Weak ferromagnetism and internal magnetoelectric effect in LiFeP ₂O ₇

The magnetic, thermodynamic, and pyroelectric properties of LiFeP ₂O ₇ single crystals are investigated with emphasis on the magnetoelectric interaction of the electrical polarization with the magnetic order parameter. The magnetic order below T _N27 K is found to be a canted antiferromagnet with a weak ferromagnetic component along the b axis. A sharp peak of the pyroelectric current at T _N proves the strong internal magnetoelectric interaction resulting in a sizable polarization decrease at the onset of magnetic order. The magnetoelectric effect in external magnetic fields combines a linear and a quadratic field dependence below T _N. Thermal expansion data show a large uniaxial magnetoelastic response and prove the existence of strong spin-lattice coupling. LiFeP ₂O ₇ is a polar compound with a strong interaction of the magnetic order parameter with the electric polarization and the lattice.