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

The specific heat and the thermal expansion under ambient pressure and the temperature dependence of magnetic susceptibility under applied pressure have been investigated for β- Mn_1-xM_x(M=Ru, Os, and Ir) alloys by taking spin fluctuations into consideration. From powder neutron diffractions for β- Mn_0.75Os_0.25alloys, the magnetic Bragg peaks was observed at 10 K, which is the first direct confirmation of a long-range magnetic order in β -Mn alloys. The relationship between the pressure dependence of the Néel temperature d T_NdP and T _Nis explained by the Ehrenfest equation. A significantly large thermal expansion coefficient in a wide range of temperature above T _Nis clearly correlated with spin fluctuations. At low and finite temperatures, the effects of spin fluctuations as a function of the Néel temperature in both the weak itinerant-electron antiferromagnetic state and intermediate antiferromagnetic state are common to β -Mn alloys, regardless of the kinds of additional elements.