Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S42: Artificially Structured or Self-Assembled Magnetic Materials - II |
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Sponsoring Units: GMAG Chair: Satish Ogale, University of Maryland Room: LACC 150B |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S42.00001: GMAG Dissertation Award: Exploring new magnetic properties in coupled magnetic nanostructures Invited Speaker: One of the basic building blocks in constructing magnetic nanostructures is a magnetic sandwich in which two ferromagnetic layers are separated by a nanometer thick nonmagnetic spacer layer. Research on coupled magnetic sandwiches has generated fruitful results such as the oscillatory magnetic interlayer coupling and giant magneto resistance. The basic question on this subject is: how does the interlayer coupling generate new magnetic properties of the sandwich? In order to single out the effect of interlayer coupling, it is necessary to measure the two magnetic layers separately. Such experimental capability becomes available after the development of x-ray magnetic circular dichroism (XMCD) technique which enables element-specific measurement. In particular, Photoemission Electron Microscopy (PEEM) technique allows the element-specific magnetic domain imaging of magnetic nanostructures. In this talk, I will discuss our recent effort in using PEEM to study coupled magnetic sandwiches. First, I will discuss results of Co/Cu/Ni and Co/Fe/Ni in which we studied the effect of interlayer coupling on the magnetic phase transitions of the Co and Ni films. We found that a coupled magnetic sandwich undergoes three types of magnetic phase transitions, depending on the two ferromagnetic films' thickness. The differences and characteristics among these three phase transitions will be discussed with the simple simulation based on an Ising model. Second, spin reorientation of coupled sandwich will be discussed. We reveal a universal dependence of the stripe domain width on the magnetic anisotropy and on the interlayer coupling. In the last, I will discuss results of FeMn/Co in which the magnetic frustration between the antiferromagnetic FeMn and ferromagnetic Co modifies the Curie and Neel temperatures of the system. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S42.00002: Oscillatory Interlayer Coupling in Co/Pt Multilayers with Perpendicular Anisotropy Jacob Knepper, Fengyuan Yang Oscillatory interlayer coupling in ferromagnet/nonmagnet multilayers has been observed in many materials with the exception of only a few metals, including Pt. Recently, Co/Pt multilayers have attracted great attention because of the perpendicular anisotropy. However, the mechanism of the magnetic coupling in Co/Pt multilayers remains unknown. To our knowledge, oscillatory interlayer coupling has only been observed in multilayers with in-plane anisotropy. We investigated the interlayer coupling in Co(0.4nm)/Pt(0-8nm) multilayers with perpendicular anisotropy and repetition from 5 to 30 made by UHV sputtering. Hysteresis loops were measured between 8 and 293 K. The interlayer coupling is always ferromagnetic, which can be readily detected by coercivity with perpendicular anisotropy. The coercivity shows an oscillatory behavior with the Pt thickness for all multilayers. The period of the oscillation is 3 nm at 80 K. The oscillation of coercivity is a clear indication of the oscillatory interlayer coupling of Co across Pt. This is the first report of oscillatory interlayer coupling in Pt and in multilayers with perpendicular anisotropy. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S42.00003: Antiferromagnetic coupling in soft amorphous ferromagnet/semiconductor multilayers Maria Velez, C. Quiros, J.I. Martin, L. Zarate, J.M. Alameda Antiferromagnetic coupling between ferromagnetic layers separated by nonmagnetic metallic interlayers has been intensively studied due to the fundamental and technological interest in such behaviour. In this work, the presence of antiferromagnetic (AF) coupling has been investigated in multilayers where the nonmagnetic interlayers are not metallic but semiconducting. The analyzed samples are amorphous (Co$_{x}$Si$_ {1-x})_{5 nm}$ /(Si)$_{d}$ multilayers obtained by co-sputtering on Si substrates, and the Si layer thickness has been varied in the range 1 nm $<$ d $<$ 15 nm. X-ray diffraction analysis has shown that the multilayered structure is well defined. The