Magnetic-phase diagram of a Mn0.9Co0.1P single-crystal. Irreversible behavior in a magnetic field

The magnetic-phase diagram of a Mn0.9Co0.1P single crystal was studied by ac susceptibility with a magnetic field (H) applied along its orthorhombic b axis. This system orders ferromagnetically at 197.5 K and below 37 K it shows a helimagnetic structure in H=0. The H-T phase diagram above 37 K is similar to the one found in pure MnP, including the presence of a Lifshitz multicritical point (HL =11.0 kOe, TL =99±1 K) at the confluence of the paramagnetic, ferromagnetic, and modulated fanlike phase. It was found that: (1) the hysteresis at the first-order transitions between the ordered phases is much larger than in MnP; (2) hysteretic behavior is observed inside the whole region of the phase diagram corresponding to the fanlike phase; (3) the second-order fan-para transition fades at low temperatures and an irreversible behavior is observed with the irreversibility field sharply increasing at lower temperatures. The last feature is in marked contrast with what is observed in pure MnP, where the second-order fan-para transition is very sharp and the transition field tends to a constant value at low temperatures.