Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F8: Focus Session: Magnetism Techniques: Temporal and Spatial Characterization |
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Sponsoring Units: GMAG Chair: Avag Sahakyan, The State Engineering University of Armenia Room: 104 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F8.00001: One-dimensional scattering of electrons and neutrons in nano heterostructures with magnetic inclusions Avag Sahakyan, Ruzan Movsesyan, Armen Kocharian The spin dependent scattering of electrons and neutrons in one dimension is investigated in systems, containing a nano size magnetic layers. Two thin systems are considered such as: a) magnetic layer with non magnetic surrounding and b) two magnetic layers divided by non magnetic layer or non magnetic surrounding. Magnetization of layers, in general, contains longitudinal and transverse components (parallel and perpendicular to the interface, respectively). It is shown that the Zeeman energy splitting of longitudinal component caused by magnetic field provides modulation of partial scattering amplitude for transmitted and reflected waves. On the other hand, the transverse field can have perceptible contribution into the backward and forward scattering phase. The latest provided an opportunity to manage continuously the scattering energy for resonance transmission. In particular, for electron system this modulation can be manifested in behavior both, conductance and reflectance. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F8.00002: Design considerations for a high sensitivity Barkhausen Noise sensor Neelam Prabhu Gaunkar, Orfeas Kypris, Cajetan Nlebedim, David Jiles Barkhausen emissions are produced due to sudden changes in magnetization when a continuously changing magnetic field is applied to a ferromagnetic material. The emissions described as Barkhausen noise can be observed as voltage signals using induction sensors. Effective capture of these emissions with high level of precision depends on several parameters influenced by the sensor design. For the magnetization unit, amongst others, the critical parameters include the magnetic field produced by the magnetizing coils, core geometry, sensor-to-specimen coupling, choice of core material, core length and operating frequency. Similarly, for the sensing unit the optimal pick-up coil material and number of winding turns need to be optimized. Enhancing these parameters will lead to improved sensitivity, reproducibility and reliability of the detected Barkhausen emissions. Using finite element analysis, this study shows design considerations for optimizing these parameters in order to achieve high accuracy in detection and analysis of Barkhausen signals especially as a tool for magnetic non-destructive evaluation. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F8.00003: Development of a new magnetic Barkhausen spectroscopy method for the non-destructive characterization of magnetic materials Orfeas Kypris, Ikenna Nlebedim, David Jiles Barkhausen emissions, which result from discontinuous, irreversible changes in magnetization, are related to the stress state, defect/inclusion sizes and microstructure of ferromagnetic materials. Time domain analysis of Barkhausen signals measured at the surface of a specimen can reveal the average magnitude of stress in the structure. Such analysis offers a powerful tool for magnetic nondestructive characterization of materials. However, determining the stress and other microstructural parameters as a function of depth still remains a challenging problem, which can be treated in the frequency domain. In this work, a model for stress-depth profiling of ferromagnets is developed. In the model, the frequency spectrum at the surface of a specimen is described in terms of two parameters; the average amplitude of Barkhausen emissions at their origin $V_{orig}$ and $\zeta$, which is proportional to the square root of magnetic permeability. A ferromagnetic structure is mathematically divided into homogeneous layers with each layer acting as a source of Barkhausen signal having a unique spectrum that is attenuated as it propagates to the surface. We show that $V_{orig}$ and $\zeta$ correlate with stress and we provide a framework for detecting stress variations as a function of depth. