Behavior of the electrical resistivity of MnSi at the ferromagnetic phase transition

The itinerant helical ferromagnet MnSi reveals a number of remarkable features, which include tricritical phenomena at the phase transition line, Fermi-liquid breakdown, and so-called partial spin order in the paramagnetic state at high pressures. These features, probably interconnected, so far have no satisfactory explanations though several ideas have been suggested. Some current ideas focus on specifics of the spin fluctuations in the paramagnetic phase of MnSi close to the phase transition line. We report here the results of electrical resistivity measurements of a single crystal of MnSi across its ferromagnetic phase transition line at ambient and high pressures. Contrary to previous work in the field we made use of compressed helium as a pressure medium. Sharp peaks of the temperature coefficient of resistivity characterize the transition line. Analysis of these data shows that at pressures to similar to 0.35 GPa these peaks have fine structure, revealing a shoulder at similar to 0.5 K above the peak. That confirms the "abnormal" spin behavior in the narrow region above the Curie point and indicates the existence of a nontrivial fluctuation mode in the paramagnetic phase of MnSi. It is symptomatic that this structure disappears at pressures higher than similar to 0.35 GPa, which was identified earlier as a tricritical point.