Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L19: Focus Session: Spin Transport & Magnetization Dynamics in Metals V |
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Sponsoring Units: GMAG DMP Chair: John D. Burton, University of Nebraska–Lincoln Room: D170 |
Tuesday, March 22, 2011 2:30PM - 3:06PM |
L19.00001: Thermal spin fluctuations in itinerant ferromagnets: Aspects of magnetic thermodynamics and transport properties Invited Speaker: The character of thermal spin fluctuations in itinerant ferromagnets is a long-standing problem. Our recent theoretical results offer new insights in this issue. First, I will discuss a classical effective spin model with both rotational and longitudinal spin fluctuations, which allows for a variable degree of itinerancy. Magnetic thermodynamics in this model was analyzed for fcc and bcc lattices using Monte Carlo simulations compared (favorably) with mean-field and generalized Onsager approximations. It was found that magnetic short-range order is relatively weak and almost independent on the degree of itinerancy. The ambiguity of the phase space measure will be emphasized. Next, I will discuss our first-principles calculations of spin-disorder resistivity (SDR) of Fe, Ni, and heavy rare-earth metals (Gd-Tm series), in which the Landauer conductance is explicitly averaged over spin disorder configurations. For Fe the SDR agrees very well with experiment. For Ni, comparison with experiment suggests that the average local moment in the paramagnetic state is reduced to $\sim $0.35 $\mu _{B}$. The effect of magnetic short-range order on SDR is found to be weak in both Fe and Ni. Overall, the results suggest that thermal spin fluctuations in Fe and Ni have an effectively classical character. While the crystallographically averaged paramagnetic SDR for rare earth metals agrees quite well with experiments, its anisotropy systematically and significantly exceeds the available measurements. This discrepancy is critically evaluated, suggesting the need for additional experiments. \\[4pt] [1] A. L. Wysocki, J. K. Glasbrenner, and K. D. Belashchenko, Phys. Rev. B \textbf{78}, 184419 (2008). \\[0pt] [2] A. L. Wysocki, R. F. Sabirianov, M. van Schilfgaarde, and K. D. Belashchenko, Phys. Rev. B \textbf{80}, 224423 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L19.00002: Theory of the Kondo Temperature in Multilevel Quantum Dots Ion Garate, Ian Affleck We develop a simple but general method to evaluate the Kondo temperature in a multilevel quantum dot that is weakly coupled to conducting leads. Our analysis reveals that the Kondo temperature is strongly enhanced when the intradot energy-level spacing is smaller than the charging energy. [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L19.00003: Role of initial quantum correlation in transient linear response Chikako Uchiyama, Masaki Aihara We study the transient linear response of a two-level system coupled with an environmental system for correlated and factorized initial conditions. We find the significant differences between the transient linear response in these cases, especially for strong system-environment interaction at intermediate temperatures. This means that we need to pay attention to the initial conditions when we analyze experiments on transient linear response. This is because the conventional factorized initial condition, in which the system-environment correlation is disregarded, results in an incorrect response.\\[0pt] Reference: C. Uchiyama and M. Aihara, Phys. Rev. A, {\bf 82}, 044104(2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L19.00004: Even-odd parity effects of a spin-1 Heisenberg chain on long-range interaction and entanglement Sangchul Oh, Mark Friesen, Xuedong Hu A strongly coupled spin chain can be used as a quantum data bus, to mediate long-range interactions and entanglement between remote qubits.By obtaining numerical solutions for finite size systems, we investigate even-odd parity effects in a spin-1 chain and their consequences for long-range interactions and entanglement. We observes some similarities with a spin-1/2 chain~$[1]$. For example, we find that an even-size chain mediates an RKKY (Ruderman-Kittel-Kasuya-Yosida)-like interaction between weakly coupled attached spins, while an odd-size chain acts as a central spin to the attached spins. On the other hand, we find that the RKKY-like interactions have a different character. For a spin-1/2 chain, the interactions decay with qubit separation as they oscillate between ferro and antiferromagnetic couplings. For a spin-1 chain, the interactions also oscillate, but they decay very little as a function of qubit separation. We believe this behavior is a manifestation of the finite Haldane gap of an integer spin chain in the thermodynamic limit.\\[4pt] $[1]$ S. Oh, M. Friesen, X. Hu, Phys. Rev. B {\bf 82}, 140403(R) (2010). [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 3:54PM |
L19.00005: Theory of Magnetization of Interacting Bloch Electrons Prasanta Misra, Gouri Tripathi We derive a theory of magnetization of interacting Bloch electrons in the paramagnetic limits. We start with a thermodynamic potential, which includes both the quasi-particle and correlation contributions. The startling result obtained by us is that the modifications brought about by the electron-electron interactions for the magnetization in the quasi-particle interaction is precisely cancelled by the contributions due to electron correlations and thus the magnetization is devoid of explicit many-body corrections. In contrast, it is well known that both the spin susceptibility and the spin Knight-shift are exchange enhanced by electron-electron interactions. This is due to second order effects in the sense that while both the spin vertices are renormalized, the renormalization of only one of the vertices is cancelled by the contribution due to electron correlations. However, there is only one spin-vertex in the expression for magnetization which is renormalized in the quasi-particle approximation. We discuss the importance of self-energy corrections on the single-particle spectrum and we have shown as how to predict that the interacting electron system is magnetic or not by considering a variant of the Hubbard Hamiltonian in the momentum space. [Preview Abstract] |
