Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N22: Focus Session: Magnetic Vortices and Exchange Biased Thin Films |
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Sponsoring Units: GMAG DMP Chair: Jian Shen, Oak Ridge National Laboratory Room: Baltimore Convention Center 319 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N22.00001: Field Dependence of Magnetic Vortex Dynamics R.L. Compton, J.P. Park, P.A. Crowell We have used time-resolved Kerr microscopy (TRKM) to investigate the dynamical behavior of micron diameter disks patterned from sputtered Permalloy (Py) films. Different growth conditions yielded grain diameters of $\sim$35~nm and $\sim$80~nm while average roughness (Ra) remained less than 1~nm for 50~nm thick films. The magnetization of the disks relaxes into a vortex ground state, in which broadband spin dynamics include a low frequency vortex translational mode (vortex mode) that is expected to be nearly independent of field, based on simulations and analytical theory. We measured the field dependence of the vortex dynamics of individual disks using 5 Oe field steps from 0 Oe through the vortex annihilation field (H$_{a}$). For a 1~$\mu$m diameter disk the vortex mode has a mean frequency of $\sim$300~MHz, but the frequency fluctuates throughout the entire field range (H$_{a} \sim$ 350~Oe) with a magnitude $\Delta f\sim$200~MHz and a characteristic period $\sim$30~Oe. The fluctuations are not symmetric about zero field and look different in detail for different disks, but are highly repeatable for the same disk. We have also observed non-linear effects including the presence of up to 3 higher harmonics of the vortex mode, with a higher harmonic occasionally dominating the spectrum. A consistent interpretation is that the vortex core samples a distribution of pinning potentials, some of which are anharmonic, as it traverses the disk under the influence of the static applied field. Supported by NSF DMR 04-06029. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N22.00002: Fractional vortices and composite domain walls in flat nanomagnets Oleg Tchernyshyov, Gia-Wei Chern We provide a simple explanation of complex magnetic patterns observed in ferromagnetic nanostructures. To this end we identify elementary topological defects in the field of magnetization: ordinary vortices in the bulk and vortices with {\em fractional} winding numbers ($\pm 1/2$) confined to the edge [1]. Domain walls found in experiments and numerical simulations in strips and rings are composite objects containing two or more of the elementary defects. Allowed compositions of a domain wall in a strip or ring are constrained by simple selection rules of topological origin: (i) An edge contains an odd number of edge defects. (ii) The net winding number of all edge and bulk defects is zero. The walls observed most frequently in experiments and simulations contain a halfvortex and an antihalfvortex (transverse walls in thin and narrow strips) or two antihalfvortices and a vortex (vortex walls in thicker and wider strips). \begin{thebibliography}{1} \bibitem{OT05} O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. {\bf 95,} 197204 (2005). \end{thebibliography} [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N22.00003: Magnetic domains in ferromagnetic particles with perpendicular anisotropy Stavros Komineas, Christoforos Moutafis, Tony Bland We derive a Derrick-like virial theorem for static states in a disc-shaped ferromagnetic particle with an axially symmetric magnetic configuration. This is applied to elementary magnetic states such as a single domain and a vortex. We calculate the vortex state in a disc-shaped particle with no anisotropy and study the very thin and very thick limits. In the very thin limit the virial relation effectively gives the vortex core radius. We also consider a particle with significant perpendicular anisotropy and show that a vortex is a static state for sufficiently thin particles. For thicker particles the vortex core expands to become comparable to the particle lateral size while the magnetization at the periphery of the particle tilts out of plane opposite to the vortex core region. In sufficiently thick particles, the magnetic state takes the form of a magnetic ``bubble'' (well-known in films) viewed here as a bidomain state. The signature of a bubble is its magnetostatic field which consists of two concentric regions of opposite sign above the particle top surface. Higher order states of multiple concentric domains of opposite magnetization are found in larger particles. We finally study the effect of an external field on magnetic bubble states. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N22.00004: Composite domain walls in flat nanomagnets: the dipolar limit Gia-Wei Chern, Hyun Youk, Kathleen Merit, Oleg Tchernyshyov Topological defects play an important role in nanoscale ferromagnets. We have previously demonstrated that domain walls in thin strips and rings are composite objects made of bulk vortices (winding numbers $n = \pm 1$) and edge defects (fractional winding numbers $n = \pm 1/2$) and given analytical solutions in the exchange limit [1]. Experimentally accessible systems are in the opposite regime where the dipolar interaction dominate. In this limit the vortex solution remains unchanged, the antivortex and antihalfvortex are deformed but survive, whereas the halfvortex acquires a high magnetostatic energy and becomes unstable. Accordingly, domain walls in this limit consist of two antihalfvortices and a vortex between them. We present a model of the domain wall in the magnetostatic limit in which the location of the vortex core is a variational parameter. As the width and thickness of a strip change, the global and local minima of the total magnetic energy yield the familiar transverse and vortex walls, as well as more exotic configurations such as the ``diagonal wall'' with a vortex hanging close to an edge. [1] O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. {\bf 95}, 197204 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N22.00005: Single vortex dynamics in patterned ferromagnetic ellipses Kristen Buchanan, Pierre Roy, Marcos Grimsditch, Frank Fradin, Konstantin Guslienko, Sam Bader, Val Novosad Measurements of low frequency dynamics of single magnetic vortices confined in elliptic ferromagnetic dots made of Permalloy with dimensions 2x1 $\mu$m$^2$ and 3x1.5 $\mu$m$^2$, 40-nm thick, have been performed using a microwave reflection method. Resonances were recorded in the sub-GHz range that can be attributed to the vortex translational mode where the vortex core follows an elliptic trajectory around its equilibrium position. The existence of single vortex states in the samples was confirmed by magnetic force microscopy. The frequency of this translational mode varies little under the influence of an in-plane static field H along the easy axis, however, it increases by more than a factor of two when H is applied along the hard axis. Micromagnetic simulations are used to explore the origin of the observed field dependence. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N22.00006: The relationship between the sign of exchange bias and the magnetization depth profile of Co/FeF$_{2}$ Michael Fitzsimmons, Brian Kirby, Sujoy Roy, Zhi-Pan Li, Igov V. Roshchin, R. Morales, S.K. Sinha, Ivan K. Schuller We have used the unique spatial sensitivity of polarized neutron beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet (Co) and an antiferromagnet (FeF$_{2})$. Our Co/FeF$_{2}$ bilayer sample is one that exhibits either positive or negative exchange bias depending upon the magnitude of the cooling field. For positive exchange bias, pinned magnetization at the Co/FeF$_{2}$ interface is directed opposite to the cooling field, while in the FeF$_{2}$ bulk, the net pinned magnetization is parallel to the cooling field. For negative exchange bias, the net pinned magnetization near the Co/FeF$_{2}$ interface is parallel to the direction of the cooling field. We propose a model that explains the cooling field dependence of the sign of exchange bias. Work at LANL and UCSD was funded by the U.S. Department of Energy, BES-DMS, and by a University of California Campus-Laboratory Collaboration grant. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:48AM |
N22.00007: Vortex State in Sub-100 nm Magnetic Nanodots. Invited Speaker: Magnetism of nanostructured magnets, which size is comparable to or smaller than ferromagnetic domain size, offers a great potential for new physics. Detailed knowledge of magnetization reversal and possible magnetic configurations in magnetic nanostructures is essential for high-density magnetic memory. Many theoretical and experimental studies are focused on a magnetic vortex which in addition to a circular in-plane configuration of spins has a core, - the region with out-of-plane magnetization. We present a quantitative study of the magnetic vortex state and the vortex core in sub-100 nm magnetic dots. Arrays of single-layer and bilayer nanodots covering over 1 cm$^2$ are fabricated using self-assembled nanopores in anodized alumina. This method allows good control over the dot size and periodicity. Magnetization measurements performed using SQUID, VSM, and MOKE indicate a transition from a vortex to a single domain state for the Fe dots. This transition is studied as a function of the magnetic field and dots size. Micromagnetic and Monte Carlo simulations confirm the experimental observations. Thermal activation and exchange bias strongly affect the vortex nucleation field and have a much weaker effect on the vortex annihilation field. Direct imaging of magnetic moments in sub-100 nm dots is extremely difficult and has not been reported