Structural Phase Transitions and Three-Dimensional Magnetic Ordering in the Nd ₂ NiO ₄ Oxide

Abstract

Neutron diffraction on polycrystalline samples of stoichiometric Nd2NiO4 shows a complex structural and magnetic behaviour as a function of temperature. The room temperature (RT) phase is orthorhombic (Bmab) and Ni2+ ions are 3D antiferromagnetically ordered (TN ≈ 320 K), with a propagation vector k = [100] and spins oriented parallel to the propagation vector, i.e. along the a-axis. The magnetic structure can be described as a gx mode (Shubnikov group BPmab'). The magnetic moment for Ni2+ is 1.57μB at 160 K. The system undergoes a structural phase transition from orthorhombic to tetragonal (P42/ncm) at T1 ≈ 130 K. The tetragonal phase allows the existence of a ferromagnetic component along c-axis in the Ni2+ spin structure, the magnetic structure can be described either as a gxcyfz mode (Shubnikov group (Pc' c' n) or as a gx + cyfz mode (Shubnikov group P42/nc' m'). At a temperature as high as 70 K the polarization of the Nd3+ ions becomes noticeable. At low temperature (TN ≈ 8 K) Nd3+ ions cooperative order takes place with a magnetic moment of 3.2μB at 1.5 K and a magnetic structure belonging to the same Shubnikov group (either Pc' c' n or P42/nc' m'). Fully oxidized samples of Nd2NiO4+δ as obtained in air atmosphere, seems to be also orthorhombic at RT and do not show static magnetic order or structural phase transitions between 1.5 K and 300 K. Partially oxidized samples (δ = 0.04) are also orthorhombic at RT, and show a similar magnetic behaviour as stoichiometric ones, but no structural phase transition at low temperatures is observed.