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Full-Text Articles in Physics
On The Ising Character Of The Quantum-Phase Transition In Lihof4, Ralph Skomski
On The Ising Character Of The Quantum-Phase Transition In Lihof4, Ralph Skomski
Ralph Skomski Publications
It is investigated how a transverse magnetic field affects the quantum-mechanical character of LiHoF4, a system generally considered as a textbook example for an Ising-like quantum-phase transition. In small magnetic fields, the low-temperature behavior of the ions is Ising-like, involving the nearly degenerate low-lying Jz = ± 8 doublet. However, as the transverse field increases, there is a substantial admixture of states having | Jz | < 8. Near the quantum-phase-transition field, the system is distinctively non-Ising like, and all Jz eigenstates yield ground-state contributions of comparable magnitude. A classical analog to this mechanism is the micromagnetic single point in magnets with uniaxial anisotropy. Since Ho3+ has …
Spin-Modified Catalysis, R Choudhary, Priyanka Manchanda, Axel Enders, Balamurugan Balamurugan, Arti Kashyap, David J. Sellmyer, Ralph A. Skomski
Spin-Modified Catalysis, R Choudhary, Priyanka Manchanda, Axel Enders, Balamurugan Balamurugan, Arti Kashyap, David J. Sellmyer, Ralph A. Skomski
Ralph Skomski Publications
First-principle calculations are used to explore the use of magnetic degrees of freedom in catalysis. We use the Vienna Ab-Initio Simulation Package to investigate both L10-ordered FePt and CoPt bulk materials and perform supercell calculations for FePt nanoclusters containing 43 atoms. As the catalytic activity of transition-metal elements and alloys involves individual d levels, magnetic alloying strongly affects the catalytic performance, because it leads to shifts in the local densities of states and to additional peaks due to magnetic-moment formation. The peak shift persists in nanoparticles but is surface-site specific and therefore depends on cluster size. Our research indicates that …
Correlations In Rare-Earth Transition-Metal Permanent Magnets, Ralph Skomski, Priyanka Manchanda, Arti Kashyap
Correlations In Rare-Earth Transition-Metal Permanent Magnets, Ralph Skomski, Priyanka Manchanda, Arti Kashyap
Ralph Skomski Publications
It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition- metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independentelectron level, the use of a single Slater determinant with broken spin symmetry introduces Hund’s rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDAþU approximations to DFT. By contrast, rareearth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. …
Intrinsic Magnetic Properties Of L10 Feni Obtained From Meteorite Nwa 6259, Eric Poirier, Frederick E. Pinkerton, Robert Kubic, Raja K. Mishra, Nina Bordeaux, Arif Mubarok, Laura H. Lewis, Joseph I. Goldstein, Ralph Skomski, Katayun Barmak
Intrinsic Magnetic Properties Of L10 Feni Obtained From Meteorite Nwa 6259, Eric Poirier, Frederick E. Pinkerton, Robert Kubic, Raja K. Mishra, Nina Bordeaux, Arif Mubarok, Laura H. Lewis, Joseph I. Goldstein, Ralph Skomski, Katayun Barmak
Ralph Skomski Publications
FeNi having the tetragonal L10 crystal structure is a promising new rare-earth-free permanent magnet material. Laboratory synthesis is challenging, however, tetragonal L10 FeNi—the mineral “tetrataenite”—has been characterized using specimens found in nickel-iron meteorites. Most notably, the meteorite NWA 6259 recovered from Northwest Africa is 95 vol.% tetrataenite with a composition of 43 at.% Ni. Hysteresis loops were measured as a function of sample orientation on a specimen cut from NWA 6259 in order to rigorously deduce the intrinsic hard magnetic properties of its L10 phase. Electron backscatter diffraction showed that NWA 6259 is strongly textured, containing L10 grains oriented along …
