Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites

We present an extensive investigation (magnetic, electric and thermal measurements and X-ray absorption spectroscopy) of the Pr0.5Ca0.5CoO3 and (Pr1-yYy)0.7Ca0.3CoO3 (y=0.0625-0.15) perovskites, in which a peculiar metal-insulator (M-I) transition, accompanied with pronounced structural and magnetic anomalies, occurs at 76 K and 40-132 K, respectively. The inspection of the M-I transition using the XANES data of Pr L3-edge and Co K-edge proofs the presence of Pr4+ ions at low temperatures and indicates simultaneously the intermediate spin to low spin crossover of Co species on lowering the temperature. The study thus definitively confirms the synchronicity of the electron transfer between Pr3+ ions and Co^(3+/4+)O3 subsystem and the transition to the low-spin, less electrically conducting phase. The large extent of the transfer is evidenced by the good quantitative agreement of the determined amount of the Pr4+ species, obtained either from the temperature dependence of the XANES spectra or via integration of the magnetic entropy change over the Pr4+ related Schottky peak in the low-temperature specific heat. These results show that the average valence of Pr3+/Pr4+ ions increases (in concomitance with the decrease of the formal Co valence) below TMI for (Pr0.925Y0.075)0.7Ca0.3CoO3 up to 3.16+ (the doping level of the CoO3 subsystem decreases from 3.30+ to 3.20+), for (Pr0.85Y0.15)0.7Ca0.3CoO3 up to 3.28+ (the decrease of doping level from 3.30+ to 3.13+) and for Pr0.5Ca0.5CoO3 up to 3.46+ (the decrease of doping level from 3.50+ to 3.27+).Comment: 19 pages, 11 figure

Interplay between mesoscopic phase separation and bulk magnetism in the layered NaxCoO2

Specific heat of the layered NaxCoO2 (x=0.65, 0.70 and 0.75) oxides has been measured in the temperature range of 3-360 K and magnetic field of 0 and 9 T. The analysis of data, assuming the combined effect of inter-layer superexchange and the phase separation into mesoscopic magnetic domains with localized spins embedded in a matrix with itinerant electronic character, suggests that the dominant contribution to the specific heat in the region of short-range ordering is mediated by quasi-2D antiferromagnetic clusters, perpendicular to the CoO2 layers

Magnetotransport properties of CaTixRu1-xO3 (x=0, 0.07)

Electrical resistance, transversal magnetoresistance and the Hall effect were studied on polycrystalline CaTixRu1-xO3 (x = 0, 0.07) samples using a conventional Quantum Design PPMS-9 equipment in the temperature range 2-300 K and magnetic field up to 9 T. Substantial differences were found between the two samples: (i) opposite to the metallic character of CaRuO3, the substituted sample has insulating-like electrical resistance; (ii) the magnetoresistance of the substituted sample changes the sign from negative to positive values with increasing temperature. The magnetoresistance of CaRuO3 is negative, the sign reversal is induced by magnetic field and only at temperatures below 15 K, such a behaviour is predicted for clustered systems; (iii) the Hall voltage in pure CaRuO3 also changes sign from negative to positive values above 35 K. This temperature coincides with the observed magnetic transition temperature, indicating that the magnetic state and the carrier character interrelate.13th Czech and Slovak Conference on Magnetism (CSMAG07), Jul 09-12, 2007, Kosice, Slovaki

Ti-induced ferromagnetism and the specific heat of CaTixRu1-xO3 (x=0, 0.005, 0.03)

The influence of Ti substitution on the specific heat of the CaTixRu1-xO3 system at low concentrations x = 0, 0.005, and 0.03 was studied in the temperature range of 2-300 K at magnetic fields tip to 9 T. Small peak was revealed in the C/T vs. T-2 dependence at around 3 K, which are field sensitive (the electronic specific heat coefficient gamma linearly decreases with the increase in magnetic field), and might be connected to some kind of magnetic ordering. The coefficient gamma is suppressed also by Ti substitution.13th Czech and Slovak Conference on Magnetism (CSMAG07), Jul 09-12, 2007, Kosice, Slovaki

Low temperature anomalies in the specific heat and magnetic susceptibility of Na0.7CoO2 samples

We present a comparative study of Na0.7CoO2 samples obtained from three different sources and prepared by different methods. The specific heat and magnetic susceptibility measurements in the temperature range 2-300 K show substantial influence on the observed anomalies, which underlines that the system is extremely sensitive to preparation protocols.13th Czech and Slovak Conference on Magnetism (CSMAG07), Jul 09-12, 2007, Kosice, Slovaki

Nevoid basal cell carcinoma syndrome (Gorlin syndrome)

Nevoid basal cell carcinoma syndrome (NBCCS), also known as Gorlin syndrome, is a hereditary condition characterized by a wide range of developmental abnormalities and a predisposition to neoplasms