Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X12: Focus Session: Anomalous Hall Effect and Spin Orbit Coupling |
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Sponsoring Units: GMAG DMP FIAP Chair: Partho Mitra, Pennsylvania State University Room: Colorado Convention Center Korbel 3C |
Friday, March 9, 2007 8:00AM - 8:36AM |
X12.00001: A Tunable Anomalous Hall effect in a Non-Ferromagnetic System Invited Speaker: The anomalous Hall effect (AHE) is a ubiquitous signature of ferromagnetism that has been known for almost as long as the Hall effect itself. Despite this, its theoretical origins nevertheless remain a subject of debate. In recent years, the physics behind the AHE has been employed to control spin transport in non-magnetic conductors via its sister phenomenon, the spin Hall effect (SHE). In this talk, I will present measurements of a magnetically-doped semiconductor quantum well that reveal a robust and tunable AHE, despite the absence of ferromagnetism. I will show that the AHE can be tuned in-situ by the application of a voltage to a nearby gate electrode and that this helps to uncover the origins of the effect. Most surprising is the fact that the parent material, ZnSe, is known to have only weak spin-orbit coupling, a property usually believed to be required for a strong AHE or SHE. This suggests that controllable semiconductor spin-transport might be realized in a larger class of materials than previously thought. Collaborators: L. S. Moore, H. T. Chou, K. C. Ku, G. Xiang, S. A. Crooker, N. Samarth, and D. Goldhaber-Gordon. See PRL 96, 196404. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X12.00002: Anomalous Hall Effect and Anomalous Nernst Effect in Ga1-xMnxAs Yong Pu, Jing Shi, Daichi Chiba, Fumihiro Matsukura, Hideo Ohno We have carried out systematic electrical and thermoelectric transport coefficient measurements on~a series of Ga1-xMnxAs samples (x from 0.01 to 0.07) with perpendicular magnetic anisotropy. 50 nm- thick Ga1-xMnxAs films were grown by molecular beam epitaxy on an InGaAs buffer layer with a tensile strain to induce the perpendicular anisotropy. Below the Curie temperature, we have observed a~non-zero transverse thermopower Sxy that accompanies the Hall resistance Rxy. Both Sxy and Rxy show abrupt jumps as the magnetization switches by an external magnetic field. The square hysteresis loops in Sxy and Rxy resemble those of the magnetization. Just as the anomalous Hall effect (AHE), the hysteresis loop in Sxy, i.e. the anomalous Nernst effect (ANE), is a consequence of the spin-orbit coupling in the ferromagnetic materials. We have measured both AHE and ANE over a wide range of temperatures in all samples, and found that the Hall resistance Rxy scales with the square of the longitudinal resistance Rxx. In contrast, the transverse thermopower Sxy is independent of the longitudinal thermopower Sxx over the same temperature range. These observations suggest that both AHE and ANE are of intrinsic or dissipationaless origin. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X12.00003: Sign change of anomalous Hall coefficient with temperature in Ga$_{1-x}$Mn$_{x}$Sb random alloys. M. Eginligil, G. B. Kim, H. Luo, B. D. McCombe We have observed sign changes as a function of temperature (T) in the anomalous Hall (AH) coefficient of ferromagnetic (FM) Ga$_{1-x}$Mn$_{x}$Sb films showing weakly localized behavior in the electrical transport. Low magnetic field measurements vs. T (below the Curie temperature, T$_{c}$, which is between 13K and 24K) show changes in the sign of the slope of the AH resistance vs. field. We attribute this unusual behavior to the movement of the chemical potential ($\mu )$ through the density of states (DOS) extrema in the spin dependent impurity band(s) as recently predicted theoretically [1]. We have developed a model based on the prediction that the AH coefficient depends on the local slope of the DOS in the hopping conduction regime. Our model uses the experimentally determined hole and Mn$_{Ga}$ concentrations to find the position of the $\mu $ vs. T. The two spin dependent impurity bands in the FM state are assumed to be gaussian. Below T$_{c}$ with increasing T, the spin-up and spin-down impurity bands move into the energy gap and converge. As T increases $\mu $ moves from its initial position on the positive slope of the low energy band (EB) through the minimum before continuing across the maximum of the higher EB. This analysis is in qualitative agreement with our experimental results. [1] Burkov and Balents, PRL, 91 (2003) Supported by NSF ESC 0224206 and University at Buffalo, SUNY [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X12.00004: Theory of Intrinsic Anomalous Hall Effect and Spin Hall Effect in Transition Metals Takuro Tanaka, Hiroshi Kontani, Jun-ichiro Inoue, Kosaku Yamada To elucidate the origin of anomalous Hall effect (AHE) in ferromagnetic transition metals, we study the intrinsic AHE based on a multi-orbital tight-binding model. A large anomalous velocity comes from the atomic $d$-orbital degrees of freedom. We derive a general expression for the intrinsic anomalous Hall conductivity (AHC) which is valid for any damping rate $\hbar/2\tau$. This expression enables us to calculate the AHC in metals with a wide range of resistivity $\rho$. The obtained AHC is almost constant with a value of $10^2\sim10^3\Omega^{-1}{\rm cm}^{-1}$ when $\rho$ is small, as found by Karplus and Luttinger. However, this relation does not hold any more in bad metals; we show that AHC is proportional to $\rho^{-2}$ when $\hbar/2\tau$ is larger than the minimum band-splitting measured from the Fermi level, $\Delta$. This crossover behavior of the intrinsic AHE, which was first derived by H. Kontani and K. Yamada [J. Phys. Soc. Jpn. {\bf 63} (1994) 2627], is recently observed in various ferromagnetic metals universally by A. Asamitsu et al. We also present the mechanism of spin hall effect in transition metal oxides. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:48AM |
