Physics Commons^™

Magnetocrystalline Anisotropy Of "-Fe2o3, Imran Ahamed, Rohilt Pathak, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

The epsilon Fe₂O₃ phase of iron oxide has been studied to understand the spin structure and the magnetocrystalline anisotropy in the bulk and in thin films of "-Fe₂O₃ and Co-doped "-Fe₂O₃. The preferential magnetization direction in the nanoparticles of "-Fe₂O₃ is along the a-axis [M. Gich et al., Chem. Mater. 18, 3889 (2006)]. Compared to the bulk band gap of 1.9 eV, the thin-film band gap is reduced to 1.3 eV in the Co-free films and to 0.7 eV in the film with partial …

Texture Development And Coercivity Enhancement In Cast Alnico 9 Magnets, Wenyong Zhang, Shah Valloppilly, Xingzhong Li, Lanping Yue, Ralph Skomski, Iver Anderson, Matthew Kramer, Wei Tang, Jeff Shield, David J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

The effect of Y addition and magnetic field on texture and magnetic properties of arc-melted alnico 9 magnets has been investigated. Small additions of Y (1.5 wt.%) develop a (200) texture for the arc-melted alnico 9 magnet. Such a texture is hard to form in cast samples. To achieve this goal, we set up a high-field annealing system with a maximum operation temperature of 1250⁰ C. This system enabled annealing in a field of 45 kOe with subsequent draw annealing for the solutionized buttons; we have been able to substantially increase remanence ratio and coercivity, from 0.70 and 1200 …

Alignment Of The (3D104S5S)3S1 State Of Zn Excited By Polarized Electron Impact, Nathan B. Clayburn, Timothy J. Gay

Timothy J. Gay Publications

We measure the integrated Stokes parameters of light from Zn (4s4p)4³P_0,1-(4s5s)5³S₁ transitions excited by a transversely polarized electron impact at energies between 7.0 and 8.5 eV. Our results for the electron-polarization-normalized linear polarization Stokes parameter P₂, between incident electron energies 7.0 and 7.4 eV, are consistent with zero, as required by basic angular-momentum coupling considerations and by recent theoretical calculations. They are in qualitative disagreement with previous experimental results for the P₂ parameter.

Three-Dimensional Nanomagnetism, Amalio Fernández-Pacheco, Robert Streubel, Olivier Fruchart, Riccardo Hertel, Peter Fischer, Russell P. Cowburn

Robert Streubel Papers

Magnetic nanostructures are being developed for use in many aspects of our daily life, spanning areas such as data storage, sensing and biomedicine. Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties. Here we review the creation of these structures and their implications for the emergence of new physics, the development of instrumentation and computational methods, and exploitation in numerous applications.

Nanosecond X-Ray Photon Correlation Spectroscopy On Magnetic Skyrmions, M. H. Seaberg, B. Holladay, J. C.T. Lee, M. Sikorski, A. H. Reid, S. A. Montoya, G. L. Dakovski, J. D. Koralek, G. Coslovich, S. Moeller, W. F. Schlotter, R. Streubel, S. D. Kevan, P. Fischer, E. E. Fullerton, J. L. Turner, F. J. Decker, S. K. Sinha, S. Roy, J. J. Turner

Robert Streubel Papers

We report an x-ray photon correlation spectroscopy method that exploits the recent development of the two-pulse mode at the Linac Coherent Light Source. By using coherent resonant x-ray magnetic scattering, we studied spontaneous fluctuations on nanosecond time scales in thin films of multilayered Fe/Gd that exhibit ordered stripe and Skyrmion lattice phases. The correlation time of the fluctuations was found to differ between the Skyrmion phase and near the stripe-Skyrmion boundary. This technique will enable a significant new area of research on the study of equilibrium fluctuations in condensed matter.

Simulation Of Alnico Coercivity, Liqin Ke, Ralph Skomski, Todd D. Hoffman, Lin Zhoue, Wei Tang, Duane D. Johnson, Matthew J. Kramer, Iver E. Anderson, C.Z. Wang

Nebraska Center for Materials and Nanoscience: Faculty Publications

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all …

Vortex Circulation Patterns In Planar Microdisk Arrays, Sven Velten, Robert Streubel, Alan Farhan, Noah Kent, Mi Young Im, Andreas Scholl, Scott Dhuey, Carolin Behncke, Guido Meier, Peter Fischer

Robert Streubel Papers

We report a magnetic X-ray microscopy study of the pattern formation of circulation in arrays of magnetic vortices ordered in a hexagonal and a honeycomb lattice. In the honeycomb lattice, we observe at remanence an ordered phase of alternating circulations, whereas in the hexagonal lattice, small regions of alternating lines form. A variation in the edge-to-edge distance shows that the size of those regions scales with the magnetostatic interaction. Micromagnetic simulations reveal that the patterns result from the formation of flux closure states during the nucleation process.

