Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J42: Focus Session: Magnetic Nanoparticles, Nanostructures & Heterostructures IV |
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Sponsoring Units: DMP GMAG Chair: Zhiqiang Qiu, UC Berkeley Room: LACC 150B |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J42.00001: Nanomagnetism on Artificially modulated Single Crystalline Substrates Invited Speaker: We have developed a new experimental technique under ultra-high vacuum to grow epitaxially the `compositionally wedged' alloys on single crystalline substrates, which can be used to produce single crystalline templates with continuously modulated lattice constants, as well as with gradually varied chemical compositions. As an example, it is applied to investigate Fe on Cu$_{x}$Au$_{1-x}$/Cu(100) grown at room temperature. Its structure and magnetism are determined as a function of compositions when the in-plane lattice constant is fixed (strained films) and when the in-plane lattice constants are variable (strain-released films). In this way, the structural and compositional contributions to the magnetic properties can be well separated and independently studied. The other applications of this new technique will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J42.00002: Domain walls in antiferromagnetically-coupled multilayer films Eric Fullerton, Andreas Berger, Olav Hellwig We report experimentally observed magnetic domain-wall structures in antiferromagnetically-coupled multilayer films with perpendicular anisotropy [1, 2]. Our studies reveal a first-order phase transition from domain walls with no net moment to domain walls with ferromagnetic cores. The transition originates from the competition between dipolar and exchange energies, which we tune by means of layer thickness. The overall dipolar fields generated by such a ferromagnetic domain wall can be reduced by having the orientation of the ferromagnetic regions reverse periodically along the domain wall. This produces a one-dimensional equivalent of stripe domains that form in ferromagnetic thin films with perpendicular anisotropy. With increasing layer thickness these one-dimensional stripe domains evolve continuously into two dimensional stripe domains that fill the sample. Although observed in a synthetic antiferromagnetic system, such domains-wall structures may be expected to occur in A-type antiferromagnets with anisotropic exchange coupling. [1]. O. Hellwig \textit{et al}., Nature Materials \textbf{2}, 112 (2003). [2]. O. Hellwig, A. Berger and E. E. Fullerton, Phys. Rev. Lett. \textbf{91}, 197203 (2003). [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J42.00003: Nanostructural effects on the magnetic anisotropy in epitaxial thin films Mukut Mitra, Suzanne te Velthuis, Rosa Alejandra Lukaszew We have observed singular behavior in the anisotropy properties during magnetization reversal of epitaxial thin films studied with longitudinal magneto-optical Kerr effect (MOKE). We have observed that annealed Ni films epitaxially grown on (001) MgO substrates exhibit additional uniaxial anisotropy superimposed to the expected four-fold anisotropy due to magnetocrystalline anisotropy. Non annealed films only show four-fold symmetry. We will show that this additional anisotropy can be explained in terms of nanostructural changes of the film surface via oxidation as well as interfacial changes. Further, annealed and non-annealed films exhibit peculiar high coercive values along the magnetization hard axes. This is particularly noticeable as ``spikes'' in a polar plot representation of the coercive field. We have proposed a model to explain these ``spikes'' in terms of a second order type of transition for the magnetization before reversal that implies enhanced domain nucleation during switching along these particular directions. In order to validate our model we have performed polarized neutron reflectometry (PNR) studies on these films. We will present our correlated MOKE and PNR studies. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J42.00004: Anomalous Minor Loop Growth in Perpendicular Co/Pt-Multilayers Andreas Berger, Olav Hellwig, Eric Fullerton We have studied the evolution of magnetic minor loops upon multiple field cycling for Co/Pt-multilayers with perpendicular anisotropy. For this purpose, we saturated the samples in a sufficiently strong magnetic field first and then measured up to 20 successive minor loops with identical field cycles. In our experiment, the minor loop field cycles were chosen to be symmetric around zero, i.e. unbiased. While most theoretical models of magnetic materials assume the existence of a stable minor loop for multiple field cycles, many experiments show a slight drift of magnetic minor loops upon repeat cycling due to thermal excitation. For the thinnest (0.4 nm Co/0.7 nm Pt)$_{3}$-multilayers, we observe such a conventional small minor loop drift. However, somewhat thicker (0.4 nm Co/0.7 nm Pt)$_{8}$-multilayers show a very different and anomalous behavior. Here, we observe a continuous growth of the magnetization amplitude for successive minor loop measurements, revealing a multi-loop memory effect. The growth is found to be limited only by approaching the full magnetization reversal, even for field amplitudes as small as 0.8*H$_{c}$. The amplitude and cycle dependence of this effect is experimentally characterized and can be consistently explained by a strong asymmetry between the nucleation and annihilation mechanism of domains in these structures. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J42.00005: Effect of disorder on itineracy and magnetism in ultra thin Fe and Fe/C$_{60}$ films Partha Mitra, Arthur Hebard We present \textit{in situ} measurements of the temperature and field dependence of the longitudinal ($R_{xx})$ and transverse ($R_{xy})$ resistance of ultra thin (d $<$ 50 {\AA}) Fe thin films and Fe/C$_{60}$ thin-film composites. The study is motivated by the question as to what happens in a band ferromagnet when the ability of the itinerant electrons to transfer spin is compromised by disorder. We use the sheet resistance $R_{xx}$ as a measure of disorder and correlate this quantity with the saturation field, the saturated moment and the carrier concentration as derived from normal and anomalous Hall effect (AHE) measurements. We find that in the weakly disordered regime ($R_{xx} \le 5000 \quad \Omega )$ where logarithmic temperature dependences dominate, there is a suppression of the saturated moment and the scaling relations $\partial R_{xx} /R_{xx} = \quad \partial R_{xy}^{AHE} /R_{xy}^{AHE} =$ $-\partial \sigma _{xx}^{AHE} /\sigma _{xx}^{AHE} $ hold. When a monolayer of C$_{60}$ is predeposited on the substrate, stable Fe/C$_{60}$ films can be made. In the strongly disordered regime where the sheet resistance at low temperatures ($\sim $5~K) is approaching 1~M$\Omega $ (a value well above the quantum limit), the saturated moment as determined by AHE measurements is still remarkably robust. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J42.00006: Ferromagnetic Switching of Nb/Ni Multilayers and Trilayers in the Superconducting and Normal States Lance De Long, Wentao Xu, S. Kryukov, E. Navarro, J. Villegas, E. Gonzalez, Jose Vicent Ni(y)[Nb(x)/Ni(y)]$_{z}$ multilayers (z = 5, 8) with x = 23, 10 nm and y = 2.5, 3.5, 5 nm, and Nb(x)/Ni(y)/Nb(x) and Ni(y)/Nb(x)/Ni(y) trilayers with x = 23, 200 nm and y = 5 nm, were investigated via SQUID magnetometry with magnetic field parallel to the film plane. The superconducting transition temperature T$_{c}$ of samples was sometimes reduced well below 8 K, depending upon the Nb layer thickness x and the total number of Ni layers. Abrupt, reproducible switching anomalies are observed for multilayers, and complex magnetization curves observed for trilayers, in the superconducting state, instead of the smooth ferromagnetic hysteresis seen in the normal state. These results suggest that a complex magnetic coupling exists between Ni layers when mediated by Nb layers in the superconducting state. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J42.00007: The Dynamic Phase Transition in Ultrathin Co/Pt-Multilayer Films: Experimental Evidence and Comparison with Simulations D.T. Robb, Y.H. Xu, O. Hellwig, A. Berger, M.A. Novotny, P.A. Rikvold The dynamic phase transition (DPT), observed in numerical simulations of magnetic systems [1,2], manifests itself by the spontaneous occurrence of a non-vanishing period-averaged magnetization (the order parameter $Q$) when the frequency $f$ of an applied alternating magnetic field exceeds a critical value $f_c$. Near $f_c$, the DPT shows all common characteristics of a second-order phase transition. Our experimental studies of ultrathin Co/Pt-multilayers provide the first strong experimental evidence of a DPT. The multilayer structure results in perpendicular anisotropy and negligible demagnetizing effects [3]. We measure out-of-plane magnetization time series by the polar-Kerr effect as a function of $f$ and an applied bias field $H_b$, observing a sharp increase in $Q$ as $f$ is increased above $f_c$. In addition, we see sharp switching of $Q$ as $H_b$ is changed from positive to negative values. The data sets allow the assembly of an experimental phase diagram. Detailed comparison with simulations of a kinetic Ising model provides strong evidence that our data represent the first unequivocal experimental observation of the DPT. [1] S.W.