Large exchange anisotropy in nanostructured Cu$_{30}$Mn$_{70}$ ribbons

Exchange bias (H$_{ex}$), arising from coupling between antiferromagnetic (AFM) and ferromagnetic (FM) materials, is a diagnostic metric of magnetic interactions in inhomogeneous systems. An extremely large H$_{ex}$ of 10 kOe is found in the rapidly-solidified alloy Cu$_{30}$Mn$_{70}$ at 10 K [1-2], but disappears at temperatures above the system blocking temperature of 123 K. X-ray diffraction reveals the presence of two well-crystallized FCC $\gamma$-phases with cubic a-parameters 3.744 and 3.750 {\AA}, unit cell volumes of 52.5 and 52.7 {\AA}$^{3}$, respectively, and a crystallite size of $\sim$30 nm. We hypothesize that the $\gamma$-phase with larger a-parameter is Mn-rich while the other is Mn-poor. The observed magnetic behavior is attributed to exchange interactions between Mn-rich regions where AFM coupling between nearest-neighbor Mn atoms dominates, and Mn-poor regions where FM coupling between next-nearest-neighbors dominates [1]. Compositional fluctuations result in an additional cluster-glass-like freezing behavior due to magnetically frustrated interactions between Mn atoms [2]. \\[4pt] [1] Kouvel, J.S., J. Appl. Phys. S31 (1961) 5, S142-147. \\[0pt][2] Mydosh, J. A. (1995), Spin Glasses, Taylor \& Francis.