Magnetoelectric properties of ${\mathrm{LiCoPO}}_4$ and ${\mathrm{LiNiPO}}_4$

In strong magnetic fields up to 20 T quasistatic measurements of the magnetic-field-induced electric polarization have been performed in ${\mathrm{LiNiPO}}_4$ single crystals between 4.2 and $\sim 25$ K. The magnetoelectric (ME) effect has been studied in detail near the Néel temperature. The temperature hysteresis of the polarization induced by a magnetic field was found. An anomalous temperature dependence of the magnetoelectric susceptibility during the phase transition and the ME “butterfly loop” were observed. The phenomenological theory of the magnetoelectric effect in ${\mathrm{LiCoPO}}_4$ and ${\mathrm{LiNiPO}}_4$ has been developed. A tentative explanation of the existence of the magnetoelectric “butterfly loop” is given. It is shown that there is a non-Lifshitz invariant in the free energy expansion which is linear in the spatial derivatives. Hence the phase transition to a space-modulated spin structure is possible. The influence of such a structure on the linear magnetoelectric effect is examined.