Two-Dimensional Spin Correlation in YFe₂O₄

Abstract

Neutron and X-ray diffraction studies were carried out on polycrystalline specimens of stoichiometric (S-) and non-stoichiometric (N-) YFe₂O₄. An asymmetric diffraction line was observed above the Néel point (T_N\simeq235 K) in S-YFe₂O₄ and both above and below T_N (T_N\simeq210 K) in N-YFe₂O₄. At above T_N, the line profile was quantitatively interpreted as due to two-dimensional critical scattering, on an assumption that the scattering cross section is a Lorentzian in a h-k plane, independent of l. The correlation length, κ, and the susceptibility at (1/3, 1/3, l), χ, obey an exponential law: κ∝(T−T_N′)^ν and χ∝(T−T_N′)^−γ, respectively, with ν=1.5±0.05, γ=3.1±0.1 and T_N′=233.0±0.5 K in S-YFe₂O₄. It was concluded that the spin correlation is two-dimensional in the hexagonal YFe₂O₄, both in the S- and N-compounds, but a crystal deformation induces a three-dimensional spin ordering in S-YFe₂O₄.