Magnetic Properties Of Fe-Doped Mnal, Priyanka Manchanda, Arti Kashyap, Jeffrey E. Shield, L. H. Lewis, Ralph Skomski
Magnetic Properties Of Fe-Doped Mnal, Priyanka Manchanda, Arti Kashyap, Jeffrey E. Shield, L. H. Lewis, Ralph Skomski
Ralph Skomski Publications
Electronic and magnetic properties of L10-ordered FexMn1−xAl alloys (x = 0, 0.0625, 0.125, 0.1875, 0.5) are investigated by first-principle supercell calculations. Pristine MnAl exhibits robust ferromagnetism involving the dense-packed Mn atoms in (001) planes of the tetragonal structure. Iron substitution for Mn significantly deteriorates the magnetization of the alloy. The reduction is a dilution effect, caused by the relatively small Fe moment of about 1.9 μB per atom, as compared to the Mn moment, which exceeds 2.4 μB. By contrast, 50% Fe substituted for Mn (x = 0.5) …
Practical Aspects Of Modern And Future Permanent Magnets, R.W. Mccallum, L. H. Lewis, Ralph Skomski, M. J. Kramer, I. E. Anderson
Practical Aspects Of Modern And Future Permanent Magnets, R.W. Mccallum, L. H. Lewis, Ralph Skomski, M. J. Kramer, I. E. Anderson
Ralph Skomski Publications
The mandate to reduce greenhouse gases will require highly efficient electric machines for both power generation and traction motor applications. Although permanent magnet electric machines utilizing Nd2Fe14B-based magnets provide obvious power-to-weight advantages over induction machines, the limited availability and high price of the rare earth (RE) metals make these machines less favorable. Of particular concern is the cost and supply criticality of Dy, a key RE element that is required to improve the high-temperature performance of Nd-based magnetic alloys for use in generators and traction motors. Alternatives to RE-based alloys do exist, but they currently lack the energy density necessary …
Berry Phase From A Randomly Fluctuating Magnetic Field, Ralph Skomski
Berry Phase From A Randomly Fluctuating Magnetic Field, Ralph Skomski
Ralph Skomski Publications
The quantum-mechanical Berry phase due to a randomly fluctuating magnetic field is calculated by exploiting an analogous random-walk problem in polymer physics. The phase depends on the time correlations of the magnetic field, despite the adiabatic nature of the Berry phase. A probability distribution for the phase is obtained, and how this phase could affect the magnetotransport in granular magnetic nanostructures is briefly discussed.
Permittivity And Permeability Of Fe(Tb) Nanoparticles And Their Microwave Absorption In The 2–18 Ghz Range, Z. Han, D. Li, M. Tong, X. Wei, Ralph Skomski, W. Liu, Z. D. Zhang, David J. Sellmyer
Permittivity And Permeability Of Fe(Tb) Nanoparticles And Their Microwave Absorption In The 2–18 Ghz Range, Z. Han, D. Li, M. Tong, X. Wei, Ralph Skomski, W. Liu, Z. D. Zhang, David J. Sellmyer
Ralph Skomski Publications
The permittivity and permeability of the Tb-doped and undoped Fe core-shell nanoparticles were investigated for frequencies from 2 to 18 GHz. The particles were synthesized by arc discharge and contain some oxygen, probably in the form of Fe2O3, Fe3O4, and Tb2O3, whereas the core material is Fe. Both the electromagnetic materials constants and the morphology of the Fe nanoparticles are changed by Tb addition, which gives rise to the shifts to higher frequencies and thinner thicknesses of the maximum microwave absorption in the Tb-doped Fe nanoparticles.
