Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U22: Focus Session: Magnetic Tunneling I |
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Sponsoring Units: GMAG FIAP DMP Chair: Jonathan Sun, IBM Room: Baltimore Convention Center 319 |
Thursday, March 16, 2006 8:00AM - 8:36AM |
U22.00001: Tunneling spin polarization in planar tunnel junctions: measurements using NbN superconducting electrodes and evidence for Kondo-assisted tunneling Invited Speaker: The fundamental origin of tunneling magnetoresistance in magnetic tunnel junctions (MTJs) is the spin-polarized tunneling current, which can be measured directly using superconducting tunneling spectroscopy (STS). The STS technique was first developed by Meservey and Tedrow using aluminum superconducting electrodes. Al has been widely used because of its low spin orbit scattering. However, measurements must be made at low temperatures ($<$0.4 K) because of the low superconducting transition temperature of Al. Here, we demonstrate that superconducting electrodes formed from NbN can be used to measure tunneling spin polarization (TSP) at higher temperatures up to $\sim $1.2K. The tunneling magnetoresistance and polarization of the tunneling current in MTJs is highly sensitive to the detailed structure of the tunneling barrier. Using MgO tunnel barriers we find TSP values as high as 90{\%} at 0.25K. The TMR is, however, depressed by insertion of ultra thin layers of both non-magnetic and magnetic metals in the middle of the MgO barrier. For ultra-thin, discontinuous magnetic layers of CoFe, we find evidence of Kondo assisted tunneling, from increased conductance at low temperatures ($<$50K) and bias voltage ($<$20 mV). Over the same temperature and bias voltage regimes the tunneling magnetoresistance is strongly depressed. We present other evidence of Kondo resonance including the logarithmic temperature dependence of the zero bias conductance peak. We infer the Kondo temperature from both the spectra width of this conductance peak as well as the temperature dependence of the TMR depression. The Kondo temperature is sensitive to the thickness of the inserted CoFe layer and decreases with increased CoFe thickness. * performed in collaboration with S-H. Yang, C. Kaiser, and S. Parkin. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U22.00002: High sensitivity of tunneling spin polarization to chemical bonding of transition metal ferromagnetic alloys at interface with insulating barrier See-Hun Yang, Hyunsoo Yang, Christian Kaiser, Stuart Parkin We report that the tunneling spin polarization (TSP) is found to be strongly influenced by the amount of oxygen used in the deposition of the tunnel barrier itself that chemical bonding at the interface between Al$_{2}$O$_{3}$ and ferromagnetic Co and Co-Pt alloys. For reactive sputter (RS) deposition of alumina using an argon-oxygen gas mixture with a low concentration of oxygen ($\sim $0.1 mTorr), much lower TSP values are found than when the alumina barrier is formed by post-plasma oxidation (PO) with $\sim $100mTorr oxygen of Al layers. X-ray absorption spectroscopy (XAS) has been used to characterize the chemical bonding at the Co or Co-Pt/Al$_{2}$O$_{3}$ interface. These studies show that Co-O bonds are much more formed for the barrier fromed by PO of Al than for that formed by RS deposition. We attribute the changes in TSP to changes in the relative tunneling probabilities from Co and Pt which are strongly influenced by oxygen bond formation.$^{1}$ $^{1}$C. Kaiser, S. van Dijken, S.-H. Yang, H. Yang, and S. S. P. Parkin, Phys. Rev. Lett. 94, 247203 (2005). [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U22.00003: Anomalous magnetic behaviors in AlO$_x$/Co$_{84}$Fe$_{16}$ Tunneling Magnetoresistance (TMR) systems induced by the interfacial oxidations K.-J. Rho, K.-J. Kim, J.-Y. Kim, J.-H. Park, J.-S. Lee, K.