Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session S6: Magneto-Optic/Electric and Strain/Shape Induced MagnetismFocus
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Sponsoring Units: GMAG DMP Chair: Anirudh Sharma, Flinders University Room: 302 |
Thursday, March 17, 2016 11:15AM - 11:27AM |
S6.00001: magnetoelectric switching and spin wave generation Bart Soree, Davide Tierno, Christoph Adelmann, Odysseas Zografos, Adrien Vaysset, Florin Ciubotaru We have investigated the dynamics of the magnetization in magnetoelectric elements for switching and generation of spin waves. The behavior of the magnetization in the magnetostrictive material coupled to the piezoelectric not only depends on the strain induced by the piezo, but also depends on the relative contribution of the different magnetic anisotropies (shape, magnetocrystalline, magnetoelastic) present in the magnetoelectric element which is coupled to a spin wave bus. Performing micromagnetic simulations allow us to draw several conclusions w.r.t. the switching behavior of magnetoelectric elements as well as conditions to generate spin waves in an effective manner. [Preview Abstract] |
Thursday, March 17, 2016 11:27AM - 11:39AM |
S6.00002: Electric field controlled strain induced reversible switching of magnetization in Galfenol nanomagnets delineated on PMN-PT substrate Hasnain Ahmad, Jayasimha Atulasimha, Supriyo Bandyopadhyay We report a \textit{non-volatile} converse magneto-electric effect in elliptical Galfenol (FeGa) nanomagnets of \textasciitilde 300 nm lateral dimensions and \textasciitilde 10nm thickness delineated on a PMN-PT substrate. This effect can be harnessed for energy-efficient non-volatile memory. The nanomagnets are fabricated with e-beam lithography and sputtering. Their major axes are aligned parallel to the direction in which the substrate is poled and they are magnetized in this direction with a magnetic field. An electric field in the opposite direction generates compressive strain in the piezoelectric substrate which is partially transferred to the nanomagnets and rotates their magnetization away from the major axes to metastable orientations. There they remain after the field is removed, resulting in non-volatility. Reversing the electric field generates tensile strain which returns the magnetization to the original state. The two states can encode two binary bits which can be written using the correct voltage polarity, resulting in non-toggle behavior. Scaled memory fashioned on this effect can exhibit write energy dissipation of only \textasciitilde 2 aJ. [Preview Abstract] |
Thursday, March 17, 2016 11:39AM - 11:51AM |
S6.00003: \textbf{Giant magnetoelectric effect in thin magnetic films utilizing inter-ferroelectric transitions}. Peter Finkel, Margo Staruch There has recently been much interest to multiferroic magnetoelectric composites based on relaxor ferroelectric single crystals as potential candidates for devices such as magnetic field sensors, energy harvesters, or transducers. Large magnetoelectric coupling coefficient is prerequisite for superior device performance in a broad range of frequencies and functioning conditions. In magnetoelectric heterostructures based on ternary relaxors Pb(In$_{\mathrm{1/2}}$Nb$_{\mathrm{1/2}})$O$_{\mathrm{3}}$-Pb(Mg$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$-PbTiO$_{\mathrm{3}}$ (PIN-PMN-PT) crystal better operational range and temperature stability as compared to binary relaxors can be achieved. Giant linear converse magnetoelectric coupling up to 2 x 10$^{\mathrm{-6}}$ s m$^{\mathrm{-1}}$ were observed in heterostructural composites with multilayered FeCo/Ag deposited on (011) PIN-PMN-PT crystals. Further enhancement of magnetoelectric coupling is demonstrated by utilizing inter-ferroelctric rhombohedral -- orthorhombic phase transitions in PIN-PMN-PT Mechanical clamping was a precondition to utilize this inter-ferroelectric transition mode to bring the crystal to a point just below its transformation threshold when very small perturbations at the input will cause large swings at the output generating a sharp uniaxial increase in strain (\textasciitilde 0.5 {\%}) and polarization change, giving rise to nonlinear effects. Details of these results and their implications will be presented. [Preview Abstract] |
