Magnetic measurements and magnetoelectric effect of pyroelectric KNiPO₄ single crystals

Abstract

Single crystals of KNiPO₄ were synthesized in a KCl flux. The crystals have the orthorhombic space group Pna2₁ at room temperature with a = 8.6333(5), b = 9.2565(5) and c = 4.9064(9) [Å]. Ferroelectric 180° domains, parallel to [001] were revealed by chemical etching. Magnetization measurements along all three orthorhombic axis, made on a SQUID magnetometer, show a magnetic phase transition to the antiferromagnetic state at T_N (= 25 ± 0.5) K with a weak ferromagnetic behaviour. All possible coefficients of the linear magnetoelectric effect have been tested, only α₁₂ and α₂₁ were found to be non zero. These components of the linear magnetoelectric effect are consistent with magnetic point group mm2 which was also found by neutron diffraction studies. However, this magnetic point group does not allow weak ferromagnetism, which is also observed. The pyroelectric nature of this compound has also been put into evidence by measuring a change of the spontaneous polarization ΔP _s = 0.61 [μC cm⁻²] between 4 and 300 K. The total pyroelectric coefficient at 300 K is p ₃ = 2.75 × 10⁻⁵ [Cm⁻² K⁻¹].

Key Words:

• magnetoelectric effect
• weak ferromagnetism
• ferroelectric domains
• pyroelectricity
• KNiPO₄