Phase Inhomogeneity of the Itinerant Ferromagnet MnSi at High Pressures

The pressure induced quantum phase transition of the weakly ferromagnetic metal MnSi is studied using zero-field ${}^{29}\mathrm{S}\mathrm{i}$ NMR spectroscopy and relaxation. Below $P^{*}\approx 1.2\mathrm{G}\mathrm{P}\mathrm{a}$, the intensity of the signal and the nuclear spin-lattice relaxation are independent of pressure, even though the amplitude of the magnetization drops by $20\%$ from the ambient-pressure amplitude. For $P>P^{*}$, the decreasing intensity within the experimentally detectable bandwidth signals the onset of an inhomogeneous phase that persists to the highest pressure measured, $P\ge 1.75\mathrm{G}\mathrm{P}\mathrm{a}$, which is well beyond the known critical pressure $P_c=1.46\mathrm{G}\mathrm{P}\mathrm{a}$. Implications for the non-Fermi liquid behavior observed for $P>P_c$ are discussed.

Figure 1

Resonant frequency ${\nu }_0$ of the NMR absorption at different pressures (solid circles). The dashed line is a guide to the eye. Diamonds represent the saturation magnetization data from Ref. 22. Inset: Susceptibility data at ambient pressure.

Figure 2

Normalized ${}^{29}\mathrm{S}\mathrm{i}$ spectrum at six pressures, from right to left: $P=0.58$, 1.15, 1.23, 1.40, 1.49, and 1.62 GPa. The $P=0$ data reproduces the results of Ref. 19. Inset: Comparison of total spectral intensity (solid circles) and the expected one (dashed line) due to pressure inhomogeneity, assuming a first order phase transition at $P_c=1.46\mathrm{G}\mathrm{P}\mathrm{a}$.

Figure 3

Spin-lattice relaxation rate $T_1^{-1}({\nu }_0)$ (circles) and spin-spin relaxation rate $T_2^{-1}({\nu }_0)$ (open diamonds) at different pressures. In the inset is a comparison of longitudinal magnetization recovery for a pressure $P<P^{*}$, and $P=1.75\mathrm{G}\mathrm{P}\mathrm{a}$. The amplitudes of the recoveries are adjusted for clarity, and the solid lines is a single-exponential recovery.

Figure 4

A phase diagram after [3] (solid line) and an onset of inhomogeneous phase between dotted lines from present NMR measurements.