Papers by Author: José Trinidad Elizalde Galindo

Abstract: The second derivative of the remission function of several magnetic materials is calculated for the parameterization of the position and intensity of the absorption bands of diffuse reflectance spectroscopy. The reflectance spectra are obtained by ultraviolet-visible spectroscopy (UV-VIS) from 400 to 1100 nm at increments of 1nm. The noise of the remission function results on errors after calculating the second derivative. Therefore, filtering of the remission function is needed before taking any action on this signal. Several methods are tested in order to calculate the second derivative. The best polynomial resulted on a second order wavelet function which is applied to the filtered remission function. Light scattering Mie theory is used to prove the behaviour of the reflected light. This research provides a method to identify and quantify magnetic particles, as well as the crystal size.

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: 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.