Thursday, March 17, 2016 11:51AM - 12:03PM |
S6.00004: Exploring Strain Induced Magnetization Effects in Metamagnetic Artificial Multiferroics using Polarized Neutron Reflectometry Steven Bennett, Andreas Herklotz, Anthony Wong, Thomas Ward, Valeria Lauter There is currently a strong drive to realize a controllable magnetic ordering transition for use in next generation spintronic based memory and computation devices. One proposed method to gain such control is the use of a changing strain in a thin film metamagnetic artificial multiferroic system. While basic concepts using electric field actuated piezoelectric strain have been recently demonstrated$^{\mathrm{1}}$, there is very little understanding of the details of strains effect on such magnetic phase transitions. Using the depth sensitive method of polarized neutron reflectometry we have been able to probe the fine details of strains contribution to the metamagnetic transition in thin films of metamagnetic FeRh$^{\mathrm{2}}$. Here we explore the effects of changing lattice strain as a function of depth using both a barium titanate substrate's structural phase transitions$^{\mathrm{3}}$ and He ion implantation. These studies have discovered a remarkably large coupling between the systems strain state and the switching behavior across the magnetostructural metamagnetic transition. $^{\mathrm{1}}$ Cherifi, R. O. et al. Nat. Mater. 31, 345--351 (2014), $^{\mathrm{2}}$ Bennett, S. P. et al. Sci. Rep. 5, 9142 (2015), $^{\mathrm{3}}$ Bennett, S. P. et al. submitted (2015) [Preview Abstract] |
Thursday, March 17, 2016 12:03PM - 12:15PM |
S6.00005: Writing magnetic phase and domain structure in FeRh by controlling lattice symmetry with strain doping T. Zac Ward, Andreas Herklotz, Anthony Wong, Steven Bennett, Valeria Lauter Low energy helium ion implantation is an effective approach to strain doping materials which allows one to expand the out-of-plane lattice parameter in epitaxial films without vacancy generation or electron/hole doping the system [1]. The ability to control crystal anisotropy and overcome Poisson's drive to conserve volume can thus offer huge dividends in controlling magnetic properties due to magnetostrictive phenomena. We present recent studies on epitaxial FeRh films which demonstrate how controlling crystal symmetry in this important intermetallic material can be used to finely control magnetic properties. We find that the first order magneto-structural phase transition from antiferromagnetic to ferromagnetic can be directly controlled through single axis lattice expansion; this effectively allows us to dictate the transition temperature anywhere between 400K and 150K. Polarized Neutron Reflectometry (PNR) data and scanning Magneto-optic Kerr effect (MOKE) measurements will be presented which demonstrate that this phase control can be confined to a specific region of the film both in depth and/or lateral position. While this holds great promise for magnetocaloric applications, many possibilities remain for devising new functionalities and gaining a deeper understanding of material properties using this technique. [1]H.W. Guo, S. Dong, P.D. Rack, J.D. Budai, A.T. Wong, A. Herklotz, P.C. Snijders, E. Dagotto, and T.Z. Ward, Phys. Rev. Lett. 114, 256801 (2015). Funded by DOE-BES-MSED. [Preview Abstract] |
Thursday, March 17, 2016 12:15PM - 12:27PM |
S6.00006: Current Control of Magnetic Anisotropy via Strain in a CoFeB Waveguide Kyongmo An, Xin Ma, Chi-feng Pai, Jusang Yang, Kevin Olsson, James Erskine, Allan Macdonald, Daniel Ralph, Robert Buhrman, Xiaoqin Li We demonstrate that in-plane charge current can effectively control the spin precession resonance in an Al$_{\mathrm{2}}$O$_{\mathrm{3}}$/CoFeB/Ta heterostructure. Brillouin Light Scattering (BLS) was used to detect the ferromagnetic resonance field under microwave excitation of spin waves at fixed frequencies. Such control originates from the modified in-plane uniaxial magnetic anisotropy field $H_{\mathrm{k}}$, which changes symmetrically with respect to the current direction. Numerical simulation suggests that the anisotropic stressintroduced by Joule heating plays an important role in controlling $H_{\mathrm{k}}$. The results provide new insights into current manipulation of magnetic properties and have broad implications on spintronic devices. [Preview Abstract] |
