Spin Reorientation Transition in Gadolinium

Abstract

Accurate measurements of the temperature derivative of the electrical resistivity (dρ/dT) are used to investigate the 2nd-order spin reorientation transition in Gd at T_r = 223 K. We confirm the negligible role played by fluctuations, but our results imply new consequences on the validity of models to describe the angle of rotation (θ) between the magnetisation and the c-axis in Gd. Quite generally, ρ(θ) = ρ(-θ) from symmetry so that dρ/dT - θ.dθ/dT near T_r. Experimentally we found dρ/dT =(T_r -T)^-1/2 for T < T_r, which implies 0 ∿ (T_r -T)^1/4. uch result is in strong disagreement withθ ∿ (T -T)^1/2 as predicted by the Landau theory with the standard free energy F = k₁ θ²+ k₂ θ⁴. Our findings rather imply F = k₁ θ² + k₃ θ⁶, which is an unexpected result. This interesting problem is critically analysed here

Work financially supported by INIC (Portugal; a) NATO Res. Grant 1481 (a, b, c) and ESIS (Belgium, b).