Neutron scattering studies of single-crystal LiNiPO4 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,±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.
@article{arxiv.cond-mat/0401441,
title = {Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO$_4$},
author = {D. Vaknin and J. L. Zarestky and J. -P. Rivera and H. Schmid},
journal= {arXiv preprint arXiv:cond-mat/0401441},
year = {2009}
}