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Nanomagnetism and Spintronics Group

 

Byoung-Chul Choi
Professor
Office: Elliot 115
Phone: (250) 721-7731
Email: bchoi(at)uvic.ca
 

 

RESEARCH ACTIVITIES

Our research activities are focused on the areas of magnetization dynamics, which occurs within a few nanoseconds after the thin film magnetic elements are driven out of their equilibrium states. Our research outcomes address the issues ranging from the fundamental question “How fast can the magnetization in small magnetic elements be switched?” to the application-related problem “How to manipulate spins for the application of high-performance/high-density/low energy-consuming magentoelectronic devices?”.
We have developed a high level of expertise in probing nonequilibrium phenomena both experimentally and numerically, and have made significant contributions to improve the understanding of the fundamentals of the magnetization process. Due to their significance in fundamental research and practical applications, many articles we published have been selected for the Virtual Journal of Nanoscale Science & Technology. The itemized summary of our research activities is below.

1. Nonequilibrium magnetization process in lithographically patterned magnetic
elements :
Over the last few years, our work focused on developing fundamental understanding of the magnetization dynamics occurring in response to sub-nanosecond transitions of the externally applied magnetic field in ferromagnetic thin film elements. Detailed knowledge of how magnets switch at very high speeds is also of relevance to extend the application of magnetic information technologies to very high data rates. The work published in the ref. [33], for example, provides a
novel example of the effects of thermally excited magnons to the complex nonequilibrium domain configuration. Recently, we have extended our magnetization dynamics study to exchange-biased ferromagnetic/antiferro-magnetic bilayer-systems. The result reported in the ref. [5], for example, includes new physics explaining the dynamic process of incoherent interaction of precessing magnetic moments with propagating spin waves.

2. Spin-wave phenomena in vortex magnetization state :
A large part of our recent research activities has been devoted to the study of vortex dynamics. In particular, we pioneered the research area of the dynamic process of vortex core switching, which led to the rich set of interesting nonequilibrium phenomena such as spin-wave generation followed by vortex-antivortex pair annihilation. Reference [27], for example, was one of the first
papers reporting the dynamic switching of vortex core and has been of great interest to magnetism community owing to the long-standing nature of the magnetization dynamics problem in vortex structures and its relevance to current applications in magnetoelectronics and magnetic storage. The paper was selected by ScienceWatch.com in 2008 as one of the 11 core papers comprising the research forefront on magnetic vortex cores (http://sciencewatch.com/ dr/erf/ maps/ 08feb_ matsci/). Recently, we reported the details of the magnetization process of the vortex imprinted ferromagnetic/antiferromagnetic bilayer square elements [5]. We experimentally observed that the resonance frequency is strongly dependent on the exchange-bias field strength. Micromagnetic modeling provided physical insight into the details of how the excitation of the spin-wave eigenmodes is affected by the lateral displacement of the remanent vortex state. The result revealed that the magnetization dynamics of exchange-displaced vortex configuration significantly differs from the collective spin-wave eigenmode excitation occurring in symmetric Landau states, which has been previously reported by many other groups.

3. Spin-transfer torque induced magnetization dynamics in multilayered nanodevice
elements :
Successful implementation of the spin-transfer torque (STT) effect for electronic devices has been considered as an essential step towards next generation spintronic devices. In our research, the dynamic response of nanomagnets in multilayered nanopillars to the spin-polarized current pulse has been investigated numerically. In ref [14], we demonstrated the controllable vortex chirality switching of the dual vortices driven by spin-polarized current pulses. The dynamics of the energy dissipation after the initial vortex chirality switching involves a series of complex magnetization processes, which includes magnetization precession and creation of additional vortex and antivortex cores. From an application point of view, our result suggests a new way to implement the vortex switching in memory device applications. The result can lead to a new integration of magnetic components into Complementary Metal–Oxide–Semiconductor (CMOS) technology, giving rise to an all-electrically controlled magneto-electronic device.
Over the last few years, we have been collaborating with the research groups at the University of Alabama (USA), the National Centre for Scientific Research (Greece), and Phillips Research Eindhoven (The Netherlands) on the fabrication of dual-vortex nanopillar elements, proposed in [24]. As of September 2012, we have successfully fabricated nanoelements with diameters ranging from 25 nm to 150 nm. Currently, we’re carrying out high-frequency magnetotransport experiments on dual-vortex nanopillar elements to investigate the vortex dynamics phenomena, such as the shift of the resonance frequencies of the spin-wave eigenmodes upon chirality switching, described in our recent numerical study [3].

