Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H43: Focus Session: Magnetic Tunnel Junctions II |
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Sponsoring Units: GMAG DMP Chair: Xiu-Feng Han, Institute of Physics, Beijing Room: LACC 150 C |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H43.00001: High resistance demagnetized state in $\rm La_{0.67}Sr_{0.33}MnO_3/SrTiO_3/La_{0.67}Sr_{0.33}MnO_3$ magnetic tunnel junctions. Eric Wertz, Qi Li Magnetic tunnel junctions were fabricated from $\rm La_{0.67}Sr_{0.33}MnO_3/SrTiO_3/La_{0.67}Sr_{0.33}MnO_3$ (LSMO) heterostructures on STO substrates. The junctions showed a tunneling magnetoresistance (TMR) $\approx$ 360\% at 5K when initially subject to a high magnetic field. However, when demagnetized, the junctions displayed a much higher resistance than that found in the standard TMR scan. Within a low magnetic field range, after demagnetization, the junctions showed a TMR $\approx$ 450\% and much sharper switching. Differences between resistance states achievable at $<$ 500 Gauss yield a TMR $\approx$ 800\%. The high resistance state may indicate the LSMO layers preferentially choose a more efficient opposite magnetic domain alignment after demagnetization. Initial demagnetization, rather than high field coercion, may be technologically applicable in magnetic tunnel junction preparation. $\rm La_{0.67}Ca_{0.33}MnO_3$/STO/LSMO junctions were also fabricated and showed anomalous TMR effects dependant on the relative angle of the applied magnetic field. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H43.00002: Colossal Magnetoresistance Based Tunnel Junctions with Magnetic and Nonmagnetic Insulating Barriers Lisa Alldredge, Rajesh Chopdekar, Brittany Nelson-Cheeseman, Yuri Suzuki We have studied magnetic and nonmagnetic insulating barrier layers in magnetic tunnel junctions to understand the effect of magnetic moments in the barrier. Previously, we have demonstrated junction magnetoresistances of up to 20{\%} at 80K in junctions with La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and Fe$_{3}$O$_{4}$ electrodes with paramagnetic CoCr$_{2}$O$_{4}$ barriers. Having shown that magnetic barriers do not completely suppress JMR, we have developed thin films of nonmagnetic Mg$_{2}$TiO$_{4}$ (MTO) as well as paramagnetic FeGa$_{2}$O$_{4}$ (FGO) and NiMn$_{2}$O$_{4}$ (NMO). We have grown MTO on (110) SrTiO$_{3}$ with pulsed laser deposition in various atmospheres. Films grown in oxygen poorer atmospheres have lower resistivities than films grown in 7mTorr of O$_{2}$, which exhibit highly insulating behavior. FGO and NMO also exhibit very insulating behavior for a wide range of deposition conditions. Magnetization measurements of trilayers (Fe$_{3}$O$_{4}$/MTO/LSMO) show switching of the two magnetic layers at coercive fields comparable to single layers of LSMO and Fe$_{3}$O$_{4}$. We will discuss transport properties of these trilayers. [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H43.00003: Magnetically Tunable Depletion Layer in Manganite-Titanate Heterojunctions Naoyuki Nakagawa, Tomofumi Susaki, Harold Y. Hwang A significant effort has been put toward the realization of oxide heterojunction devices, as well as understanding the electronic structure of interfaces between Mott insulators and band insulators. The manganite-titanate heterojunction is an attractive candidate in this context. Perovskite manganites have a versatile range of magnetic and electronic properties related to orbital, charge and spin order, such as colossal magnetoresistance (CMR) and metal-insulator transitions. Here we present rectification in this manganite-based heterojunction, demonstrating a magnetically tunable depletion layer. The magnetic field increases the junction capacitance due to the reduction of the effective depletion width across the junction. As a result of the reduction of the junction barrier, the forward bias I-V characteristics are shifted to lower voltage under a magnetic field, giving rise to exponential differential magnetoresistance. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 9:12AM |
H43.00004: Interface Controlled Tunneling Spin Polarization Invited Speaker: In recent years spin-dependent tunneling in magnetic tunnel junctions has aroused enormous interest and developed into a vigorous field of research. However, despite extensive efforts to elucidate the mechanisms of spin-dependent tunneling, a complete understanding of this phenomenon is still lacking. In this talk we will consider various factors controlling the spin polarization of the tunneling current and emphasize the decisive role of interface bonding. We will show that the tunneling spin polarization is primarily determined by the electronic and atomic structure of ferromagnet/insulator interfaces rather than bulk properties. Starting from a simple tight-binding model which demonstrates the importance of interface states that are controlled by the