Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K50: NanomagnetsFocus
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Sponsoring Units: GMAG DMP Chair: Julie Karel, Monash University Room: 397 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K50.00001: Magnetic Interactions of Nanodisk Arrays Sven Velten, Robert Streubel, Alan Farhan, Noah Kent, Mi-Young Im, Andreas Scholl, Scott Dhuey, Peter Young, Ulrich Merkt, Carolin Behncke, Guido Meier, Peter Fischer We have investigated the collective behavior of interacting magnetic nanostructures in two dimensional arrays, such as emerging circularity patterns of magnetic vortices in hexagonal and honeycomb lattices. In particular we address the impact of varying disorder. By applying magnetic fields the magnetostatic interactions in the disk arrays lead to a coupling of the individual magnetic structures. Imaging those arrays with high resolution magnetic transmission soft x-ray microscopy we observe an alternating ordering of the magnetic curling direction, the circularity, of the vortices in the honeycomb lattice. In contrast, in the hexagonal lattice, small regions of alternating lines are formed. Micromagnetic simulations reveal that the patterns arise due to flux closure states during the stabilizing process of the magnetic vortices after the magnetic field is turned off. The effect of disorder on the collective behavior is studied by X-ray Photoemission Electron Microscopy using randomly distributed soft magnetic nanoislands, forming a single domain state pointing in either of two directions. We discuss these observations in the context of spin glass behavior. [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K50.00002: Spin wave modes in ferromagnetic nano-disks, their excitation via ac currents and fields, and auto-oscillations Rodrigo Arias, Daniela Mancilla The excitation with spin polarised dc-ac currents of the linear spin wave modes of a soft ferromagnetic free layer of a nano-pillar structure is studied. These structures may be used as microwave sources when these nano-oscillators enter into auto-oscillatory regimes. The free layer is a soft ferromagnet, in the shape of a circular disk, with a very small thickness. Using a description of the magnetization dynamics in terms of a Hamiltonian for weakly interacting waves we determine the spin wave modes of the structure under two approximations: a very thin film limit, and a model with the full dipolar interaction. We consider direct and parametric excitations of different spin wave modes, i.e. when the exciting ac frequency is approximately equal to the frequency of the mode or to twice its value, respectively. The Oersted field mainly plays a role in the direct excitation of the modes. Our main conclusion is that for a dc current below the critical value necessary for the development of auto-oscillations, a very high value of the ac current is needed in order to reach the auto-oscillatory behaviour using parametric excitation. But if the out of plane component of the spin transfer torque is high enough, the ac critical current for auto-oscillations is significantly reduced. [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 8:36AM |
K50.00003: Influence of edge state on magnetic entropy in manganitesnanodisks Yanmei Wang, Jian Shao, Hanxuan Lin, Hao Liu, Yang Yu, Jirong Sun, Wenbin Wang, Lifeng Yin, Jian Shen The broken symmetry effect on CE type antiferromagnetic spin structure can lead to emerging ferromagnetic edge state in manganites systems. The edge state is expected to become more and more dominant with decreasing size of the system. In this work, we fabricate manganites nanodisks with changing diameters from epitaxial thin films to investigate how edge state affects the magnetic properties, in particular the magnetic entropy of the manganites systems. Our observe that with scaling down to the characteristic length scale of electronic phase separation(EPS), the La$_{\mathrm{0.325}}$Pr$_{\mathrm{0.3}}$Ca$_{\mathrm{0.375}}$MnO$_{\mathrm{3}}$(LPCMO) nanodisks undergo a transition from a EPS state featuring coexistence of ferromagnetic metal and charge ordering insulator phases to a single ferromagnetic metal state. The change of magnetic entropy upon this transition has been characterized by SQUID magnetometer revealing the effect of edge state on the spin ordering and thus the entropy of the system. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K50.00004: Magnetic Vortices in Nanodisks: What are the implications in macroscopic magnetic properties? Ciro Fernando Gelvez Pedroza, Edgar J. Patino The study of nanodevices is of great importance nowadays. In particular nanodisks present extraordinary properties when varying their size, shape and materials. One of the most interesting ones has been the presence of magnetic vortices which are normally not present in continuous films or bulk materials. For that reason, these constitute of great interest in potential applications such as data storage, binary logic gates or nano-plasmonics. Although there are many high cost methods for fabrication we have chosen a low cost technique based on Colloidal Lithography. Using Polystyrene Nanoparticles (100nm) nanodisks of about 180 nm in diameter have been grown using Electron Beam evaporation. The fabrication technique requires a number of steps such as spin coating, oxygen plasma and Ion Beam Etching. The samples obtained with this method were Ti/Co/Nb nanodisks with various thickness of the Co layer. Micromagnetic simulations were carried out in OOMMF giving magnetic domain structure and hysteresis loops which were later compared with those obtained experimentally using Vibrating Sample Magnetometry. Simulation results suggest a critical thickness for the appearance of magnetic vortices, revealed by hysteresis loops with substantially lower coercive fields. [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K50.00005: Fabrication and Characterization of Cobalt Microbars Diego Useche, Ciro Gelvez, Edgar J. Patino, Leonardo Basile Magnetic materials have been widely used in applications such as data storage, magnetic tunnel junctions and spin valve structures. In the present work we explore the shape anisotropy of a millimeter size array of magnetic microbars. For this study we focused on Cobalt microbars fabricated using photolithography, thermal evaporation and sputtering techniques. Magnetization measurements as a function of angle were performed on microbars samples which differ slightly in their dimensions. The measurements were carried out using vibrating sample magnetometry (VSM) and magnetic optic kerr effect (MOKE). Also, computer simulations in OOMMF were done in order to simulate the experimental results. Despite the bars being in the micrometer scale a small shape anisotropy was observed. Furthermore, when the applied field was parallel to the microbars the maximum coercivity was found and decreased slightly as the field rotated towards the perpendicular direction. It was also found a sheared hysteresis loop in one of the samples. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K50.00006: Modeling of Magnetic Behavior of Superatomic-Fullerene Assemblies Pallabi Sutradhar, Vikas Chauhan, Shiv N. Khanna, Jayasimha Atulasimha We recently carried out theoretical studies [1] on recently synthesized magnetic superatomic solids [2], consisting of magnetic Ni$_{\mathrm{9}}$Te$_{\mathrm{6}}$(Pet$_{\mathrm{3}})_{\mathrm{8}}$ clusters separated by non-magnetic C$_{\mathrm{60}}$. We studied the magnetic response of the superatomic solid by constructing the Hamiltonian for a small assembly of metal clusters interacting through the fullerene by including isotropic and anisotropic exchange interactions, magnetic anisotropy energy and Zeeman energy for the interaction between the cluster magnetic moment and the global magnetic field. Furthermore, we showed that inclusion of Dzyaloshinskii-Moriya interaction that causes spin canting was essential in order to explain the experimental trend. Here we will extend this analysis to study the magnetic response of larger Superatomic-Fullerene assemblies (supersolids) through Monte Carlo simulations to understand temperature and size effects. [1] P. Sutradhar et al., Phys. Chem. Chem. Phys., 2016 (Selected for highlight on back cover) DOI: 10.1039/c6cp05196k. [2] C.H. Lee et al., J. Am. Chem. Soc. 136, 16926, 2014. Acknowledgement: V.C. and S.N.K acknowledge Department of Energy under award no. DE-SC0006420 [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:48AM |
K50.00007: Wavevector dependent damping in nanomagnets Invited Speaker: Hans Nembach Nanomagnets are the building blocks of spintronics devices. This makes it important to understand the magnetization dynamics in nanomagnets from an application oriented view. From a more basic research orientated perspective, nanomagnets provide the opportunity to determine, if the localization of spin-wave modes influences their dynamics. We measured localized spin-wave modes in individual Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$ nanomagnets ranging from 80 nm to 400 nm in size by heterodyne magneto-optical microwave microscopy. We compared our measured spectra with micromagnetic simulations and were able to identify two spin-wave modes, the center-mode and the end-mode. We determined that the Gilbert damping for these localized spinwave modes depends on the size of the nanomagnet and on the respective spin-wave mode. We were able to exclude that the observed damping originates from an area of enhanced damping at the edge of the nanomagnets. A detailed analysis showed that