Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D28: Focus Session: Magnetic Tunnel Junctions |
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Sponsoring Units: GMAG DMP FIAP Chair: Tom Silva, National Institute of Standards and Technology Room: 205 |
Monday, March 2, 2015 2:30PM - 2:42PM |
D28.00001: Minimization of the energy costs for operating magnetic tunnel junctions Ilyas A. H. Farhat, E. Gale, A. F. Isakovic Increasing prospects of utilizing the STT-MRAM calls for the re-assessment of the overall energy (power) cost of operating magnetic tunnel junctions and related elements. This motivates our design, nanofabrication and characterization of simple tri-layer magnetic tunnel junctions which show measurable decrease in the operating energy cost. The MTJs we report about rely on nanoengineering interfaces between the insulating and magnetic layers in such a way that the area of the hysteresis loops can be controlled in one or both magnetic layers. Our TMR coefficient ranges from 45{\%} to 130{\%}, depending on the MTJ layer materials, and can be anticipated to be further increased. We also report the study of the TMR dependence on the RA product, as an important interface parameter. Lastly, we present an analysis of MTJ parameters affected by our approach and a perspective on further improvements, focusing on the device design parameters relevant for the integration of this type of MTJs. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D28.00002: Switching Properties of sub-100 nm Perpendicular Magnetic Tunnel Junctions Larysa Tryputen, Stephan Piotrowski, Mukund Bapna, Chia-Ling Chien, Weigang Wang, Sara Majetich, Caroline Ross Perpendicular magnetic tunnel junctions (p-MTJs) have great potential for realizing high-density non-volatile memory and logic devices. It is critical to solve scalability problem to implement such devices, to achieve low resistance area and to reduce switching current density while maintaining thermal stability. We present our recent results on fabrication of high resolution Ta/CoFeB/MgO/CoFeB/Ta p-MTJ devices and characterization of their switching properties as well as topography and current mapping by using nanoscale Conductive Atomic Force Microscopy. Our patterning method is based on using hydrogen silsesquioxane resist mask combined with ion beam etching. It allows to fabricate p-MTJ devices down to 40 nm in diameter while maintaining the magnetic quality of the multilayers. Repeatable, consistent switching behaviour has been observed in the obtained p-MTJ devices of 500 nm down to 40 nm with 10 -- 800 mV voltage applied. Switching field increased as device diameter decreased, from 580 Oe at 500 nm (MR $=$ 10{\%}) to 410 Oe at 80 nm (MR $=$ 9{\%}). We discuss the effect of device sizes on the switching properties. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D28.00003: Micromagnetic Study of Spin Transfer Induced Switching of an In-Plane Magnetized Layer with a Tilted Spin Polarization Gabriel Chaves-O'Flynn, Daniele Pinna, Georg Wolf, Andrew Kent Spin transfer torque switching can be enhanced by the addition of a spin-polarizing layer magnetized perpendicular to an in-plane magnetized free and reference layer. We present the results of zero temperature micromagnetic simulations of elliptical nanomagnets for which the spin current polarization is at an angle out of the free layer plane, between its easy and hard axis axes. We simulate the response of the free layer to a current pulse and record the relaxed state after the current is turned off. We show that, in agreement with a macrospin model, the presence of the polarizer enhances the reversal speed of the free layer, requiring lower current amplitude for switching to occur at a given time. However, for polarization tilts larger than a certain critical angle ($\theta_{crit}$), related to the free layer's shape, the magnetization starts to precess about the hard axis, which can lead a final state that is very sensitive to pulse conditions. We do a side-by-side comparison of our micromagnetics results with macrospin simulations. For small aspect ratios the simulations are consistent with the macrospin case. In larger ellipses the simulations show that out-of-plane precession is suppressed, we associate this with the excitation of non-uniform modes.\\[4pt] [1] Pinna et al PRB 2013 [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D28.00004: Assisted Writing in Spin Transfer Torque Magnetic Tunnel Junctions Samiran Ganguly, Zeeshan Ahmed, Supriyo Datta, Ernesto E. Marinero Spin transfer torque driven MRAM devices are now in an advanced state of development, and the importance of reducing the current requirement for writing information is well recognized. Different approaches to assist the writing process have been proposed such as spin orbit torque, spin Hall effect, voltage controlled magnetic anisotropy and thermal excitation. In this work,we report on our comparative study using the Spin-Circuit Approach regarding the total energy, the switching speed and energy-delay products for different assisted writing approaches in STT-MTJ devices using PMA magnets. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D28.00005: Giant thermal spin torque assisted magnetic tunnel junction switching Aakash Pushp, Timothy Phung, Charles Rettner, Brian Hughes, See-Hun Yang, Stuart Parkin Spin-polarized charge-currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer-torque (STT). Recently, by taking advantage of the spin-dependent thermoelectric properties of magnetic materials, novel means of generating spin-currents from temperature gradients, and their associated thermal-spin-torques (TSTs) have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fields of the MTJ. We attribute the origin of the TST to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Remarkably, we estimate through magneto-Seebeck voltage measurements that the charge-currents that would be generated due to the temperature gradient would give rise to STT that is a thousand times too small to account for the changes in switching fields that we observe. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D28.00006: Pulse-Current-Induced Switching of Ta/CoFeB/MgO with Perpendicular Magnetic Anisotropy Yu-ming Hung, Laura Rehm, Georg Wolf, Andrew D. Kent We study current-induced switching of thin magnetic layers with perpendicular magnetic anisotropy using in-plane currents and the spin-Hall effect in the quasi-static (swept current) and pulsed-current regimes. Our aim is to investigate the dynamics and efficiency of spin-transfer switching. The layer stacks consists of $\beta$-Ta(5nm)/Co$_{40}$Fe$_{40}$B$_{20}$(0.8nm)/MgO(2nm)/Ta(2nm) layers on oxidized silicon substrates. Hall bar structures with dimensions of $15 \times 180\,\mu m^{2}$ and cross shaped devices with width of $6 \,\mu m$ are investigated with DC transport and pulse measurement, respectively. In DC transport experiments, we could switch the magnetization states reproducibly by varying the in-plane field and current. In pulsed experiments, we measured the dependence of the switching probability on pulse amplitude and duration in the presence of an in-plane field. A histogram analysis indicates the existence of intermediate states and suggests incoherent magnetization switching. Nearly 100\% switching probability could be achieved at high enough pulse amplitude of 25.5 MA/cm$^{2}$ with 10 ns pulse duration and an applied field of $\sim$120 mT.\newline Supported by SRC-INDEX program. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D28.00007: Low frequency noise peak near magnon emission energy in magnetic tunnel junctions Liang Liu, Li Xiang, Huiqiang Guo, Jian Wei, Dalai Li, Z.H. Yuan, Jiafeng Feng, Xiufeng Han, J.M.D. Coey We report on the low frequency noise in magnetic tunnel junctions (MTJs) below 4 K and at low bias, where thermal activation from the bath is suppressed and magnon emission from hot tunneling electrons dominates the transport. For one CoFeB/MgO/CoFeB MTJ, within a narrow range of bias voltage around magnon emission energy, a Lorentzian shape noise spectra is observed. For one CoFeB/AlO$_{x}$/CoFeB MTJ, at similar bias voltage but within much wide bias range, a much larger Lorentzian shape noise spectra is observed and random telegraph noise is visible in the time traces. In both cases the Lorentzian spectra eventually disappears after repeated measurements, which in combination of the fitted parameters suggests magnon-assisted activation of defects as its origin. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D28.00008: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 4:06PM - 4:18PM |
D28.00009: Iterative and self-consistent quantification of nonlocal spin valves with low-resistance oxide barriers Yunjiao Cai, Yongming Luo, Chao Zhou, Chuan Qin, Shuhan Chen, Yizheng Wu, Yi Ji The standard method of quantifying nonlocal spin valves is to assume an exponential decay of the spin signal as a function of the channel length. Then the spin diffusion length and the spin polarization can be extracted via fitting. However, this method does not distinguish between the injection polarization and the detection polarization. In addition, the assumption that the channel length is the only varying parameter may not be always valid. In this work, a large number (\textgreater 50) of Py-AlOx-Cu NLSV structures on a single substrate are investigated. The standard fitting of exponential decay is initially performed but appears unsatisfactory. We then assume an additional dependence of the spin polarization on the size of the Py/AlOx/Cu junctions, and normalize the spin signals by using the actual junction sizes in individual structures. By feeding the parameters iteratively, we are able to collapse all normalized spin signals on an exponential decay curve with good correlation. The injection/detection polarization strongly depends on the size of the Py/AlOx/Cu junctions. The coexistence of large Cu resistivity and long spin diffusion length points to interesting mechanism of spin relaxation. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D28.00010: Inelastic electron tunnelling and noise spectroscopies in organic magnetic tunnel junctions with PTCDA barrier Farkhad Aliev, Isidoro Martinez, Jhen-Yong Hong, Juan Pedro Cascales, Pablo Andres, Minn-Tsong Lin The influence of internal barrier dynamics on spin, charge transport and their fluctuations in organic spintronics remains poorly understood. Here we present inelastic electron tunnelling spectroscopy (IETS) and low frequency noise (LFN) studies in magnetic tunnel junctions with thin (1.2-5nm) organic PTCDA barriers in the tunnelling regime at temperatures down to 0.3K. Shot noise is superpoissonian with a Fano factor exceeding in 1.5-2 times the maximum values reported for magnetic tunnel junctions with inorganic barriers, indicating spin dependent bunching in tunneling [1]. IETS results show energy relaxation of tunneling electrons through the excitation of collective (librons) and internal (phonons) vibrational modes of the molecules. The bias dependence of the normalised 