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F8.00004: Magneto-optic-Kerr-effect-based spin-orbit torque magnetometer Halise Celik, Xin Fan, Wenrui Wang, Jun Wu, Chaoying Ni, Kyung-Jin Lee, John Xiao, Virginia Lorenz Current-induced spin-orbit torques in heavy metal (HM)/ferromagnetic metal (FM) bilayers have attracted great attention for their potential in spintronic applications. It is essential to be able to measure the magnitude and direction of the spin-orbit torques. There have been several methods developed to measure spin-orbit torques based on second-order rectifying voltages, including spin-torque ferromagnetic resonance [1] and second-order harmonic voltage detection [2]. While these techniques have been widely used, they have their respective limits, e.g. requirement of an in-plane/out-of-plane magnetization configuration, small damping, etc. Here we present the development of a first-order spin-orbit torque magnetometer that is based on the magneto-optic Kerr effect (MOKE). The MOKE-based spin-orbit torque magnetometer is sensitive and versatile and can be used in both in-plane and out-of-plane magnetized samples. References: [1] L. Liu et al., Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect, Physical Review Letters 106, 036601 (2011). [2] J. Kim et al., Layer thickness dependence of the current-induced effective field vector in Ta\textbar CoFeB\textbar MgO, Nature Materials 12, 240-245 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F8.00005: Manipulating femtosecond magnetism through pressure: First-principles calculations Mingsu Si, Guoping Zhang Inspired by a recent pressure experiment in fcc Ni, we propose a simple method to use pressure to investigate the laser-induced femtosecond magnetism. Since the pressure effect on the electronic and magnetic properties can be well controlled experimentally, this leaves little room for ambiguity when compared with theory. Here we report our theoretical pressure results in fcc Ni: Pressure first suppresses the spin moment reduction and then completely diminishes it; further increase in pressure to 40 GPa induces a demagnetization-to-magnetization transition. To reveal its microscopic origin, we slide through the $L$-$U$ line in the Brillouin zone and find two essential transitions are responsible for this change, where the pressure lowers two valence bands, resulting in an off-resonant excitation and thus a smaller spin moment reduction. In the spin-richest $L$-$W$-$W^{'}$ plane, two spin contours are formed; as pressure increases, the contour size retrieves and its intensity is reduced to zero eventually, fully consistent with the spin-dipole factor prediction. These striking features are detectable in time- and spin-resolved photoemission experiments. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F8.00006: Magnetization process and topological plateau phase induced by circularly polarized laser Shintaro Takayoshi, Masahiro Sato, Takashi Oka One of the fundamental experiments to investigate magnetic properties of materials is a measurement of magnetization curve. Antiferromagnets with large exchange couplings, however, need high external field to achieve their saturated magnetization, and large equipment is required in experiments. We theoretically propose a new and dynamic way to realize magnetization processes of general quantum magnets without any static field. The way is to apply a circularly polarized laser to magnetic systems. We can show that the coupling between the laser and magnets is mapped to an effective static Zeeman term with a longitudinal magnetic field via a time-dependent unitary transformation or Floquet theory. It is hence expected that the magnetization curve of magnets can be realized by applying a suitable laser. We demonstrate dynamical magnetization processes by numerically solving Schr\"odinger equations for concrete quantum spin models under applied lasers. We also show that a laser-induced magnetization plateau state appears in a simple Ferro-Ferro-Antiferro spin chain model under a certain condition and it has a topological nature. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F8.00007: Circularly polarized high harmonic generation for element-selective probing of magnetic materials on a tabletop Patrick Grychtol, Emrah Turgut, Dmitriy Zusin, Dimitar Popmintchev, Tenio Popmintchev, Henry Kapteyn, Margaret Murnane, Ronny Knut, Hans Nembach, Justin Shaw, Ofer Kfir, Avner Fleischer, Oren Cohen Ultrafast short wavelength sources based on high harmonic upconversion of femtosecond lasers are unique in their ability to simultaneously probe the magnetically-sensitive M absorption edges of the 3d ferromagnets Fe, Co and Ni. This novel capability to capture the fastest spin dynamics in materials has uncovered a wealth of new fundamental understanding about spin scattering and transport on few-femtosecond timescales. However, to date these investigations have used linearly polarized higher harmonics, since it has not been possible to generate circularly polarized harmonics with sufficient flux for scientific applications. In this contribution, we present a simple setup that enables the efficient generation of circularly polarized harmonics, and demonstrates that they are bright enough for studies of magnetic materials. The fundamental and second harmonic of a Ti:sapphire laser are focused into a gas filled waveguide under good phase matching conditions, with opposite chirality circular polarizations. Thus, circularly-polarized harmonics are produced that are then used to perform magnetic circular dichroism studies in the extreme ultraviolet photon energy range. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F8.00008: Development of a microwave probe for the optical study of microwave-excited spin physics Yu-Sheng Ou, Yi-Hsin Chiu, Rohan Adur, Patrick Odenthal, Roland Kawakami, P. Chris Hammel, Ezekiel Johnston-Halperin We have developed an experimental probe that allows simultaneous broadband microwave excitation and optical excitation/detection at variable temperature and magnetic field. Specifically, we have designed a unique sample probe with a microwave stripline based sample mount that allows for direct optical access to the sample under study within a magneto- optical cryostat. This powerful combination enables optical studies of spintronic systems under microwave excitation using both CW (e.g. photo- and electro-luminescence) and time resolved (e.g. time resolved absorption/transmission and time resolved Kerr rotation, TRKR) techniques. To benchmark the capabilities of this probe we present data demonstrating simultaneous ferromagnetic resonance (FMR) and TRKR in a Fe/MgO/GaAs heterostructure. Such studies have potential applications in the study of FMR driven spin pumping and interaction of free carrier spins with native and engineered defects. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F8.00009: Optically Detected Scanned Probe Magnetic Resonance Imaging Christopher Wolfe, Vidya Bhallamudi, Hailong Wang, Chunhui Du, Sergei Manuilov, Rohan Adur, Fengyuan Yang, P. Chris Hammel Magnetic resonance is a powerful tool for studying magnetic properties and dynamics of spin systems. Scanned magnetic probes can induce spatially localized resonance due to the strong magnetic field and gradient near the magnetic tip.\footnote{K.C. Fong, M.R. Herman, P. Banerjee, D.V. Pelekhov, and P.C. Hammel, Phys. Rev. B 84, 220405(R) (2011).}$^,$\footnote{I. Lee, Y. Obukhov, G. Xiang, A. Hauser, F. Yang, P. Banerjee, D.V. Pelekhov, and P.C. Hammel, Nature 466, 845 (2010).} Nitrogen vacancy centers (NV) in diamond provide a sensitive means of measuring magnetic fields at the nanoscale. We report preliminary results towards using the high sensitivity of NV detection with a scanned magnetic probe to study local magnetic phenomena. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F8.00010: On-chip coplanar stripline for micron-scale ferromagnetic resonance and spin pumping devices Shane White, Andrew Berger, Rohan Adur, Sergei Manuilov, P. Chris Hammel Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field. While previous measurements have focused on large-scale ($\sim$mm) devices, dimensions will need to be reduced to prove useful for applications. On-chip microwave structures offer a solution to this by providing localized microwave fields that can be used to resonate small device geometries while leaving them easily accessible for electrical connections, as opposed to the ``flip-chip'' or resonant cavity methods. Using a shorted coplanar strip (CPS) waveguide, we perform broadband (6-12 GHz) FMR measurements in a permalloy bar of dimensions 20$\mu$m x 1$\mu$m x 20nm--too small to be detected by measuring microwave reflections from a cavity. FMR is detected in the permalloy strip through changes in the anisotropic magnetoresistance [1]. This scheme allows for quantitative characterization of magnetization dynamics and microwave fields. These findings demonstrate that on-chip microwave structures will enable new, smaller device geometries and measurement possibilities for a variety of spin pumping systems.\\[4pt] [1] M. V. Costache, et. al, Appl. Phys. Lett. 89, 232115 (2006). [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F8.00011: Probe-localized modes in continuous YIG thin films Rohan Adur, Chunhui Du, Sergey A. Manuilov, Chi Zhang, Denis V. Pelekhov, Hailong Wang, Fengyuan Yang, P. Chris Hammel The measurement of damping in precessing ferromagnets is obscured by the excitation of spin waves of different wavelengths due to defects and inhomogeneities in the ferromagnetic material. In order to reduce this parasitic broadening the magnetic mode can be confined to small volumes (nm to $\mu$m) either by external fields or by patterning. While nanostructures have shown size-dependent effects such as suppression of inhomogeneity when the size of the nanostructure is sufficiently small [1], it has been vital to consider the effect of imperfections in lithography that can cause edge damage and hence extrinsic linewidth broadening. In contrast, the dipolar field from a micron-sized probe magnet can be used to localize a mode in a continuous thin film without lithographic modification to the film. This technique of localized mode ferromagnetic resonance force microscopy (FMRFM) has been demonstrated in permalloy [2] at liquid helium temperature. In the present study we demonstrate probe-localized modes in a YIG thin film (t=25nm) measured at room