T. M. Mendonça

Direct evidence for Sb as a Zn site impurity in ZnO

The lattice location of ion implanted antimony in zinc oxide has been determined by means of β− emission channeling from the radioactive S124b isotope. Following 30 keV implantation of S124b into a single-crystalline ZnO sample to a fluence of 1×1014 cm−2, the angular-dependent emission rate of β− particles around several crystallographic directions was measured with a position-sensitive Si detector. The majority of Sb was found to occupy Zn sites, with the possible fraction on O sites being at maximum 5%–6%.

Local symmetry lowering in CdMn2O4 spinel

This work presents an atomic scale study of the electric field gradient (EFG) in the tetragonally distorted CdMn2O4 spinel manganite. The EFG temperature dependence at the Cd and Mn sites was followed via perturbed angular correlation measurements with the 111In and 111mCd probes, from 873 down to 12 K at Isolde-CERN. The results show that in the 12–600 K temperature range, a single Jahn-Teller distorted local phase exists. However above 100 K, a dynamic lattice distortion, evidenced by time dependent EFG fluctuations, sets in suggesting a structural instability. Above 600 K, a local MnO6 octahedra with relaxed Jahn-Teller distortions emerge and grow in the low temperature matrix, although no macroscopic tetragonal to cubic phase transition was observed.

Lattice position and thermal stability of diluted As in Ge

We present a lattice location study of the n-type dopant arsenic after ion implantation into germanium. By means of electron emission channeling experiments, we have observed that the implanted As atoms substitute the Ge host atoms and that, in contrast to several implanted metal impurities in Ge, no significant fraction of As is found on interstitial sites. The substitutional As impurities are found to be thermally stable up to 600 °C. After 700 °C annealing a strong reduction of emission channeling effects was observed, in full accordance with the expected diffusion-induced broadening of the As profile.

Dielectric and Magnetic Properties of ReMnO3 (Re = Eu, Gd) Ceramics

EuMnO 3 and GdMnO 3 ceramics have been synthesised by urea sol-gel combustion method. The dielectric and magnetic properties of high quality ceramics of both compounds were performed in the 5 K–300 K temperature range. The magnetic phase transitions in both compounds are marked by anomalies in the temperature behaviour of both dielectric constant and induced magnetization. The experimental results are discussed and compared with those ones obtained in single crystals.

Phase Separation of La0.70-xErxSr0.30MnO3 and its Effect on Magnetic and Magnetocaloric Properties

Manganites of general formula ABMnO3 (where A is a trivalent rare-earth ion and B is a divalent dopant) have interesting properties, namely colossal magnetoresistance and their applicability as materials for active magnetic regenerators. La0.70Sr0.30MnO3 (LSMO) is a ferromagnet presenting considerable magnetocaloric effect, with operating temperature TC ~ 370 K and magnetic entropy variation comparable to pure Gadolinium. The high value of TC makes LSMO unsuitable for room-temperature magnetic refrigeration applications, but by substituting La with the high-magnetic moment ion Er, TC is lowered and total magnetic entropy increases. We have found a limit of solid solubility of Er ions in LSMO, in samples prepared by either solid state or sol-gel techniques in previous works [1], in accordance with other authors [2]. We now present a more detailed study of this limit of solubility, with more samples prepared with Er substitution close to the solubility limit and SEM microscopy clearly showing the changes in microstructure caused by the formation of a secondary ErMnO3 phase, in accordance with x-ray diffraction data and TC variation along the series. The magnetocaloric properties of the series are also presented, showing the increase of Relative Cooling Power along the series, in applied magnetic fields up to 1 T.

Electrical Field Gradient Studies on La1−xCdxMnO3+δ System

A Perturbed Angular Correlation (PAC) study was performed on the La1−x CdxMnO33+δ system that presents a rich variety of structural and magnetic phase transitions as a function of the oxygen content (δ) or as a function of temperature (T). The PAC signal at room temperature allowed the determination of the Electrical Field Gradient (EFG) parameters and to correlate them with the lattice average symmetry namely the orthorhombic (O′ or O*) Pbnm and rhombohedral (R) R\( \overline 3 \)C phases measured by X-ray diffraction.