Magnetic quantum phase transition in MnSi under hydrostatic pressure

The crossover from a spin-polarized to nonpolarized state as a function of pressure (p) at low temperature (T) has been investigated in MnSi via high-precision measurements of the electrical resistivity ρ and magnetic susceptibility χ. In the magnetic phase (p<${\mathrm{p}}_{\mathrm{c}}$≃14.6 kbar), ρ∝${\mathrm{T}}^2$ at low T as expected for a Fermi liquid in a weakly polarized state. In the nonmagnetic phase (p>${\mathrm{p}}_{\mathrm{c}}$), ρ vs T is consistent with the predictions for a marginal Fermi liquid model in which nearly critical spin fluctuations of long wavelength lead to a singular quasiparticle interaction. The transition is second order for p<${\mathrm{p}}^{\mathrm{*}}$≃12 kbar and weakly first order in the range ${\mathrm{p}}^{\mathrm{*}}$<p<${\mathrm{p}}_{\mathrm{c}}$, where the transition temperature ${\mathrm{T}}_{\mathrm{c}}$ lies below a peak of χ vs T. The variation of ${\mathrm{T}}_{\mathrm{c}}$ with p and of both ρ and χ with T and p may be understood in terms of a model of quantum critical phenomena.