Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R22: Focus Session: Biomagnetism and Exchange Biased Thin Films |
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Sponsoring Units: GMAG DMP Chair: David Lederman, West Virginia University Room: Baltimore Convention Center 319 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R22.00001: Magnetic Viruses: Utilizing Self-Assembly for Biomedical Applications Invited Speaker: Magnetic nanoparticles coated with biochemical surfactants have emerged recently as an important component for enabling many biological and medical applications. We implemented a biotemplating approach to create such magnetic nanoparticles by utilizing native protein capsid shells derived in high yield from the T7 bacteriophage virus.\footnote{C.~Liu, S.-H. Chung, Q.~Jin, A.~Sutton, F.~Yan, B.~K.~Kay, S.~D.~Bader, L.~Makowski, and L.~Chen, J. Magn. Magn. Mater, in press.} The magnetic nanoparticles are grown via bio-mineralization reactions inside of hollowed-out capsids that retain their original chemical recognition properties. The resultant ``magnetic viruses'' are uniform in geometry, physical properties, and biochemical functionality. This makes these viruses ideally suited for many biomedical applications among which we investigated specifically a novel sensing scheme for target recognition based on Brownian relaxation.\footnote{S.H.~Chung, A.~Hoffmann, S.~D.~Bader, C.~Liu, B.~Kay, L.~Makowski, and L.~Chen, Appl.\ Phys.\ Lett.\ {\bf 85}, 2971 (2004); S.~H.~Chung, A.~Hoffmann, K.~Guslienko, S.~D.~Bader, C.~Liu, B.~Kay, L.~Makowski, and L.~Chen, J.\ Appl.\ Phys.\ {\bf 97}, 10R101 (2005).} For this scheme we use the {\it ac}-susceptibility of the functionalized magnetic nanoparticles suspended in liquid. Upon binding the target of interest to the particles, their Brownian relaxation time is modified, which is readily detected by a change of the frequency dependence of the magnetic susceptibility. This scheme has several advantages; (i) it requires only one binding event for sensing; (ii) there is a useful signal both in the absence and presence of the target; (iii) the signal contains information about the size of the target besides the biochemical affinity; and (iv) since the binding modifies the magnetic susceptibility of the magnetic particles there is no need for removing unbound labels. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R22.00002: Atomic model for the exchange bias in Fe/FePt(110) alloy Chanyong Hwang, W. Kim, J.S. Park, Y.P. Lee, S.C. Hong Fe overlayers on top of Pt(110) surface have been studied using STM and SMOKE. Based on our atom-resolved STM data, we have shown the atomic model for the growth of this system. One of the interesting system is the one grown at elevated temperature. In this case, Fe-Pt surface alloy is formed on top of Pt(110) surface. This surface alloy still maintains the lattice constant of Pt substrate and reveals to be an antiferromagnet. In addition to the field cooling upon additional Fe layers, our first principles calculation clear shows the antiferromagnetic order. For the unified picture of the exchange bias phenomena, we need an atomically resolved antiferromagnetic surface and we will present the possible mechanism of this exchange bias in our system. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R22.00003: Competing Mechanisms in Asymmetric Magnetization Reversal in Exchange Biased Bilayers Oleg Petracic, Zhi-Pan Li, Ivan K. Schuller A systematic micromagnetic simulation is performed to investigate the asymmetric magnetization reversal in exchange bias bilayers. When the unidirectional anisotropy is comparable with the uniaxial anisotropy of the ferromagnet, we found that the descending branch is dominated by rotation while the ascending one is dominated by domain wall motion. When the unidirectional anisotropy is large enough, the magnetization reversal occurs through FM incomplete domain walls parallel to the interface. This results in a larger transverse component in the ascending branch, opposite to the previous mechanism. These two mechanisms coexist and compete for intermediate unidirectional anisotropies and lead to very delicate situations in particular when a magnetic field