yet. Polarized grazing incidence small angle neutron scattering measurements allow dot imaging in reciprocal space. Quantitative analysis of such measurements performed on 65 nm Fe dots yields the vortex core size of $\sim 15$ nm, in good agreement with the 14 nm obtained from the simulations. \\ \\ This work is done in collaboration with Chang-Peng Li, Zhi-Pan Li, S. Roy, S. K. Sinha, (UCSD), Xavier Batlle (U. Barcelona), R. K. Dumas, Kai Liu, (UC Davis), S. Park, R. Pynn, M. R. Fitzsimmons (LANL), J. Mejia Lopez (Pontificia U. Catolica de Chile), D. Altbir, (U. de Santiago de Chile), A. H. Romero (Cinvestav-Unidad Queretaro), and Ivan K. Schuller (UCSD) and supported by AFOSR, US DOE, NSF, UC-CLE, Spanish MECD, Catalan DURSI, FONDECYT, Millennium Initiative, and Conacyt Mexico. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N22.00008: Using multiple antiferromagnet/ferromagnet interfaces as a probe of grain size dependent exchange bias in polycrystalline Co/Fe$_{50}$Mn$_{50}$ B. Bolon, M.A. Haugen, A. Abin-Fuentes, J. Deneen, C.B. Carter, C. Leighton We have used ferromagnet/antiferromagnet/ferromagnet trilayers and ferromagnet/antiferromagnet multilayers to probe the grain size dependence of exchange bias in polycrystalline Co/FeMn. X-ray diffraction and transmission electron microscopy characterization show that the FeMn grain size increases with increasing FeMn thickness in the Co (30 {\AA}) / FeMn system. Hence, in Co (30 {\AA}) / FeMn / Co (30 {\AA}) trilayers the two Co layers ``sample'' different FeMn grain sizes at the two antiferromagnet/ferromagnet interfaces. For FeMn thicknesses above $\sim $ 100 {\AA}, where simple bilayers have a thickness independent exchange bias, we are therefore able to deduce the influence of grain size on the exchange bias and coercivity (and their temperature dependence) by measuring trilayer and multilayer samples with varying FeMn thickness. Increasing the average grain size results in a large decrease in exchange bias energy. We interpret the results as being due to a decrease in uncompensated spin density with increasing grain size, further evidence for the importance of defect generated uncompensated spins. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N22.00009: Interplay between reversal asymmetry, training, and anisotropy, in epitaxial NiMn/Ni exchange biased bilayers M.S. Lund, C. Leighton We have employed electron and x-ray diffraction, x-ray reflectivity, conventional magnetometry, and polarized neutron reflectivity to probe epitaxial NiMn/Ni bilayers. Binary alloys such as NiMn often require an annealing procedure to induce AF ordering which leads to interdiffusion at the AF/ F interface and a subsequent, and poorly understood, reduction in exchange bias. Our previous work with neutron reflectivity revealed a 35 {\AA} interdiffused region that contains competing AF and F interactions resulting in a very unusual temperature dependant magnetic interface location (M.S. Lund, M.R. Fitzsimmons, S. Park, and C. Leighton, APL 2004). In this work we find that at low temperatures there is a rapid divergence of the exchange bias field, coincident with the onset of strong training, an obvious reversal asymmetry, and the appearance of higher order induced anisotropies. We show in a simple way that the rapid increase in bias field, strong training, and reversal asymmetry are all consequences of the induced anisotropies. In addition, we are able to demonstrate in a single sample that uniaxial anisotropy favors low training, while biaxial anisotropy results in large training, confirming a recent theoretical prediction (A. Hoffmann, PRL 2004). Research was supported by NSF MRSEC. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N22.00010: Effect of Hard Layer Demagnetization on the Magnetization Reversal of Epitaxial Fe/SmCo Spring Magnets Kai Liu, J.E. Davies, E.E. Fullerton, J.S. Jiang, S.D. Bader In epitaxial Fe/SmCo, a classical spring magnet, irreversible magnetization reversal is observed once the SmCo hard layer starts switching [1,2]. To distinguish the soft and hard layer reversibility separately, we studied the effect of partial SmCo layer demagnetization on the reversal behavior of the entire bilayer using the first and second order reversal curve methods (FORC and SORC, respectively). The FORC distribution [2,3] shows two distinct features during the hard layer reversal: a negative/positive pair of features and a single positive peak. The negative/positive pair is from the soft Fe layer reversal and is a manifestation of the interlayer exchange coupling. The single positive peak occurs at larger applied fields and corresponds to the reversal of the hard SmCo layer. A SORC measurements were done at several reversal fields to determine the reversibility along different FORCs. We observe that the Fe layer remains mostly reversible. The partially demagnetized