Andrés Ayuela

Structural properties of magnetic Heusler alloys

Magnetically driven actuator materials, such as the ternary and intermetallic Heusler alloys with composition , are studied within the density-functional theory (DFT) with the generalized gradient approximation (GGA) for the electronic exchange and correlation. The geometrical and electronic structures for the magnetic structure are calculated. The structures and magnetic moments at equilibrium are in good agreement with the experimental values. The structural trends with varying X and Y are explained by a d-occupation model, while a rigid-band model can account for the trends with changing M.

Ab initio study of tetragonal variants in Ni2MnGa alloy

A major challenge for the study of the magnetically shape-controlled memory alloys Ni–Mn–Ga is the structural search for their variants. We report studies of the variants of Ni2MnGa alloys from a theoretical point of view by means of density functional calculations using the generalized gradient approximation. The effect of tetragonal distortions is studied, and a metastable structure is found for c/a = 0.94, in agreement with experiments. The c/a < 1 minimum is reinforced by considering volume expansions around the experimental values for several variants. After distortion, the electron density is redistributed around the Ni atoms as shown by neutron scattering experiments. The previous disagreement between the theoretical spin density and the experimental neutron scattering is explained. In addition, elastic constants and x-ray absorption spectra are calculated in order to encourage measurements of these quantities. (Some figures in this article are in colour only in the electronic version)

First-principles investigations of homogeneous lattice-distortive strain and shuffles in Ni2MnGa

A series of first-principles calculations were performed for ferromagnetic Ni2MnGa using density functional theory and PAW potentials. Theoretically, a tetragonal crystal structure homogeneous lattice-distortive strain is stabilized around c/a = 0.94 with respect to the L21 structure when, in addition, modulation shuffles with a period of five atomic planes are taken into account. This is in agreement with the observed structures in experimental works. The modulation appears to be critically important for stability of the tetragonal structure with c/a < 1. Here, we report a new feature which is related to the optimum amplitudes of the modulation in different atomic planes. Related to this are systematic changes in the minority spin density of states near the Fermi surface, like in the formalism of a pseudo-gap.

Structural, thermal, and magnetic properties of Ni2MnGa

The two main effects underlying the magnetic shape memory effect in Ni2MnGa are martensitic transformations and magnetic anisotropy energies. Both issues are addressed here with first-principles calculations. First, we examine how the tetragonality in the martensitic phase varies with the composition. Then, the actual transformation is investigated by comparing the free energies of different phases. The transition from the cubic structure to the tetragonal structure with c/a=1.27 is driven by the vibrational free energy and occurs at a temperature of 200 K which is in the experimental range. Finally, we focus on the magnetic anisotropy energy for the tetragonal structure with c/a=0.94. It is shown to be a magnetically nearly ideal uniaxial system determined by the first-order anisotropy constant. However, it is estimated that the twinned microstructure can cause higher-order anisotropies to show up in the measured anisotropy.

First-principles calculations of spin spirals in Ni2MnGa and Ni2MnAl

We report here noncollinear magnetic configurations in the Heusler alloys Ni 2 MnGa and Ni 2 MnAl which are interesting in the context of the magnetic shape memory effect. The total energies for different spin spirals are calculated and the ground-state magnetic structures are identified. The calculated dispersion curves are used to estimate the Curie temperature which is found to be in good agreement with experiments. In addition, the variation of the magnetic moment as a function of the spiral structure is studied. Most of the variation is associated with Ni, and symmetry constraints relevant for the magnetization are identified. Based on the calculated results, the effect of the constituent atoms in determining the Curie temperature is discussed.

Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As

Donostia International Physics Centre (DIPC), POB 1072, 20018 San Sebastian, Spain(Dated: February 2, 2008)The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are investigated by all-electron total energy calculations using the projector augmented wave method within the density-functional formalism. It is found that the energetically most favorable clusters consist of Mn atomssurrounding one center As atom. As the Mn cluster grows the hole band at the Fermi level splitsincreasingly and the hole distribution gets increasingly localized at the center As atom. The holedistribution at large distances from the cluster does not depend significantly on the cluster size.As a consequence, the spin-flip energy differences of distant clusters are essentially independent ofthe cluster size. The Curie temperature T

Intrinsic hole localization mechanism in magnetic semiconductors

The interplay between clustering and exchange coupling in magnetic semiconductors for the prototype (Ga1−x,Mnx)As is investigated considering manganese concentrations x of 1/16 and 1/32, which are in the interesting experimental range. For , we study all possible arrangements of two Mn atoms on the Ga sublattice within a large supercell and find the clustering of Mn atoms at nearest-neighbour Ga sites energetically preferred. As shown by analysis of spin density and projected density of states, this minimum-energy configuration localizes further one hole and reduces the effective charge carrier concentration. Also the exchange coupling constant increases to a value corresponding to lower Mn concentrations with decreasing inter-Mn distance.

Ab initio calculations of structures and stabilities of (NaI) nNa1 and ( CsI) nCs1 cluster ions

Ab initio calculations using the Perturbed Ion model, with correlation contributions included, are presented for nonstoichiometric (NaI)_nNa+ and (CsI)_nCs+ (n=1-14) cluster ions. The ground state and several low-lying isomers are identified and described. Rocksalt ground states are common and appear at cluster sizes lower than in the corresponding neutral systems. The most salient features of the measured mobilities seem to be explained by arguments related to the changes of the compactness of the clusters as a function of size. The stability of the cluster ions against evaporation of a single alkali halide molecule shows variations that explain the enhanced stabilities found experimentally for cluster sizes n=4, 6, 9, and 13. Finally, the ionization energies and the orbital eigenvalue spectrum of two (NaI)_13Na+ isomers are calculated and shown to be a fingerprint of the structure.

Spontaneous magnetization of aluminum nanowires deposited on the NaCl(100) surface

We investigate electronic structures of Al quantum wires, both unsupported and supported on the (100) NaCl surface, using the density-functional theory. We confirm that unsupported nanowires, constrained to be linear, show magnetization when elongated beyond the equilibrium length. Allowing ions to relax, the wires deform to zigzag structures with lower magnetization but no dimerization occurs. When an Al wire is deposited on the NaCI surface, a zigzag geometry emerges again. The magnetization changes moderately from that for the corresponding unsupported wire. We analyze the findings using electron band structures and simple model wires.

Edge states and flat bands in graphene nanoribbons with arbitrary geometries

Trabajo presentado a la 14th edition of Trends in Nanotechnology International Conference, celebrada en Sevilla (Espana) del 9 al 13 de Septiembre de 2013.