Jiying Li

Spin-waves in Antiferromagnetic Single-crystal LiFePO4

Spin-wave dispersions in the antiferromagnetic state of single-crystal LiFePO4 were determined by inelastic neutron scattering measurements. The dispersion curves measured from the 0,1,0 reflection along both a * and b * reciprocal-space directions reflect the anisotropic coupling of the layered Fe 2+ S=2 spin system. The spin-wave dispersion curves were theoretically modeled using linear spin-wave theory by including in the spin Hamiltonian in-plane nearest- and next-nearest-neighbor interactions J1 and J2, inter-plane nearest-neighbor interactions J and a single-ion anisotropy D .Aw eak0,1,0 magnetic peak was observed in elastic neutron scattering studies of the same crystal indicating that the ground state of the staggered iron moments is not along the 0,1,0 direction, as previously reported from polycrystalline samples studies, but slightly rotated away from this axis.

Spin dynamics in the magnetoelectric effect compound LiCoPO4

Inelastic neutron-scattering INS experiments were performed to investigate the spin dynamics in magnetoelectric effect LiCoPO4 single crystals. Weak dispersion was detected in the magnetic excitation spectra along the three principal crystallographic axes measured around the 010 magnetic reflection. Analysis of the data using linear spin-wave theory indicates that single-ion anisotropy in LiCoPO4 is as important as the strongest nearest-neighbor exchange coupling. Our results suggest that Co 2+ single-ion anisotropy plays an important role in the spin dynamics of LiCoPO4 and must be taken into account in understanding its physical properties. High-resolution INS measurements reveal an anomalous low-energy excitation that we hypothesize may be related to the magnetoelectric effect of LiCoPO4.

Field-induced magnetic phases and electric polarization in LiNiPO4

Neutron diffraction is used to probe the (H,T) phase diagram of magnetoelectric (ME) LiNiPO4 for magnetic fields along the c axis. At zero field the Ni spins order in two antiferromagnetic phases. One has commensurate (C) structures and general ordering vectors k(C)=(0,0,0); the other one is incommensurate (IC) with k(IC)=(0,q,0). At low temperatures the C order collapses above mu H-0=12 T and adopts an IC structure with modulation vector parallel to k(IC). We show that C order is required for the ME effect and establish how electric polarization results from a field-induced reduction in the total magnetoelastic energy.

Anomalous spin waves and the commensurate-incommensurate magnetic phase transition in LiNiPO4

Detailed spin-wave spectra of magnetoelectric LiNiPO4 have been measured by neutron scattering at low temperatures in the commensurate (C) antiferromagnetic (AF) phase below T-N=20.8 K. An anomalous shallow minimum is observed at the modulation vector of the incommensurate (IC) AF phase appearing above T-N. A linear spin-wave model based on Heisenberg exchange couplings and single-ion anisotropies accounts for all the observed spin-wave dispersions and intensities. Along the b axis an unusually strong next-nearest-neighbor AF coupling competes with the dominant nearest-neighbor AF exchange interaction and causes the IC structure.