Magnetic structures and quadratic magnetoelectric effect in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>LiNiPO</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math> beyond 30 T
Ellen Fogh, Takumi Kihara, Rasmus Toft-Petersen, Maciej Bartkowiak, Yasuo Narumi, O. Prokhnenko, Atsushi Miyake, Masashi Tokunaga, Kenichi Oikawa, Michael Korning Sørensen, Julia Cathrine Dyrnum, H. Grimmer, Hiroyuki Nojiri, N. B. Christensen
Abstract
Neutron diffraction with static and pulsed magnetic fields is used to directly probe the magnetic structures in ${\mathrm{LiNiPO}}_{4}$ up to $25\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and $42\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, respectively. By combining these results with magnetometry and electric polarization measurements under pulsed fields, the magnetic and magnetoelectric phases are investigated up to $56\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ applied along the easy $c$ axis. In addition to the already known transitions at lower fields, three new ones are reported at 37.6, 39.4, and $54\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Ordering vectors are identified with ${\mathbf{Q}}_{\mathrm{VI}}=(0,\frac{1}{3},0)$ in the interval $37.6\ensuremath{-}39.4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ and ${\mathbf{Q}}_{\mathrm{VII}}=(0,0,0)$ in the interval $39.4\ensuremath{-}54\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. A quadratic magnetoelectric effect is discovered in the ${\mathbf{Q}}_{\mathrm{VII}}=(0,0,0)$ phase and the field dependence of the induced electric polarization is described using a simple mean-field model. The observed magnetic structure and magnetoelectric tensor elements point to a change in the lattice symmetry in this phase. We speculate on the possible physical mechanism responsible for the magnetoelectric effect in ${\mathrm{LiNiPO}}_{4}$.