individual (Co$_{x}$Si$_{1-x})_{5 nm}$ ferromagnetic layer presents an uniaxial anisotropy and a soft magnetic behaviour (with coercivity smaller than 1 Oe for fields applied along its easy axis), being suitable to detect the possible AF coupling in the multilayer. Magneto-optical kerr effect and alternating gradient magnetometry measurements have revealed that these multilayers do present AF coupling at room temperature for d $<$ 8 nm. Moreover, the magnetic field required to switch between antiparallel and parallel configurations is as low as 3 Oe and varies slightly with the Si layer thickness [1]. \newpage [1] C. Quiros et al., Phys. Rev. B (in press) [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S42.00004: XMCD studies of antiferromagnetically coupled Co/Pt Multilayers A. Baruth, S. Adenwalla, D.J. Keavney Previous results on multilayered structures of [Pt(5{\AA})/Co(4{\AA})]$_{3}$/NiO(t$_{NiO}${\AA}) /[Co(4{\AA})/Pt(5{\AA})]$_{3}$ show exchange coupling between the two Co/Pt layers as well as exchange bias between the Co and NiO below 200K [1]. The exchange coupling is explained through the canting of AFM NiO spins which were theoretically predicted [2] and seen using X-ray Magnetic Circular Dichroism [3]. Using XMCD we have studied the element specific magnetization of Co and NiO as functions of field and temperature (above and below the blocking temperature, 200K) in two samples with 11{\AA} and 12{\AA} NiO. At these thicknesses of NiO, both sets of Co/Pt multilayers couple antiferromagnetically, but the coupling strength for the 12{\AA} NiO sample is approximately half that of the 11{\AA}. Element specific hysteresis loops showed identical behavior for both Co and Ni implying that the AFM NiO spins at the interface cant in the direction of the Co magnetization. Photoemission electron microscope images on a virgin sample at room temperature revealed the exact correlation between FM domains in the Co and NiO layers in the strongest antiferromagnetically coupled sample. We plan to measure the AFM domain structure of NiO using Magnetic Linear Dichroism. [1] Phys. Rev. Lett. 91, 037207 (2003) [2] Phys. Rev. Lett. 92, 219703 (2004) [3] Z.Y. Liu et. al. Phys Rev B (accepted) Funded by NSF MRSEC [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S42.00005: Spin structure in exchange-biased epitaxial ferromagnetic oxide bilayers Xianglin Ke, Land Belenky, Chang-Beom Eom, Mark Rzchowski We investigate the spin structure of exchange-biased ferromagnetic bilayers for varying thicknesses near the critical thickness of the biasing layer. The epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO)/ SrRuO$_{3}$ (SRO) ferromagnetic oxide bilayers were grown on (001) SrTiO$_{3}$ single-crystal substrates by pulsed laser deposition with atomic-layer control. We find a $\sim $2Ênm critical thickness of the biasing layer for the disappearance of exchange bias. The antiferromagnetic interfacial exchange permits manipulation of the frozen-in spin structure, and the ferromagnetism of each layer enables direct magnetization measurements. From these measurements, we infer the thickness dependence of the spin structure of the biasing layer in terms of domain walls perpendicular and parallel to the bilayer interface. We find that parallel domain walls can be frozen into the biasing layer for thicknesses near the critical thickness. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S42.00006: Growth and characterization of tunable BSTO/BaM multilayers as substrates for magnetic nanoparticles N. A. Frey, R. Heindl, S. Srinath, H. Srikanth, K. R. Coffey, N. J. Dudney Multilayers of Ba0.5Sr0.5TiO3 (BSTO) and BaFe12O19 (BaM), with tunable permeability and permittivity are attractive systems for multifunctional applications. We have grown multilayers of BSTO and BaM using magnetron sputtering on Al$_{2}$O$_{3}$ and Si/SiO$_{2}$ substrates. Film growth conditions such as sputtering parameters were optimized to obtain high quality multilayers. X-ray diffraction established that both BSTO and BaM were formed and cross-sectional SEM studies showed distinct interfaces between BSTO and BaM layers. Magnetization measurements taken with a Physical Properties Measurement System (PPMS) showed a large coercivity ($\sim $2000 Oe) consistent with the hard magnetic hexaferrite component. The hysteresis loops also revealed the influence of magnetocrystalline and shape anisotropies at different temperature ranges. The