Papers by Author: José Andrés Matutes-Aquino

Abstract: Energy band structure and density of states of the compound Y(Co,Fe)5 were obtained by the augmented spherical wave method. Also the localization of the magnetic moment inside the unit cell, the effect of pressure and the substitution of Co atoms by Fe atoms were studied at the temperature of 0 K. Metallic Co showed a magnetic moment of 1.5 B while metallic Y showed its non magnetic behavior. Inside the compound YCo5 the Y atom had a contribution of –0.3 B to the total magnetization of the unit cell. A negative pressure caused the change of the Fermi energy and the modification of the magnetic moment. With a big enough positive pressure the total magnetic moment dropped to zero. The substitution of Co by Fe caused a change in the compound from strong ferromagnetic to soft ferromagnetic.

Abstract: Nanocrystalline YCo5 powders with high coercivity were prepared by mechanical milling and subsequent heat treatment at 820 °C for different annealing times, ta = 2.5, 3.0, 3.5 and 4.5 min, obtaining average crystallite sizes of  17, 19, 32 and 39 nm., respectively. The coercivity values were determined from the hysteresis loops measured at maxima fields of Hm = 5 and 20 T. The highest coercivity was obtained for the sample exhibiting  19 nm, where at room temperature and Hm = 5 T, the coercivity value is of 9.0 kOe. At 77 K and Hm = 5 T, the coercivity increase to 11.8 kOe and for Hm = 20 T, a higher value such as 13.1 kOe was found. The Ms/Mr ratio is enhanced to 0.62 indicating the occurrence of exchange interaction among nanocrystalline magnets.

Abstract: The dielectric properties of the intermetallic cubic Laves phase compound YFe2 were determined by analyzing the low loss region of the EELS spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) was obtained by Kramers-Kronig analysis. Since maxima observed in the OJDS spectra are assigned to interband transitions; these spectra can be interpreted on the basis of numerical calculations performed with the Wien2k code, using the Fully-Linearized-Augmented-Plane wave (FLAPW) method within the Local-Spin-Density Approximation. Comparison between experimental results and theory shows good agreement.

Abstract: Nanocrystalline Sm0.5Y0.5Co5 powders (average crystal size d = 12 nm) were produced by arc melting pure metals followed by mechanical milling and annealing. Different milling/annealing times and annealing temperatures were used to optimize the hard magnetic properties of these nanopowders. A noticeably enhanced coercivity and remanence (coercivity of 2.1 MA/m, and σr/σmax = 0.7 respectively) were observed in samples milled for 240 minutes and then annealed for 1 minute at Temperatures ~1200K. Such remarkable magnetic properties stem from the high magnetocrystalline anisotropy field and the homogeneous grain size of the Sm0.5Y0.5Co5 nanocrystals.