Structural, Magnetic And Magneto-Transport Properties Of Pt-Alloyed Mnbi Thin Films, Parashu Kharel, Ralph Skomski, Roger D. Kirby, David J. Sellmyer
Structural, Magnetic And Magneto-Transport Properties Of Pt-Alloyed Mnbi Thin Films, Parashu Kharel, Ralph Skomski, Roger D. Kirby, David J. Sellmyer
Ralph Skomski Publications
The structural, magnetic and magneto-transport properties of highly c-axis oriented Mn55−xPtxBi45 (x=0, 1.5, 3, and 4.5) thin films have been investigated. The coercivity of the Pt-alloyed thin films increases and the saturation magnetization decreases as the Pt concentration increases. The anisotropy field Ho increases as a function of Pt concentration, too but the coercivity increases more rapidly than the anisotropy field. This indicates an enhanced domain-wall pinning, caused by increased interstitial disorder due to the occupancy of regular Mn sites by Pt. The same mechanism explains the reduced magnetization. All samples exhibit a large …
Magnetism Of Core-Shell Ti:Tio Nanoparticles, Xiaohui Wei, Ralph Skomski, Balamurugan Balasubramanian, David J. Sellmyer
Magnetism Of Core-Shell Ti:Tio Nanoparticles, Xiaohui Wei, Ralph Skomski, Balamurugan Balasubramanian, David J. Sellmyer
Ralph Skomski Publications
Ti nanoparticles were produced using a cluster-deposition method. Ti:TiO core-shell structures were fabricated by partially oxidizing the surface of the Ti nanoparticles produced by a cluster-deposition system via multistep annealing at 250 °C in oxygen. X-ray diffraction and transmission-electron microscopy studies reveal an increase in the thickness of the TiO shell with increasing annealing time. The magnetic moment and the coercivity of the core-shell nanoparticles increase with the TiO shell thickness, which is consistent with homogenous bulk defects in TiO. The core-shell nanoparticles display an abnormal hysteresis loop, which probably reflects a combination of antiferromagnetic exchange and magnetocrystalline anisotropy.
Magnetic Antiphase Domains In Co/Ru/Co Trilayers, Zhen Li, Ralph Skomski, Steven A. Michalski, Lanping Yue, Roger D. Kirby
Magnetic Antiphase Domains In Co/Ru/Co Trilayers, Zhen Li, Ralph Skomski, Steven A. Michalski, Lanping Yue, Roger D. Kirby
Ralph Skomski Publications
Ultrathin Co/Ru/Co trilayers are investigated experimentally by magnetization curves and magnetic-force microscopy (MFM). Emphasis is on the domain-wall fine structure of antiphase domain walls in the films. The trilayers are produced by sputtering and consist of two Co layers of equal thickness (5 nm), exchange-coupled through a Ru layer of variable thickness. The sign and magnitude of the interlayer exchange are tuned by the thickness of the Ru interlayer. The exchange and its distribution are investigated by measurements of the static magnetization curves. For a Ru thickness of 0.4 nm, the exchange is predominantly antiferromagnetic and the MFM images show …
Interstitial-Nitrogen Effect On Phase Transition And Magnetocaloric Effect In Mn(As,Si) (Invited), W. B. Cui, X. K. Lv, F. Yang, Ralph Skomski, Y. Yu, X. G. Zhao, W. Liu, Z.D. Zhang
Interstitial-Nitrogen Effect On Phase Transition And Magnetocaloric Effect In Mn(As,Si) (Invited), W. B. Cui, X. K. Lv, F. Yang, Ralph Skomski, Y. Yu, X. G. Zhao, W. Liu, Z.D. Zhang
Ralph Skomski Publications
The effect of interstitial nitrogen on the phase transition and magnetocaloric behavior of MnAs1−xSixNδ (x=0.03, 0.06, and 0.09) is investigated. The interstitial nitrogen atoms cause the step-scanned x-ray diffraction peaks to shift toward lower angles and lower the Curie temperature, whereas silicon addition increases the Curie temperature to near room temperature. The thermal hysteresis is reduced to nearly 0 in MnAs1−xSixNδ, which is beneficial to practical applications. For a field change of 5 T, the largest magnetic entropy change and refrigerant capacity are 14.6 J kg−1 K−1 …
