R. E. Watson

Magnetocaloric effect in superparamagnets

The magnetocaloric effect is calculated for superparamagnetic materials as a function of temperature, field and cluster size. Assuming classical behavior, a universal curve is calculated from which an optimum cluster moment may be found for maximum entropy change upon application of a given field H at a given temperature T. Quantum effects are shown to be small for temperatures above 10 K and fields less than a few tesla. A comparison with results for a spin-72 paramagnet such as Gd3Ga5O12 (GGG) is made, which indicates that superparamagnetic materials such as magnetic nanocomposites offer the possibility of extending the upper useful temperature limit of paramagnetic materials for magnetic refrigeration.

Enthalpies of formation of AlNi: Experiment versus theory

The thermodynamic properties of theB2 AlNi phase have been revisited with calorimetric and a priori theoretical estimates of the enthalpy of formation of the stoichiometric compound. The calorimetric study has surveyed the temperature dependence of the enthalpy of formation and extrapolated it to zero temperature (for which the a priori estimates have been made), while the theoretical estimate explores the consequences of an apparent error in local density-based potentials in yielding the magnetic contribution to the reference energy of Ni metal. The present experimental value, extrapolated to 0 K, is 65.915 kJ/g-atom while the local density-based calculated value is 67.5 kJ/g-atom. These are in accord with each other and with much, but not all, the previous experimental data. An estimate of the error in the local density magnetic energy was made by comparing experimental and calculated heats for nonmagnetic Fe compounds, where the energy and its associated error are much larger, and scaling the result to Ni. This yields a “corrected” theoretical heat of 66 kJ/g-atom.

Enthalpies of formation of Ni3Al: Experiment versus theory

Using Al solution calorimetry, enthalpies of formation of Ni3Al in the L12 structure ranging from-41.3 to -42.3 kJ/mol were determined at temperatures from 300 to 1123 K. These enthalpies are substantial for an intermetallic compound and display a slight temperature dependence, which is contained within the experimental error. This temperature dependence is related to the lack of any transformations of the compound, which remains ordered up to the melting temperature. The measured enthalpies were combined with other thermodynamic data to estimate the excess entropy of formation of Ni3Al. The high degree of ordering in the compound is reflected in the large negative value of the excess entropy,whose absolute value exceeds those for other high melting temperature intermetallic compounds. The full-potential linearized augmented Slater-type orbital method (FLASTO) was used to calculate the enthalpies of formation of paramagnetic and ferromagnetic Ni3Al. These enthalpies of formation indicate the compound to be a weak ferromagnet, and they are in good agreement with the calorimetric data.

Monte Carlo and mean-field calculations of the magnetocaloric effect of ferromagnetically interacting clusters

Abstract The magnetic entropy change of a ferromagnet induced by an application of a magnetic field is greatest in the temperature region near the Curie point, and the magnitude of the effect is expected to rise monotonically with the size of the individual moments which make up the material. We explore the case of nanocomposite materials with ferromagnetically interacting clusters having large cluster magnetic moments as a function of cluster size. As cluster size increases, both Monte Carlo and mean-field calculations show a decrease in the entropy change at Tc for a given applied field and constant total magnetic moment, and an increase in the entropy change well above Tc. In addition, for the first time, we present a comparison of the results of mean-field and Monte Carlo calculations of the magnetocaloric effect in classical Heisenberg ferromagnets. Previous calculations of the magnetocaloric effect have taken the mean-field approach, which is known to underestimate the spontaneous magnetization below Tc. These issues are relevant to devices employing magnetic refrigeration.

Mossbauer studies of hydrided TiFe

The metallurgical and electronic structure of TiFeHx alloys are probed by 57Fe Mossbauer effect studies. The results are qualitatively consistent with and support Reilly and Wiswalls conclusions (1974) on the phase diagram. Volume effects and charge flow are both important in the isomer shift.

Effect of the Madelung potential on surface core-level shifts in GaAs☆

Abstract It is shown that the difference between the Madelung potentials at the (110) surface and in the bulk of GaAs is the major factor determining the core level shifts which have recently been observed and that site charge differences between surface and bulk, if significant, are such that Ga and As are less ionic at the surface than in the interior. These considerations have general relevance.

Monte Carlo simulations of the magnetocaloric effect in superferromagnetic clusters having uniaxial magnetic anisotropy

The magnetic entropy change, ΔS (hence the heating or cooling) of ferromagnetically coupled magnetic clusters induced by an application of a magnetic field is substantial and occurs over a broad temperature range. This behavior contrasts with a ferromagnetic material, where larger entropy changes can be obtained, but only over small temperature ranges close to the Curie point. Monte Carlo simulations have been carried out to obtain ΔS when the spins are subjected to uniaxial anisotropy fields. While previous calculations for interacting spins showed increases in the Curie temperature and enhancements of ΔS in the vicinity of the Curie point with increasing anisotropy, the present results for interacting clusters show similar increases in the temperature of ΔS peaks accompanied by decreases in the peak values of ΔS. These results show that the flattening of the ΔS peaks, which make superferromagnetic materials valuable for magnetic refrigeration is assisted by the presence of uniaxial anisotropy. Some effe...

Effects of madelung potentials at surfaces of ZnO on photoemission spectra and work functions

Abstract Madelung potentials have been calculated for sites in and near polar and nonpolar, primitive and reconstructed surfaces of wurtzite ZnO. Surface versus bulk potential differences result in electron binding energy differences which should be observable in photoemission spectra even on nonpolar surfaces. In polar planes, the average site charges in the outermost ion layer are “naturally” reduced relative to charges in the crystal interior, and observable effects should be larger. Results for polar surfaces of singly bonded atoms are indicative of the Madelung potential effects expected for dense layers of chemisorbed species. These potentials are especially sensitive to details of charge transfer. If a crystal has uniform site charges except for the above-mentioned charge reduction on polar surfaces, there exist surface dipoles which would cause considerably greater variation in work function from one crystal face to another than that observed experimentally; this discrepancy is resolved if, in the outermost cation-anion layer, charges are further reduced to three-quarters to one-half the “natural” values.

Where are the iron atoms and iron moments in RFEmAl12−n? A Wigner‐Seitz analysis

The ternary compounds RFenAl12−n (n=4,5,6; ThMn12 structure) form an extensive family of alloys having complicated magnetic structures, with the rare earths (R) and Fe ordering at different temperatures. This magnetic ordering has been inferred from magnetization, Mossbauer and neutron diffraction measurements. X‐ray results are in disagreement with the magnetic measurements as to which sites are occuped by the Fe, and it is not clear at which of the sites iron has the largest magnetic moment. Recently, we have suggested that the occurrence of a substantial magnetic moment at an atomic site in certain magnetic systems can be related to the occurrence of −72° disclination lines connection atoms on this site. These disclinations are bond lines joining two near‐neighbor atoms which have six nearest neighbors and can be recognized by a sixfold face appearing on the Wigner‐Seitz polyhedra of the two atoms. The Wigner‐Seitz construction and its use in the analysis of this and related problems is presented.

Enthalpies of formation of AlFe: Experiment versus theory

Abstract Enthalpies of formation for AlFe were determined by solution calorimetry over a wide concentration range (X Al =0.40–0.50) and over a wide temperature interval (T = 296 to 1029 K). The value of enthalpy of formation becomes more negative with deviations from the stoichiometric composition. The temperature dependence of the experimental enthalpy exhibits an unusual maximum of its value at intermediate temperatures. These results will be discussed in relation to ab initio calculations, with particular attention to the spin polarisation energy of elemental Fe and the influence of the magnetic properties of Fe on the correct choice of reference state for paramagnetic AlFe.