Weakly coupled antiferromagnetic planes in single-crystal ${\mathrm{LiCoPO}}_4$

Neutron-scattering and magnetic susceptibility studies of single-crystal ${\mathrm{LiCoPO}}_4$ are reported. The neutron-diffraction results indicate that in the antiferromagnetic phase the moments are not strictly aligned along the b axis, as previously reported [R. P. Santoro et al., J. Phys. Chem. $27,$ 1192 (1996)], but are uniformly rotated from this axis by a small angle $\left(\approx 4.6°\right).\mathrm{}$ This rotation breaks the mirror symmetry along the orthorhombic b axis. Symmetry considerations based on this rotation, on the magnetoelectric effect, and on a recently observed weak spontaneous magnetization along the spin direction, implying a so-far-unknown ferrimagneticlike kind of weak ferromagnetism, allow one to postulate the monoclinic magnetic point group $2^{\prime }.$ The diffraction data are analyzed in terms of weakly coupled two-dimensional Ising antiferromagnets. The large anisotropy in the susceptibility is explained in terms of the single-ion anisotropy and anisotropic exchange interactions. We argue that the alignment of the magnetic moments in the antiferromagnetic phase is determined by the single-ion anisotropy even though the exchange along this direction is the weakest.