Neutron Diffraction Study of the Magnetic Structure of BaNiF4

Abstract

The orthorhombic compounds BaMF₄ (M=Mn, Fe, Co, or Ni) have recently been found to undergo antiferromagnetic transitions at low temperatures. Susceptibility measurements on BaNiF₄ reveal that a broad maximum occurs at about 150°K similar to that observed in a number of layer‐type structures. As the temperature is lowered, χ∥ along b decreases rapidly, while χ⊥ goes through a shallow minimum around 50°K. At 77°K, no magnetic order can be detected in powder neutron diffraction patterns of BaNiF₄, but at 4.2°K there are magnetic peaks which can be indexed on a unit cell doubled along the b and c directions (A2₁am orientation). The magnetic structure consists of puckered (010) sheets within which the moments are coupled antiparallel to neighboring moments about 4‐Å apart. However, the crystal structure is such that the net interaction between adjacent sheets is zero, and ordering in the b direction depends upon interactions between second nearest sheets 14‐Å apart. This is very similar to the situation in K₂NiF₄ and pronounced two‐dimensional behavior is therefore to be expected. The moments are directed along the b axis and the ordered moment is about 2.0 μ_B per Ni ion. The magnetic symmetry is actually monoclinic, and the simplest magnetic space group is P_2a2₁.