Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N14: Focus Session: Spin-dependent Tunneling and High Magnetoresistance Devices |
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Sponsoring Units: DMP FIAP GMAG Chair: See-Hun Yang, IBM Almaden Research Center Room: 316 |
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N14.00001: Optimizing Co concentration in Co$_{x}$Fe$_{1-x}$/MgO/Co$_{x}$Fe$_{1-x}$ magnetic tunnel junctions to maximize tunneling magnetoresistance Jonathan Trinastic, Yan Wang, Hai-Ping Cheng Fe$_{1-x}$Co$_{x}$ /MgO magnetic tunnel junctions (MTJs) are of great experimental interest due to large differences in conductance between parallel and anti-parallel spin alignment in the electrodes that lead to high tunneling magnetoresistance (TMR). However, the optimal Co concentration in the electrodes that maximizes TMR is still under investigation (Bonell et al 2012,\textit{PRL,}108,176602). Using first-principles calculations, we compare the conductance and TMR of Fe$_{1-x}$Co$_{x}$/MgO junctions using 1) disordered electrodes modeled with the virtual crystal approximation (VCA), and 2) ordered Fe$_{0.75}$Co$_{0.25}$ and Fe$_{0.50}$Co$_{0.50}$ electrodes. For disordered electrodes, we find that the optimal Co concentration varies between 20 and 30 percent and TMR decreases with MgO barrier thickness. For ordered electrodes, pure Co electrodes exhibit the highest TMR for a thin MgO barrier; however, Fe$_{0.75}$Co$_{0.25}$ electrodes demonstrate the highest TMR for a thicker MgO barrier, replicating recent experimental results. In all cases, a decrease in anti-parallel transmission drives the TMR increase. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N14.00002: Large Magnetoresistance of MnBi/Bi/MnBi Spin Valve Nabil Al-Aqtash, Khaldoun Tarawneh, Renat Sabirianov Recently, a large transport spin polarization was demonstrated in MnBi films by Andreev reflection experiments [1]. Furthermore, a large magnetoresistance (MR) of 70{\%} was observed in MnBi junctions at room temperature [2]. Because of this, a spin-valve MnBi/Bi/MnBi device is promising to have large MR that can be controlled by the varying the thickness of Bi spacer. Thin films of Bi show a semimetal-semiconductor transition at reduced thicknesses. Bismuth itself shows a substantial MR and a large mean free path of electron. In this system both the electrodes and the spacer have a hexagonal unit cell. A transport magetoresistance of MnBi/Bi(6 layers)/MnBi film was calculated using density functional theory coupled with nonequilibrium Green's function method as implemented in SIESTA code. The calculations display a high transport spin polarization of MnBi. A transmission MR of the spin valve around 77{\%} is calculated, consistent with the previous experimental observation of a large magnetoresistance in MnBi contacts. Thus, MnBi is promising candidate for high MR devices with tunable spacer properties. [1] P. Kharel, P. Thapa, P. Lukashev, R. F. Sabirianov, E. Y. Tsymbal, D. J. Sellmyer, and B. Nadgorny, Phys. Rev. B 83, 024415 (2011) [2] E. Clifford, M. Venkatesan and J. M. D. Coey, J. Mag. Magn. Mater. 272-276, 1614(2004). [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N14.00003: Self-Organized Defects of Half-Metallic Nanowires in MgO-Based Magnetic Tunnel Junctions Masayoshi Seike, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida The purpose of this study is to examine the possibility of self-organization of defects and defect-induced properties in MgO-based magnetic tunnel junctions (MTJs). Using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, first-principles calculations were performed to estimate the electronic structures and total energies of MgO with various defects. From our thorough evaluation of the calculated results and previously reported experimental data, we propose that self-organized half-metallic nanowires of magnesium vacancies can be formed in MgO-based MTJs. This self-organization may provide the foundation for a comprehensive understanding of the conductivity, tunnel barriers and quantum oscillations of MgO-based MTJs. Further experimental verification is needed before firm conclusions can be drawn.