High-field magnetic phase transitions and spin excitations in magnetoelectric LiNiPO4

Abstract

The magnetically ordered phases and spin dynamics of magnetoelectric LiNiPO4 have been studied in fields up to 17.3 T along the c axis. Using neutron diffraction, we show that a previously proposed linearly polarized incommensurate (IC) structure exists only for temperatures just below the Néel temperature T N . The ordered IC structure at the lowest temperatures is shown instead to be an elliptically polarized canted spiral for fields larger than 12 T. The transition between the two IC phases is of second order and takes place about 2 K below T N . For µ 0 H > 16 T and temperatures below 10 K, the spiral structure is found to lock in to a period of five crystallographic unit cells along the b axis. Based on the neutron diffraction data, combined with detailed magnetization measurements along all three crystallographic axes, we establish the magnetic phase diagrams for fields up to 17.3 T along c and for fields up to 16 T along a and b. The spin excitations in the highfield IC spiral phase have been studied in detail by inelastic neutron scattering. A mean-field analysis shows that the spin Hamiltonian derived previously from the low-temperature spin waves at zero field predicts the transition between the linear and elliptical polarization of the IC structure, and that a generalization of the spin-wave theory, assuming the random-phase-approximation, accounts for the inelastic scattering data obtained in the commensurable uniform phase at fields below 12 T as well as those obtained in the high-field IC spiral phase.

Field-induced reentrant magnetoelectric phase in LiNiPO4

“…The associated critical fields are in rough agreement with those reported in Ref 31. The two lowest transition fields at 12 and 16 T are in agreement with our previous studies 28, 30 . The ME response P x = α xz H z of the five phases is shown in Fig.…”

“…The pulse durations (FWHM) were 5 ms and 2 ms, respectively. The absolute value of the magnetization m was scaled to previous results obtained with static fields 30 . The electrical polarization was measured using a procedure similar to that described in Refs.…”

“…28 The ratio of J 1 to J 2 leaves LiNiPO 4 near an instability to IC magnetic order. Indeed, the low-temperature commensurate phase is bordered by IC phases (with colinear and spiral spin structures, respectively) above T N in zero field, 29 and above 12 T parallel to c at 1.5 K 28, 30 . Both of these phases are magnetoelectrically inactive.…”

“…The hard axis in LiNiPO 4 is along b due to a single-ion anisotropy energy D y S 2 y with a large D y = 1.423 meV (see Ref. 30 and Appendix C). This serves to confine the magnetic moments to the ac plane.…”

Hybrid excitations due to crystal field, spin-orbit coupling, and spin waves in LiFePO4

“…In the case of the lithium-orthophosphates, it has been observed that the relative strength of the ME effect correlates with the effective total orbital moment with LiCoPO 4 (L = 3) and LiMnPO 4 (L = 0) displaying the largest and smallest ME coefficients [14][15][16][17] . Here, we report on the magnetic excitations of LiFePO 4 and expand on recent elastic and inelastic neutron studies of the lithium-orthophosphates [18][19][20] . Similar to other lithiumorthophosphates, LiFePO 4 possesses an intricate magnetic structure, where the Fe 2+ moments (S = 2; L = 2) order antiferromagnetically at T N = 50 K with moments pointing mainly along the b-direction 21,22 .…”