Neutron Scattering Study of Temperature-Concentration Phase Diagram of (Cu1-xMgx)GeO3

TL;DR

This study uses neutron scattering to investigate the phase diagram of (Cu1-xMgx)GeO3, revealing a critical doping concentration where magnetic and structural order parameters change abruptly, indicating a phase boundary.

Contribution

It provides detailed neutron scattering evidence of a sharp phase boundary at a specific doping level in (Cu1-xMgx)GeO3, clarifying the nature of magnetic and structural phase transitions.

Findings

Sudden change in antiferromagnetic moment at critical doping xc

Spin-Peierls order parameter drops sharply at xc

Phase boundary characterized by abrupt order parameter changes

Abstract

In doped CuGeO3 systems, such as (Cu1-xZnx)GeO3 and Cu(Ge1-xSix)O3, the spin-Peierls (SP) ordering (T<Tsp) coexists with the antiferromagnetic (AF) phase (T<TN<Tsp). Tsp decreases while TN increases with increasing x in low doping region. For higher x, however, the SP state disappears and only the AF state remains. These features are common for all the doped CuGeO3 systems so far studied, indicating the existence of universal T-x phase diagram. Recently, Masuda et al. carried out comprehensive magnetic susceptibility (chi) measurements of (Cu1-xMgx)GeO3, in which doping concentration can be controlled significantly better than the Zn doped systems. They found that TN suddenly jumps from 3.43 to 3.98K at the critical concentration xc sim 0.023 and that a drop in chi corresponding to the SP ordering also disappears at x>xc. They thus concluded that there is a compositional phase boundary between two distinct magnetic phases. To clarify the nature of two phases, we performed neutron-scattering measurements on (Cu1-xMgx)GeO3 single crystals with various x. Analysis of the data at fixed temperature points as a function of doping concentration has revealed sudden changes of order parameters at the critical concentration xc=0.027 +- 0.001. At finite temperatures below TN, the drastic increase of the AF moment takes place at xc. The spin-Peierls order parameter delta associated with lattice dimerization shows a precipitous decrease at all temperature below Tsp. However, it goes to zero above xc only at the low temperature limit.