Tweaking the spin-wave dispersion and suppressing the incommensurate phase in LiNiPO$_4$ by iron substitution

TL;DR

This study investigates how iron substitution in LiNiPO4 alters spin-wave dispersions, suppresses the incommensurate phase, and changes the magnetic phase transition from first to second order, revealing the role of magnetic frustration.

Contribution

It demonstrates that Fe substitution modifies magnetic interactions, suppresses incommensurate phases, and changes the nature of phase transitions in LiNiPO4, providing insights into magnetic frustration and spin dynamics.

Findings

Fe substitution suppresses the incommensurate phase.

The phase transition changes from first to second order with Fe doping.

The energy gap in spin-wave dispersion decreases with Fe content.

Abstract

Elastic and inelastic neutron scattering studies of Li(Ni$_{1-x}$Fe$_{x}$)PO$_4$ single crystals reveal anomalous spin-wave dispersions along the crystallographic direction parallel to the characteristic wave vector of the magnetic incommensurate phase. The anomalous spin-wave dispersion ({\it magnetic soft mode}) indicates the instability of the Ising-like ground state that eventually evolves into the incommensurate phase as the temperature is raised. The pure LiNiPO$_4$ system ($x=0$), undergoes a first-order magnetic phase transition from a long-range incommensurate phase to an antiferromagnetic ground state at {\it 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 inter- and in-plane nearest and next-nearest neighbor couplings reveals frustration due to strong competing interactions between nearest- and a next-nearest neighbor site, consistent with the observed incommensurate structure. The Fe substitution only slightly lowers the extent of the frustration, sufficient to suppress the IC phase. An energy gap in the dispersion curves gradually decreases with the increase of Fe content from $\sim$2 meV for the pure system ($x=0$) to $\sim$0.9 meV for $x=0.2$.