Permanent Magnetism Of Dense-Packed Nanostructures, Ralph Skomski, Yi Liu, Jeffrey E. Shield, George C. Hadjipanayis, David J. Sellmyer
Permanent Magnetism Of Dense-Packed Nanostructures, Ralph Skomski, Yi Liu, Jeffrey E. Shield, George C. Hadjipanayis, David J. Sellmyer
Ralph Skomski Publications
The effect of nanostructuring on magnetostatic interactions in permanent magnets is investigated by model calculations. Emphasis is on the energy product as a function of packing fraction of the magnetic phase, of the magnet’s macroscopic shape, and of the nanoscale feature size. The main difference between nanostructured and macroscopic magnetic bodies, namely, the transition between coherent and incoherent reversal, has a far-reaching impact on demagnetizing field and energy product. For small magnet sizes, the energy product is substantially enlarged, up to μ0Ms24 for soft magnetic materials, but this effect is difficult to exploit in real …
Magnetic Anisotropy In Itinerant Magnets, Ralph Skomski, Arti Kashyap, A. Solanki, Axel Enders, David J. Sellmyer
Magnetic Anisotropy In Itinerant Magnets, Ralph Skomski, Arti Kashyap, A. Solanki, Axel Enders, David J. Sellmyer
Ralph Skomski Publications
The structural dependence of the magnetocrystalline anisotropy of itinerant permanent magnets (or nanostructures of iron-series 3d elements) is investigated by model and tight-binding calculations. Magnetic nanostructures yield strong oscillations of the anisotropy as a function of the number of d electrons per atom, which can be tuned by alloying. While interatomic hopping is usually more important in metals than crystal-field interactions, we find substantial crystal-field corrections for some configurations, especially for the atomic square. Finally, we compare our results with Néel model.
Entropy Localization In Magnetic Compounds And Thin-Film Nanostructures, Ralph Skomski, Christian Binek, Steven A. Michalski, Tathagata Mukherjee, Axel Enders, David J. Sellmyer
Entropy Localization In Magnetic Compounds And Thin-Film Nanostructures, Ralph Skomski, Christian Binek, Steven A. Michalski, Tathagata Mukherjee, Axel Enders, David J. Sellmyer
Ralph Skomski Publications
The effect of nanostructuring on the magnetic entropy of materials for room-temperature magnetic cooling is investigated by model calculations. The materials are structurally inhomogeneous with a large number of nonequivalent crystallographic sites. In the mean-field Heisenberg model, the entropy density is a unique function of the local magnetization so that the coupled set of nonlinear mean-field equations yields not only the magnetization but also the entropy density. Since most of the entropy is localized near grain boundaries, nanomagnetic cooling requires small feature sizes. Magnetic anisotropy is a substantial complication, even on a mean-field level, but the corresponding corrections are often …
Magnetism Of Fept Surface Alloys, Jan Honolka, T. Y. Lee, K. Kuhnke, Axel Enders, Ralph Skomski, S. Bornemann, S. Mankovsky, J. Minar, J. Staunton, H. Ebert, M. Hessler, K. Fauth, G. Schutz, A. Buchsbaum, M. Schmid, P. Varga, K. Kern
Magnetism Of Fept Surface Alloys, Jan Honolka, T. Y. Lee, K. Kuhnke, Axel Enders, Ralph Skomski, S. Bornemann, S. Mankovsky, J. Minar, J. Staunton, H. Ebert, M. Hessler, K. Fauth, G. Schutz, A. Buchsbaum, M. Schmid, P. Varga, K. Kern
Ralph Skomski Publications
The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, x-ray magnetic circular dichroism, and ab initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex noncollinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk L10 magnets is not essential to achieve high-anisotropy values.
