Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L31: Focus Session: Spin Transport and Exchange Bias in Nanostructures |
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Sponsoring Units: GMAG DMP FIAP Chair: Samir Lounis, Forschungszentrum Jülich Room: 335 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L31.00001: Spin Injection and Accumulation in Metallic Lateral Spin Valves with Transparent Contacts Invited Speaker: Creation and control of spin currents is a key ingredient in spintronics, which has as a goal the use of both the spin and charge of the electron. Ferromagnetic (FM)/non-magnetic (NM) lateral spin valves are powerful devices that decouple a pure spin current from an electrical current by using a non-local geometry. We will review previous works to show how the FM/NM interface and materials control in an essential way the generation and manipulation of a spin current in non-local spin valves (NLSV). For this reason, we have studied the electrical spin injection and spin accumulation in metallic NLSV with transparent interfaces as a function of important experimental parameters such as injection current direction and magnitude, temperature, materials, and thickness. Using injected DC currents we find that the spin injection is perfectly symmetric when injecting current \textit{from} the FM or \textit{into} the FM, causing exactly the opposite spin accumulation in the NM. This provides means for a pure electrical manipulation of the spin current polarity. The change in spin accumulation with increasing injected current is produced by a temperature raise of the device due to Joule heating and confirmed by independent spin accumulation measurements as a function of temperature. Comparing experimentally measured spin accumulation in NMs with a spin-diffusion model allows us to identify the effect of surfaces on the spin diffusion length and injection efficiency, and the effect of FM electrodes on spin accumulation. These experiments have important implications for the physics of NLSV and for the development of devices based on these phenomena. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L31.00002: Large Room-Temperature Resistive Switching Behavior in Spinel Structured Nanoparticle Compacts. Tae Hee Kim, Eun Young Jang, Nyun Jong Lee, Jung-Tak Jang, Jin-Sil Choi, Jinwoo Cheon, Kyung-Jin Lee Here we report an abrupt and large bipolar switching behavior in the form of nanoparticle assembly consisting of an infinite number of monodispersed magnetic oxide single-crystallines. In the assembly of magnetite nanoparticles with size below 10 nm, we observed a room temperature current-voltage hysteresis with an abrupt and large bipolar resistive switching (switching ratio of $\sim $ 2000 {\%}). We also found that such switching behaviors can be general phenomena for nanoparticle assemblies: not limited to magnetites but also consistently observed for other kinds of spinel structured nanoparticles with different compositions. Such a huge switching phenomenon it has never been observed before in bulk powders, particularly at room temperature. Our results showed clearly that the new I-V hysteresis is dependent on the nanoparticle size, and arises from interparticle contacts. In an effort to understand and interpret the origin of the bipolar reversible switching behavior, a new theoretical model was suggested in this work. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L31.00003: Self-Assembly and Tunneling Magneto Resistance of Magnetic Nanoparticle Superlattices Chaitanya Lekshmi Indira, Concetta Nobile, Raffaella Bounsanti, Eliana D'Amone, Davide Cozzoli, Giuseppe Maruccio Template assisted self-assembly of magnetic oxide nanostructures into systematically ordered superlattices in presence of magnetic field can offer controlled interfaces and useful properties for the fabrication of magnetically engineered tunnel junctions with application in high performance magnetic random access memories. In our work we employ magnetite, an important class of half-metallic material showing super paramagnetic behavior close to room temperature and valuable coercivity at low temperatures, as nanoparticles. The self-assembly of nanoparticle superlattices on metallic non-magnetic substrates is demonstrated. Further, enhanced spin-dependent electron transport and tunneling magneto resistance in devices with crossbar geometry is discussed. \begin{enumerate} \item M. P. Pileni, J. Phys. D: Appl. Phys. 41, 134002 (2008). \item Z. M. Liao et al. Nano Lett. 6, 1087 (2006). \item K. Yakushiji et al. Nat. Mater. 