Physics Commons^™

Second-Harmonic Signature Of Chiral Spin Structures In W/Pt/Co Heterostructures With Tunable Magnetic Anisotropy, Yang Wang, Ying-Ting Chan, Xiao Wang, Tao Wang, Xuemei Cheng, Weida Wu, John Q. Xiao

Physics Faculty Research and Scholarship

Second-harmonic Hall voltage (SHV) measurement method has been widely used to characterize the strengths of spin–orbit torques (SOTs) in heavy metal/ferromagnet thin films saturated in the single-domain regime. Here, we show that the magnetic anisotropy of a W/Pt/Co trilayer can be robustly tuned from in-plane to out-of-plane by varying W, Pt, or Co thicknesses. Moreover, in samples with easy-cone anisotropy, SHV measurements exhibit anomalous ‘humps’ in the multidomain regime accessed by applying a nearly out-of-plane external magnetic field. These hump features can only be explained as a result of the formation of Néel-type domain walls, efficiently driven by nevertheless small …

Magnetism Of Novel Rare-Earth-Free Intermetallic Compounds, Haohan Wang

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Rare-earth-free magnets have drawn lots of interest because of their low cost, and the production is not limited by the shortage of rare-earth elements. This dissertation focuses on three rare-earth-free materials, Fe-Co-Ti alloys, Fe-Ni-B alloys, and Co-Si. All of them are synthesized by arc melting followed by melt-spinning. Fe_3+xCo_3−xTi₂ (x = 0, 2, 3) alloys exhibit hexagonal crystal structures and show non-collinear spin structures according to neutron diffraction. The magnetic moments have projections on both the c-axis and basal plane, and the corresponding misalignment angle exhibits a nonlinear decrease with …

Ferroelectric Polarization Control Of Magnetic Anisotropy In Pbzr0.2ti0.8o3/La0.8sr0.2mno3 Heterostructures, Anil Rajapitamahuni, L. L. Tao, Y. Hao, Jingfeng Song, Xiaoshan Xu, Evgeny Y. Tsymbal, Xia Hong

Evgeny Tsymbal Publications

The interfacial coupling between the switchable polarization and neighboring magnetic order makes ferroelectric/ferromagnetic composite structures a versatile platform to realize voltage control of magnetic anisotropy. We report the nonvolatile ferroelectric field effect modulation of the magnetocrystalline anisotropy (MCA) in epitaxial PbZr_0.2Ti_0.8O₃ (PZT)/La_0.8Sr_0.2MnO₃ (LSMO) heterostructures grown on (001) SrTiO₃ substrates. Planar Hall effect measurements show that the in-plane magnetic anisotropy energy in LSMO is enhanced by about 22% in the hole accumulation state compared to the depletion state, in quantitative agreement with our first-principles density functional theory calculations. Modeling the …

Crystal Structure And Dzyaloshinski–Moriya Micromagnetics, Ahsan Ullah, B. Balamurugan, Wuzhang Fang, Shah R. Valloppilly, Xingzhong Li, Rabindra Pahari, Lanping Yue, Andrei Sokolov, David J. Sellmyer, Ralph Skomski

Nebraska Center for Materials and Nanoscience: Faculty Publications

The relationship between atomic-scale and micromagnetic Dzyaloshinski–Moriya (DM) interactions has been investigated. By analyzing the Lifshitz invariants for different point groups, we have found that there is no unique link between the absence of inversion symmetry and DM interactions. The absence of inversion symmetry is a necessary condition for a net DM interaction in crystals, but several noncentrosymmetric point groups have zero DM interactions. In many cases, the key consideration is whether the crystals are polar and/or chiral. For example, MnSi-type spin spirals, which violate helical spin symmetry, are caused by the insertion of chiral atomic-scale building blocks into an …

Observation Of A Pressure-Induced Transition From Interlayer Ferromagnetism To Intralayer Antiferromagnetism In Sr4Ru3O10, H. Zheng, W. H. Song, J. Terzic, H. D. Zhao, Y. F. Ni, Lance E. Delong, P. Schlottmann, G. Cao

Physics and Astronomy Faculty Publications

Sr₄Ru₃O₁₀ is a Ruddlesden-Popper compound with triple Ru-O perovskite layers separated by Sr-O rock-salt layers. This compound presents a rare coexistence of interlayer (c-axis) ferromagnetism and intralayer (basal-plane) metamagnetism at ambient pressure. Here we report the observation of pressure-induced, intralayer itinerant antiferromagnetism arising from the interlayer ferromagnetism. The application of modest hydrostatic pressure generates an anisotropy that may cause a flattening and a tilting of RuO₆ octahedra. All magnetic and transport results from this study indicate these lattice distortions diminish the c-axis ferromagnetism and basal-plane metamagnetism, and induce a basal-plane antiferromagnetic state. …

