Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe

TL;DR

This study investigates the magnetic, thermodynamic, and electrical transport properties of noncentrosymmetric B20 germanides MnGe and CoGe, revealing multiple phase transitions in MnGe and contrasting electronic behaviors in CoGe.

Contribution

It provides detailed experimental characterization and comparison of MnGe and CoGe, highlighting new magnetic phase transitions and electronic properties in these noncentrosymmetric materials.

Findings

MnGe exhibits a magnetic transition at 275 K and additional low-temperature phase transitions.

MnGe is a metal with negative magnetoresistance and about 0.5 carriers per formula unit.

CoGe is a low carrier density metal with nearly temperature-independent diamagnetism.

Abstract

We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic structure calculations. In MnGe, we observe a transition to a magnetic state at $T_c=275$ K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperature of this transition which corresponds closely to the sharp peak observed in the ac susceptibility at fields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal structure parameters. Transport measurements indicate that MnGe is metal with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature independent diamagnetic susceptibility.