Effect Of Disorder On The Resistivity Of Cofecral Films, Y. Jin, R. Skomski, P. Kharel, S.R. Valloppilly, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Structural and electron-transport properties of thin films of the ferrimagnetic Heusler compound CoFeCrAl have been investigated to elucidate structure-property relationships. The alloy is, ideally, a spin-gapless semiconductor, but structural disorder destroys the spin-gapless character and drastically alters the transport behavior. Two types of CoFeCrAl films were grown by magnetron sputtering deposition at 973 K, namely polycrystalline films on Si substrates and epitaxial films on MgO (001) substrates. The resistivity decreases with increasing temperature, with relatively small temperature coefficients of –0.19 cm=K for the polycrystalline films and –0.12 cm=K for the epitaxial films. The residual resistivity of the polycrystalline films deposited …

Formation Of Mound-Like Multiscale Surface Structures On Titanium By Femtosecond Laser Processing, Edwin Peng, Alfred Tsubaki, Craig A. Zuhlke, Ryan Bell, Meiyu Wang, Dennis R. Alexander, George Gogos, Jeffrey E. Shield

Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research

Surface Functionalization Technique • Femtosecond Laser Surface Processing (FLSP) • Utilize high power, femtosecond (10-15 s) laser pulses • Produce self-organized, multiscale surface micro/nanostructures • Diverse range of applicable substrates: semiconductors, metals, polymers, & composites

Why? • What are the different types of FLSP structures on Ti? • Physical evidence needed for FLSP formation models • Optimize FLSP of Ti for biomedical & other applications

How? • Obtain evidence of mound growth processes by examining underlying microstructure • Utilize dual beam Scanning Electron Microscope-Focused Ion Beam instrument to cross section surface structures & fabricate transmission electron microscopy samples

Half-Metallic Magnetism In Ti3co5-Xfexb2, Rohit Pathak, Imran Ahamed, W. Y. Zhang, Shah Vallopilly, D. J. Sellmyer, Ralph Skomski, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

Bulk alloys and thin films of Fe-substituted Ti₃Co₅B₂ have been investigated by first-principle density-functional calculations. The series, which is of interest in the context of alnico magnetism and spin electronics, has been experimentally realized in nanostructures but not in the bulk. Our bulk calculations predict paramagnetism for Ti₃Co₅B₂, Ti₃Co₄FeB₂ and Ti₃CoFe₄B₂, whereas Ti₃Fe₅B₂ is predicted to be ferromagnetic. The thin films are all ferromagnetic, indicating that moment formation may be facilitated at nanostructural …

Cooperative And Noncooperative Magnetization Reversal In Alnicos, Raplh Skomski, Liqin Ke, Matthew J. Kramer, Iver E. Anderson, C.Z. Wang, W.Y. Zhang, Jeff E. Shield, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

It is investigated how magnetostatic interactions affect the coercivity of alnicotype magnets. Starting from exact micromagnetic relations, we analyze two limits, namely cooperative reversal processes operative on short lengths scales and noncooperative reversal processes on long length scales. In alnicos, intrawire interactions are predominantly cooperative, whereas interwire effects are typically noncooperative. However, the transition between the regimes depends on feature size and hysteresis-loop shape, and interwire cooperative effects are largest for nearly rectangular loops. Our analysis revises the common shape-anisotropy interpretation of alnicos.

Epitaxial Strain Controlled Magnetocrystalline Anisotropy In Ultrathin Ferh/Mgo Bilayers, Guohul Zheng, San-Huang Ke, Maosheng Miao, Jinwoong Kim, R, Ramesh, Nicholas Kioussis

Nebraska Center for Materials and Nanoscience: Faculty Publications

Using ab initio electronic structure calculations we have investigated the effect of epitazial strain on the magnetocrystalline anisotropy (MCA) of ultrathin FeRh/MgO heterostructures. Analysis of the energy- and k-resolved distribution of the orbital character of the band structure reveals that MCA largely arises from the spin-orbit coupling (SOC) between d_x2-y2 and d_xz/d_yz orbitlas of Fe atoms at the FeRh/MhO interface. We demonstrate that the strain has significant effects on the MCA: It not only affects the value of the MCA but also induces a switching of the magnetic easy axis from perpendicular on in-plane direction. The …

First-Principle Investigation Of Structural, Electronic And Magnetic Properties Of Co2vin And Covin Heusler Compounds, Zipporah Muthui, Rohit Pathak, Robinson Musembi, Julius Mwabora, Ralph Skomski, Arti Kashyap

