Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q7: Focus Session: Magnetic Domains and Domain Walls |
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Sponsoring Units: GMAG Chair: Randy Dumas, Gothenburg University, Sweden Room: 106 |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q7.00001: Antiferromagnetic domain dynamics observed in Fe/CoO/MgO(001) system Qian Li, Jie Zhu, Gong Chen, Tian Ping Ma, Yan Huo, Yi Zheng Wu Antiferromagnetic (AFM) domain dynamics has rarely been researched in spite of its importance in the magnetic recording application and fundamental physics studies. Usually, the magnetic properties of the AFM layer can be studied by detecting the adjacent ferromagnetic (FM) layer due to the exchange coupling at the FM/AFM interface. In this contribution, we studied the AFM domain flipping process in a FM/AFM exchange-coupled system. Single-crystalline Fe/CoO/MgO(001) was prepared by molecular beam epitaxy and its magnetic properties were measured with magneto-optic Kerr effect. With magnetic field scanning along CoO[1-10] at 143K after field cooling along CoO[110], the hysteresis loop gradually changes from a double-split loop to an easy square loop, which indicates AFM spin rotating by 90 degrees during this process. Two mechanisms, AFM domain nucleation (DN) and domain wall motion (DWM), are clearly separated by analyzing remanence signal as a function of loop sequential number, and meanwhile the flipping rates are also obtained. For the first time, AFM DN and DWM energy barriers are quantitatively determined by temperature dependence measurement. Systematic results are got that energy barriers increase linearly with CoO thickness and decrease in larger magnetic field. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q7.00002: Order and topology in antiferromagnets with surfaces Michalis Charilaou, Frances Hellman We show using Monte Carlo simulations and mean-field theory that the antiferromagnetic (AFM) magnetization, arising from uncompensated spins, exhibits a unique thermodynamic behavior that differs from that of ferromagnets or of the N\'{e}el vector. More importantly, the net uncompensated magnetization is lower than that of the surface due to finite size effects. This phenomenon can be is manifested in thin films but it is in fact the same even in infinite systems with free surfaces, suggesting a topological order in uncompensated antiferromagnets. Moreover, we investigate the effects of defects and roughness on the magnetization of AFM and show that with increasing roughness the magnetization decreases non-monotonically and reaches values of only a few percent. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q7.00003: The surface magnetization study of Cr$_{2}$O$_{3}$ by spin polarized low energy electron microscopy Shi Cao, Ning Wu, Xin Zhang, Alpha N'Diaye, Gong Chen, Andreas Schmid, Will Echtenkamp, Valeria Lauter, Christian Binek, Peter Dowben The boundary magnetization at the surface of a Cr$_{2}$O$_{3}$ single crystal has been demonstrated by using spin-polarized low-energy electron microscopy (SPLEEM), indicating net surface spin polarization. This work shows that the placement of Cr$_{2}$O$_{3}$ single crystal in the single domain state, will result in net Cr$_{2}$O$_{3}$ spin polarization at the boundary, even in the presence of a gold overlayer. There are indications that the spin-polarized low-energy electron microscopy (SPLEEM) contrast for the two polarizations states is different. In addition, the boundary magnetization protected by the symmetry exists despite of the surface roughness/softness which was studied by the non-spin neutron reflectometry and low energy electron diffraction. Unoccupied surface oxygen sites and chromium sites are possible mechanism contributing to the surface ``softness,'' which will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q7.00004: Jamming Behavior of Domain Walls in an Antiferromagnetic Film Invited Speaker: Sunil Sinha Over the last few years, attempts have been made to unify many aspects of the freezing behavior of glasses, granular materials, gels, supercooled liquids, etc. into a general conceptual framework of what is called jamming behavior. This occurs when particles reach packing densities high enough that their motions become highly restricted. A general phase diagram has been proposed onto which various materials systems, e.g glasses or granular materials, can be mapped. We will discuss some recent applications of resonant and non-resonant soft X-ray Grazing Incidence Scattering to mesoscopic science, for example the study of magnetic domain wall fluctuations in thin films. For these studies, we use resonant magnetic x-ray scattering with a coherent photon beam and the technique of X-ray Photon Correlation Spectroscopy. find that at the ordering temperature the domains of an antiferromagnetic system, namely Dysprosium metal, behave very much also like a jammed system and their associated fluctuations exhibit behavior which exhibit some of the universal characteristics of jammed systems, such as non-exponential relaxation and Vogel-Fulcher type freezing.\\[4pt] Work done in collaboration with San-Wen Chen (UCSD), Hongyu Guo (UCSD), Keoki Seu (ALS/LBL), Karine Dumesnil (Institute Jean Lamour, Universite de Lorraine, Nancy, France) and Sujoy Roy (ALS/LBL). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q7.00005: Dependence of Exchange Bias on Interfacial and Bulk Antiferromagnetic Spins Ali C. Basaran, Thomas Saerbeck, Jose de la Venta, Henning Huckfeldt, Arno Ehresmann, Ivan K. Schuller One of the key issues in exchange bias is the dependence of pinned interfacial or bulk uncompensated antiferromagnetic spins. To address this important issue, we grew simultaneously several sets of ferromagnetic/antiferromagnetic (Ni/FeF$_{\mathrm{2}})$ bilayers capped with a nonmagnetic and inert gold layer of varying thickness. He-ion irradiation was employed to selectively create defects near the Ni/FeF$_{\mathrm{2\thinspace }}$interface or in the bulk of FeF$_{\mathrm{2}}$. The penetration depth of the ions with constant energy and dose was controlled by the gold capping layer thickness. Consequently, this leads to varying depth profiles of defects in the antiferromagnetic bulk. This was confirmed by numerical simulations of the ion damage. Detailed, quantitative, structural and magnetic characterizations were compared before and after the bombardment. These studies show that the creation of defects in the antiferromagnetic bulk is playing a crucial role in exchange bias [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q7.00006: Effect of morphology on exchange bias in NiMnSn and NiCoMnIn magnetic shape memory alloys Pavel N. Lapa, James A. Monroe, Brian E. Franco, Ibrahim Karaman, Igor V. Roshchin Exchange bias (EB) is one of puzzling magnetic properties of magnetic shape memory alloys (MSMA). Despite a few attempts to explain the mechanism, there is no comprehensive model describing it. The main obstacle is the lack of information about the magnetic structure of martensitic and austenite phases. In contrast to classical EB systems where the exchange coupling happens at the interface between ferromagnetic and antiferromagnetic layers, the EB in MSMA is attributed to coexistence of ferromagnetic and antiferromagnetic regions. We report the results of structural analysis obtained using wavelength-dispersive X-ray spectroscopy (WDS) and magnetic characterization of these samples. We observe a correlation of EB with the secondary heat treatment for NiCoMnIn alloys. Comparative first order reversal curve (FORC) analysis for NiMnSn samples with different heat treatment suggests a correlation between morphology and distribution of exchange bias values. Additionally, exchange bias in these alloys can be induced even after zero-field cooling by applying a constant field for 2 hours before measuring the magnetization curve. This behavior is consistent with magnetic glassiness observed in these alloys at low temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q7.00007: Element-selective investigation of domain structure in CoPd and FePd alloys using small-angle soft X-ray scattering C. Weier, R. Adam, R. Fr\"{o}mter, J. Bach, G. Winkler, A. Kobs, H.P. Oepen, P. Grychtol, H.C. Kapteyn, M.M. Murnane, C.M. Schneider Recent optical pump-probe experiments on magnetic multilayers and alloys identified $\it{perpendicular}$ spin superdiffusion as one of possible mechanisms responsible for femtosecond magnetization dynamics. On the other hand, no strong evidence for the ultrafast $\it{lateral}$ spin transport has been reported, so far. To address this question, we studied magnetic domain structure of CoPd and FePd thin films using small-angle scattering of soft X-rays. By tuning the synchrotron-generated X-rays to the absorption edges of Fe or Co we recorded Fourier images of the magnetic domain structure corresponding to a chosen element. Applying $\it{in-situ}$ magnetic fields resulted in pronounced rearrangement of domain structure that was clearly observed in scattering images. Our analysis of both the stand-alone, as well as magnetically coupled CoPd/FePd layers provides insight into the formation of domains under small magnetic field perturbations and pave the way to better understanding of transient changes expected in magneto-dynamic measurements. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q7.00008: Graphene mediated magnetic domain formation Iori Tanabe, Yi Wang, Lingmei Kong, Christian Binek, Frank Pasquale, Yuan Cao, Bin Dong, Jeffry Kelber, Peter Dowben Both graphene on Co and graphene on Co3O4/Co samples were investigate by the Raman spectroscopy and longitudinal magneto-optic Kerr effect (MOKE). While the graphene on Co (111) bilayer thin films exhibited high remnant magnetization in plane easy axis ferromagnetism, the graphene/Co3O4/Co trilayers exhibited little remnant magnetization. The latter is due to formation of a complex multidomain state at zero applied field. The role of graphene and Co3O4 will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q7.00009: Programmable manipulation of superparamagnetic microbeads at junctions using magnetic domain walls Elizabeth Rapoport, David Bono, Geoffrey Beach There has been a steady progression in the advancement of magnetic technologies for bead manipulation in chip-based devices. Recently, we demonstrated that with curvilinear magnetic tracks, both domain wall (DW)-driven transport and detection of superparamagnetic (SPM) beads can be achieved. Here, we demonstrate that the direction of bead motion at junctions in branched curvilinear structures can be precisely selected with a vertical field. Upon exiting a junction, a single DW is split into two of opposite configuration. A vertical field strengthens the bead-DW interaction for one DW configuration, while simultaneously weakening the interaction for the other. The result is preferential bead motion with one DW over the other, allowing for the design of complex bead routing networks. Numerical work is presented in support of the theoretical basis for selective motion, and experiment reveals a threshold vertical select field for a sample of nominally identical beads. This routing technique is also shown to be able to sort a mixed population of SPM beads by simple application of a vertical field. With this work, we add an essential capability to the set of DW-mediated SPM bead handling functions required for an integrated lab-on-a-chip platform. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q7.00010: Trajectory of dynamically propagating magnetic domain walls at nanowire vertices David M. Burn, Stephanie K. Walton, Megha Chadha, Katharina Zeissler, Lesley F. Cohen, Will R. Branford Nanoscale patterning techniques can be used to fabricate magnetic nanowire structures where the behavior of individual magnetic domain walls (DWs) can be investigated. In addition to the fundamental physical understanding of magnetism, research in this area is also driven by the potential to realize novel spintronic devices for technological applications. Magnetic DWs can support a wide variety of micromagnetic structures with different magnetization, chirality and topology based on their interaction with the nanostructure geometry. These interactions can govern the field dependent domain wall trajectory and subsequent magnetization reversal that takes place within nanowire vertex structures. In this work the additional factors affecting the trajectory due to the dynamic behavior of propagating DWs are investigated. This includes the time dependent periodic changes in the DW micromagnetic structure from Walker breakdown. These results have implications for future technological applications as well as suggesting processes that may govern magnetization reversal in artificial spin ice structures. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q7.00011: Simulation of Oscillatory Domain Wall Motion Driven by Spin Waves in Nanostrip with Perpendicular Magnetic Anisotropy Shang Fan Lee, Liang Juan Chang We numerically investigate the spin waves (SW) induced domain wall (DW) oscillatory motion in a nanostrip with perpendicular magnetic anisotropy by means of micromagnetic simulation. SW carries spin angular momentum and can interact with DWs via Spin Transfer Torque (STT). Propagating SW can drive a DW motion depending on the in-plane tilt angle $\varphi $ of the wall magnetization. We calculate the instantaneous velocity of DWs as a function of $\varphi $with different SW frequency $f$. We find that the DW motion under propagating SW depends not only on the frequencies $f$, but also on the in-plane tilt angle $\varphi $. The nanostrip considered is 50 nm wide and 4000 nm long. A DW at the center is subjected to a SW source 500 nm apart on the left with amplitude in the transverse direction and varying frequency $f$. The motions of the DW induced by the SW are accompanied by in-plane rotation of magnetization of DW. Once rotated by 90 degrees, the DW shows a backward motion towards the SW source. The oscillatory amplitude and frequency of the DW motion is analyzed. A phase diagram will be presented. This study provides new perspectives for the control and manipulation of DW in a nanostrip. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q7.00012: The creation of 360 degree domain walls in ferromagnetic nanorings by circular applied magnetic fields Jessica Bickel, Spencer Smith, Katherine Aidala 360$^{\circ}$ domain walls (DWs) are the proposed transition state of ferromagnetic nanorings which are candidate devices for magnetic memory. Using micromagnetic simulations [1], we examine the formation of 360$^{\circ}$ DWs created by the application of a circular Oersted field for the transition of a 5nm thick ring from a CCW to a CW vortex. The magnetic reversal begins by canting of the magnetization either inward or outward. As the spin continues to rotate, exchange interactions result in the rotation of adjacent spins. Finally, the rotate spin aligns with the applied magnetic field, creating a transition state made of two 180$^{\circ}$ DWs of opposite winding number. As the center of the rotated domain grows, the 180$^{\circ}$ walls of adjacent domains meet. Adjacent domains cant in opposite directions to lower the magnetostatic energy relative to canting in the same direction. Therefore 180$^{\circ}$ DWs at the boundaries have the same winding number and combine to form 360$^{\circ}$ DWs. Each pair of rotated domains results in a pair of two 360$^{\circ}$ DWs of opposite winding number. This work provides better understanding of the formation of 360$^{\circ}$ DWs and may lead to the ability to control the formation of DWs via geometry. [1] http://math.nist.gov/oommf [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q7.00013: Effect of curvature on domain wall motion in elliptical nanorings Fikriye Idil Kaya, Jessica Bickel, Katherine Aidala Understanding domain wall (DW) motion in ferromagnetic nanostructures is important to realize proposed magnetic data storage and logic devices. We investigate the effect of curvature on DW pinning and motion by studying elliptical rings using micromagnetic simulations [1]. Elliptical rings with constant width have varying curvature, with the lowest curvature at the minor axis, and the greatest curvature at the major axis. DWs can be created at any angular position within the ellipse by the application of an appropriate uniform magnetic field. However, only some of these positions are stable when the field is removed. We study the stability and depinning of the DWs by applying a slowly increasing elliptical magnetic field to determine the magnitude of the field at which the DWs begin to move. By varying the major to minor axis ratio, we examine the effect of curvature on DW pinning. A larger field is required to move DWs in regions of higher curvature (near the major axis) than lower curvature (near the minor axis). Overall, we see that increasing the major to minor axis ratio of elliptical nanorings requires increasing field strength to depin the DWs along the major axis. [1] Oommf software distributed by NIST at http://math.nist.gov/oommf [Preview Abstract] |
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