 

LIST OF PUBLICATIONS

 

  1. G. Abo, Y.K. Hong, J. Jalli, J. Lee, J. Park, S. Bae, S. Kim, A. Morita, B.C. Choi, S. Erwin, and T. Tanaka, Modeling the coercivity of a spherical barium ferrite (S-BaFe) particle with uniaxial anisotropy, (submitted to) J. Appl. Phys. (2012)
  2. A. Lyle, Y.K. Hong, B.C. Choi, G. Abo, J. Jalli, R. Syslo, S. Bae, J.J. Lee, and G.W. Donohoe, Magnetic field switching of 8´8 array of Co/Cu/Py elongated and type II Pacman spin valves, (submitted to) IEEE Trans. Magn. (2012)
  3. J. Kolthammer, Th. Speliotis, Y.K. Hong, G. Abo, Q. Liu, R. Gardner, and B.C. Choi, Current Pulse Induced Toggle Switching of Dual-Vortex Magnetization in Ni80Fe20/Cu/Co Nanopillar Element, J. Appl. Phys. 112, 083928 (2012)
  4. G. Abo, Y.K. Hong, B.C. Choi, M. Donahue, S. Bae, J. Jalli, J. Park, J. Lee, M. Park, S. Gee, and S. Gee, Micromagnetic computer simulated scaling effect of S-shaped Permalloy nano-element on operating fields for AND or OR Logic, IEEE Trans. Magn. 48, 1851 (2012)
  5. H. Xu, E. Girgis, J. Rudge, Th. Speliotis, C.A. Ross, Y.K. Hong, and B.C. Choi, Magnetization dynamics in Vortex-imprinted Ni80Fe20/Ir80Mn20 square elements, IEEE Mag. Lett. 3, 3500204 (2012)
  6. G. Abo, Y.K. Hong, J. Jalli, J. Lee, J. Park, S. Bae, S.G. Kim, B.C. Choi, and T. Tanaka, Shape Dependent Coercivity Simulation of a Spherical BariumFerrite (S-BaFe) Particle with Uniaxial Anisotropy, J. Magnetics 17, 1 (2012)
  7. J. Jalli, Y.K. Hong, G. Abo, S. Bae, J. Lee, J. Park, B.C. Choi, and S.G. Kim, MFM studies of magnetic domain patterns in bulk barium ferrite single crystals, J. Magnetism & Magnetic Materials 323, 2627 (2011)
  8. H. Xu, J. Kolthammer, J. Rudge, E. Girgis, B.C. Choi, Y.K. Hong, G. Abo, Th. Speliotis, and D. Niarchos, Magnetization process in vortex-imprinted Ni80Fe20/Ir20Mn80 square elements, J. Magnetics 16, 83 (2011)
  9. J. Jalli, Y.K. Hong, J. Lee, G. Abo, T. Mewes, B.C. Choi, and S.G. Kim, Ferrimagnetic SrBaZnFeO Single Crystal with Planar Anisotropy, IEEE Mag. Lett. 2, 104 (2011)
  10. J. Lee, Y.K. Hong, S. Bae, J. Jalli, J. Park, G. Abo, G.W. Donohoe, and B.C. Choi, Integrated ferrite film inductor for power system-on-chip smart phone applications, IEEE Trans. Magn. 47, 304 (2011)
  11. J. Jalli, Y.K. Hong, S. Bae, J. Lee, G. Abo, J. Park, B.C. Choi, T. Mewes, S. Kim, S. Gee, I. Nam, and T. Tanaka, Magnetic and microwave properties of ferromagnetic Zr-substituted BaZnFeO single crystals, J. Appl. Phys. 109, 07A509 (2011)
  12. B.C. Choi, E. Girgis, C. A. Ross, Th. Speliotis, Y.K. Hong, G. Abo, D. Niarchos, and H. Miyagawa, Incoherent interaction of spin waves with precessing magnetic moments, Phys. Rev. B 81, 092404 (2010)
  13. A. Lyle, Y.K. Hong, B.C. Choi, G. Abo, S. Bae, J. Jalli, J. Lee, M.H. Park, and R. Syslo,  Spin-polarized current switching of Co/Cu/Py Pac-man type II spin-valve, J. Magnetics 15, 103 (2010) (This article has been highlighted as cover page on the issue.)
  14. J. Lee, Y.K. Hong, S. Bae, J. Park, J. Jalli, G. Abo, R. Syslo, B.C. Choi, and G.W. Donohoe, High quality factor Ni-Zn ferrite planar inductor, IEEE Trans. Magn. 46, 2417 (2010)
  15. B.C. Choi and Y.K. Hong, 2010. CMOS based Magnetic Random Access Memory, in: Nano and Microsystems  Technologies, (K. Iniewski, ed.), CRC Press. (Chapter in a research text)