the results can be understood within the model of Bar'yakhtar for damping in ferromagnets, where exchange contributions to the relaxation are considered. These additional contributions depend on the curvature of the dynamic magnetization or, in Fourier space, on k$^{\mathrm{2}}$, where k is the wavevector of the respective Fourier components of the spatial non-uniformities. We also studied the k$^{\mathrm{2}}$-damping for perpendicular standing spin-wave modes (PSSWs) in Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$ films with a thicknesses starting at 75 nm. Our results showed that any k$^{\mathrm{2}}$-damping contributions must be significantly smaller than what we have found in the nanomagnets. In order to determine if this k$^{\mathrm{2}}$-damping originates from the interface, we compared the damping in nanomagnets for 3 nm, 10 nm and 15 nm thick Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$ layers. We found, that the k$^{\mathrm{2}}$-damping for the nanomagnets decreases with increasing thickness of the ferromagnetic layer. This indicates that the k$^{\mathrm{2}}$-damping in the studied system has a strong interfacial contribution, which explains, why we were not able to measure any k2-damping for the PSSWs. 1) V. G. Bar'yakhtar et al., Zh. Eksp. Teor. Fiz. 91, 1454 (1986) 2) H.T. Nembach et al., Phys. Rev. Lett., 11, 117201 (2013) 3) M. Schoen et al., Phys. Rev. B, 91, 184417 (2015) [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K50.00008: d0 ferromagnetism in nanocrystals Yuri Dahnovsky, Vitaly Proshchenko We study $d^0$ ferromagnetism in ZnS and ZnO quantum dots (QDs) and nanowires (NW). To find the magnetization of the medium and large size nanocrystals (NC) we introduce the surface-bulk (SB) model where the separately calculated surface and bulk contributions to the total magnetic moment allows us to find the magnetization for a large nanocrystal (NC). For nanowire calculations the accuracy of the SB model varies from $0.2\%$ to $21\%$ depending on the Zn vacancy concentrations on the NC surface and in the NC core. We find that the higher the concentration of the Zn vacancies, the larger the total magnetic moment in the nanocrystal, however the magnetic moment increase faster for quantum gots rather than for nanowires. We also study the cases where the concentrations of Zn vacancies can be different on the NC surface and in the core. From the comparison of the experimental and theoretical NW magnetic moments we find that the experimental magnetization is $1.4\cdot 10^3$ smaller than the calculated one. Such a huge discrepancy can be explained from the assumption that not all magnetic moments due to Zn vacancy participate in the ferromagnetism and there are some regions with zero magnetism and uncoupled (paramagnetic) spins. [Preview Abstract] |
(Author Not Attending)
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K50.00009: Numerical Modeling of Patterned Magneto-dielectric Structures Chidubem Nwokoye Magneto-dielectric structures exhibit both electric and magnetic properties because they have relative permittivity and permeability values greater than one. Interest in magneto-dielectrics are growing due to their potential device applications, such as low profile antennas, magnetic read heads, non-volatile memories, etc [1]. We report the results of numerical modeling of the relative permittivity and permeability responses on magneto-dielectric structures with array patterns and we discuss the comparison of the results with modeling results obtained from the OOMMF software [2]. [1] W. Prellier, M.P. Singh and P. Murugavel, J. Phys. Cond. Matter 17, R803 (2005). [2] M. J. Donahue and D. G. Porter. OOMMF User’s Guide, Version 1.0. NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (Sept 1999). [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K50.00010: Magnetocrystalline anisotropy of adatoms and monolayers: an illustrative view Ondrej Sipr, Sergiy Mankovsky, Jan Minar, Hubert Ebert It has been known for decades that the magnetocrystalline anisotropy is linked to the spin-orbit coupling (SOC). Nevertheless, the mechanism how it arises for specific systems is still subject of debate. We focus on finding markers of SOC in the density of states (DOS) and on employing these markers for understanding the source of magnetocrystalline anisotropy of nanostructures, represented here by adatoms and monolayers. Fully relativistic ab-initio KKR-Green function calculations were performed for Fe, Co, and Ni adatoms and monolayers on Au(111) to investigate changes in the orbital-resolved DOS due to a rotation of magnetization. In this way one can see that for adatoms a significant contribution to the magnetocrystalline anisotropy comes from pushing of the SOC-split states above or below the Fermi level. A similar picture emerges from model crystal field Hamiltonian calculations. One of the consequences of this mechanism is that the magnetocrystalline anisotropy energy crucially depends on the position of the energy bands of the adatom with respect to the Fermi level of the substrate. We assume that these conclusions can be extended to nanodots and supported clusters. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K50.00011: Current induced incoherent magnetization dynamics in ferromagnetic/non-magnetic metallic multilayer nanowires Md Mamun Al-Rashid, Mazin Maqableh, Bethanie Stadler, Jayasimha Atulasimha High density arrays of electrodeposited nanowires consisting of ferromagnetic/non-magnetic (Co/Cu) multilayers are promising as magnetic memory devices$^{\mathrm{1-3}}$. For individual nanowires containing multiple (Co/Cu) bilayers, the stable magnetization orientations of the Co layers (with respect to each other and the nanowire axis) are dependent on the Cu layer thickness, even when the Co layer dimensions are fixed. This dependence is a result of the competition between shape anisotropy, magneto-crystalline anisotropy and intra-wire dipole coupling. However, when the nanowires are closely packed in arrays, inter-wire dipole coupling can result in complex and tunable domain structures comprising segments of multiple nanowires$^{\mathrm{4}}$. This work explores the dependence of these domain structures and their switching on the non-magnetic layer thickness and intra-wire spacing both experimentally and via rigorous micromagnetic simulation. These domain structures play a crucial role in determining the current and time required for STT switching. $^{\mathrm{1}}$Piraux et al, Appl. Phys. Lett. 65, 2484 (1994). $^{\mathrm{2}}$Maqableh et al. Nano Lett. 12, 4102 (2012). $^{\mathrm{3}}$Hernandez et al. J. Appl. Phys. 109, 07C916 (2011). $^{\mathrm{4}}$Grutter et al. MMM Conference 2016, New Orleans, LA, USA. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K50.00012: Dimensionality and stoichiometry effects on magnetic properties of Fe$_{x}$Co$_{1-x}$ nanostructures on Pt(111) from first principles Helena Petrilli, Ivan Miranda, Ricardo Igarashi, Angela Klautau We investigate the influence of dimensionality and stoichiometry changes on the behavior of local magnetic moments and exchange coupling parameters of Fe$_{x}$Co$_{1-x}$ nanostructures deposited on the fcc Pt(111) surface. The electronic and magnetic structures of these Fe$_{x}$Co$_{1-x}$/Pt(111) systems are studied using the first-principles RS-LMTO-ASA method in the framework of the DFT. We consider different configurations of linear-shaped or compact-shaped trimers and heptamers, varying the internal positions and the concentration of Fe or Co atoms. The existence of a strictly decreasing nonlinear trend of the average orbital moments with the Fe concentration for the compact clusters is demonstrated, and differs from what was found for higher-dimensional Fe$_{x}$Co$_{1-x}$ systems (monolayer/Pt(111) and bulk). Although all studied linear and compact Fe$_{x}$Co$_{1-x}$ configurations have shown to be substancially ferromagnetic between nearest Fe or Co neighbors, not all revealed a collinear ground state, presenting, in some cases, oscillating exchange parameters for further distances and a non-negligible Dzyaloshinskii-Moriya interaction, induced by the Pt strong spin-orbit coupling. [Preview Abstract] |
Wednesday, March 15, 2017 10:48AM - 11:00AM |
K50.00013: Measuring the static and dynamic magneto-optical constant across the entire M-edge in a reflection geometry using polarization scanning Phoebe M. Tengdin, Dmitriy Zusin, Christian Gentry, Adam Blonsky, Justin Shaw, Hans Nembach, Tom J. Silva, Peter Oppeneer, Henry C. Kapteyn, Margaret M. Murnane The off diagonal components of the dielectric permittivity tensor ($\epsilon_{xy}$) provide a critical link between the microscopic band structure of a ferromagnetic material and its measured magneto-optical response. However, access to the full $\epsilon_{xy}$ of a material in the extreme ultraviolet (EUV) region spanning the M shell absorption edges has remained limited due to the difficulty in analyzing the polarization of EUV light. In this work, we present a new all-optical EUV technique that allows us to measure the $\epsilon_{xy}$ tensor across the M shell absorption edges of a ferromagnet using a table-top high harmonic EUV system. When combined with recent advances in laser-driven high harmonic sources, this technique is also capable of capturing the dynamically-changing $\epsilon_{xy}$, and thus the evolving far from equilibrium band structure across the entire M-shell absorption edge of a material, with femtosecond time resolution[1]. [1]Nature Comm. 7, 12902 (2016) [Preview Abstract] |
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