Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B50: Magnetism in Curved Nanostructures and NanowiresFocus

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Sponsoring Units: GMAG DMP Chair: Amalio FernandezPacheco, University of Cambridge Room: 397 
Monday, March 13, 2017 11:15AM  11:27AM 
B50.00001: Optimizing duration of stored information in bundles of segmented magnetic nanowires Eugenio Vogel, Eduardo Cisternas, Diego Ceballos, Julian Faúndez Symbols or codes can be stored on a bunch of about a hundred thousand magnetic nanowires; in the present theoretical paper we explore ways to improve the duration of the stored information. Alumina membranes leave empty cylindrical columns which can be filled by metallic atoms. We consider the case of pores alternatively with a magnetic material for a length 2$l$, then with a nonmagnetic material (or spacer) of length $t$, to finish with another magnetic portion of eventually the same length 2$l$. These segmented nanowires are assumed to be homogeneous with diameter b, total length 2$L=$4$l+t$, with their cross sections defining a triangular lattice of interaxial distance $d$. To improve the duration of the inscribed information on the set of wires we minimize the interaction energy between any segment with all the others in both layers. The average energy per cylinder and maximum energy for any cylinder within the symbol are calculated in terms of $L, d,$ and especially$ t$. We conclude that for $t/d$ less than 10 optimum prevalence conditions are reached. Then we study the response of these systems to externally applied magnetic fields that could erase the information. Again this property is strongly dependent on $t/d$. [Preview Abstract] 
Monday, March 13, 2017 11:27AM  11:39AM 
B50.00002: Local magnetization effects on magnetotransport in networks of connected permalloy nanowires Joseph Sklenar, Brian Le, Jung Sik Park, GiaWei Chern, Cristiano Nisoli, Justin Watts, Michael Manno, David Rench, Nitin Samarth, Chris Leighton, Peter Schiffer We have performed detailed magnetotransport measurements of connected kagome artificial spin ice. To interpret our results we have performed micromagnetic simulations using MuMax3 to recreate all of the experimental configurations. We find good agreement between experiment and simulations for all inplane angular orientations of the field. In certain ranges of the applied field angle, the structure’s vertex regions control the transverse resistance. The wide array of realizable connected systems provides many vertex types, and in turn points toward the utility of artificial spin ice as a platform in which to engineer magnetoresistive effects that are sensitive to local environments. This project was funded by the US DOE under grant no. DESC0010778. This work was carried out in part in the Frederick Seitz Materials Research Laboratory at the University of Illinois at UrbanaChampaign. Work at the University of Minnesota was supported by the NSF MRSEC under award DMR1420013, and DMR1507048. CN's work is carried out under the auspices of the NNSA of the U.S. DoE at LANL under Contract No. DEAC [Preview Abstract] 
Monday, March 13, 2017 11:39AM  11:51AM 
B50.00003: Annihilation of Domain Walls in a Ferromagnetic Wire Anirban Ghosh, Kevin Huang, Oleg Tchernyshyov We study the annihilation of topological solitons in one of the simplest systems that support them: a onedimensional ferromagnetic wire with an easy axis along its length. In the presence of energy dissipation due to viscous losses, two solitons (domain walls) on the wire, when released from afar, approach each other and eventually annihilate to create a uniformly magnetized state. Starting from a class of exact solutions for stationary twodomainwall configurations in the absence of dissipation [1], we develop an effective theory that describes this annihilation in terms of four collective coordinates: a) the two zero modes corresponding to the location of the center and the average azimuthal angle of the full structure and b) their two conjugate momenta which describe the relative twist and the relative separation of the two domain walls respectively [2]. Comparison with micromagnetic simulation on OOOMF confirms that this theory captures well the essential physics of the process. We believe this work will be a good starting point for studying the annihilation of more complicated topological solitons like vortices and skyrmions in ferromagnetic thin films. [1]A.M. Kosevich, B.A. Ivanov, A.S. Kovalev, Phys. Rep. 194, 117 (1990). [2]O. Tchernyshyov, Ann. Phys. 363, 98 (2015). [Preview Abstract] 
Monday, March 13, 2017 11:51AM  12:03PM 