  16. A. Lyle, Y. K. Hong, B. C. Choi, G. Abo, M.H. Park, S.H. Gee, J. Jalli, and G. W. Donohoe, Spin-polarized current switching of Co/Cu/Py elongated Pac-man spin valve, IEEE Trans. Mag. 45, 2367 (2009) (This article has been highlighted as cover page on the issue.)
  17. S. Bae, Y.K. Hong, J. Lee, J. Jalli, G. Abo, A. Lyle, B. C. Choi, and G. W. Donohoe, High Q Ni-Zn-Cu ferrite inductor for on-chip power module, IEEE Trans. Mag. 45, 4773 (2009)
  18. J.N. Yeo, G.M. Jee, B.D. Yu, and B.C. Choi, Epitaxial strain and interfacial electronic structures at MgO/Fe(001), J. Korean Physical Society, 52, 1938 (2008).
  19. J.E. Davies, L.H. Bennet, E. Della Torre, B.C. Choi, S.N. Piramanayagam, and E. Girgis, Interaction in soft magnetic bilayers nanoscale arrays, IEEE Trans. Magn. 44, 2722 (2008) 
  20. B.C. Choi and Y.K. Hong, 2008. Spin-torque induced vortex dynamics in nanomagnets, in: Electromagnetic, Magnetostatic and Exchange Interaction Vortices in Confined Magnetic Structures, (E. Kamenetskii, ed.), Research Signpost. (Chapter in a research text)
  21. B.C. Choi, Y.K. Hong, J. Rudge, E. Girgis, Q.F. Xiao, J. Kolthammer, and G.W. Donohoe, Nonequilibrium process of magnetization switching influenced by thermal spin fluctuations, Physica Status Solidi (b), 244, 4486 (2007)
  22. B.C. Choi, Q.F. Xiao, Y.K. Hong, J. Rudge, and G.W. Donohoe, Vortex core switching dynamics in submicron Permalloy disk, IEEE Trans. Magn. 43, 2926 (2007)
  23. Q.F. Xiao, J. Rudge, E. Girgis, J. Kolthammer, B.C. Choi, Y.K. Hong, and G.W. Donohue, Dynamics of magnetic vortex core switching in Fe nanodisks by applying an in-plane magnetic field pulse, J. Appl. Phys. 102, 103904 (2007) (This article has been selected for the December3, 2007 issue of Virtual Journal of Nanoscale Science & Technology.)
  24. B.C. Choi, J. Rudge, E. Girgis, J. Kolthammer, Y.K. Hong, and A. Lyle, Spin-current pulse induced switching of vortex chirality in magnetic nanopillar, Appl. Phys. Lett. 91, 22501 (2007) (This article has been selected for the July 23, 2007 issue of Virtual Journal of Nanoscale Science & Technology.)
  25. B.C. Choi, J. Rudge, M.R. Freeman, Y.K. Hong, and Q.F. Xiao, Evolution of nonequilibrium domain structure as a function of speed of switching process in Ni80Fe20 thin film elements, IEEE Trans. Magn. 43, 2 (2007) (This article has been highlighted as cover page on the issue.)
  26. Q.F. Xiao, B.C. Choi, Y.K. Hong, J. Rudge, and G.W. Donohue, Donohue, Effect of magnetic field pulse on magnetization reversal dynamics in Ni80Fe20 elliptical nanoelements, J. Appl. Phys. 101, 24306 (2007)  (This article has been selected for the February 2007 issue of Virtual Journal of Ultrafast Science.)
  27. Q.F. Xiao, J. Rudge, B.C. Choi, Y.K. Hong, and G.W. Donohue, Dynamics of vortex core switching in ferromagnetic nanodisks, Appl. Phys. Lett. 89, 262507 (2006) 
  28. B.C. Choi, Y.K. Hong, J. Rudge, G. Donohoe, and Q.F. Xiao, Nonequilibrium domain configurations undergoing large angle rotations in mesoscopic magnetic thin film elements, J. of Magnetics 11, 61 (2006).