\ Sides et al., Phys.\ Rev.\ Lett.\ {\bf 81}, 834 (1998) [2] B.\ Chakrabarti and M.\ Acharyya, Rev.\ Mod.\ Phys.\ {\bf 71}, 847 (1999) [3] Y.Yafet and E.M.\ Gyorgy, Phys.\ Rev.\ B {\bf 38}, 9145 (1988). [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J42.00008: Spin-reorientation transition in exchange-coupled (Pt/Co)n/Sm-Co multilayers Jian Zhou, Ralph Skomski, David Sellmyer Exchange coupling through Pauli-paramagnetic spacer layers is a scientifically interesting phenomenon with applications in sensors and magnetic recording. We have investigated the interplay between interlayer exchange and competing anisotropies. Due to interface anisotropy, the (Pt/Co)$_{n}$ multilayer exhibits an out-of-plane easy axis (perpendicular anisotropy). The Sm-Co layer has in-plane anisotropy. The exchange between the (Pt/Co)$_{n}$ and Sm-Co layers is tuned by varying the thickness of the Pt spacer layer. In the (Pt5{\AA}/Co4{\AA})$_{n}$/Sm-Co40{\AA} system, we observe a spin-reorientation transition at a spacer-layer thickness of somewhat less than 5 {\AA}. Above this threshold, the Pt/Co retains its out-of-plane anisotropy, and a low-temperature coercivity of 3 kOe is obtained. Below the threshold, the Pt-Co film is exchange-coupled to the Sm-Co layer and the spin structure of (Pt/Co)$_{4}$ changes, with a low-temperature coercivity of 200 Oe. The transition is also seen in the hysteresis-loop shape. The samples exhibit a low-temperature loop shift after field cooling, which is not observed after zero-field-cooling. The behavior of the film is modeled and discussed as a function of the Pt/Co bilayer period n and of the spacer-layer thickness. This research is supported by DOE, ARO, the W. M. Keck Foundation, INSIC, and CMRA. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J42.00009: Nanofabrication as A Probe of Anisotropy Distribution in Co/Pd Multilayers Bruce Terris, Guohan Hu, Thomas Thomson Arrays of patterned magnetic islands typically exhibit switching field distributions (SFD) which are much broader than those predicted based solely on the dipolar fields from neighboring islands. The source of the broad distributions is not understood. To determine if the source of the broad island SFD arises from intrinsic film properties or extrinsic sources we have fabricated arrays of Co/Pd islands with perpendicular anisotropy and sizes ranging from 30 nm to 5$\mu $m. The islands displayed single domain remanent states after easy axis saturation. The island array's coercivity and SFD increase with decreasing island size. The observed angle dependent switching fields closely resemble the behavior predicted by the Stoner-Wohlfarth model with a minimum at 45 degrees. This angle dependence is expected for small islands which may reverse by rotation, but is surprising for the larger islands. As expected, the continuous film exhibits the 1/cos dependence as predicted for domain wall motion controlled reversal. These data lead to a model whereby the reversal of the larger islands is controlled by a nucleation event, followed by a rapid wall motion. The observed switching field of the island is thus the switching field of the softest nucleation site in the island, and the distribution of these nucleation energies in the full grown film is the source of the island SFD. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J42.00010: Ferromagnetic Phase Diagram for Nanoscale Epitaxial (Co,Mn) Alloys on GaAs(001) Li Zhang, Don Heiman, Darius Basiaga, Jim O'Brien A important goal of device and materials research is directed at discovering ways to combine the properties of ferromagnets and semiconductors. Our current work is aimed at synthesizing ferromagnetic materials which are compatible with GaAs. In a previous study, it was shown that the Heusler ferromagnet, Co$_2$MnAl can be grown epitaxially on GaAs and has a Curie temperature T$_C$=800-1000~K.