-B. Lee, S. J. Joo, K. Rhie Due to the practical application of the magnetic tunnel junction as magnetic memory cells and sensors, tunneling magnetoresistance (TMR) in the magnetic tunnel junction (MTJ) has been very actively studied, and extensive effort has been exerted in order to improve the physical properties of MTJs. As a result, it is well recognized that the performance of the MTJs strongly depends on the environment at the interface between the magnetic layer and the insulating oxide layer. In spite of such critical importance, the interface environment and its effects on the magnetic behaviors have not been well understood yet. Here we present anomalous magnetic behaviors observed in AlO$_x$/Co$_{84}$Fe$_{16}$ magnetic tunnel junction structures prepared in various oxidation processes. We examined the magnetic behaviors using the magneto-optical Kerr effect (MOKE) measurements in vacuum. The system turns out to exhibit a strong temperature dependence of the magnetic coersive field, and the behavior consistently varies with the oxidation process. The interface environments and the origin of the anomalous behaviors will be also discussed based on x-ray reflectivity and diffraction results. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U22.00004: Tunneling magnetoresistance in MgO-based magnetic tunneling junction with (001) oriented FeCo electrode fabricated by sputtering deposition. Takahiro Moriyama, Weigang Wang, John Q. Xiao MgO-based Magnetic Tunneling Junction (MTJ) is now one of the most promising structures for spintronics applications due to its very large Tunneling Magnetoresistance (TMR). However, the high TMR is generally achieved only after the annealing at high temperature that promotes the crystallization of MgO. We fabricated FeCo/MgO/FeCo MTJs on both oriented and non-oriented buffer layers by DC and RF sputtering depositions at room temperature. MgO barrier layer was formed by reactive sputtering method following a very thin Mg deposition. Without annealing, 70\% TMR ratio was observed for MTJ with oriented buffer layers, whereas 40\% TMR ratio was observed with non-oriented buffer layers. This indicates that an oriented buffer layer can enhance the TMR ratio even without high temperature annealing. We also studied the dependence of TMR on the Mg layer thickness, MgO barrier thickness, and annealing conditions. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U22.00005: Effect of interface states on spin-dependent tunneling in Fe/MgO/Fe tunnel junctions Kirill Belashchenko, Julian Velev, Evgeny Tsymbal The electronic structure and spin-dependent tunneling in epitaxial Fe/MgO/Fe(001) tunnel junctions are studied using first-principles calculations.$^1$ For small MgO barrier thickness the minority-spin resonant bands at the two interfaces make a significant contribution to the tunneling conductance for the antiparallel magnetization, whereas these bands are, in practice, mismatched by disorder and/or small applied bias for the parallel magnetization. This explains the experimentally observed decrease in tunneling magnetoresistance (TMR) for thin MgO barriers. We predict that a monolayer of Ag epitaxially deposited at the interface between Fe and MgO suppresses tunneling through the interface band and may thus be used to enhance the TMR for thin barriers. [1] K. D. Belashchenko, J. Velev, and E. Y. Tsymbal, Phys. Rev. B \textbf{72}, 140404(R) (2005). [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U22.00006: The atomic and electronic structure of the FeCoB/MgO interface J.D. Burton, S.S. Jaswal, E.Y. Tsymbal, O.N. Mryasov, O.G. Heinonen Magnetic tunnel junctions (MTJs) have recently aroused much interest due to their potential applications as random access memories and magnetic field sensors. MTJs consist of a thin insulating layer separating two ferromagnetic electrodes. Very recently FeCoB/MgO/FeCoB MTJs have shown promising results. Upon annealing, the amorphous FeCoB electrodes crystallize in a bcc structure epitaxial to the MgO(001) surface. Many groups have observed a significant increase in TMR ratios (higher than 300{\%} at room temperature [1]) after annealing. It is clear that the crystallization of the electrodes plays an important role in this increase. It is not clear, however, what happens to B after annealing and what role it plays in enhancing TMR. We present results of first-principles total energy calculations that suggest that it is energetically favorable for B to reside at the crystalline FeCoB/MgO interface rather than remain in the bulk of the FeCoB electrode. We also find that the presence of B at the interface significantly weakens bonding between the FeCoB electrode and the MgO barrier. We are investigating the presence of resonant states[2] at the FeCoB/MgO