Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B29: Spin-Dependent Transport |
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Sponsoring Units: GMAG Chair: Sergei Urazhdin, West Virginia University Room: 333 |
Monday, March 16, 2009 11:15AM - 11:27AM |
B29.00001: Spin dependent scattering in all Heusler alloy CPP-GMR nano-structures for magnetic storage applications Oleg Mryasov, Konstantin Nikolaev, Thomas Ambrose At the reduced sensor dimensions necessary for high density magnetic recording, the lower impedance of current-perpendicular- to the plane giant magnetoresistance (CPP-GMR) based read heads likely will take over the currently used tunneling MR based sensors. Main obstacle on the of realizing this transition is relatively low amplitude of the conventional CPP-GMR stacks. In this work, we investigate all-Heusler CPP-GMR spin-valves . The combination of alloys has been chosen in order to match spin states majority channel[1]. We focus on fundamentals of spin dependent interface and bulk scattering as it is affected by substitution disorder in these ternary alloys. Ab-initio electronic structure calculations employed to account for complex band structure of these ternary. We investigate interface scattering contribution within a simple model relying on the electronic band structure calculated for bulk of ferromagnetic and non-magnetic component of all Heusler CPP GMR tri-layer. We use model to investigate how to minimize impact of disorder induced states. Experimentally, these structures have been realized using conventional sputter deposition techniques and found to exhibit significant interface scattering contribution to MR signal. [1] T. Ambrose and O. Mryasov, United States Patent 6,876,522 (April 5, 2005); T. Ambrose, O. Mryasov, ``\textit{Growth and Magnetotransport Properties of Thin Co}$_{2}$\textit{MnGe Layered Structures}'', Springer Verlag Series. 2005. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B29.00002: Sub-gap cotunneling current through a spinfull quantum dot with superconducting leads Jens Paaske, Brian Andersen, Karsten Flensberg A number of recent experiments have measured the voltage driven current through a quantum dot with superconducting leads. By changing the gate voltage of the dot, several unusual properties of the sub-gap current have been revealed. For example, when the dot is occupied by an odd number of electrons, the cotunneling conductance exhibits regions of negative differential conductance as well as significant weight redistributions among the multiple Andreev scattering resonances. We have calculated the sub-gap cotunneling current within a general Hamiltonian approach, allowing us to treat the even occupied (spinless) dot exactly, and the odd occupied (spinfull) dot perturbatively. In the latter case, we present calculations of unusual sub-gap current and relate these finding to the experiments. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B29.00003: Orientation and temperature dependence of the anomalous Hall effect in hcp cobalt Ivo Souza, Eric Roman, Yuriy Mokrousov We calculate from first-principles the evolution of the intrinsic anomalous Hall conductivity vector $\vec\sigma^a$ of hcp Co as the spin magnetization direction $\hat{\bf M}$ is tilted away from the $c$-axis. We find that $\vec\sigma^a$ varies smoothly with the tilt angle $\theta$, and that its magnitude is strongly reduced, by a factor of about four, between $\theta=0$ and $\theta=\pi/2$, in good agreement with the measured anisotropy ratio of about three.\footnote{N. V. Volkenshtein {\it et al.}, Fiz. Metal. Metalloved. {\bf 11}, 152 (1961).} In addition to the anisotropic linear magnetization dependence ($\sigma^a_z/M_z\not=\sigma^a_x/M_x$) expected for any uniaxial crystal, there is a considerable nonlinearity in the dependence of $\sigma_x^a$ on $M_x=M\sin\theta$, while the relation between $\sigma_z^a$ and $M_z=M\cos\theta$ is essentially linear, as in ${\rm Mn}_5{\rm Ge}_3$.\footnote{C. Zeng {\it et al.}, Phys. Rev. Lett. {\bf 96}, 037204 (2006).} The overall angular dependence of $\vec\sigma^a$ is well-described by an expansion in terms of $l=1$ and $l=3$ spherical harmonics. From Zener's model for the influence of thermal fluctuations of $\hat{\bf M}({\bf r})$ on the temperature dependence of magnetic anisotropies,\footnote{C. Zener, Phys. Rev. {\bf 96}, 1335 (1954).