Nebraska Center for Materials and Nanoscience: Faculty Publications

Investigation of the structural, electronic and magnetic properties of full-Heusler Co₂VIn as well as half-Heusler CoVIn Cobalt based Heusler compounds using density functional theory (DFT) leads to the general conclusion that Co₂VIn and CoVIn are half-metallic materials with a gap at the Fermi level in the minority states and majority states respectively. A Hubbard-like Coulomb correlation term U has been included in the DFT (DFT+U) for the computation of the electronic and magnetic properties of the compounds. The structural properties have been calculated for the paramagnetic and ferromagnetic phases, and both Co₂VIn …

Effect Of Disorder On The Magnetic And Electronic Structure Of A Prospective Spin-Gapless Semiconductor Mncrval, P. Kharel, J. Herran, P. Lukashev, Y. Jin, J. Waybright, S. Gilbert, B. Staten, P. Gray, S. Valloppilly, Y. Huh, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

Recent discovery of a new class of materials, spin-gapless semiconductors (SGS), has attracted considerable attention in the last few years, primarily due to potential applications in the emerging field of spin-based electronics (spintronics). Here, we investigate structural, electronic, and magnetic properties of one potential SGS compound, MnCr-VAl, using various experimental and theoretical techniques. Our calculations show that this material exhibits ≈ 0.5 eV band gap for the majority-spin states, while for the minority-spin it is nearly gapless. The calculated magnetic moment for the com- pletely ordered structure is 2.9 μ_B/f.u., which is different from our experimentally measured value …

Controlling The Microstructure And Associated Magnetic Properties Of Ni0.2mn3.2ga0.6 Melt-Spun Ribbons By Annealing, Mahmud Khan, Ohud Alshammari, Balamurugan Balamurugan, Bhaskar Das, David J. Sellmyer, Ahmad Us Saleheen, Shane Stadler

Nebraska Center for Materials and Nanoscience: Faculty Publications

Here we report on the structural and magnetic properties of Ni_0.2Mn_3.2Ga_0.6 melt- spun ribbons. The as-spun ribbons were found to exhibit mixed cubic phases that transform to non-cubic structure upon annealing. Additionally, an amorphous phase was found to co-exist in all ribbons. The SEM images show that minor grain forma- tion occurs on the as-spun ribbons. However, the formation of extensive nano-grains was observed on the surfaces of the annealed ribbons. While the as-spun ribbons exhibit predominantly paramagnetic behavior, the ribbons annealed under various thermal conditions were found to be ferromagnetic with a Curie temperature …

Effect Of Fe Substitution On The Structural, Magnetic And Electron-Transport Properties Of Half-Metallic Co2tisi, Y. Jin, J. Waybright, P. Kharel, I. Tutic, J. Herran, P. Lukashev, S. Valloppilly, D. J. Sellmyer

Nebraska Center for Materials and Nanoscience: Faculty Publications

The structural, magnetic and electron-transport properties of Co₂Ti_1-xFe_xSi (x = 0, 0.25, 0.5) ribbons prepared by arc-melting and melt-spinning were investi- gated. The rapidly quenched Co₂Ti_0.5Fe_0.5Si crystallized in the cubic L2₁ structure whereas Co₂Ti_0.75Fe_0.25Si and Co₂TiFe₀Si showed various degrees of B2-type disorder. At room temperature, all the samples are ferromagnetic, and the Curie tem- perature increased from 360 K for Co₂TiSi to about 800 K for Co₂Ti_0.5Fe_0.5Si. The measured …

Measurement Of The Integrated Stokes Parameters Of Zn 468 Nm Fluorescence Excited By Polarized-Electron Impact, Nathan B. Clayburn, Timothy J. Gay

Timothy J. Gay Publications

The integrated Stokes parameters P₁, P₂, and P₃ of Zn (4³P_0,1 – 5³S₁) fluorescence resulting from transversely-spin-polarized electron impact excitation of the Zn (4s5s)5³S₁ state have been measured. This work was motivated by similar studies reported several years ago, in which non-zero values of the integrated Stokes parameter P₂ between the threshold for the (4s5s)5³S₁ excitation and the first cascading (4s5p)5³P_J threshold were measured. We observe optical excitation functions in agreement with previous experimental and theoretical results, but …

Ch3Nh3Pbi3 Perovskites: Ferroelasticity Revealed, Evgheni Strelcov, Qingfeng Dong, Tao Li, Jungseok Chae, Yuchuan Shao, Yehao Deng, Alexei Gruverman, Jinsong Huang, Andrea Centrone