On The Structural Origin Of The Single-Ion Magnetic Anisotropy In Lufeo3, Shi Cao, Xiaozhe Zhang, Tula R. Paudel, Kishan Sinha, Xiao Wang, Xuanyuan Jiang, Wenbin Wang, Stuart Brutsche, Jian Wang, Philip J. Ryan, Jong-Woo Kim, Xuemei Cheng, Evgeny Y. Tsymbal, Peter A. Dowben, Xiaoshan Xu

Evgeny Tsymbal Publications

Electronic structures for the conduction bands of both hexagonal and orthorhombic LuFeO₃ thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectra shape and the linear dichroism are observed. These differences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states has not changed, the spectra are sensitive to the changes in the local environments of the Fe³⁺ and Lu …

A Method For Measuring The Néel Relaxation Time In A Frozen Ferrofluid, Ronald J. Tackett, Jagdish Thakur, Nathaniel Mosher, Emily Perkins-Harbin, Ronald E. Kumon, Lihua Wang, Corneliu Rablau, Prem P. Vaishnava

Physics Publications

We report a novel method of determining the average Néel relaxation time and its temperature dependence by calculating derivatives of the measured time dependence of temperature for a frozen ferrofluid exposed to an alternating magnetic field. The ferrofluid, composed of dextran-coated Fe3O4 nanoparticles (diameter 13.7 nm ± 4.7 nm), was synthesized via wet chemical precipitation and characterized by x-ray diffraction and transmission electron microscopy. An alternating magnetic field of constant amplitude (H0=20H0=20 kA/m) driven at frequencies of 171 kHz, 232 kHz, and 343 kHz was used to determine the temperature dependent magnetic energy absorption rate in the temperature range from …

Using Graphene To Control Magnetic Anisotropy And Interaction Between Supported Clusters, Sanjubala Sahoo, M Fhokrul Islam, Shiv N. Khanna

Physics Publications

Stabilization of magnetic order in clusters/nanoparticles at elevated temperatures is a fundamentally challenging problem. The magnetic anisotropy energy (MAE) that prevents the thermal fluctuations of the magnetization direction can be around 1–10 K in free transition metal clusters of around a dozen atoms. Here we demonstrate that a graphene support can lead to an order of magnitude enhancement in the anisotropy of supported species. Our studies show that the MAE of supported Co5 and Co13 clusters on graphene increase by factors of 2.6 and 25, respectively. The enhancement is linked to the splitting of selected electronic orbitals that leads to …

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 …

Magnetic Anisotropy And Exchange In (001) Textured Fept-Based Nanostructures, Tom George

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

Hard-magnetic L1₀ phase FePt has been demonstrated as a promising candidate for future nanomagnetic applications, especially magnetic recording at areal densities approaching 10 Tb/in². Realization of FePt’s potential in recording media requires control of grain size and intergranular exchange interactions in films with high degrees of L1₀ order and (001) crystalline texture, including high perpendicular magnetic anisotropy. Furthermore, a write-assist mechanism must be employed to overcome the high coercivity of L1₀ FePt nanograins. The research described in this dissertation examines potential solutions to the aforementioned problems. Specifically, a nonepitaxial method of fabricating highly (001) textured …

Magnetism Of Films, Yi Liu, David J. Sellmyer

David Sellmyer Publications

Films of L10-structure Fe50–xCoxPt50 films are synthesized by co-sputtering Fe, Co, and Pt on (001) MgO substrates and Si substrates with in-situ heating at 830°C. The nanostructures and magnetic properties of the films are characterized by X-ray diffraction, transmission electron microscopy, and SQUID. The compositions of the samples Fe50–xCoxPt50 are designed to maintain an atomic (Fe+Co):Pt ratio of 50:50 while increasing the Co content in each successive sample. In all samples, the X-ray diffraction patterns from samples on MgO substrate exhibit three strong peaks, namely L10Fe50–xCoxPt50 (001), (002), and MgO (002). The X-ray and electron diffraction patterns from …

Magnetoelectric Interactions Between An Organic Ferroelectric And A Transition Metal Ferromagnet, Abhijit Mardana

Department of Physics and Astronomy: Dissertations, Theses, and Student Research

The interaction between ferromagnetic and ferroelectric films, the magnetoelectric effect, is a fascinating fundamental research area as well as having potential applications in magnetic data storage devices. We have investigated magnetoelectric coupling effects in thin film heterostructures, consists of metallic ferromagnet, cobalt, and the polymer ferroelectric [P(VDF-TrFE) 70:30]. The work described here encompasses changes in ferroelectric polarization with magnetic field as well as changes in the magnetic anisotropy with ferroelectric polarization.