Thursday, March 17, 2016 12:27PM - 12:39PM |
S6.00007: Incoherent stress-mediated magnetization reversal in shape anisotropic multiferroic nanomagnets Dhritiman Bhattacharya, Md Mamun Al-Rashid, Vimal Sampath, Noel D'Souza, Supriyo Bandyopadhyay, Jayasimha Atulasimha Strain mediated switching of multiferroic nanomagnets promises to be extremely energy efficient with dissipation per switching event of \textasciitilde 1 aJ$^{\mathrm{[1,2,3]}}$. Most theoretical approaches to studying the switching dynamics use the macrospin approximation in which all the spins in the nanomagnet are assumed to rotate coherently. However, recent experiments show that while initial and final states are well approximated by this single domain assumption, intermediate states visited during the magnetization rotation process cannot be described by it. In such cases, an interplay between the exchange, magnetostatic and stress anisotropy energies can introduce incoherent magnetization dynamics. Hence, intermediate micromagnetic configurations such as vortex states can be stabilized, particularly in nanomagnets of larger dimensions. In this work, we present rigorous micromagnetic simulations to study the peculiarities of the incoherent switching process in the context of shape anisotropic nanomagnets subjected to stress. 1.Appl. Phys. Lett., 97, 173105, 2010. 2.Appl. Phys. Lett., 99, 063108, 2011. 3.Nanotechnology, 23, 105201, 2012. [Preview Abstract] |
Thursday, March 17, 2016 12:39PM - 12:51PM |
S6.00008: Experimental manipulation of magnetic states of magnetostrictive nanomagnets using surface acoustic waves Vimal Sampath, Dhritiman Bhattacharya, Noel D'Souza, Supriyo Bandyopadhyay, Jayasimha Atulasimha The use of Surface Acoustic Waves (SAW) to assist magnetization switching in magnetostrictive nanomagnets has been theoretically studied [1] and SAW-induced magnetization rotation in micron size magnets has been experimentally demonstrated [2]. We report recent experiments on manipulation of magnetic states of Co nanoscale magnets shaped like elliptical disks (\textasciitilde 300 nm major axis, 240 nm minor axis and 10 nm thickness) delineated on bulk 128 Y-cut lithium niobate using SAW. Specifically, isolated nanomagnets that are initially in single domain states with magnetization pointing along the major axis of the ellipse are driven into a vortex state by SAW waves. However, SAW waves can trigger complete magnetization reversal in nanomagnets of moderate shape anisotropy that are dipole coupled to a highly shape anisotropic neighboring nanomagnet. [1] A.K. Biswas, S. Bandyopadhyay {\&} J. Atulasimha, Appl. Phys. Lett., 105, 072408 (2014). [2] S. Davis, A. Baruth {\&} S. Adenwalla, App. Phys. Lett., 97, 232507 (2010).The authors acknowledge the use of high voltage and high frequency pulse generator from Prof. Umit Ozgur's lab and the help of Prof. Gary Atkinson in fabrication of the IDTs for generating the SAW. [Preview Abstract] |
Thursday, March 17, 2016 12:51PM - 1:03PM |
S6.00009: \textbf{Magnetic domain response to strain generated by focused surface acoustic waves} . Uday Singh, Shireen Adenwalla The effects of strain on magnetostrictive ferromagnets include changes in the magnetization, anisotropy and domain wall velocities. A ferromagnet (FM) on the surface of a surface acoustic wave (SAW) is subjected to periodic compressive and tensile strain that has resulted in coherent rotation of the magnetization, as well as inducing ferromagnetic resonance in FM films. We describe the response of magnetic domains in Co/Pt multilayers when subjected to the high strains generated by a focused SAW. Annular interdigital transducers (AIDT) patterned on LiNbO$_{\mathrm{3}}$ form a SAW standing wave pattern with large strain amplitude at the focal center. Domains in [Co(3A)/Pt(8A)]x5 with perpendicular magnetic anisotropy were observed using a MOKE microscope within this focal region. Controlled magnetic pulses steered a magnetic domain boundary to the large strain region after nucleation.Excitation of the AIDT resulted in a reversible change in the domain wall boundary in the high strain region. We attribute this to magnetic anisotropy changes in the presence of RF strain, which results in changes in the domain configuration to minimize the free energy. We will present results showing both slow and fast magnetization changes in Co/Pt occurring in the presence of high frequency strain. This work is supported by NSF (DMR 1409622) and Nebraska MRSEC (DMR-1420645). [Preview Abstract] |
