Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H23: GMR and AMR |
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Sponsoring Units: GMAG DMP FIAP Chair: Pete Eames, NVE Corporation Room: Baltimore Convention Center 320 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H23.00001: Magnetoresistance of atomic-scale electromigrated nickel nanocontacts Zachary Keane, Lam Yu, Douglas Natelson We report measurements of the electron transport through atomic-scale constrictions and tunnel junctions between ferromagnetic electrodes. Structures are fabricated using a combination of e-beam lithography and controlled electromigration. Sample geometries are chosen to allow independent control of electrode bulk magnetizations. As junction size is decreased to the single channel limit, conventional anisotropic magnetoresistance (AMR) increases in magnitude, approaching the size expected for tunneling magnetoresistance (TMR) upon tunnel junction formation. Significant mesoscopic variations are seen in the magnitude and sign of the magnetoresistance, and no evidence is found of large ballistic magnetoresistance effects. [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H23.00002: Retention of Giant Magnetoresistance in Thin Films on Very Rough Substrates S.M. Watson, A.C. Reilly, D.M. Vlassarev, K. Smith, B.C. Holloway, W.F. Egelhoff, Jr Interfacial roughness is known to affect GMR. Past studies have measured increases , decreases or negligible changes in GMR produced by increasing interfacial roughness. Most studies have explored small scale interfacial roughness induced by changes in growth parameters or annealing$^{1}$. We present a study of the effects of large scale roughness ($\sigma _{rms}$ $>$ 5 nm) on the properties of GMR multilayers. GMR thin films were deposited on glass substrates chemically etched for different lengths of time producing a range of lateral and vertical roughness. We found that this long length scale does not have a significant detrimental effect suggesting that viable GMR multilayers can be deposited on a wide variety of surfaces, even with large roughness. M. C. Cyrille et al. Phys. Rev. B, 62, 3361 (2000), J. M. Colino et al., Phys. Rev. B, 54, 13030 (1996). $^{2}$ W. F. Egelhoff, Jr. et al., J. Appl. Phys., 82, 6142 (1997) $^{3}$ J. Alicea and S. Hershfield, J. Appl. Phys., 93, 7930 (2003) [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H23.00003: Electrical Resistance of Sputtered Chromium Thin Films Z. Boekelheide, D. Cooke, F. Hellman, D. J. Smith, M. J. Carey We have measured a large variation in the electrical resistivity of magnetron sputtered chromium thin films. The resistivity ranges from anomalously high (up to 20 times the bulk value) to comparable with the bulk value, and depends strongly on the deposition parameters, in particular the sputtering gas pressure. The range of sputtering gas pressure was 0.75 mTorr to 8 mTorr, with the resistivity increasing with pressure. We have also measured the structure and composition of the films, and the high resistivity does not appear to be caused by exotic structural phases, gross impurities, or grain size effects. The impact on the antiferromagnetism of sputtered chromium films and their role in magnetic multilayers will be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:03PM |
H23.00004: Non-saturating Magnetoresistance of Inhomogeneous Conductors Jingshi Hu, T. F. Rosenbaum, Meera M. Parish, J. B. Betts The inhomogeneous distribution of excess/deficient silver atoms, introducing spatial conductivity fluctuations with length scales independent of the cyclotron radius, lies behind the large, positive and linear transverse magnetoresistance displayed by $\mathrm{Ag}_{2\pm\delta}\mathrm{Se}$ and $\mathrm {Ag}_{2\pm\delta}\mathrm{Te}$. We report here a systematic measurement of the $\mathrm{\rho}_{xx}\mathrm{(H)}$, $\mathrm {\rho}_{xy}\mathrm{(H)}$ and $\mathrm{\rho}_{xz}\mathrm{(H)}$ components of the resistivity tensor with various sample geometries, showing clear evidence of distorted current paths as seen in theoretical simulations. Comparison of the (positive) transverse and (negative) longitudinal magnetoresistance for different sample thicknesses and electrical contact separations points to a characteristic length scale set by the spatial inhomogeneity that is as large as ten microns. [Preview Abstract] |
Tuesday, March 14, 2006 12:03PM - 12:15PM |
H23.00005: Electronic and Magnetic Structure of CrO$_2$ and CrO$_2$-RuO$_2$ Interfaces W. H. Butler, Hunter Sims, Krishna Chetry, Zhihong Lu, Sanjoy Sarker CrO$_2$ and RuO$_2$ share the same (rutile) crystal structure and have similar lattice constants. We have used Density Functional Theory within both the generalized gradient and LDA+U approximations to calculate the electronic and magnetic structure of CrO$_2$, RuO$_2$ and their interfaces. Consistent with previous calculations and experiment we find (for both GGA and LDA+U) that the CrO$_2$ Fermi energy lies in a band gap for the minority channel. RuO$_2$, in agreement with experiment, is predicted to be a metal by both types of calculations. We find relatively good matching between the majority energy bands of CrO$_2$ and either channel of RuO$_2$ in the $(100)$, $(110) $ and $(001)$ directions. The nearest neighbor exchange interaction in CrO$_2$ is calculated and used to estimate a Curie temperature of 411K. For $(100)$ interfaces of CrO$_2$ and RuO$_2$, we find that the CrO$_2$ moments remain collinear if the interfaces are abrupt, but our models of intermixed interfaces yielded non-collinear moment configurations with neighboring Cr moments making an angle of approximately $150^\circ{}$ and a small induced Ru moment oriented opposite to that of the sum of the Cr moments. We speculate that non-collinear spins at interfaces may be a common problem in oxides that may be a challenge to overcome to achieve large GMR and TMR effects in some of these materials. [Preview Abstract] |