hybridization between the atoms at the interface, we will, then, consider spin-dependent tunneling from the oxidized Co surface through vacuum [1] and in Co/Al$_{2}$O$_{3}$/Co tunnel junctions [2], where the cobalt-oxygen bonding at the interfaces play a crucial role. Our results indicate that the common argument of the dominant s-electron tunneling which is often used to explain positive values of the spin polarization in alumina-based tunnel junctions might be qualitatively incorrect. Moreover, oxygen atoms adsorbed by cobalt at the Co/Al$_{2}$O$_{3}$ interface may be a prerequisite for the positive spin polarization. The importance of the interface bonding and structure in spin-dependent tunneling makes the quantitative description of transport characteristics much more complicated; however, it broadens dramatically the possibility to engineer magnetic tunnel junctions with properties desirable for device applications. [1] K.D. Belashchenko, E.Y. Tsymbal, M. van Schilfgaarde, D. Stewart, I.I. Oleynik, and S.S. Jaswal, Phys. Rev. B 69, 174408 (2004). [2] K.D. Belashchenko, E.Y. Tsymbal, I.I. Oleynik, and M. van Schilfgaarde, submitted paper. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H43.00005: Interfacial Effects in Spin Transport in Magnetic Tunnel Junctions Tzen Ong, Barbara Jones, Annica Heyman, Shu Peng We have been looking at the spin-dependent tunneling in magnetic tunnel junctions (MTJ) using a position-dependent effective-mass Hamiltonian. Current-voltage curves and TMR values are calculated based upon analytical results, and include multiple band-to-multiple-band tunneling, to capture more realistically the main features of the band structure in cobalt and iron. The sign of the spin-current polarization is an outstanding issue in the field, and we believe that interfacial effects play a significant role. We have carried out DFT simulations of Co/Al$_{2}$O$_{3}$/Co structures, assuming Al-termination at the interface, which show significant changes in the LDOS at the interface as compared to bulk. For simulations that include disorder at the interface, there is a change in spin-polarization of the LDOS, due to interfacial scattering and screening effects, going across the interface from the Co to the Al layer. This change in spin-polarization is reflected in our calculated I-V curves and TMR values. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H43.00006: Inverse Tunneling Magnetoresistance in nanoscale Magnetic Tunnel Junctions Tae-Suk Kim We report on our theoretical study of the inverse TMR effect in the spin polarized transport through a narrow channel between two ferromagnetic metals. In the weak tunneling limit, we find the ordinary positive TMR. The TMR changes its sign as the transmission probability becomes large close to a unity. Our results might be relevant to the magnetic tunnel junction with a pinhole or the break junction with a quantum point contact. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H43.00007: BEEM and STS studies of aluminum oxide tunnel barrier layers P.G. Mather, A.C. Perrella, J.C. Read, E. Tan, R.A. Buhrman The prevalence of AlO$_{x}$ as the insulating layer in magnetic tunnel junctions and Josephson junction devices, including qubits, continues to motivate efforts to better control its electronic structure. While bulk Al$_{2}$O$_{3}$ has a band gap of $\sim $9 eV, amorphous AlO$_{x}$ layers formed at 300K exhibit a much smaller band gap, as well as extensive band tails. We have used scanning tunneling spectroscopy (STS) to examine the density of states (DOS) at the oxide surface and ballistic electron emission microscopy (BEEM) to determine how these states couple through the oxide layer. We have compared the effect of thermal annealing on AlO$_{x}$ layers to that of exposure to electron bombardment, both of which remove chemisorbed O$_{2}^{-}$ from the surface of the AlO$_{x}$ and drive the oxide to a more stoichiometric form. Both treatments increase the oxide band gap, however the electron bombardment has a less pronounced and different effect on the oxide states than 500 C anneals. The spread in the distribution of the DOS over an area of the oxide is reduced through both treatments, the greatest reduction taking place with the highest anneal temperature. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H43.00008: Temperature dependent asymmetry of the nonlocal spin injection resistance: evidence for spin non-conserving interface scattering Samir Garzon, Igor Zutic, Richard Webb We report nonlocal spin injection and detection experiments on mesoscopic Co-Al$_2$O$_3$-Cu spin valves. Spin precession in transverse fields (Hanle effect), as well as dependence of the nonlocal resistance on injector-detector magnetization alignment, have been studied. The spin diffusion length and the current spin polarization in Cu can be extracted from the measurements. We have also observed a temperature dependent asymmetry in the nonlocal resistance between parallel and antiparallel configurations of the magnetization of injector and detector. This suggests the existence of a nonequilibrium resistance that depends on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal, providing evidence for increasing interface spin scattering with temperature. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H43.00009: Dependence of giant tunneling magnetoresistance in MgO based magnetic tunnel junctions on the structure of Co electrodes See-Hun Yang, Mahesh Samant, Stuart Parkin The giant tunneling magnetoresistance recently reported at room temperature in magnetic tunnel junctions (MTJs) with crystalline MgO(100) barriers is related to the bcc structure and (100) orientation of the Co-Fe ferromagnetic electrodes[1]. We have investigated the relationship between the tunneling magnetoresistance of MTJs with ultra-thin Co layer thicknesses inserted between the CoFe and MgO barrier layers and the structure of the Co layer. The electronic structure of the Co layers was studied with x-ray emission spectroscopy (XES) and near edge x-ray absorption fine structure (NEXAFS). The structure of these layers was studied using x-ray scattering. The magnitude of the TMR effect is found to be correlated with the structure of the Co layers. [1] S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, S.-H. Yang, Nature Materials, in press (2004). [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H43.00010: Spin-dependent Transport through a Magnetic Carbon-Nanotube-Based Molecular Junction Chun Zhang, Linlin Wang, Hai-ping Cheng, Xiaoguang Zhang, Yongqiang Xue We apply a first-principles computational approach to study the transport properties of a magnetic molecular junction, which consists of two Fe-doped carbon nanotubes (CNT) (6, 0) and a C$_{60}$ molecule in the linear response regime. The conventional local spin-density functional theory (LSDFT) approach is applied to study the band structure of Fe-doped CNT. We find that for majority spin, only one band crosses the Fermi level while for minority spin, four bands cross the Fermi level. A method that combines LSDFT and non-equilibrium Green's functions technique is used to study the CNT/C60/CNT junction. For situations in which the net magnetic moments of two CNTs are parallel, we find that the conductance of minority-spin electrons is two times higher than the conductance of majority-spin electrons, which is rarely seen for the spin-dependent tunneling through layered structures. The magnetoresistance (MR) ratio is found to be 11{\%}. Our calculations suggest that CNTs have great potential in spintronics. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H43.00011: Spin-dependent tunneling conductance between a 2D quantum well and a 1D quantum wire Yun-pil Shim, S. K. Lyo* We study spin-dependent tunneling current between a quantum well (QW) and a quantum wire with in-plane magnetic field and Rashba spin-orbit interaction in the 2D quantum well. We show that the tunneling current can be spin-polarized in the presence of both an in-plane magnetic field and spin-orbit interaction. However, in the absence of the magnetic field, the isotropy of the energy spectrum makes the total tunneling current spin independent. The in-plane magnetic field changes the relative band minimum positions of the 2D QW and the wire in k-space and gives rise to a spin- polarized tunneling current. We calculate the tunneling conductance for each spin component and show that the current as spin-polarization which depends on the in-plane magnetic field. * Supported by the U.S. DOE under contract DE-AC04-94AL8500. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H43.00012: Magnetization Hysteresis and Quantum Tunneling in Lanthanide Double-Decker Complexes H. Rupp, P. M\"uller, S. Brink, O. Fuhr, M. Ruben We present magnetization measurements on single crystals of lanthanide double-deckers [Pc$_{2}$Ln]$^{-}$ TBA$^{+}$. The 4f$^{9 }$(4f$^{8})$ configuration of the Dy$^{3+}$ (Tb$^{3+})$, ion results in a J = 15/2 (J = 6) ground-state multiplett. In SQUID measurements on single crystal samples, we observed very large axial and a significant transverse anisotropy. Magnetization measurements using 2DEG ballistic Hall probes were carried out in a $^{3}$He cryostat. Hysteresis was observed for both compounds up to blocking temperatures of 4.2 and $>$10 K, respectively. The coercivity increased with decreasing temperatures and increasing sweep rate, as expected for the superparamagnet-like behaviour of a SMM. The hysteresis loops displayed step-like features characteristic for resonant quantum tunnelling of the magnetization (QTM). The step height decreased with increasing sweep rate according the Landau-Zener tunnelling mechanism. In conclusion, the lanthanide double decker molecules are SMM with the highest blocking temperatures observed to date. [Preview Abstract] |
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