Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z32: Focus Session: Magnetoresistance and Spin-dependent Transport |
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Sponsoring Units: GMAG DMP FIAP Chair: Kiran Thadani, Cornell University Room: 336 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z32.00001: Nitrogen doping in single-crystal MgO magnetic tunnel junctions. Justin Brockman, Cheng-han Yang, Mahesh Samant, Kevin Roche, Stuart Parkin Recent experiments have shown evidence for induced ferromagnetism in thin films of carbon and nitrogen-doped zinc oxide. We have discovered similar behavior in nitrogen-doped MgO films grown by plasma-assisted thermal evaporation. Here, we incorporate these films as tunneling barriers into single-crystal multilayer magnetic tunnel junctions and present experimental results showing the magnetoresistance and current-voltage characteristics for these structures. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z32.00002: Bias dependence of Fe-MgO-Fe magnetic tunnel junction devices within a single-band tight-binding model Tehseen Raza, Hassan Raza We have developed a transferable single-band tight-binding model benchmarked with the ab initio methods for Fe-MgO-Fe magnetic tunnel junction (MTJ) devices [1]. The computational complexity of our model is on the order of an effective mass one, but additionally it includes the bandstructure physics over the two-dimensional transverse Brillouin zone in an average manner. We study the bias dependence of the tunnel magnetoresistance (TMR) ratio in MTJ devices. At low bias, for both the 4-layer and 12-layer MgO barrier, the TMR is bias-independent. It is higher for the 12-layer device due to relatively a larger decrease in the AP current density. At high bias, our model predicts a sharp roll-off in TMR ratio, which is attributed to a rapid increase in the Delta{\_}1 band current density in the anti parallel (AP) configuration due to the bandedge states entering the conduction window. The TMR ultimately becomes negative when the AP current becomes higher than the P current due to the different k-states tunneling through the same barrier. [1] T. Z. Raza and H. Raza, arXiv:0804.2557 [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z32.00003: Magnetoresistance in Double Spin Filter Tunnel Junctions with Nonmagnetic Electrodes and Its Unconventional Bias Dependence Guo-Xing Miao, Martina Muller, Jagadeesh Moodera We demonstrate a large tunnel magnetoresistance (TMR) originating purely from the tuning of tunnel barrier heights in double barrier junctions with nonmagnetic electrodes. This is achieved by the spin filtering that occurs due to the selective tunneling probabilities for spin-up and -down electrons through a magnetic semiconductor barrier resulting in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to a TMR without the necessity of magnetic electrodes. This is significantly different from traditional approaches for generating TMR involving two ferromagnetic electrodes and for example, using Al2O3 or MgO barriers. We demonstrate the first realization of such unconventional tunnel junctions and TMR using EuS / EuO based spin filter barriers with nonmagnetic Al electrodes. The novel non-monotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z32.00004: The critical role of the barrier thickness in spin filter tunneling Casey Miller Spin filter tunneling is considered in the low bias limit as functions of the temperature dependent barrier parameters. We demonstrate the generation of spin polarized tunneling currents in relation to the magnetic order parameter, and discuss how an interfacially suppressed order parameter leads to a temperature dependent tunneling current asymmetry. Analyzing the full parameter space reveals that the often overlooked barrier thickness plays a critical role in spin filter tunneling. With all else fixed, thicker barriers yield higher spin polarization, and allow a given polarization to be achieved at higher temperatures. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z32.00005: Spin-Dependent Electronic Transport in Fe/MnAs/Fe Structures Kyung-Yeon Kim, Hyoung Joon Choi We have performed first-principles calculations of electronic structures and tunneling magnetoresistance of Fe/MnAs/Fe structures, which are junctions of two semi-infinite metals and a half-metal placed between them. The electronic structures are described by the Kohn-Sham density functional theory, with local spin density approximation, norm-conserving semicore pseudopotentials, and pseudo-atomic orbital basis set. The tunneling magnetoresistance is obtained by using a scattering- state method, considering different configurations of the magnetization. This work was supported by the KRF (KRF-2007-314- C00075) and by the KOSEF Grant No. R01-2007-000-20922-0. Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z32.00006: Spin dependent transport in FeCo$\vert $MgBO$\vert $FeCo magnetic tunnel junctions: Can boron in the oxide region be a good thing? Derek Stewart Recent experimental studies on FeCoB/MgO/FeCoB tunnel junctions have shown that boron can diffuse into the oxide region during rf-sputtering and result in the formation of crystalline MgBO regions[1,2]. These tunnel junctions still provide high tunneling magnetoresistance values as well as very low RA products[3]. Using a plane wave-pseudopotential density functional approach, I have examined potential Mg(B) oxides such as Mg$_{2}$B$_{2}$O$_{5}$ (both monoclinic and triclinic) as well at kotoite (Mg$_{3}$B$_{2}$O$_{6})$. Total energy calculations indicate that these oxides should be more stable than the formation of separate regions of MgO and B$_{2}$O$_{3}$. Kotoite (Mg$_{3}$B$_{2}$O$_{6})$ also has a boron concentration close to that found in the experimentally grown MgBO regions. In addition, kotoite provides a good lattice match with MgO and could act to template neighboring FeCo into crystalline bcc layers during annealing. This evidence suggests that kotoite is formed during the deposition process. I will also discuss the complex band structure of kotoite (Mg$_{3}$B$_{2}$O$_{6})$ and examine how this will also affect spin dependent transport from the FeCo leads. [1] J. Y. Bae \textit{et al.}, J. Appl. Phys. \textbf{99} 08T316 (2006) [2] J. C. Read \textit{et al.}, Appl. Phys. Lett. \textbf{90} 132503 (2007) [3] J. C. Read \textit{personal communication} [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z32.00007: Effect of thermal spin disorder on transport through magnetic tunnel junctions Aleksander Wysocki, Kirill Belashchenko We study the transport properties of Fe$_{1-x}$Co$_{x}$MgOFe$_{1-x}$Co$_{x}$ magnetic tunnel junction in the presence of spin disorder using the noncollinear density functional theory. For a given temperature the spin disorder in ferromagnetic leads is introduced by randomizing the directions of spin densities in atomic spheres according to the mean-field angular distribution function. For pure FeMg0Fe we found that even small spin disorder has a dramatic effect on transmission as compared to the zero temperature case due to the presence of interface states in the minority spin channel that are strongly affected by spin disorder. This results in a complicated temperature dependence of the tunneling magnetoresistance (TMR). On the other hand, in the case of Fe$_{1-x}$Co$_{x}$MgOFe$_{1-x}$Co$_{x}$ the interface states are less important and the main effect of the spin disorder is to decrease the spin polarization diminishing TMR. The temperature dependence of TMR is in agreement with Julliere model prediction with the spin polarization being proportional to magnetization. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z32.00008: Evolution of low-frequency resistance noise during annealing in CoFeB/MgO/CoFeB tunnel junctions Ryan Stearrett, Weigang Wang, Lubna Shah, Edmund Nowak, John Xiao We have studied the evolution of tunneling magnetoresistance (TMR) and resistance noise in magnetic tunnel junctions (MTJs) as a function of annealing time at 425$^{o}$C. Previously, we showed that short annealing times do lead to significant improvement in the MgO crystal structure and crystallization of the CoFeB electrodes, resulting in large TMR values up to 200{\%}. We also observe that the low-frequency resistance noise decreases significantly after annealing for only a few minutes. The resistance noise has a 1/f spectrum and is quantified by a Hooge-like parameter, $\alpha $, given in units of $\mu $m$^{2}$. In unannealed samples $\alpha $ is of order 10$^{-9} \quad \mu $m$^{2 }$and decreases with increasing voltage bias. Upon annealing, $\alpha $ drops to 10$^{-10} \quad \mu $m$^{2 }$and is less dependent on bias, particularly in the parallel configuration. We attribute the decrease in $\alpha $ and its bias dependence, $\alpha $(V), to a reduction of defects in and around the barrier due to annealing. The implications for optimizing the signal to noise ratio of MgO-based MTJ sensors will also be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z32.00009: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z32.00010: Current-Perpendicular-to-Plane Magnetoresistance of CoFe-based Exchange-Biased Spin-Valves Chiyui Ahn, Kyung-Ho Shin, Reza Loloee, Jack Bass, William Pratt Concentrated Co(50)Fe(50) and Co(70)Fe(30) alloys are of interest for spintronics applications. We have constrained the spin-transport properties of these alloys by measuring at 4.2 K the specific resistance (CPP area times resistance) and magnetoresistance of exchange-biased spin-valves (EBSVs) of the form (FeMn/CoFe/Cu/CoFe)---here CoFe indicates one of the two alloys of interest---where the magnetization of one CoFe layer is exchange bias pinned by the adjacent antiferromagnetic FeMn layer, and the magnetization of the other CoFe layer is free to switch from parallel (P) to anti-parallel (AP) to that of the pinned layer in a modest magnetic field. For each CoFe alloy, we have measured EBSVs where the thicknesses of both the fixed and free layers were held equal and varied together, and EBSVs where the thickness of the pinned CoFe layer was held fixed and that of the free layer was varied. From such measurements we have estimated two parameters: the bulk scattering asymmetry and the spin-diffusion length, for each of the two CoFe alloys. We will present both our data and the derived parameters. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z32.00011: Specific Resistance of Pd/Ir Interfaces Rakhi Acharyya, Hoang Yen Thi Nguyen, Reza Loloee, William P. Pratt Jr., Jack Bass, Shuai Wang, Ke Xia In electronic transport with current-flow perpendicular to the layer planes (CPP) of a metallic multilayer, the interface specific resistance AR (area A through which CPP-current flows times sample resistance R) is fundamental. Special interest focuses upon AR for metals M1 and M2 with the same crystal structure, and lattice parameters the same to within $\sim $ 1 percent, as AR can then be calculated with no free parameters. From measurements of the total AR of sputtered Pd/Ir multilayers, we obtain twice the interface specific resistance, 2AR$_{Pd/Ir}$ = 1.02 $\pm $ 0.06 f$\Omega$ m$^{2}$. For a single fcc structure with average lattice parameter of Pd and Ir, calculations including only spd orbitals give for perfect interfaces, 2AR$_{Pd/Ir}$(Perf) = 1.21 $\pm $ 0.1 f$\Omega $m$^{2}$, and for interfaces composed of two monolayers of a random 50{\%}-50{\%} alloy, 2AR$_{Pd/Ir }$(50/50) = 1.22 $\pm $ 0.1 f$\Omega $m$^{2}$. These values fall just outside the range of the experimental value. Upgrading to include f-orbitals gives 2AR$_{Pd/Ir}$(Perf) = 1.1 $\pm $ 0.1 f$\Omega $m$^{2}$ and 2AR$_{Pd/Ir}$(50-50) = 1.15 $\pm $ 0.1 f$\Omega $m$^{2}$. Within mutual uncertainties, these values are compatible with the experimental one. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z32.00012: Enhancement of Biquadratic Coupling in Co/Fe/MgO/Fe(001) Jared Wong, Yueh-Feng Chiang, Andrew Hoff, Xiaojing Tan, Yan Li, Keyu Pi, Wei Hau Wang, Harry Tom, Roland Kawakami One interesting aspect of the interlayer exchange coupling (IEC) across MgO is that in addition to the bilinear coupling, a biquadratic coupling favoring 90\r{ } magnetization alignment has been observed [1, 2, 3]. We investigate IEC in Co/Fe/MgO/Fe(001) and the affects of non-ideal aspects of the MgO-heterostructure, such as interface oxidation and impurities in the MgO, via molecular beam epitaxy (MBE) synthesis and magneto-optic Kerr effect (MOKE) measurements across wedged samples. By independently varying the oxygen content of the MgO film and the Fe/MgO interface, we find that the biquadratic coupling is correlated to the interfacial oxidation. Furthermore, the temperature dependence of the biquadratic coupling exhibits a strong increase at low temperatures. Our findings strongly support the loose spin mechanism as the origin of the biquadratic coupling across MgO[4]. 1. J. Faure-Vincent, C. Tiusan, C. Bellouard, et al., Phys. Rev. Lett. 89, 107206 (2002). 2. T. Katayama, S. Yuasa, J. Velev, et al., Appl. Phys. Lett. 89, 112503 (2006). 3. E. Snoeck, P. Baules, G. BenAssayag, et al., J. Phys.: Cond. Mat. 20, 055219 (2008). 4. J. C. Slonczewski, J. Appl. Phys. 73, 5957 (1993). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z32.00013: Magnetic tunnel junctions with a ferroelectric barrier using epitaxial La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$trilayers Shengming Guo, Ke Chen, Xiaoxing Xi, Qi Li, Yonggang Zhao We have fabricated multiferroic tunnel junctions using ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$as electrodes and ferroelectric (Ba, Sr)TiO$_{3}$ as the barrier. We have observed tunneling magnetoresistance as in a typical magnetic tunnel junction (MTJ). Since the ferroelectric barrier can be charge polarized in two opposite directions which alters tunneling conductance, we have observed large tunneling electroresistance ($\sim $ 50{\%}) when the charge polarization is switched. This has been observed for both magnetic parallel and antiparallel states. As a result, this type of junctions has four resistance states instead of two for a normal MTJ, corresponding to positive- and negative-polarized parallel and antiparallel states. The four states can be manipulated by the magnetic and electric fields. The dependence of the magnetoresistance and electroresistance as functions of magnetic field, electrical field, and bias voltage will be presented. [Preview Abstract] |
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