Tuesday, March 14, 2006 12:15PM - 12:27PM |
H23.00006: Spin-Orbital Coupling Effects on Magnetoresistance in Organic Materials Yue Wu, Zhihua Xu, Bin Hu We report the studies on magnetoresistance of organic materials based on the light-emitting diode of phosphorescent iridium complex Ir(ppy)$_{3}$ molecules dispersed in fluorescent poly(N-vinylcarbazole) (PVK). The magnetic field-dependent injection current indicates that the PVK of weak spin-orbital coupling exhibits a significant magnetoresistance while the resistance of Ir(ppy)$_{3}$ of super-strong spin-orbital coupling shows an independence of magnetic field up to 3000 Gauss. We find that the magnetoresistance from the Ir(ppy)$_{3}$/PVK composite displays a gradual decrease with increasing the concentration of Ir(ppy)$_{3}$. The magnetic field-dependent electroluminescence confirms that the dispersed Ir(ppy)$_{3}$ molecules account for the change of magnetorsistance in the Ir(ppy)$_{3}$/PVK composite. From the uniform dispersion of Ir(ppy)$_{3}$ molecules observed from transmission electron microscope, we suggest that the spin-orbital coupling is modified by the interface interaction and consequently varies the magnetoresistance in the Ir(ppy)$_{3}$/PVK composite. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H23.00007: Single domain ferromagnet/superconductor nanoscale heterostructures Andrei Ursache, James Goldbach, Thomas Russell, Mark Tuominen Nanoscopic single-domain ferromagnetic elements promise simple model behavior for future generations of spintronic devices. In this work, ferromagnetic/superconductor (Co/Pb) multilayer nanowire devices with point contact interfaces are fabricated by electrodeposition in P(S-b-MMA) diblock copolymer derived nanoporous templates, which can produce lateral dimensions as small as 10 nm. Using a single two ion species electrolyte, selective electrodeposition of Co and Pb is achieved by electrode potential control, and the process is characterized in situ by an electrochemical quartz crystal microbalance technique. This fabrication scheme provides the possibility of probing the transport spin polarization of a a single domain ferromagnetic nanowire by means of point contact Andreev reflection (PCAR) spectroscopy. Preliminary electron transport measurements will be presented. This work is supported by NSF grants DMR-0306951, DMI-0103024 and MRSEC. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H23.00008: Inverse proximity effect in Ferromagnet/ Superconductor composite Jiyeong Gu Recently, a theoretical study of spin-screening effect in ferromagnet ($F)/$superconductor ($S)$ heterojunctions has been reported [1]. From the physics involved in $F/S$ junctions it is clear that the inverse proximity effect (the magnetization of the system changes due to the presence of the superconducting correlations) is related to the properties of the conducting electron. Either bi-layer or granular ($F$ particles embedded in $S$ matrix, for example, Co particles embedded in Nb matrix) types of $F/S$ thin films have been made using co-sputtering method. We measured the magnetization as a function of temperature, especially above and below the superconducting critical temperature, to determine the screening or antiscreening effect in $F/S$ hybrid system. This experiment is useful to examine the electronic properties at the Fermi level and the distribution of magnetic moments of ferromagnetic metals. Not only inverse proximity effect but also other properties such as regular proximity effect have been investigated. [1] F. S. Bergeret and N. Garcia, Phys. Rev. B 70, 052507 (2004) [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H23.00009: Proximity effect in four-layered ferromagnet/superconductor nanostructures: decoupled superconductivity and hierarchy of critical temperatures Nail G. Fazleev, Yurii N. Proshin, Mansur G. Khusainov The four-layered nanostructure consisting of rather dirty superconducting (S) and ferromagnetic (F) metals is studied within the theory of the proximity effect taking detailed account of the boundary conditions. The F/S nanostructures with four and more F and S layers are shown to have considerably richer physics than the F/S/F trilayer and F/S superlattices. The dependence of the critical temperatures versus the F layers thicknesses is investigated. It is shown that the F/S/F$\prime $/S$\prime $ nanostructure can experience decoupled superconductivity. The latter manifests itself through a hierarchy of the critical temperature Tc, which can be different for different S and S$\prime $ layers. An optimal set of parameters is determined, for which the difference between the critical temperatures becomes significant, and the corresponding phase diagrams are constructed. A conceptual scheme of a new control device based on the F/S/F$\prime $/S$\prime $ nanostructure with superconducting and magnetic recording channels that can be controlled separately using a weak external magnetic field is proposed. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H23.00010: Giant Magneto-Refractive Effect in Pseudo-Valve Thin Films Jared D. Rokitowski, Nam H. Kim, Jian-Qing Wang Magnetic-field-induced changes in infrared transmission of Co/Cu/Co pseudo spin-valve (PSV's) thin films are studied for the Magnetorefractive Effect (MRE). Samples are deposited on a Si (100) wafer under a high vacuum using magnetron sputtering, followed by annealing at various temperatures up to 6000C to enhance the GMR effect. A Thermo Electron Nexus 670 FT-IR spectrometer is used to measure a mid-IR spectrum with wavelengths ranging from 2.5$\mu$m to 30$\mu$m. The transmitted spectra through the thin films, placed in a magnetic field, were measured by a KBr detector at room temperature. The measured GMR value for a sample made up of several layers, Fe(5nm)/Co(5nm)/Cu(3nm)/Co(2nm), that has been annealed at 2500C is about 2\% at room temperature. This sample exhibits a sharp low switching field at about 20G and a gradual high field switching at half peak height, approximately 400G. Variations of about 1.0\% in the intensity of the transmitted IR spectrum are observed for a range of magnetic field strengths up to $\pm$1300G with special attention focused in the low field response (roughly -100 to 100 Gauss). The MRE is analyzed for a particular wavelength of 4µm. The results of this graph produce a typical GMR curve. Earlier studies of Spin-vales have yielded comparable MRE results. Detailed experimental results (1) and possible explanations will be discussed. 1. S. Uran et al., Phys. Rev. B 57, 2705 (1998). [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H23.00011: Tunneling Magnetoresistance in Magnetic Tunnel Junctions with a (Zn,Cr)Te electrode. Weigang Wang, Chaoying Ni, Takahiro Moriyama, Jun Wan, Ed Nowak, John Xiao Tunnel magnetoresistance (TMR) of 21{\%} is observed at low temperature in hybrid magnetic tunnel junctions composed of a magnetic semiconductor (Zn,Cr)Te and Co electrodes separated by a Al2O3 barrier. The MTJs were deposited in a magnetron sputter system with structure as Si / (Zn,Cr)Te(50)/ Al (0.75 -- 2.75)+oxidation / Co (15) / Cu (70), where the numbers in parentheses are layer thickness in nanometers. The Cr atomic concentration in the (Zn,Cr)Te layer is controlled to be at 20{\%}. The TMR value decreases with increasing temperature but sustains up to 250K, which is a considerable improvement over the MTJs with semiconductor electrodes in previous studies. The temperature and bias dependence of TMR are understood in the context of spin polarized tunneling and spin independent hopping through impurity states. The observed zero bias anomaly in the dI/dV curve is correlated to the existing of a soft coulomb gap at the E$_{F}$ in the (Zn,Cr)Te electrode which leads to T$^{-1/2}$ dependence of logarithmic resistance, logR, at low temperature. The TMR characteristic can be significantly improved by optimizing the interface and barrier quality [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H23.00012: Giant electroresistance and giant magnetoresistance of Fe-doped amorphous carbon film on Si substrate Xiaozhong Zhang, Peng Tian, Qingzhong Xue Amorphous Fe$_{x}$-C$_{1-x}$ films are deposited on Si substrates at different temperatures using pulsed laser deposition (PLD). It is found that the resistance of Fe$_{x}$-C$_{1-x}$ films on Si (100) substrates are controlled by the measuring current within a given temperature range. Correspondingly, giant electroresistance (ER) and giant magnetoresistance (MR) were found in this material. For example, Fe$_{0.011}$-C$_{0.989}$ film has a positive MR of 138{\%} at temperature of 300K and magnetic field of 5T. It is found that the switching of the conducting channel from the Fe$_{x}$-C$_{1-x}$ film to the Si substrate plays an important role in the current-dependent resistance of Fe$_{x}$-C$_{1-x}$ films. The multilayer of Fe$_{x}$-C$_{1-x}$ films on Si (100) substrate was also prepared by PLD. This multiplayer material has a positive MR of over 30{\%} at low magnetic filed, and the MR sensitivity is comparable to the spin-vale structure of traditional GMR materials. However, this material has a novel MR mechanism. It is believed that this multiplayer material can find application in magnetic information storage such as magnetic sensors, magnetic recording head and magnetic random access memory. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H23.00013: Orbitronics: The Intrinsic Orbital Hall Effect in p-doped Silicon Taylor Hughes, Andrei Bernevig, Shou-Cheng Zhang The spin Hall effect depends crucially on the intrinsic spin-orbit coupling of the energy bands in semiconductors. However materials with small spin-orbit coupling such as silicon will only exhibit a much reduced spin Hall effect. We show that the electric field in p-doped silicon will induce a dissipationless orbital current in a fashion reminiscent of the spin Hall effect. The effect is robust against disorder because the vertex correction due to impurity scattering vanishes due to symmetry. This orbital Hall effect can lead to the accumulation of local orbital angular momentum at the edge of the sample, and can be detected by the Kerr effect. [Preview Abstract] |
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