High-pressure magnetic state of MnP probed by means of muon-spin rotation

We report a detailed muon-spin rotation study of the pressure evolution of the magnetic order in the manganese-based pnictide MnP, which has been recently found to undergo a superconducting transition under pressure once the magnetic ground state is suppressed. Using the muon as a volume sensitive local magnetic probe, we identify a ferromagnetic state as well as two incommensurate helical states (with propagation vectors $\mathbf{Q}$ aligned along the crystallographic $c$ and $b$ directions, respectively) which transform into each other through first-order phase transitions as a function of pressure and temperature. Our data suggest that the magnetic state from which superconductivity develops at higher pressures is an incommensurate helical phase.

Figure 1

(a) The $p-T$ phase diagram of MnP obtained from $\mu \mathrm{SR}$ experiments. FM, $\mathrm{Hel}-c$, and $\mathrm{Hel}-b$ denote the ferromagnetic, $\mathrm{helical}-c$, and $\mathrm{helical}-b$ magnetic orders, respectively (see text). The half solid and solid symbols correspond to the case when the $\mathrm{helical}-c$ (pink squares) or the $\mathrm{helical}-b$ (blue circles) phases occupy 10% and 90% of the sample volume, respectively. (b)–(d) The magnetic field distribution (fast Fourier transform of the ZF-$\mu \mathrm{SR}$ data) in the FM, $\mathrm{Hel}-c$, and $\mathrm{Hel}-b$ state, respectively.

Figure 2

The magnetic response of MnP at ambient pressure (0.1 MPa). (a) Temperature evolution of the total magnetic volume fraction ($f_{\mathrm{m}}$, black squares) and the $\mathrm{helical}-c$ fraction ($f_{\mathrm{Hel}}$, blue circles). (b) Temperature dependence of internal fields within the FM and $\mathrm{helical}-c$ phases. (c)–(e) ZF-$\mu \mathrm{SR}$ asymmetry spectra at $T=100\mathrm{K}$ (FM phase), $T=37\mathrm{K}$ (mixture of FM and $\mathrm{helical}-c$ phases), and $T=20\mathrm{K}$ ($\mathrm{helical}-c$ phase). Solid red lines are fits as described in the Supplemental Material Ref. [15].

Figure 3

Magnetic response of MnP at $p=2.42\mathrm{GPa}$. (a) Temperature evolution of the total magnetic volume fraction ($f_{\mathrm{m}}$, black squares) and the high-pressure ($\mathrm{helical}-b$) fraction ($f_{\text{Hel-}b}$, blue circles). (b) Temperature dependence of internal fields within the FM and $\mathrm{helical}-b$ phases. (c)–(e) ZF-$\mu \mathrm{SR}$ asymmetry spectra at $T=198.8\mathrm{K}$ (FM phase), $T=99.5\mathrm{K}$ (mixture of FM and $\mathrm{helical}-b$ phases), and $T=5.02\mathrm{K}$ ($\mathrm{helical}-b$ phase). Solid red lines are fits as described in the Supplemental Material Ref. [15].

Figure 4

Temperature dependences of ${\lambda }_{pc}$ obtained in 10 mT wTF $\mu \mathrm{SR}$ experiments (black dots) and the FM volume fraction $f_{\mathrm{FM}}$ measured in ZF-$\mu \mathrm{SR}$ experiments (red circles) at $p=2.42\mathrm{GPa}$.

Figure 5

Pressure dependence of the ordered moment $m$ in the FM state. The line is the guide for the eye.