misalignment and interfacial inhomogeneity are involved. The results are able to explain some of the confusion in the present study of the asymmetric reversal mechanisms. [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R22.00004: Magnetization Precession in Interlayer Exchange-Coupled Thin Films Steven Michalski, Jian Zhou, Ralph Skomski, Nikolay Polushkin, Roger Kirby We have studied the static and fast dynamic magnetic properties of exchange-coupled magnetic layers using both magneto-optical measurements and conventional magnetometry. The samples studied include [Co/Pt] multilayers with perpendicular magnetic anisotropy (PMA) coupled via a variable thickness intervening Cu (Pt) layer to a Co (Sm-Co) layer with in-plane magnetic anisotropy. Such systems should exhibit a wide variety of magnetic configurations, permitting control of magnetic properties through control of exchange strength and PMA. The hysteresis loops show direct evidence of ferromagnetic exchange coupling. We report here the results of magnetization precession measurements using a femtosecond laser in a pump-probe experiment with direct optical excitation. For the samples studied so far, we observe only one precession frequency for a given applied magnetic field, rather than the expected two, but the frequency varies with the thickness of the coupling layer as expected. Our results are interpreted in terms of an LLG model which includes PMA and variable exchange coupling. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R22.00005: Exchange coupling in multilayers with perpendicular magnetic anisotropy Jian Zhou, Ralph Skomski, Steven Michalski, Roger Kirby, David Sellmyer Recently much attention has been paid to the interlayer exchange coupling (IEC) through nonmagnetic layers, for especially certain applications. In this work we investigate the interlayer exchange coupling through Cu or Pt spacer layers between two magnetic phases with perpendicular or in-plane anisotropy. (Pt5{\AA}/Co3{\AA})$_{3}$ layers with perpendicular magnetic anisotropy (PMA) are exchange-coupled to an in-plane anisotropic Sm-Co 30{\AA} layer through a Pt spacer. The competing anisotropies and IEC in the system yield a spin-reorientation transition behavior with a varying Pt thickness. We also have studied (Pt5{\AA}/Co3{\AA})$_{3}$ layers which are exchange-coupled to (Co3{\AA}/Pt5{\AA})$_{3}$ through a Cu spacer. Transition from single phase hysteresis to two-phase behavior indicates a reducing IEC between the PMA layers with increasing Cu thickness. Both model calculations and micromagnetic simulations show that interlayer exchange coupling is essential in calculating the hysteresis, and indicate how the interlayer coupling can be tuned by changing the spacer thickness. * This research is supported by DOE, NSF-MRSEC, the W. M. Keck Foundation, and CMRA. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R22.00006: Spin Dynamics in Exchange Biased Co/FeMn Bilayers Jeffrey Parker, Mun Chan, Melissa Haugen, Bruce Bolon, Chris Leighton, Paul Crowell We report a study of the spin dynamics in exchange biased Co/FeMn bilayers using time resolved Kerr microscopy (TKRM) with sub-nanosecond resolution. Samples were prepared via UHV dc magnetron sputter deposition onto a Si/SiO$_x$ substrate at room temperature in an applied field of 300 Oe. Deposition of a 30nm Cu buffer layer allowed for (111) textured growth of the subsequent Co/FeMn layers. In all samples the Co thickness was held constant at 6nm, while the FeMn thickness, t, was varied between 0 and 20nm. In accordance with previous work we found a room temperature critical thickness of the FeMn layer of approximately 3.5nm and a broad peak in the coercivity at this thickness. For low FeMn thickness a noticable asymmetry occurs in the magnetization hysteresis loops only for temperatures below the blocking temperature. TRKM is used to probe the dynamic response of the magnetization to fast magnetic field pulses. The different reversal mechanisms on either side of the hysteresis loop were probed by both the static and dynamic magnetization of the films as a function of temperature (between 40 and 450 K), and applied field. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R22.00007: Exchange Bias in Fe$_{x}$Ni$_{1-x}$F$_{2}$/Co Bilayers Miyeon Cheon, Zhongyuan Liu, David Lederman FeF$_{2}$ is an ideal antiferromagnet with a tetragonal rutile crystal structure and a strong uniaxial magnetic anisotropy along the $c$-axis ([001] direction). NiF$_{2}$ also shares the rutile crystal structure with similar lattice parameters, but its magnetic anisotropy causes spins to be in the $a-b$ plane, resulting in a weak ferromagnetism due to a small tilting of the antiferromagnetic sublattices. Because several theories predict that exchange bias should be a strong function of the magnetic anisotropy in the antiferromagnet, the Fe$_{x}$Ni$_{1-x}$F$_{2}$/Co bilayer system provides a framework whereby this can be studied experimentally. We have grown several single-crystalline Fe$_{x}$Ni$_{1-x}$F$_{2 }$50 nm thick films on MgF$_{2}$ (110) substrates via molecular beam epitaxy with $x$ varying between 0 and 1.0. We will report on the exchange bias as a function of Fe concentration in the alloy. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R22.00008: Dynamics of Spontaneous Magnetization Reversal in Exchange Biased Bilayers Zhi-Pan Li, Casey W. Miller, Igor V. Roshchin, Ivan K. Schuller We observe a temperature-driven magnetization reversal of a ferromagnet (FM= Ni, Co) magnetization under a constant magnetic field, when it is exchange biased by an antiferromagnet (AF=FeF$_{2})$. We attribute this phenomenon to the competition of the AF interfacial coupling and the FM Zeeman energy. This striking behavior allows us to investigate the AF dynamics through the FM reversal behavior. We found that the magnitude of FM reversal for a given cooling field strongly depends on the cooling speed, and the wait time around the N\'{e}el temperature. This shows the magnetic viscosity of the AF around its phase transition temperature. Moreover, by thermal cycling, the FM exhibits successive spontaneous reversal. This thermal training effect further points to high energy barriers resulting in slow dynamic processes of the system. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R22.00009: Exchange bias measurements of epitaxial magnetic oxide exchange bias bilayers. Xianglin Ke, Land Belenky, Chang-Beom Eom, Mark Rzchowski It has been shown in epitaxial bilayers that antiferromagnetic exchange coupling exists at atomically abrupt interfaces between ferromagnetic oxides La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ and SrRuO$_{3}$[1,2]. We have grown the bilayers on (001) SrTiO$_{3}$ substrates by pulsed laser deposition with atomic layer control. By analyzing the in-plane magnetization loops of the LSMO layer, and the total magnetization of the (frozen) SRO layer, we determine the exchange field of the bilayer and the domain structure of the SrRuO$_{3}$ biasing layer as a function of cooling field. This data shows evidence for domain walls both parallel and perpendicular to the interface. We support this conclusion with a simple energy minimization including contributions from the interfacial exchange, dipole interaction with the cooling field, domain wall energies, and magnetostatic energy. We also pattern the samples with e-beam and x-ray lithography techniques. Size effect on coercivities of both LSMO and SRO electrodes and exchange bias in the bilayers will be discussed. [1] X. Ke, M. S. Rzchowski, L.J. Belenky, C. B. Eom$,$ \textit{Appl. Phys. Lett}, \textbf{84}, 5458 (2004). [2] X. Ke, L.J. Belenky, C. B. Eom, M. S. Rzchowski, \textit{J. Appl. Phys}. \textbf{97}, 10K115 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R22.00010: Quantitative Depth Profiling of Interfacial Moments in Paramagnetic CoO in Py/CoO bilayer. Sujoy Roy, X. Liu, S.K. Sinha, B.J. Taylor, M.B. Maple, Y. Tang, Jung-IL Hong, A.E. Berkowitz, T. Leo, D.J. Smith, S. Park, M.R. Fitzsimmons, C. Sanchez-Hanke, C.