### Session X14: Focus Session: Spin-Polarized Transport

 Friday, March 9, 2007 8:00AM - 8:36AM X14.00001: Tuning the Exchange Bias in Spin Valves by an Electric Current Invited Speaker: Maxim Tsoi An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted [1] that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. This talk will discuss our recent experiments [2] that demonstrate the transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density ($\sim$1012 A/m2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias could be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications. [1] A. S. Nunez, R. A. Duine, P. Haney, and A. H. MacDonald, Phys. Rev. B 73, 214426 (2006). [2] Z. Wei et al., cond-mat/0606462. Friday, March 9, 2007 8:36AM - 8:48AM X14.00002: Spin transfer in exchange biased magnetic nanopillars Nickolas Anthony , Sergei Urazhdin We present a study of the effect of current on the magnetic state of nanopatterned ferromagnetic/antiferromagnetic bilayers. We show that the magenetic state of the antiferromagnet can be affected by a high current density. First, the exchange bias can be altered by applying a pulse of current. The change is accompanied by an increase of coercivity. Additionally, the magnetic anisotropy depends on the value and the direction of the applied current. Our findings cannot be explained by the Joule heating, and indicate that a spin transfer effect similar to that previously demonstrated for ferromagnets is also responsible for the current-induced effects in nanopatterned antiferromagnets. Friday, March 9, 2007 8:48AM - 9:00AM X14.00003: Specific Resistance and Scattering Asymmetry of Py/Pd, Fe/V, Fe/Nb, and Co/Pt Interfaces Amit Sharma , Tony Romero , Nikoleta Theodoropoulou , Reza Loloee , William Pratt Jr. , Jack Bass The properties of interfaces between normal (N) and ferromagnetic (F) metals, described by specific resistance, AR* (A = area, R = resistance), and scattering asymmetry, $\gamma$, are of interest to optimize current-perpendicular-to-plane (CPP) magnetoresistance (MR) and current-induced magnetization-switching (CIMS) in nanopillars. Sputtered standard Py/Cu, Co/Cu, and Fe/Cr interfaces have 2AR* $\sim$ 1 f$\Omega$m$^{2}$ and $\gamma \quad \sim$ 0.7. Recently, sputtered F/Al interfaces with F = Py, Co, Fe, and Co(91)Fe(9) were found to have very large 2AR$^{\ast }$ $\sim$ 9 f$\Omega$m$^{2}$, but small $\gamma \quad \le$ 0.1 [1]. In hopes of finding interfaces with both large 2AR* and larger $\gamma$ than for F/Al, we have determined 2AR* and $\gamma$ at 4.2K for sputtered Py/Pd, Fe/V, Fe/Nb, and Co/Pt pairs, where we've matched crystal structures of the F and N metals. We will present our data and our derived values of 2AR* and $\gamma$. [1] N. Theodoropoulou et al., J. Appl. Phys. \textbf{99}, 08G502 (2006); \textit{ibid}., IEEE Trans. on Magn. (Submitted). Friday, March 9, 2007 9:00AM - 9:12AM X14.00004: Current-perpendicular-to-plane magnetoresistance of multilayered Co/Cu nanocolumns by scanning tunneling microscope P. Morrow , A. Kar , X. Tang , T. Parker , G. -C. Wang , T. -M. Lu , J. Y. Dai In this work we present a method to measure the current-perpendicular-to-plane magnetoresistance (CPP-MR) of a small number of multilayered nanocolumns using a nonmagnetic STM. Samples were grown on Au-coated Si substrates by oblique angle thermal deposition from separate Co and Cu sources. We set the layer thicknesses and column lengths at 5-15 nm and 200-700 nm, respectively. SEM images show column diameters of about 25-100 nm, and the multilayer structure is confirmed by EELS and HR-TEM. VSM analysis gives coercivities of a few tens to several hundred G. In the MR measurement, mechanical contact was established between the STM tip and a small number of as-deposited nanocolumns, and dynamic hysteresis loops of resistance vs. magnetic field (up to 2.5 kG) were then collected in air at room temperature. The observed MR ratio for most samples was on the order of 1{\%}, which is posited to be due to the same physical mechanism as the GMR effect, but with significantly less efficacy. Cautions in the experiment and factors that may facilitate higher MR are also discussed. Friday, March 9, 2007 9:12AM - 9:24AM X14.00005: Large Magnetoresistance in Co/Ni/Co Ferromagnetic Single Electron Transistors Ruisheng Liu , Hakan Pettersson , Lukasz Michalak , Carlo Canali , Dmitry Suyatin , Lars Samuelson We report on magnetotransport investigations of Co/Ni/Co ferromagnetic single electron transistors, fabricated using a high-precision alignment procedure invoked during e-beam writing. As a result of reduced size, the devices exhibit single-electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal TMR traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18{\%} is observed within a small source-drain bias regime. The TMR decreases rapidly with increasing bias, which we primarily attribute to the excitation of magnons in the central island. Friday, March 9, 2007 9:24AM - 9:36AM X14.00006: Ballistic Anisotropic Magnetoresistance in Electrodeposited Co Nanocontacts Andrei Sokolov , Chunjuan Zhang , Evgeny Y. Tsymbal , Jody Redepenning , Evgeny Kiriranov , Bernard Doudin As dimensions of a metallic conductor is reduced, spin-dependent conductance quantization in units of $e^{2}$/$h_{ }$leads to unusual magnetoresistive phenomena. One of them is ballistic anisotropic magnetoresistance (BAMR), a quantized change in the ballistic conductance according