Ni-Mn-Ga thin films produced by pulsed laser deposition

Polycrystalline Ni-Mn-Ga thin films have been deposited by the pulsed laser deposition (PLD) technique, using slices of a Ni-Mn-Ga single crystal as targets and onto Si (100) substrates at temperatures ranging from 673 K up to 973 K. Off-stoichiometry thin films were deposited at a base pressure of 1×10-6-Torr or in a 5 mTorr Ar atmosphere. Samples deposited in vacuum and temperatures above 823 K are magnetic at room temperature and show the austenitic {220} reflection in their x-ray diffraction patterns. The temperature dependences of both electrical resistance and magnetic susceptibility suggest that these samples exhibit a structural martensitic transition at around 260 K. The magnetoresistance ratio at low temperature can be as high as 1.3%, suggesting the existence of a granular structure in the films

Magnetic field induced entropy change and magnetoelasticity in Ni-Mn-Ga alloys

The magnetocaloric effect that originates from the martensitic transition in the ferromagnetic Ni-Mn-Gashape-memory alloy is studied. We show that this effect is controlled by the magnetostructural coupling at boththe martensitic variant and magnetic domain length scales. A large entropy change induced by moderatemagnetic fields is obtained for alloys in which the magnetic moment of the two structural phases is not verydifferent. We also show that this entropy change is not associated with the entropy difference between themartensitic and the parent phase arising from the change in the crystallographic structure which has beenfound to be independent of the magnetic field within this range of fields.

First-principles study of the neutral molecular metal Ni(tmdt)2.

The electronic structure of the molecular solid Ni(tmdt)2, the only well characterized neutral molecular metal to date, has been studied by means of first-principles density functional calculations. It is shown that these calculations correctly describe the metallic vs semiconducting behavior of molecular conductors of this type. The origin of the band overlap leading to the metallic character and the associated Fermi surfaces has been studied.

Ab initio study of MF2 (M=Mn, Fe, Co, Ni) rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the ground-state structural, electronic, and magnetic properties of the rutile-type compounds MF2 (M=Mn, Fe, Co, and Ni). All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the [001], [111], and [100] (or [010]) directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed.

Systematic study of the temperature dependence of the saturation magnetization in Fe, Fe-Ni and Co-based amorphous alloys

Magnetization versus temperature in the temperature interval 2-200 K was measured for amorphous alloys of three different compositions: Fe 81.5B14.5Si4, Fe40Ni38 Mo4B18, and Co70Fe5Ni 2Mo3B5Si15. The measurements were performed by means of a SQUID (superconducting quantum interference device) magnetometer. The aim was to extract information about the different mechanisms contributing to thermal demagnetization. A powerful data analysis technique based on successive minimization procedures has demonstrated that Stoner excitations of the strong ferromagnetic type play a significant role in the Fe-Ni alloy studied. The Fe-rich and Co-rich alloys do not show a measurable contribution from single-particle excitations.

Controlling the spin of metal atoms adsorbed on oxide surfaces: Ni on regular and defective sites of the MgO(001) surface

We have analyzed the relative energy of nonmagnetic and magnetic low-lying electronic states of Ni atoms adsorbed on regular and defective sites of the MgO(001) surface. To this end cluster and periodic surface models are used within density functional theory. For Ni atoms adsorbed on oxygen vacancies at low coverage, the interaction energy between the metal and the support is much larger than on regular sites. Strong bonding results in a diamagnetic adsorbed species and the energy required to reach the high-spin state increases. Moreover, a correlation appears between the low-spin to high-spin energy difference and the interaction energy hypothesizing that it is possible to prepare the surface to tune the high-spin to low-spin energy difference. Magnetic properties of adsorbed thin films obtained upon increasing coverage are more difficult to interpret. This is because the metallic bond is readily formed and dominates over the effect of the atoms directly bound to the vacancy.

Los determinantes sociales del pensamiento: ni anomalismo ni funcionalización

El principal objetivo del presente trabajo es indagar si las habituales tentativas por descifrarel pensamiento desde un punto de vista sociológico, a pesar de su vehemente compromisoantirreduccionista, podrían aceptar en último término la reducción de ideas, argumentos y doctrinasa sus presuntos determinantes sociales. En una primera etapa analizamos el ¿sociologismoanómalo¿ de Martin Kusch, señalamos algunas de sus lagunas, y establecemos que la panopliaconceptual en losofía de la mente es fértilmente aplicable al ámbito racional-social. A continuaciónconsideramos el funcionalismo reduccionista de Jaegwon Kim, nos centramos en latesis de que los estados intencionales de la mente, al contrario de lo que ocurre con los qualia,son causalmente funcionalizables, e intentamos establecer sus posibles repercusiones en la interpretaciónsocial del pensamiento. La aplicación de la metodología funcionalista al horizonteracional-social, de todos modos, parece venir desmentida por la evidencia, ya que las propiedadesdel pensamiento equiparables a los qualia son de hecho eminentemente funcionalizablesen la medida que fundamentan la efectividad social de los recursos gurativos y retóricos. Enun sentido análogo, la obra de Hans Blumenberg demuestra que los inescrutables ingredientes gurativos de todo pensamiento que él denomina ¿metáforas absolutas¿ son tan decisivos paracualquier proceso abstractivo que los conceptos nunca llegan a agotar su alcance determinador.Con lo cual los conceptos y las ideas más generales, antes considerados del todo funcionalizables,ahora dependen de algunas metáforas básicas, aun cuando por analogía con los qualia selas creía inmunes a toda funcionalización. Por tanto parece que la reducción no puede resolverlas di cultades que plantean los presuntos ¿determinantes sociales del pensamiento¿. Al mismotiempo, y con rmando así algunos emergentismos, los aspectos causal-funcionales del ámbitoracional-social dan la impresión de ser inconciliables con la enigmática estructura causal queponen de mani esto las aproximaciones losó cas al problema mente-cuerpo.