temperature. Using FMRFM we explore the spatial and size dependence of inhomogeneity and damping of a localized mode within a continuous film. [1] C Hahn et al, 58th MMM conference BC-09 (2013) [2] I Lee et al, Nature 466, 845 (2010) [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F8.00012: Scanned Spin-Precession Microscopy: Progress towards cantilever based approach Vidya Bhallamudi, Christopher Wolfe, Vivek Amin, Helena Reichlova, Andrew Berger, David Stroud, Jairo Sinova, P.Chris Hammel The principal spin microscopy tools for spintronic materials are primarily based on optical detection and are thereby limited to certain materials. There is a need for imaging tools that can address a wider range of materials. Towards this end we recently developed Scanned Spin-Precession Microscopy [1, 2], where we demonstrated the ability to extract local spin properties from a spatially-averaged signal. This is enabled by the modification of the precessional behavior of the spins in a small region by the strongly inhomogeous magnetic field from a micromagnetic probe. We will discuss this novel imaging tool and our recent efforts towards a cantilever-based approach for wider applicability, especially for electrical spin-based devices.\\[4pt] [1] V. P. Bhallamudi et.al., PRL 111, 117201 (2013).\\[0pt] [2] V. P. Bhallamudi et.al., JAP. 111, 013902 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F8.00013: Soft x-ray coherent diffraction imaging on magnetic nanostructures Xiaowen Shi, James Lee, Shrawan Mishra, Daniel Parks, Tolek Tyliszczak, David Shapiro, Sujoy Roy, Steve Kevan Coherent soft X-rays diffraction imaging enable coherent magnetic resonance scattering at transition metal L-edge to be probed so that magnetic domains could be imaged with very high spatial resolution with phase contrast, reaching sub-10nm. One of the overwhelming advantages of using coherent X-rays is the ability to resolve phase contrast images with linearly polarized light with both phase and absorption contrast comparing to real-space imaging, which can only be studied with circularly polarized light with absorption contrast only. Here we report our first results on high-resolution of magnetic domains imaging of CoPd multilayer thin film with coherent soft X-ray ptychography method. We are aiming to resolve and understand magnetic domain wall structures with the highest obtainable resolution here at Advanced Light Source. In principle types of magnetic domain walls could be studied so that Neel or Bloch walls can be distinguished by imaging. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F8.00014: Deterministic propagation of nanomagnetic logic observed by time-resolved XMCD-PEEM Mark Nowakowski, Zhang Gu, Brian Lambson, Jeongmin Hong, Ralph Storz, Patrick Bennett, David Carlton, Weilun Chao, Scott Dhuey, Anthony Young, Andrew Doran, Matthew Marcus, Andreas Scholl, Jeffrey Bokor Nanomagnetic logic (NML) is a low-power logic architecture that relies on the dipolar coupling of closely spaced (30 nm) magnets (450x150 nm) to flow binary information down lithographically defined chains. A majority logic gate selects an output based on the magnetic orientation of three intersecting NML chains, permitting logic functions without requiring electrical currents like those used in Si-based transistors. The repeatable and reliable flow of magnetic signal propagation down a chain, a critical feature of this technology, has not been experimentally demonstrated, however computational models have predicted NML signal flow and have postulated a better performance from lithographically engineered magnets with configurational anisotropy. Using the PEEM-3 microscope at the Advanced Light Source, we perform an XMCD pump-probe measurement and observe signal propagation along a chain of 13 magnets with configurational anisotropy. We resolve successive individual magnets flipping on 100 ps time scales and complete signal propagation down the chain after 1-2 ns. This behavior is consistent with previous computational models. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F8.00015: Real Space Visualization of Mott Gap and Magnon Excitations Yao Wang, Chunjing Jia, Brian Moritz, Thomas Devereaux Real-space and time information plays a significant role in understanding inhomogeneous physical and chemical processes at the nano-scale. Experimentally, inelastic light scattering promises to become an important tool for characterizing the spatio-temporal properties of complex systems. To demonstrate the power of this technique, we perform a theoretical study of real-space charge and spin density response functions in the Hubbard model to track time-dependent Mott gap and magnon excitations. Carrier doping is found to affect the evolution of the charge and spin response with distinct timescales and real-space patterns appearing for n- or p-type materials. [Preview Abstract] |
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