SmCo layer is the main source of irreversibility, particularly when the applied field approaches the hard layer nucleation/saturation field. [1] Fullerton, PRB 58, 12193 (1998). [2] Davies, et al, APL 86, 262503 (2005). [3] Davies, et al, PRB 70, 224434 (2004); PRB 72, 134419 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N22.00011: Electrically controlled exchange bias for spintronic applications Xi He, Srinivas Polisetty, Christian Binek Electrically controlled exchange bias (EB) is proposed for novel spintronic applications [1]. Basic effects of electrically controlled EB and its magnetoelectric (ME) switching are studied in a Cr$_{2}$O$_{3}$(111)/(Co/Pt)$_{3}$ heterostructure. Exchange coupling between the ME antiferromagnet Cr$_{2}$O$_{3}$ and a ferromagnetic CoPt multilayer exhibits perpendicular EB. The latter is controlled by applied axial electric fields inducing excess magnetization at the interface. The enhancement of this hitherto weak tuning effect is explored when replacing ME bulk pinning systems by epitaxal thin films. Recently, the sign of the EB field has been tuned via field cooling the system in either parallel or antiparallel axial magnetic and electric fields [2].$_{ }$Here, the crossover from bulk to thin film ME pinning systems is studied and spintronic applications are suggested based on the electrically controlled EB. Pure voltage control of magnetic configurations of tunneling magnetoresistance spin valves is proposed as an alternative to current-induced magnetization switching. In addition we suggest an XOR operation realized in a MEally pinned giant magneto resistance structure. [1] Ch. Binek, B.Doudin, J. Phys. Condens. Matter\textbf{ 17}, L39 (2005). [2] P. Borisov et al., Phys. Rev. Lett. \textbf{94}, 117203 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N22.00012: Exchange bias training effect in coupled all ferromagnetic bilayer structures Srinivas Polisetty, Xi He, Christian Binek, Andreas Berger We study exchange coupled bilayers of soft and hard ferromagnetic (FM) thin films by means of Alternating Gradient Force Magnetometry. A CoCr thin film realizes the magnetically soft layer (SL) which is exchange coupled via a Ru-interlayer with a hard CoPtCrB pinning layer (HL). This new class of all FM bilayers shows remarkable analogies to conventional antiferromagnetic (AF)/FM exchange bias (EB) heterostructures. Not only do these all FM bilayers exhibit a tunable EB effect, they also show a distinct training behavior upon cycling the SL through consecutive hysteresis loops. Training resembles the cycle dependent evolution of the bias field and is to a large extend analogous to the gradual degradation of the EB field observed upon cycling the FM top layer of a AF/FM EB heterostructure through consecutive hysteresis loops. However, in contrast to these conventional EB systems, our all FM bilayer structures allow the observation of training induced changes in the bias-setting HL by means of simple magnetometry. Our experiments show unambiguously that the training effect is driven by deviations from equilibrium in the pinning layer. A comparison of the experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N22.00013: Alignment-Sensitive Reversal Mechanisms of Epitaxial-FeF$_{2}$/Polycrystalline-Ni Exchange Biased Thin Films* Justin Olamit, Kai Liu, Zhi-Pan Li, Ivan K. Schuller Magnetization reversal mechanisms of epitaxial-FeF$_{2}$/polycrystalline-Ni exchange biased thin films were investigated with vector magnetometry and a First Order Reversal Curve (FORC) technique [1]. The FORCs were measured \textbf{\textit{without}} remounting the sample after the vector magnetometry measurements, ensuring consistency between the two methods. Samples were exchange biased by field cooling along the FeF$_{2}$ spin axis. When the applied field is aligned with the spin axis, the transverse hysteresis loop is flat and FORC analysis shows that the magnetization switching is highly irreversible ($\sim $80{\%}), indicating that domain nucleation and motion is the reversal mechanism. With a misalignment of 5\r{ }, the transverse hysteresis loop shows that the reversal is predominantly by rotation [2] and FORC analysis shows that the majority of the magnetic switching is by a reversible mechanism (only $\sim $40{\%} irreversible). These results demonstrate that the magnetization reversal mechanisms are \textbf{\textit{extremely sensitive}} to the alignment of the applied field with the antiferromagnet spin axis and the exchange bias direction [3]. .1. J. E. Davies, et al., Phys. Rev. B \textbf{70}, 224434 (2004); Phys. Rev. B \textbf{72}, 134419 (2005). 2. J. Olamit, et al., Phys. Rev. B \textbf{72}, 012408 (2005). 3. A. Tillmans et al, cond-mat/0509419. *Work supported by ACS-PRF, Alfred P. Sloan Foundation, and DOE. [Preview Abstract] |
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