X12.00005: Semi-classical theory of the Anomalous Hall Effect Invited Speaker: The new semi-classical theory of the anomalous Hall effect will be presented. Its goal is to derive all contributions to the Hall conductivity via a strongly simplified approach, based only on the semi-classical Boltzmann equation and wave packet equations of motion. This approach operates only with gauge invariant quantities and thus all expressions acquire a clear physical interpretation. In the diffusive regime the semi-classical approach leads to the same quantitative predictions as the formally exact approach based on the Kubo-formula. I will also discuss the 1-1 correspondence among different techniques applied to the problem of the anomalous Hall effect. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X12.00006: Anomalous Hall Effect in Superparamagnetic Co-(La,Sr)TiO$_{3}$ Thin Films Shixiong Zhang, Weiqiang Yu, Satish B. Ogale, Sanjay R. Shinde, Darshan C. Kundaliya, Joshua S. Higgins, Ranjan Sahu, Richard L. Greene, Thirumalai Venkatesan, Wangkong Tse, Shengyu Young, Lourdes G. Salamanca-Riba We performed a systematic study of the magnetic properties and the Hall effect on 5{\%} cobalt doped (La, Sr)TiO$_{3 }$thin films grown by pulsed laser deposition. The superparamagnetic nature of the system is established by several protocols of magnetic measurements. Nevertheless, the anomalous Hall effect (AHE) is observed in the system, the Hall resistivity vs magnetic field loops being found to be identical to the magnetic hysterisis loops. This once again (Phys. Rev. Lett. 92, 166601 (2004)) highlights the limitations of AHE as a tool to test the intrinsic nature of ferromagnetism in a diluted magnetic system. Possible reasons for the origin of the AHE in our system are discussed. [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X12.00007: ABSTRACT HAS BEEN MOVED TO H12.00012 |
Friday, March 9, 2007 10:12AM - 10:24AM |
X12.00008: Mobility of Charge Carriers and Magnetoresistance of Dilute Magnetic Semiconduc. Michael Foygel, James Niggemann, Andre Petukhov We studied electrical transport in dilute magnetic semiconductors, which is determined by scattering of free carriers by localized magnetic moments. In our calculations of the scattering time and the mobility of the majority and minority-spin carriers we took into account both the effects of thermal spin fluctuations and of built-in spatial disorder of the magnetic atoms. These effects are responsible for the magnetic-field dependence of the mobility of the charge carriers. The application of the external magnetic field suppresses the thermodynamic spin fluctuations thus increasing the mobility and promoting negative magnetoresistance. Depending on the type of the carriers and on parameters of the impurity potential, scattering by built-in spatial fluctuations of the atomic spins increases or decreases with the magnetic field. The latter effect is due to the change in the magnitude of the random local Zeeman splitting with the magnetic field. Under certain circumstances it may promote positive magnetoresistance. We discuss the role of the above effects on mobility and magnetoresistance of semiconductors where magnetic impurities are electrically active or neutral. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X12.00009: Interplay of disorder and spin-orbit coupling in diluted magnetic semiconductors Xavier Cartoix\`a, Byounghak Lee, Nandini Trivedi, Richard M. Martin We address two open questions: (i) upon doping GaAs with Mn, where do the holes go? (ii) how does the disorder arising from the random Mn locations affect the carrier polarization? Using a realistic multi-band tight-binding model with disorder effects included exactly, we find upon examining the nature of the eigenstates that the system is composed of two components: for a given doping of Mn atoms, a fraction of holes are localized in the midgap region, whereas the rest hybridizes with the host valence band generating extended states. These fractions change in a systematic way with doping. We further find that while spin-orbit coupling reduces the spin polarization by mixing different spin states in the valence bands, disorder from the random locations of the Mn ions enhances the spin polarization. This is primarily because the additional field from the ferromagnetically aligned impurities polarizes the itinerant carriers. We also show that the ferromagnetic coupling between localized Mn moments exhibits a strong real space anisotropy. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X12.00010: Boundary conditions for spin diffusion Victor Galitski, Anton Burkov, Sankar Das Sarma We introduce a general method of deriving boundary conditions for spin-charge coupled transport in disordered systems with spin-orbit interactions and derive boundary conditions for spin diffusion in the Rashba model for various types of boundaries. Due to the surface spin precession, the boundary conditions are generally non-trivial and may contain terms, which couple different components of the spin density. We argue that boundary conditions and the corresponding electric-field-induced spin accumulation depend on the nature of the boundary and therefore the spin Hall effect in a spin-orbit coupled system can be viewed as a non-universal edge phenomenon. [Preview Abstract] |
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