B50.00004: Electricfield modulation of interface magnetic anisotropy and spin reorientation in (Co/Pt)$_{\mathrm{3}}$/ PMNPT heterostructure YOU BA, Ying Sun, Wenbo Wang, Wei He, Xiaoli Zheng, Lvkuan Zou, Qinghua Zhang, Lin Gu, Zhaohua Cheng, Jianwang Cai, Weida Wu, Cewen Nan, Yonggang Zhao Electricfield control of magnetism in multiferroic heterostructures, composed of ferromagnetic (FM) and ferroelectric (FE) materials, has attracted much attention due to its interesting physics as well as applications. So far, most of the work on electricfield control of magnetism in FM/FE heterostructures focused on FM films with an inplane magnetic anisotropy and the work on FM films with a perpendicular magnetic anisotropy (PMA), is rather limited. Moreover, electricfield control of the interface magnetic anisotropy, which favors the PMA, has not been demonstrated. (Co/Pt)$_{\mathrm{3}}$ multilayers is a model system of PMA, which originates from the interface magnetic anisotropy. We study electricfield control of magnetism of (Co/Pt)$_{\mathrm{3}}$ multilayers with different thicknesses grown on PMNPT(011) FE substrates. Electricfield driven spin reorientation transition was observed. We also determined electric field induced changes of the bulk and interface magnetic anisotropies. Our analysis shows that electricfield modulation of interface magnetic anisotropy plays an essential role in driving the spin reorientation transition. Our work is also helpful for electricfield modulation of DzyaloshinskiiMoriya interaction and Rashba effect originated from interface to create new phenomena and functionalities. [Preview Abstract] 
Monday, March 13, 2017 12:03PM  12:15PM 
B50.00005: Voltage Control of Magnetic Anisotropy Guanhua Hao, Shi Cao, Nick Noviasky, Carolina Ilie, Andre Sokolov, Yuewei Yin, Xiaoshan Xu, Peter Dowben Pd/Co/Gd$_{2}$O$_{3}$/Si heterostructures were fabricated via pulsed laser deposition and ebeam evaporation. Hysteresis loops, obtained by longitudinal magnetooptical Kerreffect (MOKE) measurements, indicates an initial inplane magnetic anisotropy. Applying a perpendicular voltage on the sample, the differences between the polar and longitudinal MOKE and anomalous Hall effect data indicates there is a reversible change in magnetic anisotropy, from inplane to outofplane, with applied voltage. Prior work by others suggests that the change in magnetic anisotropy is due to redox reactions at the Co/Gd$_{2}$O$_{3}$ interference. Voltage controlled magnetism can result from changing interfacial chemistry and does not always require a magnetoelectric coupling tensor. [Preview Abstract] 
Monday, March 13, 2017 12:15PM  12:27PM 
B50.00006: Strong perpendicular magnetic anisotropy at FeCoB/MgO interface with an ultrathin HfOx insertion layer Yongxi Ou, Daniel Ralph, Robert Buhrman The realization of robust perpendicular magnetic anisotropy (PMA) in heavy metal(HM)/FeCoB/MgO thinfilm heterostructures has enabled a pathway for the implementation of high density memory elements based on perpendicularly magnetized tunnel junctions, and also provides a platform for the study and control of domain walls and of novel magnetic chiral structures such as skyrmions in nanowire structures. Here we report on the achievement of more robust PMA in Ta/FeCoB/MgO heterostructures by the insertion of an ultrathin HfOx passivation layer at the FeCoB/MgO interface. This is accomplished by depositing one to two atomic layers of Hf onto the FeCoB before the subsequent rf sputter deposition of the MgO layer, which fully oxidizes the Hf layer as confirmed by Xray photoelectron spectroscopy measurements. The result is a strong interfacial perpendicular anisotropy energy density as large as 1.7 erg/cm\textasciicircum 2 without any postfabrication annealing treatment. Similar results have been achieved with the use of W and Pt HM base layers. This work broadens the class and enhances the capabilities of PMA HM/FM heterostructures for spintronics research and applications. [Preview Abstract] 
Monday, March 13, 2017 12:27PM  12:39PM 
B50.00007: Frequency spectra of magnetization noise in Pt/Co/Pt trilayers near the spin reorientation transition Andrew Balk, Ian Gilbert, John Unguris, Scott Crooker We use scanning optical Kerr magnetometry to measure the stochastic magnetization noise of ferromagnetic Pt/Co/Pt trilayers near an outofplane to inplane spin reorientation transition. We find the magnetization noise spectra extend to MHz frequencies. As the spin reorientation transition is crossed from the outofplane to inplane side, the frequency spectra exhibit a crossover from power law to broad spectrum behavior. On the outofplane side, the power law exponent is robust to changes in temperature, magnetic anisotropy, and applied magnetic fields. Measurements of the noise as a function of both inplane and outofplane applied fields show symmetry consistent with a magnetic easy axis canted from the surface normal. The canting direction is strongly dependent on position, an observation which is consistent with the cone state that arises near the spin reorientation transition in similar materials. [Preview Abstract] 
