Magnetic and atomic short range order in Fe1−xCrx alloys

Abstract

We study the magnetic short range order (MSRO) in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}0.15)$ where an inversion of atomic short range order (ASRO) occurs at ${x}_{C}=0.11(1)$. Our combination of neutron diffuse scattering and bulk magnetization measurements offers a comprehensive description of these local orders at a microscopic level. In the dilute alloys $(xl0.04)$, the Cr atoms bear a large moment ${\ensuremath{\mu}}_{\mathrm{Cr}}=\ensuremath{-}1.0(1)\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, antiparallel to the Fe ones $({\ensuremath{\mu}}_{\mathrm{Fe}})$. They fully repel their Cr first and second neighbors, and perturb the surrounding Fe moments. With increasing $x$, near neighbor Cr-Cr pairs start to appear and the Cr moment magnitude decreases, while ${\ensuremath{\mu}}_{\mathrm{Fe}}$ shows a rounded maximum for ${x}_{1}=0.06(1)l{x}_{C}$. Above ${x}_{C}$, ASRO turns to local Cr segregation, thereby increasing magnetic frustration. First-principles calculations reproduce the observed moment variations but overestimate the magnitude of the Cr moment. In order to reconcile theory with experiment quantitatively, we propose that the magnetic moments start canting locally, already above ${x}_{1}$. This picture actually anticipates the spin glasslike behavior of Cr-rich alloys. The whole study points out the subtle interplay of MSRO and ASRO, yielding an increasing frustration as $x$ increases, due to competing Fe-Cr and Cr-Cr interactions and Cr clustering tendency.