Disorder-induced coexistence of itinerant low-spin and localized high-spin states of Mn in Rh-doped MnSi

Despite the 60-year history of research on band magnetism in MnSi, the field remains a vibrant area of study. This area is still of great interest although the physics of weak itinerant magnetism is complicated because of small magnetic moments and an uncertain role of local interactions. This work presents Rh-doped MnSi compounds in which a high-spin (HS) state of Mn magnetic moments has been detected in $^{55}\mathrm{Mn}$ nuclear magnetic resonance (NMR) measurements. The doping of MnSi with Rh results in a transition to the HS state for ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}\mathrm{Si}$ at $x={x}_{c}\ensuremath{\approx}0.025$ with two Mn magnetic moments $\ensuremath{\approx}1.3$ and $2.2\phantom{\rule{0.16em}{0ex}}{\textmu{}}_{B}$, which are ordered just below 200 K. This transition occurs only in part of the Mn atoms, while the other Mn atoms remain in a low-spin (LS) state. Concurrently, the Dzyaloshinsky-Moriya (DM) interaction for LS helical states of Mn moments is preserved up to $x\ensuremath{\approx}0.13$. Furthermore, variations in the Rh concentration result in discernible alterations in the magnetic field-temperature phase diagrams. In this case, it was observed that the temperature range of existence of the A phase, host skyrmion lattice, was markedly increased in presence of Rh doping, up to $x=0.025$ at least. Small-angle neutron scattering has evidenced the existence of a skyrmion lattice in the helicoidal magnetic phase of ${\mathrm{Mn}}_{0.98}{\mathrm{Rh}}_{0.02}\mathrm{Si}$. The Rh-doped MnSi compound thus demonstrates the coexistence of the HS and LS states of Mn. Our DFT calculations has indicated that this behavior can only be the case when Rh occupies not only Mn but also Si positions in the MnSi compound. Furthermore, our findings indicate that Ir doping of MnSi does not result in the formation of a high-temperature phase, but rather in the suppression of the DM interaction. Although Rh and Ir belong to the same column of Mendeleev's periodic table, they exhibit disparate behaviors upon MnSi doping.