interface and will discuss the effects of interfacial B on them. [1] J. Hayakawa \textit{et al}., 2005 MMM Conference. [2] Belashchenko \textit{et al}., \textit{Phys. Rev. B}~\textbf{72}, R140404 (2005). [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U22.00007: Structural and magnetic properties of Fe/MgO/Ge(100) heterostructures and Fe/MgO/Fe(100) magnetic tunnel junctions Yueh-Feng Chiang, Kyle Pi, Yan Li, Roland Kawakami MgO is one of the most attractive materials for spintronic devices due to a novel spin filtering effect that dramatically increases spin polarization. We utilize molecule beam epitaxy (MBE) to synthesize ferromagnet (FM)/MgO/semiconductor heterostructures for efficient spin injection and detection in Si, Ge and GaAs lateral devices and MgO-based magnetic tunnel junctions (MTJs) for MRAM applications. Initial studies have focused on optimizing the growth of Fe/MgO/Ge(100) heterostructures and Fe/MgO/Fe MTJs and investigating their magnetic properties. Results indicate high-quality layer-by-layer growth with roughness at the atomic scale. \textit{In situ} reflection high energy electron diffraction (RHEED) is utilized for investigating surface roughness during growth. We observe streaky RHEED patterns and intensity oscillations for the homoepitaxial growth of Ge on Ge(100) at 370$^{o}$C, which indicates an atomically flat Ge buffer layer. Epitaxial MgO layers grown on top of the Ge buffer at room temperature also showed streaky RHEED patterns and atomic force microscopy (AFM) images revealed the rms roughness to be 0.2 nm ($\sim $1 atomic layer) for a 3 nm thick MgO film. High remanance and small coercive field have been observed in epitaxial Fe (5nm)/MgO(3nm)/Ge(100) heterostructures via magneto-optic Kerr effect (MOKE) illustrating good magnetic properties. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U22.00008: XPS studies of MgO based magnetic tunnel junction structures John Read, Phil Mather, Eileen Tan, Robert Buhrman The very high tunneling magnetoresistance (TMR) obtained in MgO magnetic tunnel junctions (MTJ)$^{(1,2)}$ motivates the investigation of the electronic properties of the MgO barrier layer and the study of the ferromagnetic metal - MgO interface chemistry. Such large TMR values are predicted by theory due to the high degree of order apparent in the barrier and electrode materials. However, as grown ultra-thin MgO films generally contain defects that can influence electron transport properties through the creation of low energy states within the bulk MgO band-gap. We will report the results of x-ray photoelectron spectroscopy (XPS) studies of (001) textured ultra-thin MgO layers that are prepared by RF magnetron sputtering and electron beam evaporation on ordered ferromagnetic electrodes and in ordered MTJ structures with and without post growth vacuum annealing. XPS spectra for both MgO deposition techniques clearly indicate a surface oxygen species that is likely bound by defects in the oxide$^{(3)}$ in half-formed junctions and improvements in MgO quality after counter electrode deposition. We will discuss our results regarding the chemical properties of the oxide and its interfaces directed towards possibly providing guidance to engineer improved MgO MTJ devices. [1] S.S.P. Parkin et. al., Nature Materials, \textbf{3}, 862 (2004). [2] S. Yuasa et. al., Nature Materials, \textbf{3}, 868 (2004). [3] E. Tan et. al. , Phys. Rev. B. , \textbf{71}, 161401 (2005). [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U22.00009: Interlayer Exchange Coupling in Fe$\vert $MgO$\vert $Fe Magnetic Tunnel Junctions T. Katayama , S. Yuasa, Julian Velev, S. Jaswal, E. Tsymbal Fully epitaxial Fe$\vert $MgO$\vert $Fe(001) films with wedge-shaped MgO layer were prepared on single-crystal MgO(001) substrates using MBE technique [1]. Structure of the films is Fe-free-layer(15, 20, 30 nm)/MgO(0.3-1.8 nm)/Fe-pinned-layer(10 nm)/Ir-Mn. The interlayer exchange coupling (IEC) energy was obtained at room temperature from a unidirectional shift of the Kerr hysteresis loop. The IEC was found to be antiferromagnetic for small MgO thickness but changed sign at 8.5{\AA}. In order to explain this behavior we performed \textit{ab-initio} calculations of IEC in Fe$\vert $MgO$\vert $Fe(001) MTJs with and without oxygen vacancies in MgO. Our results