Magnetic phase diagram of MnSi in the high-field region

Abstract

The high-field region of the magnetic phase diagram of MnSi is probed by magnetization, resistivity, and magnetoresistance measurements carried out in the temperature range 1.8--300 K for magnetic fields up to 8 T. It is shown that the phase boundary between the paramagnetic (PM) phase and the spin-polarized (SP) phase has no positive slope as was suggested previously, and appears to be practically vertical at the transition temperature ${T}_{c}\ensuremath{\sim}$ 30 K. We argue that the broad maxima of the resistivity and magnetization derivative, which develop in the range $T>{T}_{c}$, are determined by the specific form of functional dependences of these quantities in the PM phase and do not correspond to any ``diffuse'' SP-PM transition. A universal relation between magnetoresistance and magnetization $\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}=\ensuremath{-}{a}_{0}{M}^{2}$ found in the PM phase of MnSi is shown to hold in a wide temperature and magnetic field range where magnetoresistance varies by more than two orders of magnitude. The analysis of the transport and magnetic resonance data favors the explanation of the magnetic properties of MnSi by Heisenberg-type localized magnetic moments rather than within the magnetism of itinerant electrons. A low-temperature anomaly at $T\ensuremath{\sim}$ 15 K corresponding to the features of magnetoresistance and $g$ factor is reported.