Temperature Dependence of the Paramagnetic Resonance Line-widths in RbNiF₃ and RbMnF₃

Abstract

Temperature dependences of the paramagnetic resonance line-widths ΔH of ferrimagnetic (very nearly ferromagnetic) RbNiF₃ and antiferromagnetic RbMnF₃ have been observed in order to investigated the dynamics of the spin fluctuation near the critical points. Reflecting the importance of the anisotropy energy, by which the damping of K=0 transverse mode is influenced, no anomalous increase in widths occurs in isotropic antiferromagnet RbMnF₃ on passing through the critical point. But in the case of uniaxial ferrimagnet RbNiF₃, great narrowing of the width occurs with decreasing temperatures. The width is minimum at (1.025∼1.035)T_c and increases slightly with further decreasing temperatures. The product ΔH·x_⊥, where x_⊥ is the perpendicular susceptibilty, shows almost exactly the same temperature variation as that of MnF₂ with the critical exponent −2⁄3, supporting the theory of Mori and Kawasaki pretty well.