Phase transitions of MnP for a field parallel to the hard-magnetization direction: A possible new Lifshitz point

The phase transitions and phase diagram of MnP in the presence of a magnetic field $\overrightarrow{\mathrm{H}}$ parallel to the $a$ axis were investigated with the use of differential susceptibility, magnetostriction, thermal expansion, magnetization, and ultrasonic measurements. Three ordered phases were observed. One is the ferromagnetic phase. The other two are tentatively identified as fan and cone phases. The disordered (paramagnetic) phase, and the ferromagnetic and fan phases, meet at the paramagnetic-ferromagnetic-fan (PFF) triple point, located at $T_t=121\mathrm{}$ K and $H=51\mathrm{}$ kOe. The phase diagram near this triple point has a striking similarity to that near the corresponding point in the phase diagram for $\overrightarrow{\mathrm{H}}\parallel b$. The fan-paramagnetic and ferromagnetic-paramagnetic transitions are of second order, but the ferromagnetic-fan transitions are of first order. The qualitative features of the phase diagram near the PFF triple point and the crossover exponent $\phi =0.64\mathrm{}\pm 0.02$ are consistent with those expected for a Lifshitz point. The cone, ferromagnetic, and fan phases meet at another triple point, located at $T\simeq 60$ K and $H=41\mathrm{}$ kOe. The ferromagnetic-cone transitions are of first order. The cone-fan transitions are weakly first order at high temperatures, but there is an uncertainty concerning the order of these transitions at low temperatures. An anomaly in the magnetostriction and differential susceptibility was observed at low magnetic fields (several kOe). This anomaly is attributed to imperfections of the crystals.