4, 57 (2005). \end{enumerate} [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L31.00004: Conductive atomic force microscopy measurements of nanopillar magnetic tunnel junctions E. R. Evarts, C. Hogg, J. A. Bain, S. A. Majetich Magnetic tunnel junctions have been studied extensively for their magnetoresistance and potential uses in magnetic logic and data storage devices, but little is known about how their performance will scale with size. Here we examined the electronic behavior of 12 nm diameter magnetic tunnel junctions fabricated by a novel nanomasking process. Scanning electron microscopy images indicated feature diameter of 12 nm, and atomic force microscopy showed a height of 5 nm suggesting that unmasked regions have been milled on average to the oxide barrier layer, and areas should have the remnants of the free layer exposed with no remaining nanoparticle. Electrical contact was made to individual nanopillars using a doped-diamond-coated atomic force microscopy probe with a 40 nm radius of curvature at the tip. Off pillar we observed a resistance of 8.1 x 10$^{5}$ $\Omega $, while on pillar we found a resistance of 2.85 x 10$^{6} \quad \Omega $. Based on the RA product for this film, 120 $\Omega -\mu $m$^{2}$, a 12 nm diameter cylinder with perfect contact would have a resistance of 1.06 x 10$^{6} \quad \Omega $. The larger experimental value is consistent with a smaller contact area due to damaging the pillar during the ion milling process. The magnetoresistance characteristics of these magnetic tunnel junctions will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L31.00005: Spin filtering in transport through single-molecule magnet Mn12 Kyungwha Park, Salvador Barraza-Lopez, Jaime Ferrer, Victor Garcia-Suarez We investigate electronic transport through a single-molecule magnet Mn12 in a two-terminal set up using the Green's function method in conjunction with density-functional theory. Our transport calculation will provide crucial information on the effect of interfaces and molecular geometry on transport, and complement theoretical studies based on many-body model Hamiltonians. We consider a single-molecule magnet Mn12 bridged between Au electrodes via thiol group and alkane chains such that its magnetic easy axis is normal to the transport direction. The electrodes are treated semi-infinite and the transport calculation is performed self-consistently within density-functional theory. We present a spin-filtering effect through Mn12 and coupling strength of the Mn12 and electrodes. We also discuss the effect of additional electron correlations on the transport. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L31.00006: Antiferromagnetic exchange coupling measurements on single Co clusters W. Wernsdorfer, D. LeRoy, C. Portemont, A. Brenac, R. Morel, L. Notin, D. Mailly We report on single-cluster measurements of the angular dependence of the low-temperature ferromagnetic core magnetization switching field in exchange-coupled Co/CoO core-shell clusters (4 nm) using a micro-bridge DC superconducting quantum interference device ($\mu$-SQUID). It is observed that the coupling with the antiferromagnetic shell induces modification in the switching field for clusters with intrinsic uniaxial anisotropy depending on the direction of the magnetic field applied during the cooling. Using a modified Stoner-Wohlfarth model, it is shown that the core interacts with two weakly coupled and asymmetrical antiferromagnetic sublattices. Ref.: C. Portemont, R. Morel, W. Wernsdorfer, D. Mailly, A. Brenac, and L. Notin, Phys. Rev. B 78, 144415 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L31.00007: The exchange bias effect in Ni/NiO and NiO nanoparticles Angela Kou, Mikhail Feygenson, Lauren Kreno, Jonathan Patete, Amanda Tiano, Fen Zhang, Stanislaus Wong, Meigan Aronson We used magnetic measurements, X-ray diffraction, and HRTEM to study the exchange bias field in Ni/NiO and NiO nanoparticles made by a modified wet chemistry method. We oxidized re-dispersed powders of bare Ni nanoparticles in air at 400$^{o}$C and 900$^{o}$C. HRTEM showed that annealing at 900$^{o}$C of bare Ni nanoparticles led to the formation of exceptionally high quality NiO nanoparticles, resembling perfect bulk-like crystalline order. To our knowledge, there are no reports of NiO particles of such quality in the literature. The loop shift was 1000 Oe at 300K for the NiO nanoparticles, while it was only 120 Oe at 10K for the Ni/NiO nanoparticles. The difference is explained by the different origins of the loop shift in Ni/NiO and NiO nanoparticles. In Ni/NiO nanoparticles, the loop shift is associated with exchange interactions between ferromagnetic Ni and antiferromagnetic NiO. In NiO nanoparticles, however, the origin of the shift is an uneven number of ferromagnetic sublattices present in NiO nanoparticles, which interact differently with an applied magnetic field (Kodama, 1999). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L31.00008: Correlation between bias fields and magnetoresistance in CoPt biased FeNi/Ta/FeNi GMR heterosystems Yi Wang, S. Sahoo, W. Echtenkamp, Ch. Binek Exchange coupled magnetic hard layer (HL)/ soft layer (SL) thin films show SL biasing in close analogy to conventional exchange bias systems with antiferromagnetic pinning.$^{1}$ Here we study CoPt(35nm)/FeNi450nm/Ta(d)/FeNi450nm heterostructures with d between 0.7 and 5nm. The CoPt films have in-plane magnetic anisotropy and pin the adjacent FeNi SL films. The latter are exchange coupled from top via Ta spacer layers with FeNi in a GMR-type architecture. We use AGFM and SQUID magnetometry to study the FeNi magnetization reversal with (CoPt) and without (vacuum) pinning layer proximity. The two minor FeNi hysteresis loops of the GMR trilayer reveal different biasing effects due to the distinct exchange interaction at the respective interfaces. The FeNi/CoPt coupling is systematically tuned via a series of set fields which allow partial demagnetization of the pinning layer. Moreover we study the correlation between the overall and minor magnetization reversals and the corresponding magnetoresistance effects for various temperature between T=20 and 400K. $^{1}$Ch. Binek, S. Polisetty, Xi He and A. Berger, Phys. Rev. Lett. \textbf{96}, 067201 (2006). Financial support by NSF through Career DMR-0547887, MRSEC DMR-0820521 and the NRI. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L31.00009: From Exchange Bias to Magnetic Memory Karine Chesnel, Steve Kevan, Eric Fullerton, Matt Carey, Jeff kortright, Brian Wilcken, Joseph Nelson A better understanding and control of magnetic domain morphology and reversal processes in magnetic thin films is useful in the realm of perpendicular magnetic recording technology. We found the possibility to create magnetic domain memory in thin ferromagnetic films by inducing a spatially varying exchange coupling interactions. We evidenced this phenomenon in a perpendicular exchange bias film made of [Co/Pd] IrMn mutilayers. Our coherent X-ray magnetic scattering speckle correlation study shows that the film exhibits no memory at room temperature but acquires a very high degree of magnetic memory, above 95{\%} with subsequent field cycling when the sample is zero field cooled below the blocking temperature of the IrMn layers (T$<$275K). [1]. We present here the memory's dependency with magnetic field, temperature, and cooling conditions. We also discuss the spatial dependency by analyzing finely the local speckle correlation as a function of scattering angle, thus indicating variations of memory with different spatial scales in the domain pattern. [1] Chesnel et al, PRB, 78, 132409 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L31.00010: Detection of Bottom Electrode Oxidation in Magnetic Tunnel Junctions via Exchange Bias Effect Wei Chen, Nam Dao, Kevin West, David Kirkwood, Jiwei Lu, Stuart Wolf The oxidation of the bottom ferromagnetic (FM) electrode in a magnetic tunnel junction (MTJ) is detrimental for high tunneling magnetoresistance (TMR). This has long been a tricky problem for the fabrication of MTJs. We propose a method to detect such oxidation by measuring the exchange bias effect from the CoO/FM system if the FM surface is oxidized and CoO is formed. Along with the moderate exchange bias even more significant training effect and increased FM coercivity are observed at low temperature that depend on the oxidation level. All of these effects help in the detection of the FM surface oxidation. MTJs with MgO and vanadium oxide as tunnel barrier candidates are tested by this technique with the purpose of optimizing the barrier quality for best TMR performance. [Preview Abstract] |
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