\\[4pt] References:\newline [1] K. Sato et al.: Rev. Mod. Phys. 82, 1633 (2010).\newline [2] M. Seike et al.: Jpn. J. Appl. Phys. 50, 090204 (2011). \newline [3] M. Seike et al.: Physica B 407, 2875 (2012).\newline [4] M. Seike et al.; Jpn. J. Appl. Phys. 51, 050201 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N14.00004: Mechanisms of perpendicular magnetic anisotropy and interlayer exchange coupling in MgO-based tunnel junctions Invited Speaker: Mairbek Chshiev Magnetic tunnel junctions (MTJ) comprising ferromagnetic (FM) electrodes with MgO spacer have been an object of high interest for spintronics due to Bloch states symmetry spin filtering leading to high tunnel magnetoresistance (TMR) [1] and due to observation of antiferromagnetic (AF) coupling between FM electrodes across MgO spacer [2]. This attention have been strongly reinforced in a view of a huge interest in MTJs with perpendicularly magnetized magnetic layers (p-MTJs) originating from large values of interfacial perpendicular magnetic anisotropy (PMA) first observed at Pt\textbar Co\textbar MOx interfaces (M$=$Ta, Mg, Al, Ru\textellipsis ) [3,4] and later reported for Co\textbar MgO [4,5] and CoFeB\textbar MgO p-MTJs [6]. In this talk we will elucidate mechanisms responsible for the PMA from first-principles [7] and report the effect of interfacial oxidation conditions on the PMA in Fe(Co)\textbar MgO p-MTJs. In particular, we found very large PMA values for MTJs with pure interfaces in agreement with recent experiments [4,6]. Furthermore, it will be demonstrated that oxidation conditions strongly affect the PMA which strongly correlates with TMR in agreement with experiments [7,8]. Finally, we will discuss the origin of AF coupling in Co\textbar MgO p-MTJs which oscillates as a function of FM layer thickness in agreement with theories of interlayer exchange coupling in MTJ [5].\\[4pt] [1] W.H. Butler et al, \textit{Phys. Rev. B} 63,054416(2001); \textit{IEEE Trans. Magn. }41,2645(2005).\\[0pt] [2] J. Faure-Vincent et al, \textit{Phys. Rev. Lett.} 89,107206(2002); T. Katayama et al, \textit{Appl. Phys. Lett.} 89,112503(2006); H.-X. Yang et al, \textit{Appl. Phys. Lett.}~ 96,262509(2010).\\[0pt] [3] S. Monso et al, \textit{Appl. Phys. Lett.} 80,4157(2002); B. Rodmacq et al, \textit{J. Appl. Phys.} 93,7513(2003).\\[0pt] [4] L. Nistor et al, \textit{Appl. Phys. Lett.} 94,012512(2009).\\[0pt] [5] L. Nistor et al, \textit{IEEE Trans. Magn.} 45,3472(2009); \textit{Phys. Rev. B} 81,220407(2010).\\[0pt] [6] S. Ikeda et al, \textit{Nature Mat.,} 9,271(2010).\\[0pt] [7] H.-X. Yang et al, \textit{Phys. Rev. B}~84,054401(2011).\\[0pt] [8] L. Nistor et al, \textit{IEEE Trans. Magn.} 46,1412(2010). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N14.00005: Point-contact study of soft magnetic CoSiBFeNb amorphous alloys Heidi Seinige, Cheng Wang, Valerii Tsoi, Maxim Tsoi We study magnetotransport in nanoscale point contacts to soft magnetic CoSiBFeNb ribbons. Such ultrasoft amorphous alloys attracted considerable attention previously because they exhibit Giant Magnetoimpedance (GMI) effect - large variations in the electrical impedance as a function of an external magnetic field [see, for instance, M.-H. Phan, H.-X. Peng, Prog. Mater. Sci. 53, 323 (2008) and references therein]. GMI is attributed to the field-induced variations in alloy permeability and has been established through ac measurements on bulk samples which revealed a strong dependence on ac frequency and amplitude but did not show any variations in dc resistance at all. In our experiments, we use nanocontacts to probe magnetotransport in amorphous CoSiBFeNb at the nanoscale. We use point contacts to inject both ac and dc currents into the alloy ribbons prepared by a melt-spinning technique. Measurements with ac currents revealed GMIs similar to those in macroscopic samples. Interestingly, we also observe a dc magnetoresistance which may be attributed to magnetic domain reorientations in a small contact region. Effects of high dc densities on the magnetoresistance are discussed in terms of spin-transfer torque (STT) effect. We thank A. Serebryakov for providing ribbon samples. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N14.00006: Measurement of the transport spin polarization of Ag/Fe$_3$O$_4$ bilayers using point-contact Andreev reflection Michael Osofsky, Priyanga Jayathilaka, Casey W. Miller The development of point-contact Andreev reflection (PCAR) has provided a relatively simple method for determining the spin polarization of various ferromagnetic materials. This technique utilizes point contact tunneling from a superconducting tip into a ferromagnet (FM) as a probe of the spin-polarization of the FM. Quantitative information can be extracted from the conductance data through a modified Blonder, Tinkham, Klapwijk (BTK) model of supercurrent conversion at a superconductor-metal interface (Andreev reflection) which includes the spin-polarization of the normal metal. The 100{\%} spin polarized oxide, Fe$_{3}$O$_{4}$, which is insulating at low temperature, is of great interest for spintronics applications. In order to use PCAR to measure the spin polarization of this system, it is necessary to provide a conducting layer. In this talk we will describe the results of PCAR measurements of Ag/Fe$_{3}$O$_{4}$ bilayers as a function of Ag thickness. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N14.00007: 3D Effect in Determination of Spin Polarization using Andreev Reflection Spectroscopy Jessica Gifford, Charles Snider, Jonny Martinez, Tingyong Chen Andreev Reflection Spectroscopy (ARS) has been utilized to measure spin polarization of magnetic materials, as well as the superconducting gap of superconductors. These values are extracted by a modified Blonder-Tinkham-Klapwijk (BTK) model or the more recent Chen-Tesanovic-Chien (CTC) model. Both consider the F/S interface as one dimensional (1D). However, a tip may have a point angle with three dimensional (3D) effects. We present both theoretical and experimental studies of the 3D effects in the determination of spin polarization. We have found that for an ideal interface without interfacial scattering (Z), the 3D ARS spectra are the same as 1D spectra. But for non-ideal interfaces the 3D effect can drastically change the conductance spectra depending on the point angle of the tip. The 3D spectra can be well described by the 1D model with a different interfacial scattering factor and a slightly different inelastic scattering factor. The spin polarization and superconducting gap is the same as the1D model, demonstrating that 1D ARS model can be utilized to determine spin polarization as long as Z is not of any concern. Finally, we apply the both the 1D and the 3D models to a set of ARS data and show that the extracted spin polarization value is the same for both models. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N14.00008: Graphene-based magnetic tunnel junctions Invited Speaker: Enrique Cobas Graphene's in-plane transport has been widely researched and has yielded extraordinary carrier mobilities of 10$^{5}$ cm$^{2}$/Vs and spin diffusion lengths of exceeding 100$\mu $m. These properties bode well for graphene in future electronics and spintronics technologies. Its out-of-plane transport has been far less studied, although its parent material, graphite, shows a large conductance anisotropy. Recent calculations [1,2] show graphene's interaction with close-packed ferromagnetic metal surfaces should produce highly spin-polarized transport out-of-plane, an enabling breakthrough for spintronics technology. In this work, we fabricate and measure FM/graphene/FM magnetic tunnel junctions using CVD-grown single-layer graphene. The resulting juctions show non-linear current-voltage characteristics and a very weak temperature dependence consistent with charge tunneling transport. Furthermore, we study spin transport across the junction as a function of bias voltage and temperature. The tunneling magnetoresistance (TMR) peaks at two percent for single-layer graphene junctions and exhibits the expected bias asymmetry and a temperature dependence that fits well with established spin-polarized tunneling models. [3] Results of mutli-layer graphene tunnel junctions will also be discussed.