple mechanism for a positive exchange bias

TL;DR

This paper presents a simple mechanism explaining how interface coupling in FM/AF bilayers can produce positive exchange bias, influenced by cooling field strength, interface roughness, and quantum effects, with implications for magnetic device design.

Contribution

It introduces a straightforward model linking interface coupling and cooling field to the sign of exchange bias in FM/AF bilayers, including effects of roughness and quantum interactions.

Findings

Low cooling field induces negative HE via AF spin polarization.

High cooling field results in positive HE by reversing AF spin alignment.

Interface roughness and quantum effects influence HE magnitude and sign.

Abstract

We argue that the interface coupling, responsible for the positive exchange bias (HE) observed in ferromagnetic/compensated antiferromagnetic (FM/AF) bilayers, favors an antiferromagnetic alignment. At low cooling field this coupling polarizes the AF spins close to the interface, which spin configuration persists after the sample is cooled below the Neel temperature. This pins the FM spins as in Bean's model and gives rise to a negative HE. When the cooling field increases, it eventually dominates and polarizes the AF spins in an opposite direction to the low field one. This results in a positive HE. The size of HE and the crossover cooling field are estimated. We explain why HE is mostly positive for an AF single crystal, and discuss the role of interface roughness on the magnitude of HE, and the quantum aspect of the interface coupling.