Two-dimensional spin correlation in YFe₂O₄

Neutron and X-ray diffraction studies were carried out on polycrystalline specimens of stoichiometric (S-) and non-stoichiometric (N-) YFe ₂O₄. An asymmetric diffraction line was observed above the Neel point (T_N≃235 K) in S-YFe₂O₄ and both above and below T_N (T_N≃210 K) in N-YFe ₂O₄. 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 /. The correlation length, k, and the susceptibility at (1/3, 1/3, /), χ, obey an exponential law: k∝(T-T′_n) ^v and χ∝T-T′_n)^-y, respectively, with v=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.