Dzyaloshinski–Moriya Micromagnetics Of Magnetic Surface Alloys, Ralph Skomski, Jan Honolka, S. Bornermann, H. Ebert, Axel Enders
Dzyaloshinski–Moriya Micromagnetics Of Magnetic Surface Alloys, Ralph Skomski, Jan Honolka, S. Bornermann, H. Ebert, Axel Enders
Ralph Skomski Publications
The magnetism of magnetic surface alloys such as Fe–Pt on Pt(997) has been investigated by micromagnetic model calculations. A unique feature of some of these systems is that interatomic exchange, magnetocrystalline anisotropy, and Dzyaloshinski–Moriya (DM) interactions are all of comparable magnitude, of the order of 0.5 meV. This leads to an intriguing nanoscale interplay between ordinary magnetization states and noncollinear spin structures. We identify two cases where the latter dominate. First, for sufficiently strong DM interaction, the zero-field spin structures change from uniform to a canted state with an incommensurate wave vector. Second, a similar transition occurs for weak DM …
Magnetic Correlations In Nanocomposite Fept:Au And Fept:C Films, Thomas A. George, Ralph Skomski, David J. Sellmyer
Magnetic Correlations In Nanocomposite Fept:Au And Fept:C Films, Thomas A. George, Ralph Skomski, David J. Sellmyer
Ralph Skomski Publications
The nanostructure and magnetism of L10-ordered FePt crystallites in matrices of Au and C are investigated as a function of the Au and C contents. Emphasis is on the relation between hysteresis-loop shape, magnetic correlation length, and structural disorder, as relevant to magnetic recording media. The highly (001) textured thin films are produced by annealing [Fe&#;&#;Pt&#;X]n multilayers. The coercivity and the hysteresis-loop slope at coercivity increase and decrease, respectively, with increasing volume fraction of the matrix. The magnetic force microscopy images show that the slope decrease is accompanied by a reduction in the magnetic correlation length. These …
Magnetism Of Tio And Tio2 Nanoclusters, Xiaohui Wei, Ralph Skomski, Balamurugan Balasubramanian, Z. G. Sun, Stephen Ducharme, David J. Sellmyer
Magnetism Of Tio And Tio2 Nanoclusters, Xiaohui Wei, Ralph Skomski, Balamurugan Balasubramanian, Z. G. Sun, Stephen Ducharme, David J. Sellmyer
Ralph Skomski Publications
Nanoclusters of rocksalt TiO, anatase TiO2, and rutile TiO2 were produced by cluster deposition and examined with transmission-electron microscopy, x-ray diffraction, and magnetization measurements. The clusters are all magnetic at room temperature, but the magnetization is structure-dependent. The hysteresis loops show coercivities that are of the order of 100 Oe and all films show a preferential in-plane magnetization direction. The size dependence of the magnetization was investigated for rutile clusters with average sizes from about 15 to 40 nm. The analysis of the measurements indicates that the magnetism is predominantly located near the surface of the clusters …
Graded Permanent Magnets, Ralph Skomski, J. Hadjipanayis, David J. Sellmyer
Graded Permanent Magnets, Ralph Skomski, J. Hadjipanayis, David J. Sellmyer
Ralph Skomski Publications
The effect of semihard magnetic phases and interfaces on the performance of nanostructured two-phase permanent magnets is investigated by model calculations. In addition to the trivial coercivity increase due to the replacement of soft regions by semihard regions, there is a coercivity enhancement even if the volume-averaged anisotropy is kept constant during the introduction of the semihard phase. A variational approach is used to derive analytical results for representative anisotropy profiles. The improvement is operative on length scales slightly larger than that of the soft phase in hard-soft composites, but the main challenge is to find semihard light or heavy …
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 …
Structural Phase Transition And Ferromagnetism In Monodisperse 3 Nm Fept Particles, Chuan-Bing Rong, Narayan Poudyal, Girija S. Chaubey, Vikas Nandwana, Ralph Skomski, Y. Q. Wu, Matthew J. Kramer, J. Ping Liu
Structural Phase Transition And Ferromagnetism In Monodisperse 3 Nm Fept Particles, Chuan-Bing Rong, Narayan Poudyal, Girija S. Chaubey, Vikas Nandwana, Ralph Skomski, Y. Q. Wu, Matthew J. Kramer, J. Ping Liu
Ralph Skomski Publications
FePt nanoparticles with a size of 3 nm and thermally stable room-temperature ferromagnetism are investigated. The monodisperse nanoparticles were prepared by chemical synthesis and a salt-matrix annealing technique. Structural and magnetic characterizations confirmed the phase transition from the disordered face-centered cubic structure to the L10 structure with the chemical ordering parameter of 0.62±0.05. Analysis in blocking temperature and fitting of temperature dependence of switching field reveals that the transformed 3 nm nanoparticles have a magnetic anisotropy constant of (2.8±0.2) x 106 J/m3, smaller than those for the bigger particles and the fully ordered L1 …