} we predict that the $l=3$ terms give rise to an appreciable increase with temperature of the anisotropy ratio. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B29.00004: Determination of spin polarization of amorphous ferromagnet CoFeB by Point-Contact Andreev Reflection Sunxiang Huang, Tingyong Chen, Chia-Ling Chien Amorphous ferromagnet CoFeB plays a key role in spintronic devices. Larger tunneling magnetoresistance (TMR) is resulted when amorphous CoFeB is incorporated into either AlO$_{x}$ or MgO magnetic tunnel junctions (MTJs) than those with CoFe. The critical switching current density in spin transfer torque devices with CoFeB as the free layer is significantly less than that with NiFe. The TMR of MgO-based MTJs are also noticeably different using Co$_{20}$Fe$_{60}$B$_{20}$ and Co$_{40}$Fe$_{40}$B$_{20}$. These phenomena indicate a substantial spin polarization of CoFeB, whose value and compositional dependence should be determined. We have recently determined the spin polarization of amorphous Co$_{x}$Fe$_{80-x}$B$_{20 }(x$=20, 40, 60) using the point-contact Andreev reflection technique [1]. The spin polarization of amorphous CoFeB has been found to be as high as 65{\%}. In contrast to the large enhancement of TMR during crystallization of CoFeB in MgO-based MTJs, the spin polarization of crystallized CoFeB is in fact much reduced. Very recent theoretical studies [2] using density functional theory indicate an enhanced spin polarization in amorphous CoFeB, in good agreement with our measurements. [1] S. X. Huang \textit{et al}., APL, \textbf{92}, 242509 (2008). [2] P.V. Paluskar \textit{et al.,} PRL, \textbf{100}, 057205 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B29.00005: Structural and orientation dependence of the anomalous Hall effect in cobalt crystals Eric Roman, Ivo Souza Co undergoes a structural phase transition at $660$~K from a ferromagnetic hcp phase to a ferromagnetic fcc phase. We present a first-principles study of the anomalous Hall conductivity (AHC) in hcp, fcc, and fct cobalt crystals. We find that the AHC in the fcc phase is about half that of the hcp phase, in good agreement with experiment. \footnote{I.A. Tsoukalas, Phys. Stat.Sol. (a) {\bf 23}, K41 (1974).} By expressing the AHC as the Kramers-Kronig transform of the magnetic circular dichroism (MCD) spectrum, we relate the change in the AHC to differences between the infrared MCD spectra of the two phases. In particular, there is a large, negative spin-flip contribution in fcc, which is absent in hcp. We also describe the effect of tetragonal distortions on the AHC of Co, by studying its evolution along the fcc-bcc Bain path. These distortions alter the dependence of the AHC on the magnetization direction, an effect which is also significant for the orbital moment. \footnote{ O. Hjortstam, K. Baberschke, J. M. Wills, B. Johannson, and O. Eriksson, Phys. Rev. B \textbf{55}, 15026 (1997). } [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B29.00006: Measurement of the transport spin polarization of disordered metals using point-contact Andreev reflection M. Osofsky, G. Woods, J. Sanders, H. Srikanth, S. Kolesnik, T. Maxwell, B. Dabrowski Point contact Andreev reflection (PCAR) studies were done on bulk, polycrystalline SrRu$_{1-x}$(TM)$_{x}$O$_{3}$ (TM = Cr, Mn, Ti) and SrRu$_{0.92}$O$_{3}$ with a high degree of disorder. The curves are typical of many other materials studied, except that the conductance is not constant at large voltages as is commonly observed in most metals. This result is most likely due to the effect of disorder on the density of states (DOS) that produces the well known square-root of V anomaly. After the problem of the V$^{1/2 }$behavior at large V was eliminated by proper normalization, the PCAR spectra for samples were analyzed using the modified BTK model. Pure SrRuO3 undergoes ferromagnetic ordering at a Curie temperature of $T$C $\sim $ 160 K and has a relatively high spin polarization ($\sim $0.6). Our results indicate that, when the lattice is disordered from either the presence of Ru lattice site defects or the substitution of a transition metal for the Ru, the curie temperature, $T_{C}$ changes by a factor of two while the spin polarization is almost unchanged. