Pressure Dependence of the Magnetic Transitions in Fe-Rh Alloys

The pressure coefficients of the Curie temperature $\theta$ and the first-order transition temperature $T_0$ have been measured for two binary and three ternary alloys containing Fe and Rh. The coefficients are constant with pressure for all the alloys up to 25 kbars. The existence of a triple point in the $P-T$ plane is demonstrated for these alloys. At pressures above the critical pressure, there is no ferromagnetic state, only antiferromagnetic and paramagnetic, consistent with Hargitai's theory that Rh atoms have no moment in the antiferromagnetic state. The pressure data and Kouvel's field-dependence data are used for comparison with the ratio of volume change to magnetization change as predicted from thermodynamics. Exchange inversion is discussed and ruled out as an acceptable model for the magnetic behavior. The effects of heat treatment on $T_0$ and $\frac{d{T}_0}{\mathrm{dP}}$ and the relationship between $T_0$ and $\frac{d{T}_0}{\mathrm{dP}}$ are also discussed.