Alexei Gruverman Publications

Ferroelectricity has been proposed as a plausible mechanism to explain the high photovoltaic conversion efficiency in organic-inorganic perovskites; however, convincing experimental evidence in support of this hypothesis is still missing. Identifying and distinguishing ferroelectricity from other properties, such as piezoelectricity, ferroelasticity, etc., is typically nontrivial because these phenomena can coexist in many materials. In this work, a combination of microscopic and nanoscale techniques provides solid evidence for the existence of ferroelastic domains in both CH₃NH₃PbI₃ polycrystalline films and single crystals in the pristine state and under applied stress. Experiments show that the configuration of CH …

Tunneling Magnetoresistance Sensors With Different Coupled Free Layers, Yen-Fu Liu, Xiaolu Yin, Yi Yang, Dan Ewing, Paul J. De Rego, Sy-Hwang Liou

Nebraska Center for Materials and Nanoscience: Faculty Publications

Large differences of magnetic coercivity (HC), exchange coupling field (HE), and tun- neling magnetoresistance ratio (TMR) in magnetic tunnel junctions with different coupled free layers are discussed. We demonstrate that the magnetization behavior of the free layer is not only dominated by the interfacial barrier layer but also affected largely by the magnetic or non-magnetic coupled free layers. All these parameters are sensitively controlled by the magnetic nanostructure, which can be tuned also by the magnetic annealing process. The optimized sensors exhibit a large field sensitivity of up to 261%/mT in the region of the reversal synthetic ferri- magnet at …

Anomalous Photovoltaic Effect In Organic-Inorganic Hybrid Perovskite Solar Cells, Yongbo Yuan, Tao Li, Qi Wang, Jie Xing, Alexei Gruverman, Jinsong Huang

Alexei Gruverman Publications

Organic-inorganic hybrid perovskites (OIHPs) have been demonstrated to be highly successful photovoltaic materials yielding very-high-efficiency solar cells. We report the room temperature observation of an anomalous photovoltaic (APV) effect in lateral structure OIHP devices manifested by the device’s open-circuit voltage (V_OC) that is much larger than the bandgap of OIHPs. The persistent V_OC is proportional to the electrode spacing, resembling that of ferroelectric photovoltaic devices. However, the APV effect in OIHP devices is not caused by ferroelectricity. The APV effect can be explained by the formation of tunneling junctions randomly dispersed in the polycrystalline films, which allows …

Theoretical Approach To Electroresistance In Ferroelectric Tunnel Junctions, Sou-Chi Chang, Azad Naeemi, Dmitri E. Nikonov, Alexei Gruverman

Alexei Gruverman Publications

In this paper, a theoretical approach comprising the nonequilibrium Green’s function method for electronic transport and the Landau-Khalatnikov equation for electric polarization dynamics is presented to describe polarization-dependent tunneling electroresistance (TER) in ferroelectric tunnel junctions. Using appropriate contact, interface, and ferroelectric parameters, the measured current-voltage characteristic curves in both inorganic (Co/BaTiO₃/La_0.67Sr_0.33MnO₃) and organic (Au/PVDF/W) ferroelectric tunnel junctions can be well described by the proposed approach. Furthermore, under this theoretical framework, the controversy of opposite TER signs observed experimentally by different groups in Co/BaTiO₃/La_0.67Sr_0.33MnO₃ systems is …

Domain Formation Mechanism Of The Si(110) “16 × 2” Reconstruction, N. K. Lewis, Nathan B. Clayburn, Evan M. Brunkow, Timothy J. Gay, Y. Lassailly, J. Fujii, I. Vobornik, W. R. Flavell, E. A. Seddon

Timothy J. Gay Publications

The main factor that determines which of the two domains forms upon reconstruction of the Si(110) “16 × 2” surface has been investigated. Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) images showed that the domain orientation was independent of the heating current direction used to induce the Si(110) “16 × 2” reconstruction. Reciprocal-space lattice models of the reconstruction allowed for the correct identification of domain orientations in the LEED images, and they confirmed that the reconstruction is two dimensionally chiral. It is proposed that the domain orientation upon surface reconstruction is determined by the direction of monatomic steps …

Comment On “Electronic Structure Of Mo(1−X)Rex Alloys Studied Through Resonant Photoemission Spectroscopy”, Prescott Evans, Peter A. Dowben

Peter Dowben Publications

Further analysis of the resonant photoemission data, found within Sundar et al (2016 J. Phys.: Condens. Matter 28 315502), show the intensities do not follow the elemental composition in the Mo_1−xRe_x alloy. Similar trends are observed in the published data for Gd_1−xNi_x alloy films. The analysis of the resonant photoemission intensities suggests that Mo in the Mo_1−xRe_x alloy and Gd in the Gd_1−xNi_x alloy have nearest neighbor bonds to Re and Ni respectively. This means the A–B bond is favored over the average of …