RESUMO
Magnetic neutron scattering data for Sm (SmB6, Sm(Y)S) and Eu (EuCu2Si2-x Ge x ) intermediate-valence compounds have been analysed in terms of a generalized model of the intermediate-radius exciton. Special attention is paid to the correlation between the average ion's valence and parameters of the low-energy excitation in the neutron spectra, such as the resonance mode, including its magnetic form factor. Along with specific features of the formation of the intermediate-valence state for Sm and Eu ions, common physical mechanisms have been revealed for systems based on these elements from the middle of the rare-earth series. A consistent description of the existing experimental data has been obtained by using the concept of a loosely bound hole for the Eu f-electron shell in the intermediate-valence state, in analogy with the previously established loosely bound electron model for the Sm ion.
RESUMO
Multiferroic, CoCr2O4 bulk material undergoes successive magnetic transitions such as a paramagnetic to collinear and non-collinear ferrimagnetic state at the Curie temperature (TC) and spiral ordering temperature (TS) respectively and finally to a lock-in-transition temperature (Tl). In this paper, the rich sequence of magnetic transitions in CoCr2O4 after mixing the octahedral site with 10% of iron are investigated by varying the size of the particle from 10 to 50 nm. With the increasing size, while the TC increases from 110 to 119 K which is higher than the TC (95 K) of pure CoCr2O4, the TS remains unaffected. In addition, a compensation of magnetization at 34 K and a lock-in transition at 10 K are also monitored in 50 nm particles. Further, we have examined the magnetic-ordering temperatures through neutron scattering using a polarized neutron beam along three orthogonal directions after separating the magnetic scattering from nuclear-coherent and spin-incoherent contributions. While a sharp long-range ferrimagnetic ordering down to 110 K and a short-range spiral ordering down to 50 K are obtained in 50 nm particles, in 10 nm particles, the para to ferrimagnetic transition is found to be continuous and spiral ordering is diffused in nature. Frequency-dependent ac susceptibility (χ) data fitted with different phenomenological models such as the Neel-Arrhenius, Vogel-Fulcher and power law, while ruling out the canonical spin-glass, cluster-glass and interacting superparamagnetism, reveal that both particles show spin-glass behavior with a higher relaxation time in 10 nm particles than in 50 nm. The smaller spin flip time in 50 nm particles confirms that spin dynamics does not slow down on approaching the glass transition temperature (Tg).
RESUMO
We report the inelastic neutron scattering study of spin dynamics in EuCu(2)(Si(x)Ge(1-x))(2) (x = 1, 0.9, 0.75, 0.6), performed in a wide temperature range. At x = 1 the magnetic excitation spectrum was found to be represented by the double-peak structure well below the energy range of the Eu(3+) spin-orbit (SO) excitation (7)F(0)â(7)F(1), so that at least the high-energy spectral component can be assigned to the renormalized SO transition. Change of the Eu valence towards 2 + with increased temperature and/or Ge concentration results in further renormalization (lowering the energy) and gradual suppression of both inelastic peaks in the spectrum, along with developing sizeable quasielastic signal. The origin of the spectral structure and its evolution is discussed in terms of excitonic model for the mixed valence state.
RESUMO
Peculiarities in the lattice dynamics of the Kondo insulator Y bB(12) have been studied by inelastic neutron scattering. Selected phonon modes were traced above and below the temperature region (T ~ 50 K) where the gap opens in the electron density of states. The intensities of some low-energy modes exhibit an anomalous temperature dependence for q vectors close to the Brillouin zone boundary, suggesting a renormalization of the phonon eigenvectors. This effect is thought to arise from a coupling with magnetic excitations of the same symmetry, which exist at nearby energies. It is argued that this magnetovibrational coupling may in turn play a role in the steep temperature crossover existing in Y bB(12) between the low-temperature (Kondo insulator) and high-temperature (incoherent spin-fluctuation) regimes, which is rapidly suppressed by lighter Zr substitution.
RESUMO
Inelastic neutron scattering experiments have been performed on the archetype compound YbB(12), using neutron polarization analysis to separate the magnetic signal from the phonon background. With decreasing temperature, components characteristic for a single-site spin-fluctuation dynamics are suppressed, giving place to specific, strongly Q-dependent, low-energy excitations near the spin-gap edge. This crossover is discussed in terms of a simple crystal-field description of the incoherent high-temperature state and a predominantly local mechanism for the formation of the low-temperature singlet ground state.