(Author Not Attending)
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S6.00010: Orientation dependences of surface morphologies and energies of iron-gallium alloys Marcio Costa, Hui Wang, Jun Hu, Ruqian Wu, Suok-Min Na, Hyunsuk Chun, Alison B Flatau Magnetostrictive Fe-Ga alloys (Galfenol) are very promising rare-earth free materials for applications in sensors, actuators, energy-harvesters and spintronic devices. Investigation on surface energies of Galfenol based on density functional calculations (DFT) and contact angle measurements may provide fundamental understandings and guidance to further optimize the performance of Galfenol. DFT calculations predict that Ga-covered (110) surface of Galfenol is more stable in Ga-rich condition, while Ga-covered (001) surface of Galfenol surface become more favorable in Ga-poor condition. Moreover, a full Ga overlayer tends to form on top of Gafenol surfaces regardless their orientation, both in agreement with the experimental observation. Further studies on Ga segregation in the Fe bcc matrix demonstrate that the Fe-Ga separation is unlikely to occur since Ga diffusion toward the surface is effectively self-stopped once the Ga overlayers form on the facets. [Preview Abstract] |
Thursday, March 17, 2016 1:15PM - 1:27PM |
S6.00011: Fingerprinting Morphology of Magnetic Shape Memory Alloys Using First Order Reversal Curves (FORC) and Neutron Scattering. Igor V. Roshchin, Pavel N. Lapa, Kathryn L. Krycka, Brian B. Maranville, James A. Monroe, Brian E. Franco, Ibrahim Karaman In Ni-Mn-In- and Ni-Mn-Sn-based alloys, two magnetic phases with ferromagnetic and antiferromagnetic exchange couplings between two nearest Mn atoms can coexist. The interaction between these phases results in exchange bias (EB). The EB field depends on the cluster sizes. Using the first order reversal curve (FORC) analysis of magnetization for Ni-Co-Mn-Sn and Ni-Co-Mn-In samples with different heat treatment, we can obtain information about cluster sizes of the structural phases in these alloys. This is especially important for polycrystalline alloy samples where dark-field images showing different phases are hard to obtain. Such a Ni-Co-Mn-Sn polycrystalline sample was characterized with small angle neutron scattering (SANS). Analyses of the scattering as a function of wavevector transfer in 50 Oe and 15 kOe applied field yield the average magnetic domain size of 21.2$\pm$6.6 nm and a polydispersity of 0.32$\pm$0.02 at 300 K, in good agreement with our prediction. The temperature evolution of the domain size will be discussed. Using an off-specular reflectometer in transmission geometry, the same sample was measured at a field of 270 Oe and 5.15 kOe. The fit of the 270 Oe data yields grain sizes of approximately 0.11--0.12 $\mu$m with polydispersities between 0.98 and 1.27. [Preview Abstract] |
Thursday, March 17, 2016 1:27PM - 1:39PM |
S6.00012: Detecting an in-plane rotation of magnetization in GdFeCo films Farzaneh Hoveyda, Serban Smadici It is often important to distinguish between magnetization reversal by coherent rotation in different planes and domain wall motion. Magnetization curves were measured at different temperatures with magneto-optical Kerr Effect in longitudinal (L-MOKE) and polar (P-MOKE) geometries on sputter-deposited Gd$_{\mathrm{x}}$Fe$_{\mathrm{y}}$Co$_{\mathrm{1-x-y\thinspace }}$(GFC) films of variable thickness. Depending on the probed region, the L-MOKE signal measured with decreasing external field H$_{\mathrm{ext}}$ was found to be lower than the signal observed with increasing H$_{\mathrm{ext}}$ (negative remanence magnetization). We show that this is due to a contribution to the signal of$\thinspace M_{\bot }$, the magnetization component perpendicular to the scattering plane. This identifies the type of reversal in these GFC films as in-plane coherent rotation of magnetization. $M_{\bot }$ is also proportional to the torque. Azimuthal measurements on Co$_{\mathrm{2}}$FeAl samples showed a regular variation of the MOKE signal, in one possible application of these observations to torque measurements. [Preview Abstract] |
