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

Abstract

Elastic and inelastic neutron-scattering studies of Li͑Ni 1−x Fe x ͒PO 4 single crystals reveal anomalous spinwave dispersions along the crystallographic direction parallel to the characteristic wave vector of the magnetic incommensurate phase. The anomalous spin-wave dispersion ͑magnetic soft mode͒ indicates the instability of the Ising-type ground state that eventually evolves into the incommensurate phase as the temperature is raised. The pure LiNiPO 4 system ͑x =0͒ undergoes a first-order magnetic phase transition from a long-range incommensurate phase to an antiferromagnetic ͑AFM͒ ground state at T N = 20.8 K. At 20% Fe concentrations, although the AFM ground state is to a large extent preserved as that of the pure system, the phase transition is second order, and the incommensurate phase is completely suppressed. Analysis of the dispersion curves using a Heisenberg spin Hamiltonian that includes interplane and in-plane nearest-and next-nearest-neighbor couplings reveals frustration due to strong competing interactions between nearest-and next-nearest-neighbor sites, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the incommensurate phase. An energy gap in the dispersion curves gradually decreases with the increase in Fe content from ϳ2 meV for the pure system ͑x =0͒ to ϳ0.9 meV for x = 0.2.

Antiferromagnetic order in Li(Ni1−xFex)PO4(Zimmermann1, Sondermann2, Li3 et al. 2013Phys. Rev. BSelf Cite2060View full textAdd to dashboardCite

“…We thus confirm earlier results obtained by neutron diffraction: 35 A substitution of 6% is too small for exerting fundamental changes in the magnetic structure. The magnetic properties of (iv) The data point to an unusually large single-ion anisotropy of the Fe 2+ ions which is consistent with other observations on the LiMPO 4 system.…”

“…This is in agreement with experiments on LiMnPO 4 where a Fe substitution of 30% (or less) achieves this purpose. 44 Furthermore, the Fe substitution readily suppresses the incommensurate magnetic phase of LiNiPO 4 with spin rotation in the yz plane 21,35 which is also consistent with a dominating Fe 2+ single-ion anisotropy.…”

“…42 Although the NN superexchange is strongest, the NNN interactions cannot be neglected and promote frustration. 35 The…”

Strong crystalline field at the Fe site and spin rotation in olivine LiNi0.99 F57e0.01 PO4 material by Mössbauer spectroscopy

“…4,5 The LiNiPO 4 compound exhibits the exceptionally large ME coefficients and the hysteresis of the "butterfly" type at low temperature. [7][8][9] Neutron scattering studies have shown that the spin system associated with the Ni 2+ ions in this single crystal LiNiPO 4 undergoes a commensurateincommensurate ͑IC͒ magnetic phase transition near Néel temperature. Olivine LiNiPO 4 crystallizes in the Pnma space group, and consist of distorted NiO 6 , LiO 6 , and PO 4 units.…”

“…[1][2][3] In the olivine phosphate materials, LiMPO 4 ͑M =Ni,Fe,Co,Mn͒, a strong magnetoelectric ͑ME͒ effect has been observed in the antiferromagnetic ͑AF͒ phases. [6][7][8] The NiO 6 octahedra is cornershared and cross-linked with the PO 4 tetrahedra, forming a three-dimensional network, with tunnels along the ͓010͔ and ͓001͔ directions occupied by Li ions. A general mechanism leading to this ME effect in LiNiPO 4 has not yet been established.…”