Coupled nature of magnetic and structural transition in MnNiGe under pressure

Differential-thermal-analysis investigation in MnNiGe under pressure (8 kbar) shows that (i) the first-order TiNiSi-to-Ni₂In-type structural transition point TD decreases with increasing hydrostatic pressure (dTDʰdP=-12.2 and dTDᶜdP=-10.0 K/kbar); (ii) the second-order helimagnetic-to-paramagnetic transition point TN situated below TD, increases with pressure (dTNdP=+2.3 K/kbar); (iii) TN and TD coincide to draw a new first-order transition line TCOL(P) above a triple point PTRI=3-4 kbar and that the pressure slope (dTCOLʰdP=dTCOLᶜdP=-5.4 K/kbar) is intermediate in value between those for TD and TN. 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.