Beyond the blocking model to fit nanoparticle ZFC/FC magnetisation curves.

We consider the probability of a magnetic nanoparticle to flip its magnetisation near the blocking temperature, and use this to develop quasi-analytic expressions for the zero-field-cooled and field-cooled magnetisation, which go beyond the usual critical energy barrier approach to the superparamagnetic transition. The particles in the assembly are assumed to have random alignment of easy axes, and to not interact. We consider all particles to be of the same size and then extend the theory to treat polydisperse systems of particles. In particular, we find that the mode blocking temperature is at a lower temperature than the peak in the zero-field-cooled magnetisation versus temperature curve, in agreement with experiment and previous rate-equation simulations, but in contrast to the assumption many researchers use to analyse experimental data. We show that the quasi-analytic expressions agree with Monte Carlo simulation results but have the advantage of being very quick to use to fit data. We also give an example of fitting experimental data and extracting the anisotropy energy density K.

A two-population analysis of the magnetic dipolar interaction in superparamagnetic systems: a Monte Carlo study.

To understand the influence of the magnetic dipolar interaction on the blocking temperature (T(B)) of superparamagnetic systems, usual models treat the dipolar energy as an additional term to the single-particle anisotropy energy barrier. However, such approaches cannot describe non-monotonic T(B)(c) dependences as reported both experimental and theoretically. Therefore, alternative approaches should be explored. For such a purpose, in this work we investigate a simple approach based on splitting the total population of the system into two subgroups, depending on its relative orientation (parallel- or antiparallel-aligned) with respect to the applied magnetic field direction. The suitability of such approach was explored by means of a Monte Carlo technique that provided us with a good insight into the properties of the system. Our results indicate that this two-population analysis can be a promising way to understand the SPM behaviour of interacting nanomagnetic systems.

Ab initio study of the influence of nanoscale doping inhomogeneities in the phase separated state of La1-xCaxMnO3.

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.

A first-principles study of the influence of helium atoms on the optical response of small silver clusters.

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.

Density functional study of the magnetic properties of Bi4Mn clusters: discrepancy between theory and experiment.

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.

Magnetocaloric effect in magnetic nanoparticle systems: how to choose the best magnetic material?

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.

Density functional study of the oxidation of small neutral and charged silver clusters.

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.

Role of the dipolar interaction on the macroscopic state of magnetic nanoparticle systems: a Monte Carlo study.

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.

Small Au clusters oxidation: an ab initio study.

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.

Fermiology and magnetism in weak itinerant ferromagnet CoS2: an ab initio study.

Electronic structure calculations have been performed on the compound CoS(2), an itinerant ferromagnet whose magnetic properties can be understood in terms of spin fluctuation theory. We have identified nesting features in the Fermi surface of the compound, active for long wavelength spin fluctuations. The electronic structure of the material is close to a half-metal. We show the importance of introducing spin-orbit coupling (SOC) in the calculations, which partially destroys the half-metallicity of the material. By means of transport properties calculations, we have quantified the influence of SOC in the conductivity at room temperature. Analyzing the effect of SOC helps in understanding the negative magnetoresistance of the material, whose conductivity varies within a few per cent with the introduction of small perturbations in the states around the Fermi level.

Enhanced dimerization of TiOCl under pressure: spin-Peierls to Peierls transition.

We report x-ray diffraction and magnetization measurements under pressure combined with ab initio calculations to show that high-pressure TiOCl corresponds to an enhanced Ti3+-Ti3+ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between Ti3+ ions along the b axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.

Studies of Fe-Cu microwires with nanogranular structure.

We report on the fabrication, and structural and magnetic characterization of Cu(63)Fe(37) microwires with granular structure produced by rapid quenching, using the Tailor-Ulitovsky method, from the immiscible alloys. X-ray diffraction study demonstrated that the structure consists of small (6-45 nm) crystallites of Cu and body centred cubic α-Fe. Magnetic properties have been measured in the range of 5-300 K using a SQUID (superconducting quantum interference device) magnetometer. The temperature dependences of the magnetization measured in a cooling regime when no external magnetic field is applied (zero-field cooling) and in the presence of the field (field cooling) show considerable difference below 20 K. This difference could be related to the presence of small α-Fe grains embedded in the Cu matrix. Those α-Fe grains appear to be blocked at temperatures below that at which the maximum of the magnetization is observed in the low temperature range. Significant magnetoresistance (about 7%) has been found in the samples studied. The shape of the observed dependences is typical of a giant magnetoresistance effect.

Homopolar bond formation in ZnV2O4 close to a metal-insulator transition.

Electronic structure calculations for spinel vanadate ZnV2O4 show that partial electronic delocalization in this system leads to a structural instability, with the formation of V-V dimers along the [011] and [101] directions, and readily accounts for the intriguing magnetic structure of this material. The formation of V-V bonds is a consequence of the proximity to the itinerant-electron boundary and is not related to orbital ordering. We discuss how this mechanism naturally couples charge and lattice degrees of freedom in magnetic insulators close to such a crossover.

Enhanced pressure dependence of magnetic exchange in A2+[V2]O4 spinels approaching the itinerant electron limit.

We report a systematic enhancement of the pressure dependence of T(N) in A(2+)[V(2)]O(4) spinels as the V-V separation approaches the critical separation for a transition to itinerant-electron behavior. An intermediate phase between localized and itinerant-electron behavior is identified in Zn[V(2)]O(4) and Mg[V(2)]O(4) exhibiting mobile holes as large polarons. Partial electronic delocalization, cooperative ordering of V-V pairs in Zn[V(2)]O(4) below T(s) approximately T(N) and dT(N)/dP<0, signals that lattice instabilities associated with the electronic crossover are a universal phenomenon.

Beam modes in graded-index media and topological phases.

The change in the polarization plane of light vector beams retaining their shape (beam modes) under propagation through gradient-index media is evaluated as a topological phase acquired by cyclic and noncyclic evolutions of these beams on their projective Hilbert space (momentum sphere). The polarization changes are evaluated by means of the characteristic parameters of the light beam selected.