Monday, March 13, 2017 12:39PM  12:51PM 
B50.00008: High quality TmIG films with perpendicular magnetic anisotropy grown by sputtering C. N. Wu, C. C. Tseng, S. L. Yeh, K. Y. Lin, C. K. Cheng, Y. T. Fanchiang, M. Hong, J. Kwo Ferrimagnetic thulium iron garnet (TmIG) films grown on gadolinium gallium garnet substrates recently showed stressinduced perpendicular magnetic anisotropy (PMA), attractive for realization of quantum anomalous Hall effect (QAHE) of topological insulator (TI) films via the proximity effect. Moreover, current induced magnetization switching of Pt/TmIG has been demonstrated for the development of room temperature (RT) spintronic devices.$^{\mathrm{1}}$ In this work, high quality TmIG films (about 25nm) were grown by sputtering at RT followed by postannealing. We showed that the film composition is tunable by varying the growth parameters. The XRD results showed excellent crystallinity of stoichiometric TmIG films with an outofplane lattice constant of 1.2322nm, a narrow film rocking curve of 0.017 degree, and a film roughness of 0.2 nm. The stoichiometric films exhibited PMA and the saturation magnetization at RT was 109 emu/cm3 (RT bulk value 110 emu/cm3) with a coercive field of 2.7 Oe. In contrast, TmIG films of Fe deficiency showed inplane magnetic anisotropy. The high quality sputtered TmIG films will be applied to heterostructures with TIs or metals with strong spinorbit coupling for novel spintronics. 1. C. O. Avci et al., Nat. Mater. (2016, in press) [Preview Abstract] 
Monday, March 13, 2017 12:51PM  1:03PM 
B50.00009: Magnetocrystalline effects in narrow ferromagnetic patterns Gabriel Chaves, Cyrill Muratov We introduce a reduced micromagnetic framework for the study of narrow ferromagnetic patterns on thin films with cubic magnetocrystalline anisotropy. The relevant patterns are curves of arbitrary shape that could be described as ‘curves with a width’, such as annuli and magnetic tracks. In these geometries, the magnetization is constant in the directions perpendicular to the curve and changes smoothly in the longitudinal direction. The curve makes an angle $\alpha$ with the easy axis and the magnetization is described by its tilt from the easy axis, $\theta$. We define a parameter $\Delta$ that measures the ratio of crystalline to shape anisotropy energies and study the existence of stable configurations (energy minimizers) as $\Delta$ varies. For $\Delta<0.5$ the magnetization is closely aligned along the longitudinal direction and only $\pi$ domain walls produce discontinuities in $\theta$. These $\pi$ walls are charged and interact magnetostatically. For $0.5<\Delta$ there are chargefree domain walls whenever $\alpha$ makes a $\pi/4$ angle with the easy axis. We apply this framework on nanorings and find the global energy minimizers in the presence on nonnegligible anisotropy. [Preview Abstract] 
Monday, March 13, 2017 1:03PM  1:15PM 
B50.00010: Surfacepatterned ferromagnetic thin films: an easy way to generate inverse hysteresis loops. Beatriz Mora, Nastassia Soriano, Carolina Redondo, David Navas, Rafael Morales Inverted hysteresis loops (i.e., unusual magnetic behavior characterized by negative remanence and negative coercivity) have so far been reported in antiferromagnetically coupled multilayers or bilayers grown with orthogonal anisotropies. In this work we demonstrate that inverted hysteresis loops can be achieved by surface patterning of a single ferromagnetic material [1]. Trenchpatterned Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$ (Permalloy, Py) films were fabricated by interference lithography (IL). We started from Py thin films 100 nm thick. Then, line patterns of Ti hard masks were transferred onto the Py films by plasma etching. Different trench depths were obtained by varying the etching time. Surface patterned Py films with trench depths between 15{\%}25{\%} of the total thickness exhibited inverted hysteresis loops for a wide range of applied magnetic field orientations. A theoretical approach based on the StonerWohlfarth model was proposed to explain the physical origin of this unusual magnetic behavior in patterned nanostructures. The model accounts for the experimental results and predicts the conditions that magnetic and geometric parameters of trenchpatterned films must satisfy to yield inverted hysteresis loops. [1] B. Mora et al. Nano Res. 9 (2016) 2347. DOI 10.1007/s122740161121y. [Preview Abstract] 
