Short-range structural and magnetic order in rapidly-solidified Ag- Mn alloys

Abstract

Short-range structural and magnetic order in rapidly solidified nanostructured AgMn alloys featuring nominal large Mn content (x = 25, 32, 40 at.%), well above the maximum equilibrium concentration (x ~ 14 at.%), are quantified using X-ray, neutron and magnetic probes. These inhomogenous alloys contain coexisting Mn-rich and Mn-deficient nanoscopic face-centered-cubic-type phases. Interaction between these regions provides large exchange bias at low temperature, with the magnitude of the unidirectional anisotropy field increasing gradually with increased Mn concentration. Neutron diffraction data reveal no evidence of long-range antiferromagnetic ordering but instead identifies short-range structural order from the presence of a diffuse scattering peak that evolves with Mn content. Moderate annealing temperatures (T = 350 °C) promote homogenization of the Mn content in the alloys with subsequent suppression of exchange bias, confirming the above model. A strong interfacial exchange density between Mn-deficient and the Mn-rich nanocrystalline phases exists that is determined to be of the same order of magnitude as that found in other exchange-biased ferromagnetic/antiferromagnetic systems. The intriguing behavior of these AgMn alloys as a function of temperature, of Mn content and of annealing condition highlights options to manipulate and tailor their magnetic attributes such as interface exchange and magnetic anisotropy.