Coexistence of spin frustration and spin unfrustration induced spontaneous exchange bias in Heusler alloys

TL;DR

This study investigates the coexistence of spin frustration and unfrustration in Mn2Ni1.5Al0.5 Heusler alloys, revealing their roles in spontaneous exchange bias and how magnetic transitions influence the blocking temperature.

Contribution

It uncovers the coexistence of frustrated and unfrustrated AFM states in Heusler alloys and links interface coupling and magnetic transitions to SEB.

Findings

Coexistence of spin frustrated and unfrustrated AFM states confirmed.

Frustrated AFM transitions to superferromagnet state under magnetic field.

Internal magnetic field controls the blocking temperature of SEB.

Abstract

The mechanism of spontaneous exchange bias (SEB) and the dominant factor of its blocking temperature are still unclear in Heusler alloys. Here, the related investigations are performed in Mn2Ni1.5Al0.5 Heusler alloys with SEB. The results of both magnetic measurements and first-principles calculations confirmed that spin frustrated and unfrustrated antiferromagnetic (AFM) states coexist there and they have different magnetic anisotropies, which are essential for SEB. Based on a series of measurement strategies, we demonstrate that the frustrated AFM state undergoes a first-order magnetic transition to the superferromagnet (SFM) state with the help of an external magnetic field, and SFM is retained due to the first-order property of the magnetic transition. SEB originates from the interface coupling of multiple sublattices between the unfrustrated AFM state and SFM state. By analyzing the Arrott plot using the Landau model, we found that the internal field of the system dominates the blocking temperature of SEB, which paves the way for improving the blocking temperature.