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University of Nebraska - Lincoln

2000

David Sellmyer Publications

Articles 1 - 12 of 12

Full-Text Articles in Physics

Sm–Co–Cu–Ti High-Temperature Permanent Magnets, Jian Zhou, Ralph Skomski, C. Chen, George C. Hadjipanayis, David J. Sellmyer Sep 2000

Sm–Co–Cu–Ti High-Temperature Permanent Magnets, Jian Zhou, Ralph Skomski, C. Chen, George C. Hadjipanayis, David J. Sellmyer

David Sellmyer Publications

A class of promising permanent-magnet materials with an appreciable high-temperature coercivity of 8.6 kOe at 500 °C is reported. The Sm–Co–Cu–Ti magnets are prepared by arc melting and require a suitable heat treatment. Magnetization measurements as a function of temperature and x-ray diffraction patterns indicate that the samples are two-phase mixtures of 2:17 and 1:5 structures. Depending on heat treatment and composition, some of the magnets exhibit a positive temperature coefficient of coercivity. The promising high-temperature behavior of the coercivity is ascribed to the temperature dependence of the domain-wall energy, which affects the curvature ...


Magnetic Localization In Transition-Metal Nanowires, Ralph Skomski, H. Zeng, M. Zheng, David J. Sellmyer Aug 2000

Magnetic Localization In Transition-Metal Nanowires, Ralph Skomski, H. Zeng, M. Zheng, David J. Sellmyer

David Sellmyer Publications

Magnetization reversal in transition-metal nanowires is investigated. Model calculations explain why magnetization reversal is localized, as opposed to the sometimes assumed delocalized coherent-rotation and curling modes. The localization is a quite general phenomenon caused by morphological inhomogenities and occurring in both polycrystalline and single-crystalline wires. In the polycrystalline limit, the competition between interatomic exchange and anisotropy gives rise to a variety of random-anisotropy effects, whereas nearly single-crystalline wires exhibit a weak localization of the nucleation mode. Model predictions are used to explain the coercive and magnetic-viscosity behavior of Co (and Ni) nanowires electrodeposited in self-assembled alumina pores.


Nanostructure And Magnetic Properties Of Composite Copt:C Films, M. Yu, Yi Liu, David J. Sellmyer May 2000

Nanostructure And Magnetic Properties Of Composite Copt:C Films, M. Yu, Yi Liu, David J. Sellmyer

David Sellmyer Publications

The nanostructure and magnetic properties of composite CoPt:C films at room temperature were investigated as a function of annealing temperature, carbon concentration, and film thickness. CoPt:C films with a variety of carbon concentrations were fabricated by cosputtering Co, Pt, and C onto water-cooled Si(100) substrates followed by annealing. X-ray diffraction and transmission electron microscopy analyses indicate that nanocrystallites of face-centered-tetragonal (fct) CoPt phase, which has a uniaxial magnetocrystalline anisotropy constant of about 5×107 erg/cm3, can be formed in carbon matrix when the annealing temperature is higher than 600 °C. The grain sizes of ...


Temperature Dependence Of Magnetic Hysteresis Of RcoX:Co Nanocomposites (R=Pr And Sm), J. Ping Liu, Ralph Skomski, Yi Liu, David J. Sellmyer May 2000

Temperature Dependence Of Magnetic Hysteresis Of RcoX:Co Nanocomposites (R=Pr And Sm), J. Ping Liu, Ralph Skomski, Yi Liu, David J. Sellmyer

David Sellmyer Publications

The temperature dependence of magnetic hysteresis of RCox:Co (R=Pr and Sm) nanocomposite films is reported. These films are prepared by dc and rf sputtering and subsequent thermal processing. It is found that the squareness of the hysteresis loops deteriorates with decreasing temperature. This is attributed to the enhanced magnetic anisotropy of the hard phases at low temperatures. The analysis of the magnetic reversal process shows that the anisotropy enhancement leads to a transition from cooperated to independent behavior. This analysis agrees well with the experimental results.


Structure And Magnetic Properties Of N-Containing Nd–Fe–B Alloys Prepared By Mechanical Alloying, W. Liu, Z.D. Zhang, J. Ping Liu, X.K. Sun, David J. Sellmyer May 2000

Structure And Magnetic Properties Of N-Containing Nd–Fe–B Alloys Prepared By Mechanical Alloying, W. Liu, Z.D. Zhang, J. Ping Liu, X.K. Sun, David J. Sellmyer

David Sellmyer Publications

The structure and magnetic properties of Nd2Fe14BNδ -based alloys prepared by mechanical alloying have been investigated. For the Nd14Fe78B8Ny series, a large amount of NdN, α-Fe, and a small Nd2Fe14B phase are observed in x-ray diffraction patterns. With increasing Nd content, the amount of the Nd2Fe14BNδ phase increases and α-Fe decreases gradually. When x≥25 in the NdxFe92-xB8Ny systems, some amount of the N-containing, Nd-rich phase appears. Meanwhile, the content of the ...


