B. Alascio

Electron spin resonance and magnetization in perovskite and pyrochlore manganites

We have performed electron spin resonance (ESR) and dc susceptibility, χdc(T), measurements in La0.7Ca0.3MnO3 perovskite up to 1000 K. This is adequately described in the paramagnetic regime in the constant coupling approximation assuming an effective Heisenberg-like pair interaction. The ESR linewidth is given by ΔHpp(T)=ΔHpp(∞)[C/Tχ(T)]. The ESR intensity follows the T dependence of χdc(T). We conclude that the relaxation mechanism is essentially the same for the whole studied paramagnetic range. The observed behavior reflects the progressive importance of magnetic clustering on approaching Tc. We have found similar T dependence for La0.7Sr0.3MnO3 and La0.7Pb0.3MnO3 perovskites, and also for the pyrochlore In2Mn2O7.

On the anomalous susceptibility of small gap magnetic semiconductors (SmS, SmSe, SmTe)

Abstract The magnetic susceptibility of the Sm chalcogenides shows a large dependence on the nature of the anion. In the three compounds (which under pressure become metallic) the energy needed to promote an electron from the 4f shell to the conduction band is very small. We present here a mechanism based on the hybridization between f states and band states that relates the anomalies of the susceptibility with the energy gaps.

The electronic structure of CaMnOx with 2.66 ≤ x ≤ 3.00

Abstract We studied the electronic structure of CaMnOx with 2.66 ≤ x ≤ 3.00 by means of soft X-ray absorption spectroscopy (XAS). The Mn 2p XAS spectra present changes in the shape of the multiplets and chemical shifts. The chemical shifts indicate a decrease of the Mn 3d count as the oxygen content increases. The Mn 2p XAS spectra are calculated projecting the atomic multiplet on the appropriate crystal field. The O 1s XAS spectra show an increase in the absorption intensity just above the bottom of the conduction band. This effect indicates an increase of unoccupied O 2p states for larger oxygen contents. The O 1s XAS spectra are calculated using cluster model calculations which include charge transfer processes. We conclude that the doped holes in the CaMnOx system contain a mixture of metal and ligand character.

Valence, electronic specific heat and magnetic susceptibility of alpha-cerium

Abstract A sixfold degenerate Anderson Hamiltonian, which includes the Coulomb repulsion between localized and band states in the mean field approximation following Ramirez and Falicov, is treated in the limit of infinitely correlated 4ƒ states and to second order in the mixing parameter. Assuming a line width Γ ≈ 0.02eV we obtain at low temperatures and 11 kbar, a valency of 3.76 and sizeable contributions from the 4ƒ levels to the electronic specific heat and magnetic susceptibility of alpha Ce. Thus we show, that the highly correlated picture necessary to the explanation of the alpha—gamma transition by Ramirez and Falicov needs not to be abandoned in order to explain the anomalies in alpha Ce.

Correlation effects in the electronic structure of high Tc superconductors

Abstract We model Cu-O high Tc superconductors by a generalized Hubbard Hamiltonian. We obtain the ground state for finite linear chain systems exactly. This allows us to calculate physical quantities to show that: a) those systems can change from a highly ionic to a highly metallic state as holes are added. b) In the metallic state holes tend to form pairs in next nearest neighbour sites.

Spin dynamics in perovskites, pyrochlores, and layered manganites

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing C...

Possibility of a distortion induced gap in intermediate valence compounds

Abstract It is shown that in some stoichiometric intermediate valence compounds a symmetry breaking distortion leading to a hybridization gap in the electronic density of states can exist. We study this possibility for the high pressure phase of SmS and the paramagnetic phase of TmSe.

Statistical mechanics of magnetic non magnetic transitions of rare earth ions in metals

Abstract Using a simple model, and by means of a phenomenological extension of the Greens function method, it has been possible to give a more precise mathematical footing to the ideas put forward by Hirst, Maple, Wohleben and Coles regarding the demagnetization of rare earth ions in metals.

Magnetic re-entrance in intermediate valence compounds

Abstract We explore the possibility of magnetic re-entrance in intermediate valence compounds. Using a simplified Anderson-Lattice model we obtain the pressure-temperature magnetic phase diagram. This diagram shows that for some value of the microscopic parameters the temperature induced two transitions (non-magnetic to magnetically ordered to paramagnetic). We calculate the magnetization and the average occupation number of the localized state. Estimations of the observability of the effect in systems like CeAl2 are made.

Effect of f-d Coulomb repulsion in the thermodynamic properties of anomalous rare-earth impurities

Abstract The effect of the Coulomb repulsion between f and d electrons in the thermodynamic properties of the Anderson Model is studied. The magnetic susceptibility is calculated by means of the renormalization group approach. It is shown that the properties of the model with f-d repulsion can be mapped into the properties of a model without f-d repulsion and renormalized parameters. A discussion of the effect of direct exchange between localized and extended states is also included.