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B29.00007: Anomalous Hall effect in Y$_2$Fe$_{17-x}$Co$_x$ single crystals Jolanta Stankiewicz, Konstantin Skokov We study experimentally the Hall resistivity of Y$_2$Fe$_{17-x}$Co$_x$ single crystals ($x\leq 8$) for wide temperature and applied magnetic field ranges, and for various magnetic field orientations with respect to the easy-magnetization axis. We find a large anomalous Hall effect (AHE) anisotropy in this system for $x\leq 2$. The AHE resistivity $\rho_{xy}$, measured with an applied magnetic field $H\perp {\it c}$-axis, is nearly one order of magnitude larger than the one for $H$ along the hard magnetization direction ($H\parallel {\it c}$-axis). Furthermore, the former is very large and varies linearly with the longitudinal resistivity $\rho$, whereas the latter follows $\rho^2$ for $T < 150$ K. We tentatively interpret the behavior of $\rho_{xy}$ for $H\parallel \it c$-axis in terms of an intrinsic effect related to the inter-orbital hopping between degenerate {\it d}-orbitals. Such hopping is allowed for high symmetry points at the crystallographic dumb-bell sites in this configuration. On the other hand, there is no inter-orbital hopping for $H\perp \it c$-axis. However, a huge amplitude of the AHE resistivity for this configuration, which follows from skew scattering, is puzzling. Both the AHE anisotropy and the large skew scattering go away as more Fe is substituted by Co. We attribute this to variations in the electronic structure of the Y$_2$Fe$_{17-x}$Co$_x$ system when Co atoms start to occupy the dumb-bell crystallographic sites. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B29.00008: Signal propagation in time-dependent spin transport Yao-Hui Zhu, Hans Christian Schneider Signal propagation in magnetic multilayers is studied using a macroscopic theory of time-dependent spin transport. Our analysis shows that time-dependent spin transport possesses a wave-diffusion duality, i.e., it is wave like for fast signal modulation and reduces to the diffusion equation for slow modulation [1]. The wave-like characteristics allow us to extract a finite spin signal-propagation velocity, which cannot be done using the spin diffusion equation. Applications to different switching scenarios for collinear and noncollinear spin transport through magnetic multilayers will be discussed.\newline [1] Y.-H. Zhu, B. Hillebrands, H. C. Schneider, Phys.\ Rev.\ B \textbf{78}, 054429 (2008) [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B29.00009: The Anomalous Hall Effect in Ultra-Thin Amorphous CNi$_3$ Films Yimin Xiong, Philip Adams We present anomalous Hall effect(AHE) measurements in ultra-thin CNi$_3$ films. Films with sheet resistances in the range $R\ll $$R_Q$ to $R\sim R_Q$ were studied in fields up to 9 T and temperatures down to 2 K. We find that in addition to scattering processes, the AHE in high resistance films is strongly influenced by disorder-enhanced electron-electron interaction effects. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B29.00010: Electronic structure and spin-filter effect of $\gamma$-Fe$_{2}$O$_{3}$ (maghemite) Hiroyoshi Itoh, Syuta Honda, Jun-ichiro Inoue, Hideto Yanagihara, Eiji Kita We theoretically study the electronic structure and spin-dependent transport of spinel-like $\gamma$-Fe$_{2}$O$_{3}$ (maghemite) which is one of candidates for spin-filter devices. By performing first principles calculations (GGA+$U$) for iron vacancy ordered Fe$_{64}$O$_{96}$, the spin-dependent band-gap of the maghemite in the ferrimagnetic insulating ground state is determined. It is also shown that excess of Fe and O atoms significantly affects on the band gap of the minority spin state. In the light of obtained electronic structure, the spin-filter effect and variable range hopping of the maghemite are discussed. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B29.00011: Scanning Probe Microscopy of Quantum Chains Israel Mandel, Gabriel Cwilich, Fredy Zypman Quantum strongly correlated spin systems exhibit energy spectra related to their underlying classical dynamics. For integrable systems, the level-spacing is Poissonian, while for chaotic dynamics, it follows a Wigner-Dyson distribution, corresponding to localized and metallic phases [1,2] Linear spin chains and braids have been considered recently [3] showing that a certain degree of disorder or the presence of impurities along the chain induces a transition from one statistic to the other. We consider a quantum Heisenberg system of two perpendicular linear chains (T-shaped) interacting through the points of closest approach between them. The motivation is to model the tip of a Magnetic Force Microscope, the separation between tip and sample playing the role of an impurity (a different hopping parameter). We study the level-spacing statistics of the whole system as a function of tip-sample separation, observing a transition in level statistics, inducing a change in the total energy, detectable by monitoring the force between tip and sample. We propose a tool to search for signatures of an integrable-to-chaotic transition by looking at the force-separation curves in Scanning Probe Microscopy. [1] Shklovskii B I, Shapiro B, Sears B R, Lambrianides P and Shore H B 1993 \textit{Phys. Rev. }B \textbf{47 }11487 [2] Braun D, Montambaux G and Pascaud M 1998 \textit{Phys. Rev. Lett. }\textbf{81 }1062 [3] Lea F. Santos, Phys Rev E78, 031125 (2008) [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B29.00012: Kondo decoherence: finding the right spin model for iron impurities in gold and silver J. von Delft, T.A. Costi, L. Bergqvist, A. Weichselbaum, T. Micklitz, A. Rosch, P. Mavropoulos, P. Dederichs, F. Mallet, L. Saminadayar, C. B\"auerle We exploit the decoherence of electrons due to magnetic impurities, studied via weak localization, to resolve a longstanding question concerning the classic Kondo systems of Fe impurities in the noble metals gold and silver: which Kondo-type model yields a realistic description of the relevant multiple bands, spin and orbital degrees of freedom? Previous studies suggest a fully screened spin $S$ Kondo model, but the value of $S$ remained ambiguous. We perform density functional theory calculations that suggest $S = 3/2$. We also compare previous and new measurements of both the resistivity and decoherence rate in quasi 1-dimensional wires to numerical renormalization group predictions for $S=1/2,1$ and 3/2, finding excellent agreement for $S=3/2$. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B29.00013: Electronic Interactions between Au Films and the Prussian Blue Analog Co$_{3}$[Os(CN$_{6})$]$_{2}$ T. Wellington, M. Hilfiger, A. Ford, C. Avendano, K. Dunbar, W. Teizer The Prussian blue analog Co$_{3}$[Os(CN$_{6})$]$_{2}$ exhibits photoinduced changes of magnetic behavior as well as charge transfer induced spin transitions at low temperature. Magnetic measurements on the bulk material show an increased magnetic susceptibility after illumination with red light, as the analog exhibits an abrupt spin transition due to enhanced cooperativity. We are exploring electronic interactions between this Prussian blue analog and gold films of varying thickness. Low-temperature measurements of the magnetoresistance of the gold films, with and without a surface layer of the analog, are performed. The study focuses on how the presence of the analog on the surface affects the transport properties within the gold film. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B29.00014: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 2:03PM - 2:15PM |
B29.00015: Enhancement of positive magnetoresistance at the metamagnetic transition in Tb$_{5}$Si$_{3}$ E.V. Sampathkumaran, S. Narayana Jammalamadaka, Niharika Mohapatra It is well-known that, in the antiferromagnetic systems, at the metamagnetic transition field (H$_{t})$, negative MR [defined as {\{}$\rho $(H)-$\rho $(0){\}}/$\rho $(0) where $\rho $ is the electrical resistivity] is observed. Here we present evidence for the opposite behavior of MR in an intermetallic compound, viz., Tb$_{5}$Si$_{3}$, known to form in Mn$_{5}$Si$_{3}$-type hexagonal structure (space group: P6$_{3}$/mcm). We observe a field-induced ferromagnetic transition in the magnetically ordered state (T$_{N}$= 69 K). The key experimental finding is that, below T$_{N}$, $\rho $ is dramatically enhanced at all temperatures (resulting in large ``positive'' MR) at H$_{t}$, in sharp contrast to the expectation based on common knowledge. This finding bears significant relevance to electron scattering phenomena in general, and in particular in metamagnetic systems. [Preview Abstract] |
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