Effects Of Ga Substitution For Fe On The Structure And Magnetic Properties Of Nd8.4Fe87.1-XGaXB4.5 (X = 0–2.2) Alloys Prepared By Mechanical Alloying, B.Z. Cui, X.K. Sun, W. Liu, Z.D. Zhang, D.Y. Geng, X.G. Zhao, J. Ping Liu, David J. Sellmyer May 2000

Effects Of Ga Substitution For Fe On The Structure And Magnetic Properties Of Nd8.4Fe87.1-XGaXB4.5 (X = 0–2.2) Alloys Prepared By Mechanical Alloying, B.Z. Cui, X.K. Sun, W. Liu, Z.D. Zhang, D.Y. Geng, X.G. Zhao, J. Ping Liu, David J. Sellmyer

David Sellmyer Publications

The effects of a partial substitution of Ga for Fe on the structure and magnetic properties of mechanically alloyed Nd8.4Fe87.1B4.5 were studied. With increasing Ga content, the magnetic properties of the nanocomposite magnets first improve, reach maxima at 1.1 at.% Ga, and then degrade. The addition of Ga helps to control the morphology, especially the grain sizes of both α -Fe and Nd2Fe14B.


Curie Temperature Of Multiphase Nanostructures, Ralph Skomski, David J. Sellmyer May 2000

Curie Temperature Of Multiphase Nanostructures, Ralph Skomski, David J. Sellmyer

David Sellmyer Publications

The Curie temperature and the local spontaneous magnetization of ferromagnetic nanocomposites are investigated. The macroscopic character of the critical fluctuations responsible for the onset of ferromagnetic order means that there is only one Curie temperature, independent of the number of magnetic phases present. The Curie temperature increases with the grain size and is, in general, larger than predicted from the volume averages of the exchange constants. However, the Curie-temperature enhancement is accompanied by a relative reduction of the spontaneous magnetization. Due to the quadratic dependence of the permanent-magnet energy product on the spontaneous magnetization, this amounts to a deterioration of ...


Magnetic Properties Of Cluster-Beam-Synthesized Cobalt: Noble-Metal Films, J.M. Meldrim, Y. Qiang, Yi Liu, H. Haberland, David J. Sellmyer May 2000

Magnetic Properties Of Cluster-Beam-Synthesized Cobalt: Noble-Metal Films, J.M. Meldrim, Y. Qiang, Yi Liu, H. Haberland, David J. Sellmyer

David Sellmyer Publications

A cluster-beam deposition technique has been used to produce magnetic clusters embedded in a nonmagnetic matrix. We report here on films with cobalt clusters of average diameter of 5.5 nm embedded both in Cu and Ag. Volumetric concentrations of Co ranged from 10% to 50%. Magnetization and low temperature hysteresis loops, both field cooled (FC) and zero-field cooled (ZFC), have been measured between 4.2 and 300 K. The FC and ZFC magnetization bifurcate at or above room temperature with the clusters having a nonzero remanence at room temperature. Low temperature hysteresis loops exhibit a two-phase nature with one ...


Structure And Magnetic Properties Of Smco7-XTiX With Tbcu7-Type Structure, Jian Zhou, I.A. Al-Omari, J. Ping Liu, David J. Sellmyer May 2000

Structure And Magnetic Properties Of Smco7-XTiX With Tbcu7-Type Structure, Jian Zhou, I.A. Al-Omari, J. Ping Liu, David J. Sellmyer

David Sellmyer Publications

The SmCo7-xTix, x = 0–0.56 bulk samples are prepared by arc melting. X-ray diffraction indicates that samples with 0.2<x7-type structure phase and other minor phases appear for other values of x, which indicates that Ti helps stabilize the 1-7 phase. The lattice parameters ratio (c/a) increases with increasing Ti concentration. Room temperature saturation magnetization and Curie temperature decrease with increasing x. X-ray diffraction and magnetization measurements on aligned samples show that all samples studied have uniaxial anisotropy. The anisotropy field is found to increase with increasing x reaching a maximum of 175 ...


Magnetic Properties Of Self-Assembled Co Nanowires Of Varying Length And Diameter, H. Zeng, M. Zheng, Ralph Skomski, David J. Sellmyer, Yi Liu, L. Menon, Supriyo Bandyopadhyay May 2000

Magnetic Properties Of Self-Assembled Co Nanowires Of Varying Length And Diameter, H. Zeng, M. Zheng, Ralph Skomski, David J. Sellmyer, Yi Liu, L. Menon, Supriyo Bandyopadhyay

David Sellmyer Publications

Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal.


Magnetic And Structural Properties Of Smco7-XCuX Alloys, I.A. Al-Omari, Y. Yeshurun, Jian Zhou, David J. Sellmyer May 2000

Magnetic And Structural Properties Of Smco7-XCuX Alloys, I.A. Al-Omari, Y. Yeshurun, Jian Zhou, David J. Sellmyer

David Sellmyer Publications

We report the structural and magnetic properties of SmCo7-xCux, where x= 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7. X-ray diffraction shows that these alloys from the disordered hexagonal TbCu7-type structure. For large values of x (x≥0.8) the hexagonal TbCu7-type structure cannot be formed. X-ray diffraction on magnetically aligned samples show that these samples have uniaxial anisotropy. The lattice parameters (a and c) are dependent on the Cu concentration, and the unit cell volume is found to increase with x. The saturation magnetization decreases with ...


Hysteresis-Loop Overskewing In The Light Of A Novel Nucleation Mode, Ralph Skomski, J. Ping Liu, David J. Sellmyer May 2000

Hysteresis-Loop Overskewing In The Light Of A Novel Nucleation Mode, Ralph Skomski, J. Ping Liu, David J. Sellmyer

David Sellmyer Publications

It is demonstrated that the magnetostatic demagnetizing factor is not the only sample-shape dependent contribution to the hysteresis loop. There are also exchange contributions, which lead to interesting effects, particularly in nanostructures. An example is the recently discovered bulging mode in two-phase nanomagnets. The bulging mode exhibits the angular symmetry of the coherent mode, but it is incoherent due to its radial variation and gives rise to negative demagnetizing factors.