Synchrotron x-ray-scattering study of magnetic-field-induced transitions in Cu-1-x(Zn,Ni)(x)GeO3

We have studied the high-field phase behavior of the spin-chain compound Cu-1-x(Zn, Ni)(x)GeO3 by synchrotron x-ray scattering in magnetic fields up to 13 T. The rich magnetic phase diagram of this system includes uniform, low-field commensurate spin-Peierls (C), high-field incommensurate spin-Peierls (IC), and Neel phases. For the pure system (x=0), we have determined the shape of the phase boundaries as well as the incommensurability and harmonic content of the incommensurate lattice distortion in the IC phase as a function of field and temperature. The results are in good qualitative agreement with predictions based on the soliton lattice model of the IC phase, but significant quantitative discrepancies are found. Dilution of the copper oxide spin-1/2 chains with very small amounts (x similar to 0.01) of spin-0 (Zn) or spin-1 (Ni) impurities results in a short-range-ordered IC phase with an anisotropic correlation length comparable to the average impurity separation. This presumably reflects strong pinning of the magnetic solitons to the impurities. For larger x the long-range order is disrupted also in the C phase, and the system undergoes a Neel transition at low temperatures. Aided by supplementary neutron-diffraction measurements, we construct magnetic phase diagrams for the diluted systems. For these systems we also present detailed measurements of the incommensurabilities and correlation lengths as a function of both field and temperature. The data are interpreted in terms of a simple phenomenological model based on lifetime broadening of the spin excitations in finite-sized chain segments.