\\[4pt] References:\\[0pt] [1] Karpan et al., Phys. Rev. Lett. 99, 176602, 2007.\\[0pt] [2] Yazyev and Pasquarello, Phys. Rev. B. 80, 035408, 2009.\\[0pt] [3] Cobas et al., Nano Letters 12, 3000, 2012. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N14.00009: Magnetic Tunnel Junctions with a Graphene Tunnel Barrier Wan Li, Daniel Ralph We have fabricated ferromagnet/graphene/ferromagnet (FM/Gr/FM) junctions in which current flows vertically so that the graphene acts as a single-atom-thick barrier. In contrast to previous work, we utilize a fabrication process that avoids oxidation of the magnetic electrodes. We measure typical resistance-area products significantly lower than previously reported. We will present an analysis of whether this difference is due to the absence of a magnetic oxide or to defects in the graphene. We will also discuss the magnetoresistance of these devices, and how both the resistance and the magnetoresistance depend on the quality of the graphene. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N14.00010: Magnetoresistance and negative differential resistance in Ni/Graphene/Ni vertical heterostructures driven by finite bias voltage: A first-principles study Kamal K. Saha, Anders Blom, Kristian S. Thygesen, Branislav K. Nikolic Using the nonequilibrium Green function formalism combined with density functional theory, we study finite-bias quantum transport in Ni/Gr$_n$/Ni vertical heterostructures where $n$ graphene layers are sandwiched between two semi-infinite Ni(111) electrodes. We find that recently predicted pessimistic magnetoresistance of 100\% for $n \ge 5$ junctions at zero bias voltage $V_b \rightarrow 0$, persists up to $V_b \simeq 0.4$ V, which makes such devices promising for spin-torque-based device applications. In addition, for parallel orientations of the Ni magnetizations, the $n=5$ junction exhibits a pronounced negative differential resistance as the bias voltage is increased from $V_b=0$ V to $V_b \simeq 0.5$ V. We confirm that both of these nonequilibrium transport effects hold for different types of bonding of Gr on the Ni(111) surface while maintaining Bernal stacking between individual Gr layers. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N14.00011: Large spin accumulation due to spin-charge coupling across a break-junction Shuhan Chen, Han Zou, Siu-Tat Chui, Yi Ji We investigate large spin signals in break-junction nonlocal spin valves (NLSV). The break-junction is a nanometer-sized vacuum tunneling gap between the spin detector and the nonmagnetic channel, formed by electro-static discharge. The spin signals can be either inverted or non-inverted and the magnitudes are much larger than those of standard NLSV. Spin signals with high percentage values (10{\%} - 0{\%}) have been observed. When the frequency of the a.c. modulation is varied, the absolute magnitudes of signals remain the same although the percentage values change. These observations affirm the nonlocal nature of the measurements and rule out local magnetoresistive effects. Owing to the spin-charge coupling across the break-junction, the spin accumulation in a ferromagnet splits into two terms. One term decays on the charge screening length (0.1 nm) and the other decays on the spin diffusion length (10 nm nm). The magnitude of the former is proportional to the resistance of the junction. Therefore a highly resistive break-junction leads to a large spin accumulation and thereby a large spin signal. The signs of the spin signal are determined by the relationship between spin-dependent conductivities, diffusion constants, and density of states of the ferromagnet. [Preview Abstract] |
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