Structural and magnetic transition in YbVO3: a neutron diffraction study

Abstract

Polycrystalline samples of YbVO₃ have been studied by neutron powder diffraction (NPD), specific heat and magnetization measurements. YbVO₃ becomes magnetically ordered below T_N = 104 K. Neutron diffraction measurements revealed that only the V³⁺ magnetic moments (S = 1) participate in the magnetic ordering and the magnetic unit cell coincides with the chemical one (k = 0). The magnetic structure is defined by the basis vectors (0, A_y, F_z), which implies that the magnetic moments are antiferromagnetically coupled in the a–c plane, and adjacent planes are also antiferromagnetically coupled along the b direction. At T = 1.5 K, the ordered magnetic moment is 1.37(10) μ_B per V atom, with a value of the ferromagnetic component of 0.6(2) μ_B. The magnetic structure remains stable from T_N down to 1.5 K. There are no indications that the Yb³⁺ moments adopt a long distance ordering, at least down to 1.5 K, although they present short range correlations at low temperature. A magnetization reversal effect is detected as a strong decrease of the susceptibility immediately below T_N, down to 75 K. For lower temperatures the paramagnetic contribution of Yb³⁺ ions accounts for the increase of the susceptibility down to 1.5 K. A high-resolution crystallographic study demonstrated that, above T_N, the crystallographic structure is orthorhombic, space group Pnma, showing an orbital ordering effect that sets at around 170 K; below T_N, in a wide temperature range down to 65 K, a magnetostrictive crystallographic transition occurs simultaneously with the appearance of the magnetic ordering, and YbVO₃ becomes monoclinic, space group P2₁/n11. The changes observed in the V–O bond lengths across the transition suggest the adoption of a more severe orbital ordering below T_N.