Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO$_4$

TL;DR

This study reveals a first-order commensurate-incommensurate magnetic phase transition in LiNiPO$_4$, with incommensurate phases mediating between paramagnetic and antiferromagnetic states, influenced by spin dimensionality crossover.

Contribution

It provides the first neutron scattering evidence of a spontaneous first-order incommensurate magnetic transition in LiNiPO$_4$, linking it to spin dimensionality crossover and magnetoelectric anomalies.

Findings

Observation of satellite peaks indicating incommensurate magnetic order.

Wavelength of magnetic modulation varies with temperature.

Incommensurate phases are due to spin crossover from continuous to Ising states.

Abstract

Neutron scattering studies of single-crystal LiNiPO$_4$ reveal a spontaneous first-order commensurate-incommensurate magnetic phase transition. Short- and long-range incommensurate phases are intermediate between the high temperature paramagnetic and the low temperature antiferromagnetic phases. The modulated structure has a predominant antiferromagnetic component, giving rise to satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg reflection, and a ferromagnetic component giving rise to peaks at small momentum-transfers around the origin at $(0,\pm Q,0)$. The wavelength of the modulated magnetic structure varies continuously with temperature. It is argued that the incommensurate short- and long-range phases are due to spin-dimensionality crossover from a continuous to the discrete Ising state. These observations explain the anomalous first-order transition seen in the magnetoelectric effect of this system.