Irreversibility of the magnetic state of Tm1 xTbxCo2 revealed by specific heat, electrical resistivity, and neutron diffraction measurements

The substitution of Tb for Tm in the Laves phase compound Tm Co2 leads to appearance of a magnetic moment on the Co atoms through the metamagnetic transition in the itinerant d -electron subsystem and gives rise to long-range ferrimagnetic order in Tm1-x Tbx Co2 at x≥0.15. The magnetic state of the compound Tm0.9 Tb0.1 Co2, i.e., just below the critical Tb concentration, is characterized by the presence of large regions with short-range magnetic order and localized spin fluctuations (LSFs) induced in the Co 3d -electron subsystem by the fluctuating f-d exchange due to the Tm-Tb substitution. The peculiar magnetic state of this compound is strongly influenced by an external magnetic field which produces a first-order magnetic phase transition to a long-range ferrimagnetic state with the magnetic moment on the Co atoms up to (0.7-0.8) μB. This field-induced transition in Tm0.9 Tb0.1 Co2 is found to be irreversible. It is accompanied by a giant and irreversible reduction of the electrical resistivity (Δρ ρ∼-45%), specific heat (by about 3.7 times at 2 K), and intensity of magnetic neutron scattering. Such behavior is associated with the field-induced metamagnetic transition in the itinerant d -electron subsystem mediated by the f-d exchange. Significantly enhanced values of the residual resistivity and the coefficient γ of the T -linear contribution to the specific heat in the compound with x=0.1 as well as their unusual behavior with temperature and under application of the magnetic field is ascribed to the presence of LSF. © 2006 The American Physical Society.This work was partly performed at the Swiss Spallation Neutron Source SINQ, Paul Scherrer Institute (PSI), Villigen, Switzerland. This work was supported by the Russian Foundation for Basic Research (Grant No. 04-02-96060) and by the Swiss National Science Foundation (SCOPES Project No. IB7420-110849)

Disorder and relaxation mode in the lattice dynamics of PbMg1/3_{1/3}Nb2/3_{2/3}O3_3 relaxor ferroelectric

The low-energy part of vibration spectrum in PbMg$_{1/3}$Nb$_{2/3}$O$_3$ relaxor ferroelectric was studied by inelastic neutron scattering. We observed the coexistence of a resolution-limited central peak with strong quasielastic scattering. The line-width of the quasielastic component follows a $\Gamma_0+Dq^2$ dependence. We find that $\Gamma_0$ is temperature-dependent. The relaxation time follows the Arrhenius law well. The presence of a relaxation mode associated with quasi-elastic scattering in PMN indicates that order-disorder behaviour plays an important r\^ole in the dynamics of diffuse phase transitions