Anomalous spin waves and the commensurate-incommensurate magnetic phase transition inLiNiPO4

Abstract

Detailed spin-wave spectra of magneto-electric LiNiPO4 have been measured by neutron scattering at low temperatures in the commensurate (C) antiferromagnetic (AF) phase with TN = 20.8 K. An anomalous low-energy mode is observed at the modulation vector of the incommensurate (IC) AF phase appearing above TN . 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.

Magnetic phase diagram of magnetoelectric LiMnPO4

“…9, an investigation of the spin excitations revealed that there are three relatively strong exchange couplings in the bc plane (J bc ∼ 0.5 meV) and two much weaker out-of-plane couplings (J a ∼ 0.05 meV) as in the other lithium orthophosphates. 10,11 As expected, a very weak single ion anisotropy (D ≈ 0.008 meV) was found, compared to the strongest in-plane nearest-neighbor exchange interactions. Thus, LiMnPO 4 is a pseudo-2D Heisenberg system and has been found to exhibit a spin-flop [12][13][14][15] transition, which is a nearly 90…”

Tweaking the spin-wave dispersion and suppressing the incommensurate phase inLiNiPO4by iron substitution

“…An unusual minimum in the spinwave dispersions in the AFM commensurate ground state was observed at the modulation vector of the IC phase and was explained as the precursor of the C-IC phase transition that originates from a trade off between competing Heisenberg interactions of nearest-and next-nearest-neighboring Ni 2+ ions and an extra lock-in energy at lower temperatures originating from the strong single-ion anisotropies found in the system. 18 We have recently reported the spin dynamics and magnetic properties of the isostructural LiFePO 4 , LiCoPO 4 , and LiMnPO 4 systems and found no evidence for an IC phase and no anomalous spin-wave dispersions. [19][20][21] In the present study we have substituted Fe for Ni to form LiNi 1−x Fe x PO 4 single crystals with up to x = 0.2 to compare with the magnetic behavior of the parent material and shed further light on the IC phase in LiNiPO 4 .…”

“…6. The small canting of the Ni 2+ spins has a negligible influence on the spin-wave model, 18 so for simplicity we have assumed a ground state with spins pointing strictly along the c axis. The in-plane NN coupling ͑J 1 ͒ is mediated by an oxygen through a Ni 2+ -O-Ni 2+ bond.…”

“…A recent neutron-scattering study 18 investigated the spin dynamics of pure LiNiPO 4 to determine the spin Hamiltonian and identify other features that characterize the aforementioned second group of IC systems, particularly, looking for a soft-magnetic mode, the analog of the soft mode in structurally IC systems. An unusual minimum in the spinwave dispersions in the AFM commensurate ground state was observed at the modulation vector of the IC phase and was explained as the precursor of the C-IC phase transition that originates from a trade off between competing Heisenberg interactions of nearest-and next-nearest-neighboring Ni 2+ ions and an extra lock-in energy at lower temperatures originating from the strong single-ion anisotropies found in the system.…”

“…18 using the linear Holstein-Primakoff spinwave theory 18,34 to calculate the eigenvalues as a function of wave vectors. The interaction parameters determining the eigenvalues are then refined by a nonlinear-least-square fit to the measured dispersion curves.…”