show that without O vacancies the IEC is ferromagnetic and decays exponentially with MgO thickness. However, in the presence of O vacancies the IEC is antiferromagnetic for thin barriers and changes sign with increasing barrier thickness. This behavior is consistent with our experimental observations and is explained by the resonance contribution to the IEC due to localized defect states [2]. [1] S. Yuasa \textit{et al}., Nature Mater. \textbf{3}, 868 (2004). [2] M. Y. Zhuravlev \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 026806 (2005). [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U22.00010: Inelastic electron tunneling spectroscopy of MgO$_{x}$F$_{y}$ barriers Jun Hyung Kwon, Tesu Kim, Jino Lee, Kookrin Char Recent development in TMR junction with MgO barrier attracts a great deal of attention. It is reported that the junctions with MgO barrier exhibit higher TMR with lower RA value. Combined with the spin-transfer switching that has been demonstrated, the future MRAM architecture will to incorporate the MgO barrier TMR junctions. The~ device parameters for MRAM will require the RA value of about 100 $\Omega -\mu $m$^{2}$, corresponding to about 1 nm thick MgO barrier layer. In order to understand the electrical properties of MgO barrier, we have fabricated Mg/MgO/Mg tunneling junctions as the function of oxidation time of the Mg metal layer. These Mg/MgO/Mg cross-strip junctions are deposited using stencil masks without a vacuum break, and the size of junction area is about 130 $\mu $m by 160 $\mu $m. When measuring d$^{2}$I/dV$^{2}$-V, namely the inelastic tunneling spectroscopy, we observed the peaks correponding to MgO bonds, indicating that the MgO barrier is a stable and good insulator. Using the IETS measurement technique, we will present the interface properties between the ferromagnetic electrode and the MgO barrier layer.~Moreover, we will report on our MgO$_{x}$F$_{y}$ tunnel barrier made by our fluorine-doping method. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U22.00011: Spin torque and spin current in magnetic tunnel junctions Mairbek Chshiev, William H. Butler, Alan Kalitsov, Nick Kioussis In recent years, current-induced spin torque [1] has attracted strong interest both because it may advance our understanding of fundamental physics and because it may have useful applications. We present a study of non-equilibrium spin currents and the corresponding spin torques in magnetic tunnel junctions with non-collinear moments. Calculations are based on the Keldysh formalism in which the non-equilibrium Green functions are calculated within a tight-binding model using the technique of Caroli et al. [2]. The properties of spin torque and spin currents are studied as a function of applied bias, barrier thickness and lattice structure type. In addition, the exchange coupling between ferromagnetic layers in magnetic tunnel junctions is investigated via its relation to the current induced spin torque. [1] J. C. Slonczewski, J. Magn. Magn. Mat. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996); E. Myers, D. Ralph, J. Katine, R. Louie, R. Buhrman, Science, 285, 867 (1999). [2] C. Caroli, R. Combescot, P. Nozieres, D. Saint-James, J. Phys. C: Solid St. Phys., 4, 916 (1971). [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U22.00012: Shot noise in Cr-doped and undoped Co/Al$_{2}$O$_{3}$/Py magnetic tunnel junctions Farkhad Aliev, Ruben Guerrero, Patrick Le Clair, Jagadeesh Moodera We have found that shot noise in Co(80{\AA}) /Al$_{2}$O$_{3}$(14{\AA})/Py(100{\AA}) magnetic tunnel junctions (MTJs) is reduced with respect to Poisonian value. The Fano factor, obtained at frequencies (100$<$f$<$1000Hz), temperatures (T$<$10K) and biases (below 150 meV) where the shot noise dominates, varies between F $\approx $0.8 and 0.65, indicating correlated electron tunnelling. Doping of the insulating barrier with Cr inclusions suppresses the conductivity and tunnelling magnetoresistance, and restores the Fano factor to a value corresponding to uncorrelated transport (F$\approx $1). These results indicate an enhanced cross-correlation between electrons due to trapping or spin- flip assisted tunnelling in the undopped MTJs, and a possible Coulomb blockade in the Cr doped MTJs. [Preview Abstract] |
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