  29. M.H. Park, Y.K. Hong, B.C. Choi, M.J. Donahue, S.H. Gee, and H. Han, Vortex head-to-head domain walls and their formation in onion-state ring elements, Phys. Rev. B 73, 94424 (2006)
  30. Q.F. Xiao, J. Rudge, B.C. Choi, Y.K. Hong, and G.W. Donohue, Dynamics of ultrafast magnetization reversal in submicron elliptical Permalloy thin films, Phys. Rev. B 73, 104425 (2006) (This article has been selected for the April 2006 issue of Virtual Journal of Ultrafast Science.)
  31. B.C. Choi, Q.F. Xiao, Y.K. Hong, S.H. Gee, J. Jabal, H. Han, K.J. Hass, and G.W. Donohoe, Numerical simulation study of magnetization precession dynamics in submicron elliptical Ni80Fe20 thin film elements, IEEE Trans. Magn. 42, 42, 3216 (2006)
  32. K.J. Hass, G.W. Donohoe, Y.K. Hong, and B.C. Choi, Magnetic latches for space applications, IEEE Trans. Magn. 42, 2751 (2006)
  33. B.C. Choi, J. Ho, G. Arnup, and M.R. Freeman, Nonequilibrium domain pattern formation in mesoscopic magnetic thin film elements assisted by thermally excited spin fluctuations, Phys. Rev. Lett. 95, 237211 (2005). (This article has been selected for the December 12, 2005 issue of Virtual Journal of Nanoscale Science & Technology and for the January 2006 issue of Virtual Journal of Ultrafast Science.)
  34. B.R. Pujada, B.C. Choi, Y.K. Hong, M.H. Park, H. Han, S.H. Gee, and G.W. Donohoe, Micromagnetic configurations and switching mechanism in Pacman-shaped submicron Ni80Fe20 magnets, J. Appl. Phys. 97, 63905 (2005)
  35. B.C. Choi, Y.K. Hong, M.H. Park, H. Han, S.H. Gee, and G.W. Donohoe, Magnetization switching dynamics depending on as-patterned magnetization state in magnetic thin film elements, IEEE Trans. Magn. 41, 2709 (2005)
  36. B.C. Choi, B.R. Pujada, Y.K. Hong, M.H. Park, H. Han, S.H. Gee, and G.W. Donohoe, Micromagnetic domain structures and magnetization switching mechanism in submicron thin film elements, IEEE Trans. Magn. 41, 3109 (2005)
  37. H. Han, Y.K. Hong, M.H. Park, B.C. Choi, S.H. Gee, J. Jabal, G. Abo, A. Lyle, B. Wong, and G.W. Donohoe, Interaction effect on switching behaviors of patterned Pac-man array, IEEE Trans. Magn. 41, 4341 (2005)
  38. B.C. Choi, A. Krichevsky, and M.R. Freeman, 2005. Magnetization Dynamics using Time-Resolved Magneto-Optic Microscopy, in: Novel Techniques for Characterizing Magnetic Materials, (Y. Zhu, ed.), Kluwer Academic. (Chapter in a research text)
  39. M.H. Park, Y.K. Hong, S.H. Gee, D.W. Erickson, T. Tanaka, and B.C. Choi, Effect of shape anisotropy on switching behaviors of Pac-man Ni80Fe20 submicron elements. J. Appl. Phys. 95, 7019 (2004)
  40. B.C. Choi, G. Arnup, M. Belov, and M.R. Freeman, Novel phenomena in dynamic domain configurations in mesoscopic magnetic thin film elements. J. Appl. Phys. 95, 6540 (2004)
  41. J. Ho, F.C. Khanna, and B.C. Choi, Radiation-spin interaction, Gilbert damping, and spin torque, Phys. Rev. Lett. 92, 97601 (2004) (This article has been selected for the March 15, 2004 issue of Virtual Journal of Nanoscale Science & Technology.)
  42. B.R. Pujada, J. Svendsen, K. O. Chipeniuk, B.C. Choi, M.H. Park, Y.K. Hong, S.H. Gee, and D.W. Erickson, Magnetic switching depending on as-patterned magnetization state in Pac-man shaped Ni80Fe20 submicron elements, J. Appl. Phys. 96, 4362 (2004)