Thursday, March 17, 2016 1:39PM - 1:51PM |
S6.00013: Nanoscale Confinement of All-Optical Magnetic Switching in TbFeCo TianMin Liu, Tianhan Wang, Alexander Reid, Matteo Savoini, Xiaofei Wu, Benny Konene, Patrick Granitzka, Catherine Graves, Daniel Higley, Zhao Chen, Gary Razinskas, Markus Hantschmann, Andreas Scherz, Joachim Stohr, Arata Tsukamoto, Bert Hecht, Alexey Kimel, Andrei Kirilyuk, Theo Rasing, Hermann Durr Gold two-wire antennas structures are placed upon the surface of the all-optical switching film TbFeCo. They resonate with the optical field and create a field enhancement in its vicinity, which is used to confine the area where optical switching can occur. It is demonstrated that single femtosecond optical laser pulses can reverse magnetization in a controllable fashion by such confinement. The magnetic states are imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the material's heterogeneity. [Preview Abstract] |
Thursday, March 17, 2016 1:51PM - 2:03PM |
S6.00014: Measurements of the Domain Magnetization Direction and its Effects on the Sensitivity of Magneto-optic Field Sensors. Mannix Shinn, Anthony Garzarella, Dong Ho Wu, Rongjia Tao Bismuth doped, rare earth iron garnet (Bi:RIG) thick films exhibit a large magneto-optic response to external magnetic fields while exhibiting low optical insertion loss, making them ideal candidates for polarimetric magnetic field sensors. It was generally found that the Faraday rotation and overall sensitivity of the sensors depends on the orientation of the local domain magnetization relative to the direction of laser propagation. In arrayed Bi:RIG sensors, it is critical that the optical path of the laser is perpendicular to the easy-axis of each film of the array, in order to avoid magnetically-induced optical incoherence (MIOI). Therefore a precise, localized measurement of the magnetization vector within the films is necessary. Since traditional magnetization measurement techniques do not provide adequate resolution, several new approaches to precisely measure the easy axis were developed and will be described in this presentation. These approaches involve measurements of the directionality of the Faraday response, incoherence in the Malus curves, and damping in the domain wall motion. Such measurements have been instrumental in constructing and optimizing arrayed Bi:RIG sensors, which currently have a sensitivity of 6 pT/Hz^{1/2}. [Preview Abstract] |
Thursday, March 17, 2016 2:03PM - 2:15PM |
S6.00015: Dynamic Interplay of Coherent Rotations and Domain Wall Motion in Faraday Rotators based on Ferromagnetic Crystals. Anthony Garzarella, Dong Wu, Mannix Shinn Under small, externally-applied magnetic fields, the Faraday rotation in magneto-optic material containing ferromagnetic domains is driven primarily by two principal mechanisms: domain wall motion and coherent domain rotations. Domain wall motion yields a larger Faraday responsivity but is limited by magnetically induced optical incoherence and by damping effects. Coherent domain rotation yields smaller Faraday rotations, but exhibits a flatter and broader frequency response. The two mechanisms occur along orthogonal principal axes and may be probed independently. However, when probed along an oblique angle to the principal axes, the relationship between the Faraday rotation and the external field changes from linear to tensorial. Although this may lead to more complicated phenomena (e.g. a sensitivity axis that depends on RF frequency), the interplay of domain rotation and domain wall motion can be exploited to improve responsivity or bandwidth. The detailed experimental data can be understood in terms of a quantitative model for the magnitude and direction of the responsivity vector. Applications to magnetic field sensors based on arrayed bismuth doped iron garnet films will be emphasized in this presentation. [Preview Abstract] |
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