Partial spin ordering and complex magnetic structure in BaYFeO4: a neutron diffraction and high temperature susceptibility study.

The novel iron-based compound, BaYFeO4, crystallizes in the Pnma space group with two distinct Fe(3+) sites, that are alternately corner-shared [FeO5](7-) square pyramids and [FeO6](9-) octahedra, forming into [Fe4O18](24-) rings, which propagate as columns along the b-axis. A recent report shows two discernible antiferromagnetic (AFM) transitions at 36 and 48 K in the susceptibility, yet heat capacity measurements reveal no magnetic phase transitions at these temperatures. An upturn in the magnetic susceptibility measurements up to 400 K suggests the presence of short-range magnetic behavior at higher temperatures. In this Article, variable-temperature neutron powder diffraction and high-temperature magnetic susceptibility measurements were performed to clarify the magnetic behavior. Neutron powder diffraction confirmed that the two magnetic transitions observed at 36 and 48 K are due to long-range magnetic order. Below 48 K, the magnetic structure was determined as a spin-density wave (SDW) with a propagation vector, k = (0, 0, (1)/3), and the moments along the b-axis, whereas the structure becomes an incommensurate cycloid [k = (0, 0, ∼0.35)] below 36 K with the moments within the bc-plane. However, for both cases the ordered moments on Fe(3+) are only of the order ∼3.0 μB, smaller than the expected values near 4.5 μB, indicating that significant components of the Fe moments remain paramagnetic to the lowest temperature studied, 6 K. Moreover, new high-temperature magnetic susceptibility measurements revealed a peak maximum at ∼550 K indicative of short-range spin correlations. It is postulated that most of the magnetic entropy is thus removed at high temperatures which could explain the absence of heat capacity anomalies at the long-range ordering temperatures. Published spin dimer calculations, which appear to suggest a k = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure.