Purpose - The purpose of this paper is to show how the geometrical information of Maxwell's equations is coded into the constitutive equations. Design/methodology/approach - The Maxwell's equations have been written with the tensorial algebra into a three-dimensional Euclidean space and compared with the usual four-dimensional relativistic approach. Findings - This simple geometry allows the finding of the relativistic information coded on the electric and magnetic fields, showing that they are not independent as relativity affirm obtaining their transformation for a moving inertial observer. Originality/value - The main value of the paper is to present a simple mathematical tool which enables the engineers or applied physicists to obtain the relativistic transformations of the fields without using four-dimensional geometries and the more sophisticated mathematical techniques.

The chemical influence in the phase separation phenomenon that occurs in perovskite manganites is discussed by means of ab initio calculations. Supercells have been used to simulate a phase separated state, that occurs at Ca concentrations close to the localized itinerant crossover. We have first considered a model with two types of magnetic ordering coexisting within the same compound. This is not stable. However, a non-isotropic distribution of chemical dopants is found to be the ground state. This leads to regions in the system with different effective concentrations, that would always accompany the magnetic phase separation at the same nanometric scale, with hole-rich regions being more ferromagnetic in character and hole-poor regions being in the antiferromagnetic region of the phase diagram, as long as the system is close to a phase crossover.

The series of V spinels [A2+] V2 O4 (A = Cd, Mn, Zn, Mg) provides an opportunity to tune the V-V distance continuously, in the frustrated pyrochlore lattice of the spinel. This system has been shown to approach the metallic state when V-V distance is reduced. The proximity to the transition leads to a dimerized structure in ZnV2 O4 caused by lattice instabilities. A different manner to tune the V − V distance of this structure is to fix the A2+ cation (in our case, Zn) and apply pressure. We have analyzed the evolution of the electronic structure of the system in the dimerized state. Such structure prevents the system to present a metallic phase at moderate pressures. We have also calculated the transport properties in a semiclassical approach based on Boltzmann transport theory. Our results support the validity of this structural distortion by providing a nice fit with experimental measurements.

We have performed collinear and noncollinear calculations on neutral Bi(4)Mn and collinear ones on ionized Bi(4)Mn with charges +1 and -1 to find out why theoretical calculations will not predict the magnetic state found in the experiment. We have used the density functional theory to find a fit between the theoretical prediction of the magnetic moment and the experimental value. Our calculations have consisted in a structural search of local energy minima, and the lowest energy magnetic state for each resulting isomer. The geometry optimization found three local minima whose fundamental state is the doublet spin state. These isomers could not be found in previous theoretical works, but they are higher in energy than the lowest-lying isomer by ≈1.75 eV. This magnetic state could help understand the experiment. Calculations of noncollinear magnetic states for the Bi(4)Mn do not lower the total magnetic moment. We conclude arguing how the three isomers with doublet state could actually be the ones measured in the experiment.

Optical excitation spectra of Ag(n) and Ag(n)@He(60) (n = 2, 8) clusters are investigated in the framework of the time-dependent density functional theory (TDDFT) within the linear response regime. We have performed the ab initio calculations for two different exact exchange functionals (GGA-exact and LDA-exact). The computed spectra of Ag(n)@He(60) clusters with the GGA-exact functional accounting for exchange-correlation effects are found to be generally in a relatively good agreement with the experiment. A strategy is proposed to obtain the ground-state structures of the Ag(n)@He(60) clusters and in the initial process of the geometry optimization, the He environment is simulated with buckyballs. A redshift of the silver clusters spectra is observed in the He environment with respect to the ones of bare silver clusters. This observation is discussed and explained in terms of a contraction of the Ag-He bonding length and a consequent confinement of the s valence electrons in silver clusters. Likewise, the Mie-Gans predictions combined with our TDDFT calculations also show that the dielectric effect produced by the He matrix is considerably less important in explaining the redshifting observed in the optical spectra of Ag(n)@He(60) clusters.

The quasi-one-dimensional cobalt oxide Sr6Co5O15 is studied using first-principles electronic-structure calculations and Boltzmann transport theory. We have been able to describe the electronic structure, characterized by the structural one-dimensionality and a particular type of charge ordering, with unexpected electronic structure of the different Co atoms. The origin of the large unquenched misaligned orbital angular momenta comes out naturally from a correct description of the different crystal-field environments. The evolution with the on-site Coulomb repulsion (U) of the electronic structure and the transport properties is discussed, with a best agreement with experiment found for the smallest value of U that allows to converge the correct in-chain ferrimagnetic ground state.

We have performed Monte Carlo simulations to treat the effect of the dipolar interaction in assemblies of superparamagnetic nanoparticles. Our simulations reproduce correctly the increase of the blocking temperature (T(B)) as the concentration increases, as observed experimentally. Interestingly, we have observed a progressive displacement of the M2 versus H/M isotherms (Arrott plots) from the origin as the concentration of nanoparticles increases. Moreover, the curvature of the isotherms at T > T(B) changes from positive to negative slope at high sample concentrations, resembling the shape of a first order phase transition. These results are surprisingly similar to that found in a conventional magnetic phase transition under the effect of a random anisotropy or a random field.

We have studied the energetic and structural stability of the interaction of molecular oxygen with small neutral, anionic and cationic silver clusters, Ag(n) (3 < or = n < or = < 8). The calculations have been carried out using a linear combination of atomic Gaussian-type orbitals within the density functional theory as it is implemented in the demon-ks3.5 code. The O2 molecule has been placed in different positions surrounding the cluster, in order to increase the configurational space of the structural minima. We have found that the oxidized cation and neutral clusters undergo a 2D-3D structural transition even before than the nonoxidized counterparts. Moreover, our results show that the adsorption energies on the cationic and neutral silver oxide clusters manifest an odd-even alternation pattern. Likewise, the average magnetic moment of the O2 radical in the charged and neutral silver environment tends to be greater than the charged and neutral bare diatomic oxygen molecule.

Magnetic nanoparticles with controlled magnetocaloric properties are a good candidate to lower the temperature of nanosized systems: they are easy to manipulate and to distribute into different geometries, as wires or planes. Using a Monte Carlo technique we study the entropy change and refrigerant capacity of an assembly of fine magnetic particles as a function of their anisotropy and magnetization, key-parameters of the magnetic behavior of the system. We focus our attention on the anisotropy energy/dipolar energy ratio by means of the related parameter c0 = 2K/M(S)2, where K is the anisotropy constant and M(S) is the saturation magnetization of the nanoparticles. Making to vary the value of co parameter by choosing different K-M(S) combinations, allows us to discuss how the magnetocaloric response of an assembly of magnetic nanoparticles may be tuned by an appropriate choice of the magnetic material composition.

We report a study of the dopant distribution in the lattice of La1-xCaxMnO3, at concentration x = 0.375, in the ferromagnetic region of the phase diagram. The symmetry of the compound allows for four possible inequivalent Ca2+ distributions. We calculate within the LDA+U approximation (by full-potential ab initio methods) the optimized geometries for each structure and the influence of the dopant homogeneity on the electronic structure of the material. We have analyzed the electronic structure for each of the distributions of Ca2+ considered, using the density of states and the partial Mn density of states. The system is half-metallic for all configurations, due to the large Hund's rule exchange on the Mn3+ cations. The analysis shows that the electronic structure remains largely invariant under different Ca2+ distributions in the lattice. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electronic structure calculations on the low dimensional spin-1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g orbital occupied (dyz) with a large hopping integral along the b direction of the crystal. The most important magnetic coupling is Ti-Ti along the b axis. The transition temperature (Tc) has a linear evolution with pressure, and at about 10 GPa this Tc is close to room temperature, leading to a room temperature spin-Peierls insulator-insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures, with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.

We have performed ab initio calculations in the Density Functional Theory framework on unsupported small gold clusters with size ranging from three to seven atoms. In our calculations we have introduced a single O2 molecule on different places around the cluster surface, and in both parallel and perpendicular position with respect to the cluster surface. We have found that the oxygen molecule bonds in-plane with the bidimensional Au cluster when the number of Au atoms is even, and it will be adsorbed off-plane if the number of Au atoms is odd. The latter case, despite not presenting a true chemical bonding, has great stability due to spin pairing and electrostatic interactions, and the structures will be distorted respect to the geometry of their pure Au cluster equivalents.

Electronic structure calculations were performed for ZnV2O4, a material close to a metal-insulator transition. Structural optimization leads to the formation of V-V dimers along the off-plane chains. A strong spin-lattice coupling is expected close to the transition to itinerancy. No orbital ordering is observed in such a structure, and the experimentally found magnetic structure is naturally explained. (C) 2008 Elsevier B. V. All rights reserved.

Surprising enhancement of the magnetic moments recently observed in dilute Co-Mn alloy clusters is explained using ab initio electronic structure calculations. The calculated magnetic moments generally agree with the reported experimental data. An equation for calculating the magnetic moments of the Co-Mn alloy clusters has been derived to correct the deviations predicted by the rigid-band model and the virtual bound states approximation. A strategy is proposed to obtain the ground-state structures of the Co-Mn clusters and it was also put to the test of the experiment.

The magnetocaloric properties of a fine magnetic particle system are studied by means of a Monte Carlo technique, focusing on the role played by the magnetic anisotropy. By varying the anisotropy of the particles, keeping fixed their size and magnetization, we have found that for a fixed concentration the entropy grows for smaller values of the anisotropy. It is also observed that, when comparing different concentrations, the entropy change for low anisotropy is larger at low concentrations, while for high anisotropy it follows the opposite trend. It is also observed that the blocking temperature increases with increasing values of the anisotropy, as it is expected with the enhancement of the anisotropy energy barriers of the particles. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A superparamagnetic nanoparticle system is a good candidate for implementing the magnetocaloric effect, due to its suitable properties as a frozen ferrofluid able to follow a thermodynamic cycle. For such aim, we have concentrated on the study of the entropy dependence on both the particle size and sample concentration using a Monte Carlo simulation. We have found that for a given sample concentration there exists a particle size for the larger entropy increase, and reciprocally for a given particle volume there exists a sample concentration able to produce the larger entropy change.

We have studied the influence of the applied magnetic field on the blocking temperature (TB) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective TB as a function of H. We have focused our study on the limit H→HK (where HK is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of TB. The classical expression predicts TB to disappear for H⩾HK, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions TB exists even for fields H>HK.

On the basis of experimental thermoelectric power results and ab initio calculations, we propose that a metal–insulator transition takes place at high pressure (approximately 6GPa) in MgV2O4.

V. Pardo, 2, 3, ∗ P. Blaha, M. Iglesias, 3 K. Schwarz, D. Baldomir, 3 and J.E. Arias 1 Departamento de F́ısica Aplicada, Facultad de F́ısica, Universidad de Santiago de Compostela, E-15782 Campus Sur s/n, Santiago de Compostela, Spain 2 Institute for Materials Chemistry, Vienna University of Technology, Getreidemarkt 9/165, A-1060 Vienna, Austria 3 Instituto de Investigaciones Tecnológicas, Universidad de Santiago de Compostela, E-15782, Santiago de Compostela, Spain (Dated: February 2, 2008)

We have performed ab initio calculations on the lowest energy structure of eight-atom gold clusters, a tetracapped square with D4h symmetry, locking it to several total magnetic moments. We have found that for nonzero values of the total magnetic moment, the D4h symmetry is not stable. We have also found that for a fixed nonzero total magnetic moment, the stable structure is a distortion of the tetracapped square, along the square’s diagonals. This structure has a D2h symmetry. The rest of possible structures for this cluster have been calculated as well, and no magnetism-dependent deformation was found, except for the tetracapped tetrahedron with symmetry Td, which is unstable for nonzero total magnetic moment. In this case, the favored structure is a bicapped octahedron with symmetry D2d. The structural change due to magnetic constraints is relevant since the surface of gold clusters has shown a catalytic behavior highly dependent on the structure.