to the direction of magnetization. Here we present a first observation of BAMR in Co electrodeposited nanocontacts by \textit{in-situ }investigation of their spin-dependent transport properties. We compare the results from electrochemically synthesized and break junction contacts. By measuring the conductance as a function of the applied magnetic field direction at saturation, we find the step-wise variation of the ballistic conductance, signature of the BAMR effect. Our results show that BAMR can be positive and negative, and have symmetric and asymmetric angular dependence. This behavior is explained using a simple tight-binding model in terms of the effect of the spin-orbit interaction on the electronic band structure of nanocontacts. Friday, March 9, 2007 9:36AM - 9:48AM X14.00007: ABSTRACT WITHDRAWN Friday, March 9, 2007 9:48AM - 10:00AM X14.00008: First principles calculations of anisotropic magnetoresistance in ferromagnetic nanocontacts D. Jacob , J. Fernandez-Rossier , J.J. Palacios Here we present {\it ab initio} transport calculations of ferromagnetic nanocontacts [1] including the spin-orbit (SO) coupling for the very first time. Due to the SO coupling the conductance of the nanocontact changes with the direction of the magnetization giving rise to the so-called anisotropic magnetoresistance (AMR). We investigate the magnitude of the AMR effect while going from the atomic contact regime (BAMR) [2] to the tunneling regime (TAMR) [3]. Our work is motivated by recent experiments on ferromagnetic nanocontacts [4,5] which report much larger AMR values than those usually obtained for bulk materials in agreement with recent electronic structure calculations of ideal monatomic Ni chains [2].\\ {\bf References:}\\ $[1]$ D. Jacob, J. Fern\'andez-Rossier, and J. J. Palacios, Phys. Rev. B {\bf 71}, 220403(R) (2005).\\ $[2]$ J. Velev et al., Phys. Rev. Lett. {\bf 94}, 127203 (2005).\\ $[3]$ L. Brey, C. Tejedor, and J. Fern\'andez-Rossier, Appl. Phys. Lett. {\bf 85}, 1996 (2004).\\ $[4]$ M. Viret et al., Eur. Phys. J. B {\bf 51}, 1 (2006).\\ $[5]$ K. I. Bolotin and F. Kuemmeth, and D. C. Ralph, Phys. Rev. Lett. {\bf 97}, 127202 (2006). Friday, March 9, 2007 10:00AM - 10:12AM X14.00009: Superconductivity suppression by ferromagnetism in bi- and tri-layers of La$_{0.7}$Ca$_{0.3}$MnO$_3$\ ferromagnets and high-T$_c$\ YBa$_2$Cu$_3$O$_{7-\delta}$ Norbert M. Nemes , Flavio Y. Bruno , Mar Garcia-Hernandez , Axel Hoffmann , Suzanne G. E. te Velthuis , Cristina Visani , Carlos Leon , Zouhair Sefrioui , Jacobo Santamaria Large magnetoresistance occurs in F/S/F trilayers of highly spin-polarised La$_{0.7}$Ca$_{0.3}$MnO$_3$\ ferromagnet and high-T$_c$\ superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ for antiferromagnetic (AF) alignment of the manganite layers. We discuss the relative importance of spin diffusion across the superconductor, proximity effect at the F/S interface and stray fields due to domain walls of the ferromagnet based on magnetoresistance and magnetisation measurements of bilayers and trilayers of various thickness of superconductor and ferromagnet. Friday, March 9, 2007 10:12AM - 10:24AM X14.00010: Spin injection and imbalence in ferromagnet/ superconductor/ ferromagnet double tunnel junctions. Hyunsoo Yang , See-Hun Yang , Stuat Parkin The interplay between magnetism and superconductivity can be explored in double tunnel junctions (DTJs) with a superconducting (SC) middle electrode, CoFe/ MgO/ Al/ MgO/ CoFe, prepared using metal shadow masks and magnetron sputter deposition. Due to the strong competition between magnetism and superconductivity induced by the accumulation of spin polarized electrons in the SC, the superconducting gap is reduced with increasing bias voltage for anti-parallel alignment of the two ferromagnetic electrodes, as predicted theoretically [1]. We find that a large inverse (or negative) TMR is observed around gap energy and almost negligible TMR at zero bias. [1] S. Takahashi, H. Imamura, and S. Maekawa, Phys. Rev. Lett. \textbf{82}, 3911--3914 (1999). Friday, March 9, 2007 10:24AM - 10:36AM X14.00011: Cooper pair phase oscillation in thin Al superconductor induced by effective Zeeman splitting from spin injection Guo-xing Miao , Jochem Nietsch , Jagadeesh Moodera By placing a superconductor (S) and a ferromagnet (F) in close contact, the superconductivity proximity effect induces transient Cooper pairs in F leading to FFLO [1,2] state, while the ferromagnetic proximity effect will populate the S region with non-equilibrium parallel spins. In our experiment, the spins are induced from both sides of the superconductor symmetrically through thin Al$_2$O$_3$ tunnel barriers. By toggling the two F layers between parallel and anti-parallel, we can effectively turn on and off the spin imbalance in the Al layer creating $>$ 1000\% MR. The Tc of Al layer is shifted between parallel and antiparallel states as a net result of the non-equilibrium spin population. Such Tc shift is observed to oscillate with Al layer thickness, which is a clear evidence that the effective Zeeman splitting caused by parallel spin population can also induce FFLO states in superconductors. The CPP conductance in such a structure also show dramatic difference between the two states. 1. P. Fulde and R.A. Ferrel, Phys. Rev. 135, A550 (1964) 2. A. I. Larkin and Y. N. Ovchinnikov, Sov. Phys. JEPT 20, 762 (1965) Friday, March 9, 2007 10:36AM - 10:48AM X14.00012: ABSTRACT WITHDRAWN