multilayer structures can be made even more versatile with the inclusion of magnetic nanoparticles to help achieve a greater degree of tunability and frequency agility under applied electric and magnetic fields. Fe$_{3}$O$_{4}$ nanoparticles were deposited on the multilayer film surface using the Langmuir-Blodgett technique. Images of the resulting structures will be presented. Work at USF supported by DARPA/ARO through Grant No. DAAD 19-03-1-0277 [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S42.00007: Temperature Dependent Magnetization Reversal of [Co/Pt]/Ru Multilayers Olav Hellwig, Joseph E. Davies, Kai Liu, Eric E. Fullerton Magnetization reversal in antiferromagetically coupled [Co/Pt]/Ru multilayers have been shown to be either laterally or vertically correlated, depending on the layer thicknesses.$^{1}$ Here we report on our investigation of the magnetization reversal as a function of temperature using the First Order Reversal Curve (FORC) method.$^{2}$ At high temperatures the vertically correlated reversal dominates, and we observe the reversal behavior similar to that of just a Co/Pt multilayer.$^{2}$ At low temperatures the effect of antiferromagnetic interlayer coupling, and consequently laterally correlated reversal, becomes more prominent. At intermediate temperatures the two modes coexist and can be tuned by the applied magnetic field, leading to exotic reversal behavior where the FORCs could exist outside of the major hysteresis loop. This is due to the delicate balance among the magnetostatic, domain wall, and interlayer exchange coupling energies. \newline \newline $^{1}$O. Hellwig, et al., Nature Mat. \textbf{2}, 112 (2003). \newline $^{2}$ J. E. Davies, et al., Phys. Rev. B \textbf{70}, (22), Dec. 1$^{st}$ (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S42.00008: Growth and structure-property correlations in perpendicular exchange biased magnetic multilayers Xiaosong Ji, Honglyoul Ju, Kannan Krishnan Si/Pt$_{200\mbox{{\AA}}}$/(Co$_{6\mbox{{\AA}}}$/Pt$_{20\mbox{{\AA}}})_{\times 5}$/FeMn/Pt$_{20\mbox{{\AA}}}$ multilayers with strong perpendicular anisotropy and large exchange bias field were deposited by ion-beam sputtering. It was found that the microstructure and magnetic properties were greatly affected by the growth parameters such as different substrates, buffer layer thickness, multilayer thicknesses, number of bilayers, substrate temperatures, and most critically, by the ion-beam energies which was studied for the first time. The structure-property correlations of the multilayers, as a function of ion beam energy and other sputtering parameters, were investigated with emphasis on the role of interdiffusion at the interfaces. By comparing the structural properties of the multilayers deposited with different ion-beam energies, especially the degree of interdiffusion measured by x-ray reflectivity, with the magnetic properties, we show that lower ion-beam energy deposition has stronger perpendicular anisotropy and exchange bias field due to the better control of interdiffusion. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S42.00009: Diffraction MOKE on multilayer magnetic nanodisks Kristen Buchanan, Marcos Grimsditch, Konstantin Guslienko, Samuel Bader, Valentyn Novosad Diffraction magneto-optical Kerr effect magnetometry (DMOKE) is an excellent tool for investigating magnetization reversal in complex systems as it provides additional information not contained in bulk hysteresis measurements. The hysteresis loops measured on the diffraction beams can be understood in terms of field dependent magnetic form factors. Here the DMOKE technique was employed to investigate the magnetization reversal process in stacked ferromagnetic nano-disks, separated by a non-magnetic layer. The disks interact strongly via magnetostatic interactions and also via interlayer exchange coupling governed by the thickness and composition of the spacer layer. Micromagnetic simulations indicate that the disks will each support vortices of opposite chirality at remanence and reverse through coordinated nucleation, displacement, and annihilation of vortices when interlayer exchange is important. For thin, well separated disks, however, the magnetostatic interactions can be comparable to the vortex nucleation field and the reversal is quite different. Diffraction hysteresis loops for Permalloy (Ni$_{80} $ Fe$_{20}$ ) dots with thicknesses of up to 40 nm and radii of 250-1250 nm separated by a Cu spacer (1-20 nm) will be compared with loops calculated from micromagnetic simulations. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S42.00010: Magnetic stripe melting and metastable bubble domains in Fe/Ni/Cu(001) J. Choi, C. Won, Y.Z. Wu, T. Owens, J. Wu, A. Scholl, A. Doran, W. Kim, X.F. Jin, Z.Q.Qiu Qiu Spin reorientation transition (SRT) of Fe/Ni/Cu(001) was investigated using photoemission electron microscopy (PEEM). Stripe domains were imaged as a function of temperature to monitor the domain melting process. We found that the stripe domains melt in a narrow thickness range of the SRT region. This result indicates that the Curie temperature at the SRT point is lowered by the reduction of the magnetic anisotropy. In addition to the stripe domains, we observed a metastable phase of magnetic bubble domains in the SRT region, which may suggests the importance of higher order magnetic anisotropy in the SRT. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S42.00011: Tunable Thermal Hysteresis in CoNi/Gd Nanolayers Maria R. Hossu, Sezen Demirtas, Ali R. Koymen, Robert E. Camley For the first time we present experimental results proving that artificial ferrimagnetic multilayers show magnetic thermal hysteresis in the total magnetic moment. CoNi/Gd multilayers, a typical ferrimagnetic system, grown by dc magnetron sputtering has \textbf{bow-tie} shaped magnetic hysteresis with temperature over a wide range of layer thicknesses and external magnetic fields. The magnetic phase transition occurs at different temperatures during the heating and cooling cycles. Our results show that we can control the width of the magnetic thermal hysteresis loop over a temperature range of 90K. These results are in good agreement with theoretical calculations. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S42.00012: Field cooling dependence of the anisotropy in exchange biased FeF$_{2}$/Co films A.K. Alsmadi, S.G.E. te Velthuis, Hongtao Shi, David Lederman Using polarized neutron reflectometry we have studied the magnetization reversal in exchange biased single-crystal FeF$_{2} $/Co films grown on MgF$_{2}$. A recent study showed that the anisotropy of the antiferromagnetic FeF$_{2}$ plays an important role in determining the magnitude and effective direction of the exchange bias field H$_{E}$[1]. After field cooling perpendicular to the c-axis (easy axis of FeF$_{2}$), the magnetization curve determined with the applied field parallel to the c-axis, shows a double loop, one with positive bias, and one with negative bias. This behavior suggests that the antiferromagnet is simply split into two types of domains, inducing opposite H$_{E}$ along the c-axis. However, our observation of spin-flip reflectivity in the field regions separating the two loops indicates that some rotation of the Co magnetization also occurs during reversal, implying that H$_{E}$ does not lie exclusively along the c-axis. \\[4pt] [1] Hongtao Shi, David Lederman, Phys. Rev. B 66, 094426 (2002). [Preview Abstract] |
Wednesday, March 23, 2005 5:18PM - 5:30PM |
S42.00013: Oscillatory Exchange Bias in Fe/Cr Bilayers J.S. Parker, L. Wang, P.A. Crowell, C. Leighton We have measured the magnetization of Fe{(001),50 \AA}/Cr{(001),t$_{Cr}$(\AA)} bilayers grown on MgO(001) using the magneto-optical Kerr effect (MOKE). Samples were grown by UHV dc magnetron sputtering at 400 C with a wedge of Cr (t$_{Cr}$ = 0-1000 \AA) on top of a 50 \AA Fe layer. Structural characterization of the films was performed by grazing incidence X-ray diffraction (GIXRD), wide angle XRD, and atomic force microscopy (AFM). We observe small ($\sim$1 Oe) periodic oscillations ($\sim$70 K period) in the exchange bias field (H$_E$) as a function of temperature below the blocking temperature (T$_B$ $\sim$ 300K), which is attributed to variations in the wavelength, $\Lambda$, of the incommensurate spin density wave (SDW) spin structure in the antiferromagnetic Cr layer. Using scanning MOKE we have measured and compared the change in oscillation period for different t$_{Cr}$ in a single bilayer. We observe a peak in the coercive field as a function of temperature for thicker Cr layers. This effect may be related to the spin-flip transition between longitudinal and transverse spin density waves in the Cr layer. *Research supported by the NSF MRSEC program under DMR-0212032. [Preview Abstract] |
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