Line of multicritical Lifshitz points in the phase diagram of MnP

Lifshitz critical behavior has been reported in the orthorhombic magnetic system MnP which exhibits uniaxial Ising-type Lifshitz points (LP's). Two LP's have been previously identified in the phase diagram of MnP. One when the external magnetic field H was applied along the intermediate anisotropy axis b, the other when H was along the hard axis a. Both LP's occurs roughly at the same temperature (TL{~}120 K) but at different values of H because of the difference in anisotropy along these axes. In this work we report on measurements made with H applied within the ab plane, at 20^∘{} and 45^∘{} from the b axis. The observed phase diagram shows, near the confluence of the modulated and ferromagnetic phases with the paramagnetic phase, the same characteristics of the phase boundaries found for H{∥}a and H{∥}b. An analysis of the crossover exponent {φ}, obtained from the shape of the phase boundaries near the confluence point gave {φ}=0.61±{}0.02, and TL=119.9 K. These values are consistent with those found when H is along the a and the b axes. This result indicates that there is a line of Lifshitz multicritical points connecting the two previously found LP's. {} 1996 The American Physical Society.