$^{[1]}$ In the present work, nanoscale alloys of Co$_{1-x}$Mn$_x$ were grown epitaxially on GaAs(001) substrates. Nanometer thick layers of (Co,Mn) are found to be ferromagnetic over a wider range of x-values than for bulk materials --- x$\leq$0.8 for 5-10~nm thick films, compared to only x$\leq$0.32 for bulk materials. The Curie temperature of (Co,Mn) nanometer films decreases linearly for increasing x, from T$_C$=800~K at x=0.35 to T$_C$=0 at x=0.8. The extended range of x for ferromagnetism is attributed to the strained lattice which is in registration with the GaAs lattice. Work supported by NSF-DMR-0305360. [1] Y. Chen, D. Basiaga, J.R. O'Brien, and D. Heiman, \emph{Anomalous magnetic properties and Hall effect in ferromagnetic Co$_2$MnAl epilayers}, Appl. Phys. Lett. \textbf{84}, 4301 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J42.00011: Magnetocapacitance and surface magnetism in Pd/Fe/Pd trilayer structures R.P. Rairigh, A.F. Hebard For sufficiently thin insulator spacing in metal-insulator-metal (M-I-M) trilayer structures, the capacitance can be dominated by the interface of the dielectric and the electrodes. If one or both of the electrodes are ferro- or paramagnetic, the screening length is influenced by a difference in the spin-up and spin-down densities of states, and the resulting magnetic-field induced changes in capacitance (magnetocapacitance) become a sensitive measure of surface magnetism. We have grown Pd (200 {\AA})/Fe~(1.5 {\AA})/Pd ($x~${\AA}) trilayer structures, where $x$ was varied from 50~{\AA} to 2~{\AA}. All of the films are ferromagnetic having similar saturation magnetizations at 10~K. However, as $x$ is decreased, there is a significant increase in the coercive field ($H_{c})$ from $H_{c}\sim $7~Oe for $x$~=~50~{\AA} to $H_{c}\sim $30~Oe for $x$~=~2 {\AA}. The sensitivity of magnetic properties to the proximity of the interface reflects a cross over from bulk to surface-dominated magnetism. We will correlate this crossover with magnetocapacitance measurements on M{\-}I{\-}M capacitors where the Pd/Fe/Pd trilayer is the base electrode and $x$ is the separation of the Fe layer from the interface with the dielectric of the capacitor. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J42.00012: Low temperature magnetism in Sb$_{2-x}$V$_x$Te$_3$ thin film grown on sapphire substrate by MBE Zhenhua Zhou, Yi-Jiunn Chien, Ctirad Uher Ferromagnetic semiconductors containing transition ions have received considerable attention since they are promising materials for spintronics application. However, their Curie temperatures need to be increased in order to be useful. In this work, good quality ferromagnetic semiconductor Sb$_{2-x}$V$_{x}$T$_{3}$ thin films have been grown on sapphire (0001) substrate by non-equilibrium MBE technique. The change from diamagnetism to ferromagnetism with the increasing concentration of V in the Sb$_{2}$Te$_{3}$ matrix has been studied via temperature dependent magnetic susceptibility and magnetotransport measurements. The solubility limit of V in the Sb$_{2}$Te$_{3}$ thin films has been studied by X-ray diffraction and EPMA (Electron Probe Microanalysis). The anisotropic dependence of the magnetization in Sb$_{2-x}$V$_{x}$T$_{3}$ thin films has been studied. The underlining physical mechanism that leads to the long-range magnetic order in Sb$_{2-x}$V$_{x}$T$_{3}$ will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J42.00013: Buried-interface characterization in magnetic nanostructures using standing wave-excited x-ray emission and resonant inelastic x-ray scattering Masamitsu Watanabe, Brian Sell, See-Hun Yang, Bongjin Mun, Norman Mannella, Long Pham, Stephenie Ritchey, Akira Nambu, Farhad Salmassi, Jinghua Guo, Jeffrey Kortright, S.S.P. Parkin, Charles Fadley Yang et al. (J. Phys. Cond. Matt. \underline {14}, L406 (2002)) have discussed a new method for studying buried interfaces using soft x-ray standing waves created by Bragg reflection from a multilayer mirror, combined with a wedge-shaped sample profile. Prior work has been based on photoemission, a photon-in/electron-out spectroscopy. We will here discuss the first experimental results of applying this method via more bulk-sensitive photon-in/photon-out spectroscopies: x-ray emission (XES) and resonant inelastic x-ray scattering (RIXS). We have measured XES and RIXS intensities from an Fe/Cr bilayer that is a prototypical giant magnetoresistance combination via both sample-scanning and rocking-curve methods. Magnetic circular dichroism has also been measured in Fe RIXS spectra. Combining this data with x-ray optical calculations permits determining the compositional and magnetic structure of the Fe/Cr interface. Work supported by DOE Off. of Science, Basic Energy Sciences, Mat. Sci. Div. [Preview Abstract] |
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