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Full-Text Articles in Physics
Temperature Dependence Of Half-Metallic Hybridization Gaps, Ralph Skomski
Temperature Dependence Of Half-Metallic Hybridization Gaps, Ralph Skomski
Ralph Skomski Publications
The temperature dependence of the spin polarization of half-metallic ferromagnets is investigated by model calculations. Emphasis is on changes in the density of states, especially on the closure of the hybridization gap. The Stoner-type calculation yields a simple one-electron expression for the gap as a function of temperature and a corresponding transition temperature T*, above which the spin polarization is drastically reduced. The small parameter determining the finite-temperature breakdown of the spin splitting is the width of the hybridization gap relative to other bandwidth parameters. The model predictions are consistent with the experiment and with first-principles electronic structure calculations, …
Hysteresis Of Ultrasmall Fe–Pt Particles, Ralph Skomski, J. Ping Liu, C. B. Rong, David J. Sellmyer
Hysteresis Of Ultrasmall Fe–Pt Particles, Ralph Skomski, J. Ping Liu, C. B. Rong, David J. Sellmyer
Ralph Skomski Publications
The magnetization reversal in very small FePt particles is investigated by analytical and numerical calculations. The modeling focuses on particles with diameters from 3 to 15 nm, as produced by a salt-matrix annealing technique. Experiment shows that the particles exhibit a certain degree of structural inhomogenity, which has a far-reaching effect on the magnetic hysteresis. In particles larger than about 10 nm, the magnetization-reversal mode is strongly inhomogeneous, and there are several scenarios that depend on the symmetry of inhomogenity. Small particles reverse nearly coherently, and the coercivity is essentially equal to the volume-averaged anisotropy. In this case, nonrectangular hysteresis …
Nucleation And Wall Motion In Graded Media, Ralph Skomski, Thomas A. George, David J. Sellmyer
Nucleation And Wall Motion In Graded Media, Ralph Skomski, Thomas A. George, David J. Sellmyer
Ralph Skomski Publications
Magnetization reversal in graded magnetic-recording media and its effect on the areal density are investigated by model calculations. By choosing suitable solid-solution materials it is conceptually straightforward, though practically challenging, to achieve arbitrarily low write fields. The writing process involves both the nucleation of reverse domains and their propagation along elongated particles. The performance of the medium is optimized for pinning and nucleation fields of comparable size, and the two fields can be tuned by adjusting the length of the elongated particles (pillars) and the anisotropies of the hard and soft ends. However, the write-field reduction negatively affects the areal …
Temperature- And Field-Induced Entropy Changes In Nanomagnets, Ralph Skomski, Christian Binek, Tathagata Mukherjee, Sarbeswar Sahoo, David J. Sellmyer
Temperature- And Field-Induced Entropy Changes In Nanomagnets, Ralph Skomski, Christian Binek, Tathagata Mukherjee, Sarbeswar Sahoo, David J. Sellmyer
Ralph Skomski Publications
Room-temperature magnetic-entropy changes in nanostructures for magnetic refrigeration are investigated by model calculations. Using a mean-field approach, the magnetic entropy is calculated as a function of temperature, magnetic field, particle size, anisotropy, and interaction strength. Both isotropic (Heisenberg) and uniaxial (Ising and XY) anisotropies are considered. The nanoparticle entropy strongly depends on the character of the anisotropy, in contrast to atomic ferromagnetism, where the anisotropy energy is much smaller than the interaction energy. Most promising are isotropic particles and particles with weak easy axis anisotropy, as well as easy-plane particle with the field in the plane. The optimum nanoparticle size …
Band-Structure And Correlation Effects In The Co(111) Planes Of Coo, Ralph Skomski, Xiaohui Wei, David J. Sellmyer
Band-Structure And Correlation Effects In The Co(111) Planes Of Coo, Ralph Skomski, Xiaohui Wei, David J. Sellmyer
Ralph Skomski Publications
