Unexpected effect of Ru-substitution in lightly doped manganites

In this Communication we report about the unexpected effect of ruthenium doping in sodium ligthly-doped manganites. This effect seems to be in contrast with the usual model applied to describe the effect of this magnetic ion into the manganite structure. We propose a possible compensation mechanism which seems also able to describe other peculiar features encountered in these materials.Comment: 3 pages, 2 Figures to appear in ChemCom

High Pressure X-ray Diffraction Study of MgMn2O4 Tetragonal Spinel

The phase stability of the MgMn2O4 spinel has been studied by means of high-pressure X-ray diffraction for pressures up to 30 GPa. Two samples with different inversion degrees have been considered. Both spinels undergo a phase transition towards an orthorhombic structure (CaMn2O4-type). For the more inverted sample the transition pressure is at least 1 GPa lower with respect to that of the less inverted spinel. Also the volume contraction, relative compressibility and density trends are different for the two samples. These variations have been explained according to differences in the cation distribution. and electronic properties of the samples.Comment: 12 pages; 4 Figures presented at the SRMS-

Effects of Cation Vacancy Distribution in Doped LaMnO3+d Perovskites

In this paper we report studies on the correlation between the presence and distribution of cation vacancies in doped manganites (La,M)MnO3+delta (where M = Na, Ca) and their magnetic properties. Results indicate that cation vacancies are distributed differently for the different crystal structures and dopant ion type. In particular it is shown that knowledge of the total vacancies concentration alone is not enough to fully characterize the physical properties of manganites and that their distribution between the A and B sites of the perovskite structure plays a crucial role which should be taken into account in future studies.Comment: 27 pages, 5 figure. To appear in J. Solid State Che

Structure and Magnetism of HoBaCo2O5+delta Layered Cobaltites with 0.02<delta<0.22

In this paper we have studied, by means of high-resolution neutron powder diffraction and magnetic susceptibility, the structural and magnetic features of selected samples of the HoBaCo2O5+delta layered cobaltite in the low oxygen content range (0.02<delta<0.22). The results shows a strong antiferromagnetic contribution at room temperature coupled to an intermediate spin state of the Co3+ ions.Comment: 16 pages, 4 figures, submitted to Solid State Com

Strong electronic correlations in LixZnPc organic metals

Nuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk LixZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 and of the static uniform susceptibility chi_S on approaching room temperature are characteristic of a Fermi liquid. Moreover, while for x around 2 the electrons are delocalized down to low temperature, for x-> 4 a tendency towards localization is noticed upon cooling, yielding an increase both in 1/T1 and chi_s. The x dependence of the effective density of states at the Fermi level D(EF) displays a sharp enhancement for x=2, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that LixZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develo

Strong electronic correlations in LixM(Pc,Nc) organic conductors near half flling

EPR and NMR measurements in LixM(Pc,Nc) (M=Zn,Mg; Pc=phthalocyanine, Nc=naphthalocyanine) are presented. The changes of the electronic properties induced by the substitution of the metal M and by the expansion of the organic core are discussed for the half filling of the conduction band, i.e. for x around 2. The comparison with Li2ZnPc, which behaves as a Fermi liquid, suggests that the substitution of Zn with Mg does not lead to substantial effects. On the contrary in Li2:2MgNc, where a more two-dimensional packing of the molecules is attained, a stronger tendency to localization and a low temperature phase transition towards a spin density wave ground state are observed

Thermal treatment and light illumination effects in EPR spectra of KtaO3:Cu

In order to understand the manner of incorporation of oxygen vacancies into perovskite structure oxides, KTaO3 crystals containing Cu2+ impurity ions were investigated following thermal treatments in atmosphere of H-2, air, and vacuum. The results of thermal treatments were controlled by EPR. The influence of light illumination on copper centres in these crystals was studied also. The thermal treatment and light illumination effects in EPR spectra of Cu impurity centres are studied for the incipient perovskite-like ferroelectric KTaO3 single crystals. It was found that thermal treatment at similar to 950 degrees C in hydrogen atmosphere and in vacuum strongly depresses EPR signal of both types of Cu2+(1) and Cu2+(2) axial centres substituting Ta5+ in the B-sublattice positions. Subsequent high-temperature treatment in air restores EPR signals of Cu2+ centres, but changes the ratio between Cu2+(1) and Cu2+(2) centres concentration. These effects, together with UV light illumination effects observed for T Cu+ thermal and photo-induced transformations of the Cu impurity centres

Enhancement of room temperature ferromagnetism in N-doped TiO2-x rutile: Correlation with the local electronic properties

The magnetic and electronic properties of ferromagnetic undoped and N-doped TiO2−x rutile have been probed by soft x-ray spectroscopies. Upon N doping, a fivefold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancies, rather to additional in-gap states, arising from the replacement of O with N atoms in the rutile structure that can provide more favorable conditions for the onset of ferromagnetic orderin