1/f noise studied up to 350mV reveals that the excitation of some phonon modes has a strong impact on LFN with over a 10-fold reproducible increase near some specific biases. The dependence of the IETS and LFN anomalies with the relative magnetic alignment of the electrodes will also be discussed. [1] J.P.Cascales, et al., submitted to Applied Physics Letters [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D28.00011: Direct Observation of Magnetoresistance Variation in Molecular Junctions Induced by Electrode Geometry Xiangmin Fei, Guangfen Wu, Vanessa Lopez, Gang Lu, Hong-Jun Gao, Li Gao Spin-polarized electron transport in the Co/C60/Co/Ni molecular junctions, in which the fullerene (C60) molecule is in electrical contact with electrodes, has been investigated using an ultra-high vacuum cryogenic scanning tunneling microscope (STM). By combining spin-polarized STM and current-displacement measurements, the spin-polarized contact conductance of molecular junctions has been measured at 5 K. Large tunnel magnetoresistance (TMR) values higher than -60{\%} have been observed. Depending on electrode geometry, the measured TMR values vary by a factor of $\sim$ 1.5. The atomic-scale geometry of the electrode apex strongly impacts the spin-polarization of the electrodes and that of the interfacial hybrid molecular states. Our findings suggest that atomic-scale engineering of electrodes represents a new and effective approach to tuning the magnetotransport in molecular spintronic devices. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D28.00012: Memristive behavior in tunnel junctions with graphene oxide barrier Mirko Rocci, Ana Perez-Mu\~noz, Javier Del Valle, Jose Luis Vicent, Carlos Leon, Zouhair Sefrioui, Jacobo Santamaria, Francesco Perrozzi, Luca Ottaviano, Michele Nardone, Sandro Santucci, Emanuele Treossi, Vincenzo Palermo Resistive switching in Graphene Oxide (GO) structures has shown its potential for future nonvolatile memory applications. Here we report on GO (2-20 layers thick) as tunnel barriers in combination with half-metallic La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ (LSMO) manganites, Ag, and Ni as electrodes. Hybrid LSMO/GO/Ag junctions show a memristive-like behavior with more than 5 orders of magnitude resistance change (between high and low states) at low temperature. We explain the resistance switching in terms of (redox) generation of oxygen vacancies at the GO metal interfaces and their diffusion through the GO layer under the large applied electric fields (10$^{\mathrm{8}}$ V/m). Magnetic tunnel junctions fabricated with Ni (instead of Ag) show a significant tunnelling magnetoresistance (TMR) combined with the nonvolatile memristor response. The sign of the TMR changes from positive to negative upon resistive switching of the GO. We interpret the sign inversion as due to changes in the Ni surface bonding state occurring as the result of the oxygen accumulation (depletion) at its surface. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D28.00013: ABSTRACT MOVED TO G14.00004 |
Monday, March 2, 2015 5:06PM - 5:18PM |
D28.00014: Fluorine Functionalized BNNT as a Spin Filter Kamal Dhungana, Ranjit Pati Spin filtering is a phenomenon that allows one to generate spin-polarized carriers in a circuit comprised of a magnetic channel sandwiched between two non-magnetic electrodes. In recent years, the quest for a novel low-dimensional metal-free magnetic channel that would exhibit both magnetism at a higher temperature and excellent spin filtering property has been intensively pursued. Herein, using a first-principles approach, we study the magnetic property of fluorine functionalized boron nitride nanotube (F-BNNT). A long range ferromagnetic spin ordering is found to occur in the F-BNNT at temperature much above the room temperature. Our spin polarized transport study shows that the fluorine functionalization in BNNT not only enhances its conductance by more than two orders, which is in excellent agreement with the experimental report, but also makes it a perfect spin filter. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D28.00015: Antiferromagnet controlled tunneling anisotropic magnetoresistance Cheng Song, Yuyan Wang, Feng Pan We investigate tunneling anisotropic magnetoresistance (TAMR) in antiferromagnets (AFM)-based junctions, where Co/Pt magnetization drives partial rotation of AFM moments with the formation of exchange-spring [1]. The existence of exchange-spring is further confirmed by element specified x-ray magnetic dichroism [2]. Because of superior thermal tolerance of perpendicular exchange coupling and the stability of moments of $\sim$ 6 nm-thick IrMn in [Pt/Co]/IrMn/AlO$_{\mathrm{x}}$/Pt junctions, TAMR gets significantly enhanced up to room-temperature [1]. The TAMR behavior in [Pt/Co]/IrMn/AlO$_{\mathrm{x}}$/metal junctions is insensitive to the top metal electrodes [3]. The situation turns out to be different when the top electrode is replaced by AFM. TAMR is observed in IrMn/AlOx/IrMn junctions, where the resistance states are governed by the relative arrangement of the AFM moments adjacent to AlO$_{\mathrm{x}}$ [4]. Our findings would advance the process towards practical AFM spintronics. \\[4pt] [1] Y. Y. Wang, et al. Phys. Rev. Lett., 109, 137201 (2012).\\[0pt] [2] Y. Y. Wang, et al. New J. Phys., in press.\\[0pt] [3] Y. Y. Wang, et al. Appl. Phys. Lett., 103, 202403 (2013).\\[0pt] [4] Y. Y. Wang, et al. Adv. Funct. Mater. doi: 10.1002/adfm.201401659. [Preview Abstract] |
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