Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T31: Focus Session: Magnetic, Electric, and Photo-Induced Magnetization Reversal |
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Sponsoring Units: DMP GMAG Chair: Axel Enders, University of Nebraska Room: 335 |
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T31.00001: Ultrafast magnetic imaging of nanostructures Invited Speaker: Today's technology advances into smaller and more complex structures for information processing. As structures get smaller, many processes of interest become faster as the propagation speed of excitations couple the length scale with the time scale. Microscopic techniques with a spatial resolution reaching the atomic level are being developed with very impressive success. On the other hand, time resolved techniques based on ultrafast laser systems allow us to explore processes on the femtosecond time scale. The focus of this talk is to unite the two worlds, the ultra-fast and the ultra-small. A powerful approach to time resolved microscopy is based on x-ray techniques. The wavelength of x-rays offers a spatial resolution in the nanometer range. Ultrafast x-ray techniques are currently being developed based on synchrotron sources as well as free electron lasers. The talk will demonstrate ultrafast microscopy techniques on imaging magnetization reversal dynamics in spin transfer devices. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T31.00002: Magnetization Dynamics of Magnetic Nano-Elements on Ultrafast Time Scales. J.P. Davis, J.A.J. Burgess, Z. Liu, R.D. Sydora, M.R. Freeman Recently, our group has systematically studied the switching of a \textit{single} permalloy nano-disk (160 nm diameter) between the vortex and quasi-single domain ground states as a function of applied bias field, including the observation of real-time switching at particular fields [1]. This was performed using the time-resolved magneto-optical Kerr effect (TR-MOKE), which allows dynamics to be studied on sub-nanosecond time scales. To accurately simulate the switching behavior, it was necessary to take into account the domed shape of the nano-disks, which result from the lift-off fabrication. Because of the sensitivity of the magnetization dynamics to the shape of the disks, we have begun fabrication of nano-disks using a shadow mask procedure [2] with a collimated deposition source under ultra-high vacuum (UHV) to produce high quality nano-elements. Magnetization dynamics of these UHV fabricated nano-disks will be discussed, with complementary scanning probe microscopy characterization of the disks.\\ 1. Z. Liu, R.D. Sydora and M.R. Freeman, PRB \textbf{77}, 174410 (2008).\\ 2. M.M. Deshmukh, D.C. Ralph, M. Thomas and J. Silcox, Appl. Phys. Lett. \textbf{75}, 1631 (1999). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T31.00003: Ultrafast switching of a nanomagnet by a combined in-plane and out-of-plane polarized spin-current pulse Oukjae Lee, V.S. Pribiag, P.M. Braganca, P.G. Gowtham, E.M. Ryan, D.C. Ralph, R.A. Buhrman For fast write operation of a spin-torque (ST) magnetic storage device, the exertion of a strong initial torque can switch the nanomagnet moment without the help of the thermal fluctuations. Use of an out-of-plane polarized reference layer can very quickly excite large free layer motion but reliable reversal requires precise ST pulse timing. The combination of strong in-plane and out-of-plane polarized spin currents can substantially relax this pulse-timing requirement. We have fabricated CPP spin-valve devices that incorporate both an out-of-plane polarizer, and an in-plane polarizer to quickly excite and reverse the moment of an in-plane polarized free layer. For pulse currents ranging between 100ps -- 10ns, the reversal speeds are notably faster and much less thermally distributed than for a conventional spin-valve with the same pulse current amplitude. We will discuss the details of the short-pulse behavior of these device structures and the optimization of this approach for high-speed magnetic memory. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T31.00004: Total angular momentum conservation in laser-induced femtosecond magnetism Guoping Zhang, Yihua Bai, Thomas F. George Spin momentum is not a classical quantity [1,2]. It is unclear how the conservation law affects spin momentum change in laser-induced femtosecond magnetization [3]. In solids, the rotational symmetry is lifted by the translational symmetry, and the spin and orbital momenta components of different total angular momenta mix to some extent. This mixing is the origin of the time-dependent total angular momentum in experiments. The remaining unmixed portion accounts for an extra spin change in three independent circularly-polarized laser experiments [4]. \\[0pt] [1] G. P. Zhang, Y. Bai, W. H\"ubner, G. Lefkidis, and T. F. George, J. Appl. Phys. {\bf 103}, 07B113 (2008). [2] G. P. Zhang and W. H\"ubner, Phys. Rev. Lett. {\bf 85}, 3025 (2000). [3] G. P. Zhang, Phys. Rev. Lett. {\bf 101}, 187203 (2008). [4] G. P. Zhang and T. F. George, Phys. Rev. B {\bf 78}, 052407 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T31.00005: Evolution of magnetic states in ferromagnetic nanorings in an applied azimuthal field Abby Goldman, Katherine Aidala, Tianyu Yang, Mark Tuominen Ferromagnetic nanorings form unique magnetic states that hold tremendous potential for maximizing data storage densities. One such state is the closed-flux vortex state, in which the magnetic field is completely enclosed within the ring, thus minimizing the magnetostatic energy, but also keeping the exchange energy low as adjacent magnetic moments are mostly aligned. A natural way to generate this state is through an external azimuthal field, as if from a current carrying wire passing through the center of the ring. We perform micromagnetic simulations to investigate the evolution of magnetic states in an external azimuthal field. For some applied current, the chirality of the ring will reverse, often into an intermediate state that evolves into a perfect vortex at higher current. Thin, wide rings have significantly lower switching currents than thick, narrow rings. We examine the dependence of the switching current and intermediate states on geometric properties such as the diameter, thickness, asymmetry and width of the ring. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T31.00006: Polarity reversal of magnetic vortex core by in-plane non-resonant pulsed magnetic field Xuemei Cheng, David Keavney, Kristen Buchanan Magnetic vortices have been of great interest because of their potential applications in non-volatile data storage. Recently, Van Waeyenberge et al. demonstrated vortex core reversal in permalloy squares using an in-plane r.f. excitation field with the frequency close to the translational-mode eigenfrequency. In this work we report polarity reversal of magnetic vortex core by non-resonant in-plane field pulses. The core polarity was first determined by watching the sense of vortex core gyration in a 6 micron permalloy disk imaged by time-resolved x-ray photoemission electron microscopy (TR-PEEM) with 1mT excitation field pulses. After the waveguide was pulsed at 5mT, we determined core polarity at 1mT again. We demonstrate that the core polarity can be switched back and forth by pulsing at 5mT. The micromagnetic simulations and TR-PEEM images confirm that when the core is displaced beyond 25{\%} of disk radius, distortions of the core region occur, promoting a transient domain state involving a complex cross-tie wall, and subsequent reversal of the core polarity. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T31.00007: Hybridization of quantum spin waves in small structures Valentyn Novosad, S.T. Chui, Sam Bader We apply the Holstein-Primakoff and Bogoliubov transformations to compute the spin wave states of small magnetic structures including the effect of the dipolar interaction. We found that as the film gets thicker, states with a significant q=0 component are hybridized with states with higher Fourier components. In the presence of a static magnetic field opposite to the magnetization direction, surface states that are responsible for magnetization reversal are coupled to the extended states. The response function is increased by an order of magnitude. This suggests an intriguing scenario for assisted switching of the magnetization with an additional external a.c. field. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T31.00008: Electric field-induced modification of magnetism in thin CoPd films. Mikhail Zhernenkov, Michael Fitzsimmons, Jerzy Chlistunoff, Jarek Majewski, Ioan Tudosa, Eric Fullerton Recently, M. Weisheit et al., [Science \textbf{315}, 349 (2007)] reported modification of magnetic properties of thin-film ferromagnets by applying a large electric field at the surface of a ferromagnet. For an applied voltage of -0.6 V, the coercivities of 20 {\AA} thick FePt and FePd films were changed by -4.5 and +1{\%}, respectively. Here, we report polarized neutron reflectometry measurements of the magnetization depth profile of a 180 {\AA} thick Co$_{50}$Pd$_{50}$ film immersed in an electrolyte as a function of applied electric field in an external magnetic field. The measurements were done at two values of applied electric potential of -0.6 volts and -1.2 volts and at zero volts (open-circuit potential). The applied magnetic field was 3 kG. We found a linear increase of the film magnetization with electric field. The change of magnetization occurred in the region of the film within 72 {\AA} of the electrolyte/Co$_{50}$Pd$_{50}$ interface. The magnetization of the top part of the layer is increased by 2{\%} (-0.6 V) and 3.6{\%} (-1.2 V) compared to the open circuit potential result. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T31.00009: Electric-field-driven spin resonance of a Mn dopant in GaAs V.R. Povilus, J.-M. Tang, M.E. Flatt\'e All-electric manipulation of the ground-state spin system of a Mn dopant in GaAs using a static electric field in combination with a transverse dynamic electric field requires careful positioning of two gates on the nanoscale[1]. Here we propose a method of efficiently controlling the spin of a Mn dopant using parallel static and dynamic electric fields, but adding a small static magnetic field. In a scalable geometry this would permit full control of the ground-state J=1 spin of a Mn dopant using a single electric gate. The energy states and eigenfunctions of the Mn dopant system under the influence of both an electric and magnetic field cannot be simply described using an effective electric-field-dependent g tensor, as would be done in g tensor modulation resonance[2]. However, the dynamical equations for the spin can be numerically solved and exhibits high-visibility Rabi oscillations. For example, with a static electric field of 500 V/cm, a dynamic electric field of 300 V/cm with frequency 9.1 GHz, and a magnetic field of 0.1 Tesla, all oriented in the [113] direction, the Rabi oscillation period is 2.7 ns.\\[0pt] [1] J.-M Tang, Jeremy Levy, and M. E. Flatt\'e, Phys. Rev. Lett. 97, 106803 (2006).\\[0pt] [2] Y. Kato, et al., Science 299, 1201 (2003). [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T31.00010: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T31.00011: Photoinduced Magnetism of Ternary Transition Metal Prussian Blue Analogs D.M. Pajerowski, M.W. Meisel, J.E. Gardner, D.R. Talham The magnetism of Prussian blue analog materials (PBAs) can be tuned with external stimuli such as temperature, pressure, and light. Recently, novel effects have been seen in PBAs with substitutionally mixed ternary and quaternary transition metals, rather than the usual binary analogs. One noteworthy material we have studied is a Na$_{a}$Ni$_{1-x}$Co$_{x}$[Fe(CN)$_{6}$]$_{b}$$\cdot$nH$_{2}$O powder, which can show either a photoinduced increase or \emph{decrease} in magnetization depending upon the Ni substitution, the applied magnetic field, and the temperature. This result is the first example of a photoinduced decrease in magnetization while generating new spins via a charge transfer induced spin transition (CTIST) in a bulk material. Constrastingly, the photodecrease observed in PBA binary thin films has different microscopic origins [1-2]. Insight into the underlying mechanisms can be obtained by using mean field models, which qualitatively reproduce the experimental data. SQUID magnetometer, FT-IR, TEM, and EDS data will be presented.\newline [1] J.-H. Park, \emph{et al.}, Appl. Phys. Lett. \textbf{85}, 3797 (2004). \newline [2] F. A. Frye \emph{et al.}, Chem. Mater. \textbf{20}, 5706 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T31.00012: Bismuth in strong magnetic fields: unconventional Zeeman coupling and correlation effects Jason Alicea, Leon Balents Recent experiments on bismuth have uncovered remarkably rich magnetization structure at fields well beyond the regime in which all carriers are expected to reside in the lowest Landau level. Motivated by these findings, we start from a microscopic tight-binding model and derive a low-energy Hamiltonian for the holes and three Dirac electrons pockets in bismuth. We find that an unconventional electron Zeeman effect, overlooked previously, suppresses the quantum limit for the electrons dramatically, giving rise to the observed anomalous magnetization structure. We further study interaction effects near fields at which the 2nd Landau level for one electron pocket empties, where magnetization hysteresis was observed. Here we find instabilities towards both charge density wave and Wigner crystal phases, and propose that hysteresis arises from a first-order transition out of the latter. [Preview Abstract] |
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