Suppression of the bulk high spin–low spin transition by doping the chiral magnet MnGe

In the MnGe chiral magnet, the helimagnetic order and local moment collapse in two steps, showing the succession of high spin (HS) and low spin (LS) states as pressure increases. Here, we use high-pressure neutron diffraction to study the doped compounds ${\mathrm{Mn}}_0.86{\mathrm{Co}}_0.14\mathrm{Ge}$ and ${\mathrm{Mn}}_0.9{\mathrm{Rh}}_0.1\mathrm{Ge}$, and show that the evolution of their microscopic magnetic properties is instead continuous. It means that the bulk HS-LS transition is a unique feature of pure MnGe, very sensitive to small changes of the band structure and easily suppressed by chemical substitution. On the other hand, the helimagnetic correlations appear to be strengthened by doping and survive up to larger pressures ($\ensuremath{\approx}19\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$, to be compared with $\ensuremath{\approx}13\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$). We discuss these results in the light of other disordered systems with remarkable properties, the so-called Invar alloys.