-C Kao We have employed soft x-ray resonant magnetic reflectometry to determine the depth dependence of the net ferromagnetic moment in a Permalloy/CoO bilayer \textit{above} the N\'{e}el point of the antiferromagnetic CoO at 300K. \textit{Quantitative element specific }depth dependent charge and magnetization density profiles have been determined by analyzing the specular reflectivity data at the L$_{3}$ edges of Co and Ni using resonant magnetic scattering theory in the Distorted Wave Born Approximation. We have found that a thin interfacial layer with charge density different from either the Permalloy (Py) or CoO forms at the Py/CoO interface. This layer is magnetic even at room temperature and has a \textit{different} temperature dependence of magnetization compared to Py. We have put the depth profile of magnetization in an \textit{absolute scale} by combining the results of reflectivity measurements and SQUID magnetometry. Work of SKS and MBM supported by DOE. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R22.00011: Effects of coupling on domain structure of [Pt(6{\AA})/Co(4{\AA})]$_{3}$/NiO(t$_{NiO})$/[Co(4{\AA})/Pt(6{\AA})]$_{3}$ multilayers with oscillatory coupling Andrew Baruth, S.H. Liou, S. Adenwalla, D.J. Keavney Multilayered structures of Pt(6{\AA})/Co(4{\AA})]$_{3}$/NiO(t$_{NiO})$/[Co(4{\AA})/Pt(6 {\AA})]$_{3}$ show oscillatory exchange coupling between the two Co/Pt layers [1]. One possible mechanism for the exchange coupling may be via the propagation of canted NiO spins at the interface, an effect that can reproduce the oscillatory coupling [2] and that has been seen using X-ray Magnetic Circular Dichroism [3]. Domain images on virgin samples taken with XMCD-PEEM and MFM reveal a direct correlation between domain size and absolute coupling strength. The coupling strength was varied by changing both NiO and Pt thicknesses. MFM images show that the coupling in both FM and AFM coupled samples occurs domain by domain. FM domains form within the AFM domain wall due to a slight relative shift of the domains in the upper and lower multilayers. These FM regions contain visible stripes corresponding to opposite net magnetizations in the domain wall. [1] Phys. Rev. Lett. 91, 037207 (2003) [2] Phys. Rev. Lett. 92, 219703 (2004) [3] Phys. Rev. B 70, 224423 (2004) [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R22.00012: Fabrication and characterization of lateral all-magnetic superlattices Nikolay Polushkin, Steven Michalski, Lanping Yue, Roger Kirby Conventionally, patterned magnetic elements produced by lithographic methods are surrounded by nonmagnetic spacers. We describe a technique for \textit{direct laser fabrication }of 1D and 2D superlattices composed of two different magnetic materials with \textit{variable} properties. The approach is based on the phase transformations induced locally by interfering laser beams in thin alloyed layers of transition metals (Fe-V, Fe-Cr, Co-Pt). Using magnetic force microscopy and methods of magnetometry, we demonstrate that the structures patterned even in the submicron regime possess significant contrast in the magnetizations, with sharp interfaces between the constituents. We have used time-resolved magneto-optical Kerr effect measurements to further characterize the magnetic properties of these samples. These data allow us to obtain the magnetizations and gyromagnetic ratios of the constituents. Possible application to microwave nanophotonics of such all-magnetic superlattices with \textit{tunable }magnonic band gaps is briefly discussed. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R22.00013: Ferromagnetism of CoAl(100) induced by oxidation at 300 K Volker Rose, Klaus Brueggemann, Rudolf David, Harald Ibach In situ magneto-optical Kerr effect measurements were performed to investigate the magnetic behavior of CoAl(100) upon oxidation at room temperature. As an ideal Co$_{50}$Al$_{50}$ crystal is known to be paramagnetic at room temperature, our studies show a transformation into a ferromagnetic phase during increasing exposure to O$_{2}$. Oxidation of CoAl(100) at 300 K leads to the growth of an amorphous Al$_{2}$O$_{3}$ film [1], causing a Co excess at the metal/oxide interface that is sufficient to provide ferromagnetism. Annealing of the oxidized sample brings about a drastic increase of the coercive field, generated by diffusion of Al into the Co film. But, after annealing at 550 K, the ferromagnetism vanishes completely. [1] V. Rose, V. Podgurski, I. Costina, R. Franchy, Surf. Sci 541 (2003) 128. [Preview Abstract] |
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