Coupling of pinned magnetic moments in an antiferromagnet to a ferromagnet and its role for exchange bias

Abstract

The interaction between uncompensated pinned magnetic moments within an antiferromagnetic (AFM) layer and an adjacent ferromagnetic (FM) layer responsible for the existence of exchange bias is explored in epitaxially grown trilayers of the form FM2/AFM/FM1 on Cu3Au(0 0 1) where FM1 is ~12 atomic monolayers (ML) Ni, FM2 is 21–25 ML Ni, and AFM is 27 ML or 50 ML Ni~25Mn~75. Field cooling for parallel or antiparallel alignment of the out-of-plane magnetizations of the two FM layers does not make a difference for the temperature-dependent coercivity (H C), magnitude of exchange bias field (H eb), AFM ordering temperature (T AFM), and blocking temperature for exchange bias (T b). We explain this by a model in which the uncompensated pinned magnetic moments distributed within the volume of the AFM layer interact with both of the FM layers, albeit with different strength. Parallel and antiparallel coupling between the magnetization of the pinned moments and the FM layers equally exists. This leads to the experimentally observed independence of H C, H eb, as well as of T AFM and T b on the magnetization direction of the FM layers during field cooling. These results provide new and detailed insight into revealing the subtle and complex nature of the exchange bias effect.

Growth, Structure, and Magnetic Properties of Artificially Layered NiMn in Contact to Ferromagnetic Co on Cu3Au(001)

“…Exchange bias originates from uncompensated pinned magnetic moments within the AFM, which couple to the FM layer. Since these pinned magnetic moments are distributed throughout the AFM layer, [23,[37][38][39][40][41] their coupling to the FM layer and thus the exchange-bias field depends on the intra-AFM exchange interaction. A smaller average vertical atomic spacing enhances the exchange interaction in the AFM layer.…”

Tuning the exchange bias effect via thermal treatment temperature in bulk Ni50Mn42In3Sb5 Heusler alloys

“…As previous reported, the EB effect in some Heusler alloys can be attributed to the exchange coupling between the SG and the AFM matrix, since the short-range ordered FM clusters are assumed to be formed in the SG state during application of an external field in the cooling process. 29) Recently, it was found that H EB depends strongly on the size of the FM clusters in the AFM matrix. 30) The size of the FM clusters in the SG and SSG systems are different at varying temperatures under ZFC and FC processes, resulting in a huge difference in the EB effect.…”