Atomistic Simulation of Surface Effects on Magnetic Properties of Alloy Nanomaterials

In the magnetic recording industry, L10 ordered CoPt and FePt nanoparticles have been considered as promising material candidates to advance the recording density beyond 1Tbit/in2. Compared with their bulk form, these alloy nanoparticles exhibit inferior magnetic properties. Surface effects, which are much more pronounced in nanometer scale, have been suggested to contribute to the deteriorated properties. In this work, surface related phenomena in these alloys are explored using atomistic simulation method. Density Functional Theory (DFT) calculations on the surface segregation effect have been performed in cuboidal, cuboctahedral nanoparticles and the related low index surfaces of L10 ordered CoPt alloy. Pt surface segregation to the outermost surface is found thermodynamically favorable in both nanoparticles and crystallographic surfaces. This segregation causes directly the break in structural, chemical ordering and accordingly the reduction in magnetic moment and change in magnetic anisotropy. Under 2nd order perturbation theory, the magnetic anisotropy energy on surface slabs has been associated with the change in d_(z^2 ) state density of surface Co atoms in the minority spin channel. Moreover, the magnetic properties of CoPt and FePt nanoparticles are demonstrated to be affected by particle shape using DFT calculations. This shape dependent magnetism is found correlated with the contraction in atomic spacing and local chemical composition. In addition, the surface spin canting mechanisms are identified for CoPt and FePt cuboctahedral nanoparticles. The different spin canting fashions for these two materials have been reproduced by micromagnetic simulation using Néel’s surface anisotropy model. The relationship between magnetoelastic coupling and Néel’s anisotropy constant in tetragonal lattice has been established. Through the calculation of Néel’s anisotropy constant from first principles, the different spin canting mechanisms have been explained. Finally, the effect of doping Cu, Ag and Au atoms on CoPt and FePt surfaces has been investigated. The Pt surface segregation has been found suppressed by the impurity atoms and the magnetic moment of surface Co/Fe atoms is restored up to the value of corresponding bulk-terminated surface. These additive atoms are proved to be beneficial for the improvement of magnetic properties on CoPt (001) surface and FePt (100) surface

0++0^{++} scalar glueball in finite-width Gaussian sum rules

Based on a semiclassical expansion for quantum chromodynamics in the instanton liquid background, the correlation function of the $0^{++}$ scalar glueball current is given, and the properties of the $0^{++}$ scalar glueball are studied in the framework of Gaussian sum rules. Besides the pure classical and quantum contributions, the contributions arising from the interactions between the classical instanton fields and quantum gluons are come into play. Instead of the usual zero-width approximation for the resonance, the Breit-Wigner form for the spectral function of the finite-width resonance is adopted. The family of the Gaussian sum rules for the scalar glueball in quantum chromodynamics with and without light quarks is studied. A consistency between the subtracted and unsubtracted sum rules is very well justified, and the values of the decay width and the coupling to the corresponding current for the $0^{++}$ resonance, in which the scalar glueball fraction is dominant, are obtained.Comment: 18pages, 9figure