The origin of the striking magnetic anisotropy of cobaltous oxide (CoO) is investigated by model calculations. A key feature of the electronic structure of the antiferromagnetic compound is the accommodation of Co 3d electrons in the (111) Co planes, with parallel and antiparallel intra- and interplane spin orientations and reminiscent of Cu 3d electrons in the CuO2 planes in high-temperature superconductors. The strong correlations of the Co 3d electrons lead to a picture intermediate between traditional crystal-field and itinerant-electron descriptions of the magnetic anisotropy. Using a simple configuration-interaction approach, we analyze the effect of 3d …
Substrate-Controlled Growth And Magnetism Of Nanosize Fe Clusters On Pt, Ralph Skomski, J. Zhang, V. Sessi, Jan Honolka, K. Kern, Axel Enders
Substrate-Controlled Growth And Magnetism Of Nanosize Fe Clusters On Pt, Ralph Skomski, J. Zhang, V. Sessi, Jan Honolka, K. Kern, Axel Enders
Ralph Skomski Publications
The growth and magnetism of nanosize Fe clusters on Pt and other metal surfaces are investigated. Fe clusters have been fabricated directly on the substrates by buffer layer assisted growth under ultrahigh vacuum conditions. The mean cluster diameter and the average cluster spacing were controlled by the Fe coverage and the buffer layer thickness. The enhanced magnetic anisotropy of such clusters of diameters between 0.5 and 10 nm with respect to bulk is discussed. Interface anisotropy contributions are compared with direct dipolar cluster-cluster interaction and indirect interactions mediated by the substrate, including preasymptotic ferromagnetic interaction. It is found that this …
Proteresis In Co:Coo Core-Shell Nanoclusters, X.-H. Wei, Ralph Skomski, Zhiguang Sun, David J. Sellmyer
Proteresis In Co:Coo Core-Shell Nanoclusters, X.-H. Wei, Ralph Skomski, Zhiguang Sun, David J. Sellmyer
Ralph Skomski Publications
The magnetism of ultrasmall Co:CoO core-shell nanoclusters is investigated. The structures, produced by cluster-beam deposition, have Co core sizes ranging from 1 to 7 nm and a CoO shell thickness of about 3 nm. Hysteresis loops as well as zero-field-cooled and field-cooled magnetization curves have been measured and a striking feature of the nanostructures is the existence of proteretic (clockwise) rather than hysteretic loops in the core-size range from 3 to 4 nm. The proteretic behavior and its particle-size dependence reflect a subtle interplay between various anisotropies and exchange interactions in the Co and CoO phases and at the Co–CoO …
Nanostructure And Magnetic Properties Of L10 Fept:X Films, Thomas A. George, Zhen Li, Minglang Yan, Yingfan Xu, Ralph Skomski, David J. Sellmyer
Nanostructure And Magnetic Properties Of L10 Fept:X Films, Thomas A. George, Zhen Li, Minglang Yan, Yingfan Xu, Ralph Skomski, David J. Sellmyer
Ralph Skomski Publications
Nonepitaxial FePt: X films (X =Cu,Au,CuAu) with tunable magnetic properties are fabricated and investigated. Emphasis is on controlling and adjusting the magnetic properties of high-density perpendicular recording media through exchange decoupling and anisotropy. The films are initially deposited as multilayers with the structure [FePt&#;X]n and have individual thicknesses from about 0.06 to 1.1 nm. To create an (001)-oriented granular L10 structure, the films are then annealed at temperatures of 600 °C for 5 min and 550 °C for 10 min. The data indicate that Cu enters the L10 lattice whereas Au segregates at the …
Ordered Layers Of Co Clusters On Bn Template Layers, J. Zhang, V. Sessi, C. H. Michaelis, I. Brihuega, Jan Honolka, K. Kern, Ralph Skomski, X. Chen, Geoffrey Rojas, Axel Enders
Ordered Layers Of Co Clusters On Bn Template Layers, J. Zhang, V. Sessi, C. H. Michaelis, I. Brihuega, Jan Honolka, K. Kern, Ralph Skomski, X. Chen, Geoffrey Rojas, Axel Enders
Ralph Skomski Publications
The synthesis of highly ordered monolayers of metallic nanoclusters by a buffer-layer assisted growth (BLAG) route is introduced and investigated. The focus is on clusters of Co deposited onto mechanically stable, periodically corrugated boron-nitride layers by repeated BLAG cycles. The approach combines the advantages of well-established preparation methods for surface-supported clusters; namely, the versatility of cluster deposition from the gas phase and the positional accuracy of the directed growth on template surfaces. The particle coverage and geometry are obtained from scanning tunneling microscopy experiments, and analyzed with analytic models and by Monte Carlo simulations. The model shows that the approach …