Monday, March 13, 2017 1:15PM  1:27PM 
B50.00011: Curvaturestabilized magnetic skyrmions Volodymyr Kravchuk Magnetic skyrmion is striking example of a topological soliton, it is a particlelike (localized) excitation in perpendicularly magnetized ferromagnetic films. Due to the topological stability, the skyrmion is considered as a key element for the next generation data storage and logic spintronic devices. In the current study\footnote{V.~Kravchuk et al., Phys. Rev. B \textbf{94}, 144402 (2016)} we consider skyrmion on a curvilinear magnetic film. First, we show that the skyrmion topological charge experiences a shift, which is determined by genus of the surface. Thus, the skyrmion on a spherical shell is topologically trivial, as a consequence, it can be induced by uniform external magnetic field, what is not possible for a planar film. Performing the topological analysis we derive the general expression for the gyrocoupling vector for an arbitrary curvilinear surface. It is important for any further collective variable description of dynamics of topological solitons on curvilinear surfaces. Second, we demonstrate that the magnetic skyrmion can be stabilized by curvature effects only, even when the intrinsic DzyaloshinskiiMoriya interaction (DMI) is absent. This in contrast to planar films, where the DMI is of principle importance for the skyrmion stabilization. [Preview Abstract] 
Monday, March 13, 2017 1:27PM  1:39PM 
B50.00012: CurvatureInduced Anisotropy in Thin Ferromagnetic Films Oleg Tretiakov, Massimiliano Morini, Sergiy Vasylkevych, Valeriy Slastikov The large curvature effects on micromagnetic energy of a thin ferromagnetic film with nonlocal dipolar energy are considered. We predict that the dipolar interaction and surface curvature can produce perpendicular anisotropy which can be controlled by engineering a special type of periodic surface shape structure. Similar effects can be achieved by a significant surface roughness in the film. We show that in general the anisotropy can point in an arbitrary direction depending on the surface curvature. We provide simple examples of these periodic surface structures to demonstrate how to engineer particular anisotropies in the film. [Preview Abstract] 
Monday, March 13, 2017 1:39PM  2:15PM 
B50.00013: Magnetism in curved geometries Invited Speaker: Robert Streubel Deterministically bending and twisting twodimensional structures in the threedimensional (3D) space provide means to modify conventional or to launch novel functionalities by tailoring curvature and 3D shape. The recent developments of 3D curved magnetic geometries, ranging from theoretical predictions over fabrication to characterization using integral means as well as advanced magnetic tomography, will be reviewed. Theoretical works predict a curvatureinduced effective anisotropy and effective DzyaloshinskiiMoriya interaction resulting in a vast of novel effects including magnetochiral effects (chirality symmetry breaking) and topologically induced magnetization patterning. The remarkable development of nanotechnology, e.g. preparation of highquality extended thin films, nanowires and frameworks via chemical and physical deposition as well as 3D nano printing, has granted first insights into the fundamental properties of 3D shaped magnetic objects. Optimizing magnetic and structural properties of these novel 3D architectures demands new investigation methods, particularly those based on vector tomographic imaging. Magnetic neutron tomography and electronbased 3D imaging, such as electron holography and vector field electron tomography, are wellestablished techniques to investigate macroscopic and nanoscopic samples, respectively. At the mesoscale, the curved objects can be investigated using the novel method of magnetic Xray tomography. In spite of experimental challenges to address the appealing theoretical predictions of curvatureinduced effects, those 3D magnetic architectures have already proven their application potential for life sciences, targeted delivery, realization of 3D spinwave filters, and magnetoencephalography devices, to name just a few. Reference: R. Streubel et al.: “Magnetism in curved geometries”, J. Phys. D: Appl. Phys. 49, 363001 (2016). [Preview Abstract] 
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