Similarity and Difference between Magnetic- and Nonmagnetic-Impurity Effects in Spin-Peierls Cuprate CuGeO_3

TL;DR

This study investigates how magnetic (Ni) and nonmagnetic (Mg) impurities affect the spin-Peierls transition in CuGeO_3, revealing a universal phase transition between dimerized-antiferromagnetic and uniform-antiferromagnetic states.

Contribution

It provides experimental evidence that doping with magnetic or nonmagnetic impurities induces a universal first-order phase transition in CuGeO_3.

Findings

Disappearance of the spin-Peierls transition at x≈0.020

First-order phase transition between D-AF and U-AF phases

Discovery of a new phase with easy axis in the a-c plane

Abstract

We have measured the magnetic susceptibility of single-crystal Cu_{1-x}Ni_xGeO_3 to study doping effects of the magnetic impurity (Ni^{2+} has S=1 spin) on the inorganic spin-Peierls (SP) material CuGeO_3. We observed the disappearance of the SP transition and the abrupt increase of the Neel temperature from 2.5 to 3.4 K at x nearly equal to 0.020, which indicates the first-order phase transition between the dimerized-antiferromagnetic (D-AF) and the uniform-antiferromagnetic (U-AF) phases, as was discovered in Mg-doped (Mg^{2+} is nonmagnetic) system [T. Masuda et al., Phys. Rev. Lett. 80, 4566 (1998)]. This indicates that this transition is universal for the doping to Cu site. We also found another phase with the easy axis in the a-c plane in low x region.