Spin fluctuation, thermal expansion anomaly, and pressure effects on the Néel temperature ofβ-MnM (M=Ru, Os, and Ir) alloys

Abstract

The specific heat and the thermal expansion under ambient pressure and the temperature dependence of magnetic susceptibility under applied pressure have been investigated for $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Mn}}_{1\ensuremath{-}x}{M}_{x}$ ($M=\mathrm{Ru}$, Os, and Ir) alloys by taking spin fluctuations into consideration. From powder neutron diffractions for $\ensuremath{\beta}\ensuremath{-}{\mathrm{Mn}}_{0.75}{\mathrm{Os}}_{0.25}$ alloys, the magnetic Bragg peaks was observed at 10 K, which is the first direct confirmation of a long-range magnetic order in $\ensuremath{\beta}$-Mn alloys. The relationship between the pressure dependence of the N\'eel temperature $d{T}_{\mathrm{N}}∕dP$ and ${T}_{\mathrm{N}}$ is explained by the Ehrenfest equation. A significantly large thermal expansion coefficient in a wide range of temperature above ${T}_{\mathrm{N}}$ is clearly correlated with spin fluctuations. At low and finite temperatures, the effects of spin fluctuations as a function of the N\'eel temperature in both the weak itinerant-electron antiferromagnetic state and intermediate antiferromagnetic state are common to $\ensuremath{\beta}$-Mn alloys, regardless of the kinds of additional elements.