  43. J. Ho, F.C. Khanna, and B.C. Choi, Combination of dyanmical invariant method and radiation-spin interaction to calculate magnetization damping, Phys. Rev. B. 70, 172402 (2004)
  44. H.C. Shin, J.H. Oh, B.C. Choi, and S.C. Choi, Design of an energy conversion system with decomposition of H2O and CO2 using ferrites, Phys. Stat. Sol. (c) 1, 3748 (2004)
  45. B.C. Choi and M.R. Freeman, 2004. Nonequilibrium Spin Dynamics by Time-Resolved Magneto-Optical Kerr Microscope, in: Ultrathin Magnetic Structures (B. Heinrich and J.A.C. Bland, ed.) Vol. 4, Springer Verlag, Berlin. (Chapter in a graduate level teaching text)
  46. B.C. Choi, A. Krichevksy, and M.R. Freeman, Ultrafast Magnetization Imaging. Proceedings of the IEEE, Vol. 91, No. 5, 781 (2003) (Special issue on Spintronics, an invited review article on the topic of spintronics).
  47. M.H. Park, Y.K. Hong, S.H. Gee, D.W. Erickson, and B.C. Choi, Magnetization configuration and switching behavior of newly shaped submicron NiFe elements: Pac-man shape. Appl. Phys. Lett. 83, 329 (2003)
  48. W.Y. Lee, K.H. Shin, B.C. Choi, and J.A.C. Bland, Magnetization reversal dynamics in Ni80Fe20 thin films. J. Magn. Magn. Mater. 239, 103 (2002)
  49. B.C. Choi and M.R. Freeman, 2002. Time domain optical imaging of ferromagnetodynamics, in: Magnetic Microscopies of Nanostructures (H. Hopster and H.P. Oepen, ed.), Springer Verlag, Berlin. (Chapter in a research text)
  50. B.C. Choi, M. Belov, W.K. Hiebert, G.E. Ballentine, and M.R. Freeman, Ultrafast magnetization reversal dynamics investigated by time domain imaging. Phys. Rev. Lett. 86, 728 (2001) (This article has been highlighted by the journal, "Movies show quick magnetic flips", Physical Review Focus 7, Story 3, 19 January 2001.)
  51. B.C. Choi, G.E. Ballentine, M. Belov, and M.R. Freeman, Bias field dependence of the spatiotemporal evolution of magnetization reversal in a mesoscopic Ni80Fe20 element. Phys. Rev. B 64, 144418 (2001)
  52. M.R. Freeman and B.C. Choi, Advances in magnetic microscopy. Science 294, 1484 (2001) (Invited article on the topic of magnetic imaging techniques).
  53. B.C. Choi, G.E. Ballentine, M. Belov, W.K. Hiebert, and M.R. Freeman, Ultrafast magnetization reversal dynamics on a micrometer-scale thin film element studied by time domain imaging. Mat. Res. Soc. Symp. Proc., 648, P4.9 (2001)
  54. B.C. Choi, G.E. Ballentine, M. Belov, W.K. Hiebert, and M.R. Freeman, Pico-second time-resolved magnetization reversal dynamics in Ni80Fe20 microstructure. J. Appl. Phys. 89, 7171 (2001).
  55. W.Y. Lee, A. Samad, T.A. Moore, J.A.C. Bland, and B.C. Choi, Dynamic hysteresis behavior in epitaxial spin-valve structures. J. Appl. Phys. 87, 6600 (2000)
  56. S. Hope, B.C. Choi, P.J. Bode, and J.A.C. Bland, Direct observation of the stabilization of ferromagnetic order by magnetic anisotropy. Phys. Rev. B 61, 5876 (2000)
  57. W.Y. Lee, A. Samad, T.A. Moore, J.A.C. Bland, and B.C. Choi, Magnetization reversal dynamics in epitaxial spin-valve structures. Phys. Rev. B 61, 6811 (2000)
  58. B.C. Choi, A. Samad, C.A.F. Vaz, S. Langridge, J. Penfold, and J.A.C. Bland, Layer selective determination of magnetization vector configurations in an epitaxial double spin valve structure: Si(001)/Cu/Co/Cu/FeNi/Cu/Co/Cu. Appl. Phys. Lett. 77, 892 (2000)