Coupled nature of magnetic and structural transition in MnNiGe under pressure

Abstract

Differential-thermal-analysis investigation in MnNiGe under pressure (8 kbar) shows that (i) the first-order TiNiSi-to-${\mathrm{Ni}}_{2}$In-type structural transition point ${T}_{D}$ decreases with increasing hydrostatic pressure ($\frac{d{T}_{D}^{h}}{\mathrm{dP}}=\ensuremath{-}12.2$ and $\frac{d{T}_{D}^{c}}{\mathrm{dP}}=\ensuremath{-}10.0$ K/kbar); (ii) the second-order helimagnetic-to-paramagnetic transition point ${T}_{N}$ situated below ${T}_{D}$, increases with pressure ($\frac{d{T}_{N}}{\mathrm{dP}}=+2.3$ K/kbar); (iii) ${T}_{N}$ and ${T}_{D}$ coincide to draw a new first-order transition line ${T}_{\mathrm{COL}}(P)$ above a triple point ${P}_{\mathrm{TRI}}=3\ensuremath{-}4$ kbar and that the pressure slope ($\frac{d{T}_{\mathrm{COL}}^{h}}{\mathrm{dP}}=\frac{d{T}_{\mathrm{COL}}^{c}}{\mathrm{dP}}=\ensuremath{-}5.4$ K/kbar) is intermediate in value between those for ${T}_{D}$ and ${T}_{N}$. This is the first example of a collaborating phase transition in which two distinctly different physical properties can cause either a simultaneous transition or two separate ones. A simple phenomenological theory, based upon a Landau-like free-energy expression, provides an understanding of the collaborating magnetic and structural transition in MnNiGe. A mechanism for this transition is proposed.