In samples of Co overlayers on P(VDF-TrFE), in which the Co is not constrained by the substrate, the polarization shows a large change on application of a perpendicular magnetic …

Ferroelectric Control Of Magnetic Anisotropy, Abhijit Mardana, Stephen Ducharme, Shireen Adenwalla

Stephen Ducharme Publications

We demonstrate unambiguous evidence of the electric field control of magnetic anisotropy in a wedge-shaped Co film of varying thickness. A copolymer ferroelectric of 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE) overlays the Co wedge, providing a large switchable electric field. As the ferroelectric polarization is switched from up to down, the magnetic anisotropy of the Co films changes by as much as 50%. At the lowest Co thickness the magnetic anisotropy switches from out-of-plane to inplane as the ferroelectric polarization changes from up to down, enabling us to rotate the magnetization through a large angle at constant magnetic field …

Fabrication And Magnetic Properties Of Fe Nanostructures In Anodic Alumina Membrane, J H. Lim, W S. Chae, H O. Lee, L Malkinski, S G. Min, J B. Wiley, J H. Jun, S H. Lee, J S. Jung

Physics Faculty Publications

Several Fe nanostructures with different lengths, diameters, and separations of the constituting magnetic components have been synthesized using anodized alumina membranes (AAMs) to understand the influence of these parameters on their magnetic properties. Fe nanostructures with high crystallinity and (110) orientation were synthesized by electrodeposition at room temperature in regular AAMs and mild-hard AAM (Mi-Ha AAM). Fe nanostructures with different aspect ratios (1:1, 1:10, and 1:75) in the form of nanodots, nanorods, or nanowires were synthesized in regular AAMs with the 100 nm interpore distance. Mi-Ha AAMs with two different pore sizes (70 and 120 nm) and 250 nm interpore …

Morphology, Magnetic And Dynamic Properties Of Artificial Structures Assembled By Acoustic Vibrations, K B. Paul, L Malkinski

Physics Faculty Publications

Cobalt micromagnetic particles with an average size of ≈ 1.6 μm and varying total mass of the powder were assembled on patterned media with perpendicular magnetization by acoustic vibrations onto designed shapes reflecting the primary material. The replicas were studied with scanning electron microscopy, vibrating sample magnetometry, and ferromagnetic resonance spectroscopy. Their properties were significantly influenced by the shape anisotropy induced through the parent molds. A tendency in the development of the physical characteristics of the replicas was observed as their geometrical parameters changed.

The Temperature Dependence Of Hysteretic Processes In Co Nanowires Arrays, I Astefanoaei, I Dumitru, A Diaconu, L Spinu, A Stancu

Physics Faculty Publications

In this paper, the temperature dependence of the hysteretic processes of Co nanowires, squarelly ordered in an array prepared by electrodeposition in nanopores of alumina membranes was analyzed. Both the magnetostatic interactions induced in the nanowires arrays and the thermal stresses (radial, azimuthal and axial stresses), which appear during the cooling of the system (nanowire and alumina template) from room temperature to 3 K was evaluated. The analysis of thermal induced stresses provides useful informations concerning the magnetic anisotropy in the Co nanowires. The temperature dependence of the remanent magnetization and coercitive field as an effect of the induced thermal …

Microwave Absorption Of Patterned Arrays Of Nanosized Magnetic Stripes With Different Aspect Ratios, Leszek M. Malkinski, Minghui Yu, Andriy Y. Voyk, Donald J. Scherer Ii, Leonard Spinu, Weillie Zhou, Scott Whittenburg, Zachary Davis, Jin-Seung Jung

Physics Faculty Publications

Arrays consisting of nanosized stripes of Permalloy with different length-to-width ratios have been fabricated using electron beam nanolithography, magnetron sputtering, and lift-off process. These stripes have a thickness of 100 nm, a width of 300 nm, and different lengths ranging from 300 nm to 100 μm. The stripes are separated by a distance of 1 μm. Magnetization hysteresis loops were measured using a superconducting quantum interference device susceptometer. Microwave absorption at 9.8 GHz was determined by means of ferromagnetic resonance technique. The dependence of the resonant field on the angle between the nanostructure and the in-plane dc magnetic …

Simulating The Maximum Domain Wall Speed In A Magnetic Nanowire, Andrew Kunz

Physics Faculty Research and Publications

The dynamics of domain wall motion in permalloy nanowires have been simulated utilizing the Landau-Lifshitz-Gilbert (LLG) equation of motion. The simulation results are presented in terms of the domain wall speed for ranges of the Gilbert damping parameter alpha and nanowire width. The maximum domain wall speed is independent of alpha. The speed of the domain wall can be increased by increasing the nanowire width, but this lowers the critical field. For applied fields below the critical field, the wall moves uniformly along the wire and the speed of the wall increases with increases in the driving field. This behavior …

Normal Mode Mixing And Ferromagnetic Resonance Linewidth, Andrew Kunz, R. D. Mcmichael

Physics Faculty Research and Publications

The normal modes of an inhomogeneous thin film are obtained by diagonalization of the perturbed Hamiltonian. The resulting modes are mixtures of the spin-wave modes and the uniform mode. We find that the ferromagnetic resonance intensity spectrum of the diagonalized system has a Lorentzian profile, and that the results correspond to the two-magnon model for weak perturbations. For stronger perturbations, the density of states is smoothed, and the spectrum becomes asymmetric due to the low-frequency cutoff of the spin-wave manifold. The technique is expected to be valid for perturbation amplitudes that are large enough to invalidate the assumptions of the …