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-) YFe 2 O 4 . An asymmetric diffraction line was observed above the Néel point ( T N ≃235 K) in S-YFe 2 O 4 and both above and below T N ( T N ≃210 K) in N-YFe 2 O 4 . 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 ′ ) v 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-YFe 2 O 4 . It was concluded that the spin correlation is two-dimensional in the hexagonal YFe 2 O 4 , both in the S- and N-compounds, but a crystal deformation induces a three-dimensional spin ordering in S-YFe 2 O 4 .