Tuesday, March 22, 2011 3:54PM - 4:06PM |
L19.00006: Exact solution for permeability of a ferromagnet under parametric bichromatic irradiation Adil-Gerai Kussow, Alkim Akyurtlu If two parametrically coupled electromagnetic fields are applied to a ferromagnet, a non-linearity of the equation of precession of magnetic moment strongly affects the permeability. If the coupling constant $\vert \beta \vert <1$ between the probe (p) wave and the support (s) wave is small, the permeability $\hat {\mu }(\omega)$, at the frequency of the probe wave $\omega =\omega _p =2\omega _s$, is still monochromatic-like, with the re-normalized resonance spin waves frequency $\Omega _r \to \Omega (1+\beta )$. If a coupling is strong, $\beta \;\,\le -\,\,1$, unusual response effects are possible (the magnetic transparency and profoundly non-monochromatic permeability $\hat{\mu}(\omega)$. Possible optical applications to the homogeneous negative refractive index materials are discussed. [Preview Abstract] |
Tuesday, March 22, 2011 4:06PM - 4:18PM |
L19.00007: Element specific analysis of magnetic anisotropy in practical Mn-based antiferromagnetic alloys from first principles Khmelevskyi Sergii, Alexandr B. Shick, Peter Mohn Magnetic Anisotropy Energy (MAE) and element specific contribution to MAE has been studied for practical Mn-based antiferromagnetic alloys with layered L1$_{0}$ structure in the framework of the Local Spin Density Approximation and fully relativistic torque method. It is found that the contribution to the total MAE from non-magnetic 3d and 4d-elements in MnNi and MnPd alloys is comparable to the contribution of the magnetic Mn atoms. In the 3d-5d MnIr alloy the Ir contribution is found to be dominating. The origin of this contribution from the elements with total zero atomic spin moment is linked to the calculated non-trivial spin density distributions on the corresponding atom, which gives a zero moment only on average. We have also found and discuss a strong dependence of the total and element specific contribution to MAE on the state of the magnetic order. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L19.00008: Theoretical search for new permanent magnets with no rare earth atoms Liqin Ke, Vladimir Antropov We performed the extensive computational search for better permanent magnets containing no rare earth atoms. Our initial studies are concentrated on the intrinsic properties of magnetic materials such as magnetization, the Curie temperature and magnetic anisotropy. A computational tool based on the electronic structure methods has been developed to describe these physical properties as a function of electronic concentration. The application of this technique allowed us to provide several possible directions to perform a search for new materials. We discuss some physical limitations of required properties in iron based materials using the analysis of their electronic structure and simplified magnetic models. The issues of chemical substitutions, modification of geometry and changing the dimensionality of systems will be discussed as well. The specific results will be shown for Fe and Co based systems with the additions of N, C, W, Al and other atoms. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L19.00009: Ab initio study of d$^{\bigcirc }$ magnetism in CaC Hadi Akbarzadeh, Zahra Nourbakhsh, S. Javad Hashemifar The half-metallic ferromagnetism in d$^{0}$ ionic compounds has attracted considerable attention in the spintronics community [Phys. Rev. B 73 024404, 2006]. In this work we employ density functional theory to study electronic, magnetic, and mechanical properties of the high ionic CaC compound in the Zinc Blend (ZB), Rock Salt (RS), B$_{2}$, Wurtzite (WZ), NiAs, and tetragonal structures.The observed ferromagnetic equilibrium state in the RS, NiAs, WZ, tetragonal and ZB structures is attributed to the partially filled sharp p band of carbon around the Fermi level. Half-metallicity has been found in the equilibrium ZB and expanded WZ structures with a magnetic moment of 2 $\mu _{B}$/formula units. Comparing the Gibbs free energy of various structures indicates favorability of the half metallic phases in negative pressures. Topological analysis of the electronic charge density reveals topological character of the paramagnetic-ferromagnetic phase transition of the revised NiAs and B$_{2}$ structures as well as the geometrical character of the magnetic phase transitions of the RS and ZB structures. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L19.00010: Greigite and Spinorbitronics Baomin Zhang, Gilles de Wijs, Rob de Groot Greigite(Fe$_{3}$S$_{4})$ and magnetite(Fe$_{3}$O$_{4})$ are isoelectronic and isostructural ferrimagnets. In biology, the motility of magnetotactic bacteria is based on any or both of them. Not much work is known on greigite. Unlike half-metallic magnetite, greigite is a normal metal. Although the constituent elements are light, the complex Fermi-surface of greigite is remarkably sensitive to relativistic effects. The existence of several Fermi-surface sheets is dependent on the direction of the magnetization. This implies spintronics based on a homogeneous material rather than a device. Since this effect is intrinsically relativistic, spin-contamination is irrelevant here. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L19.00011: Experimental and theoretical investigations into the twinning energy of an FSMA system P.K. Mukhopadhyay, Madhuparna Karmakar, Rajini Kanth B., S.N. Kaul Ferromagnetic shape memory alloys (FSMA) are smart materials with largest magnetic field induced strain below austenite - martensite transformations. The lower temperature martensitic state is characterized by the presence of structural twins that have this exceptional magnetoelastic coupling. To understand this behavior, we carried out sound velocity and attenuation measurements on a typical FSMA material, NiFeAl system, and determined the Young's moduli under various stresses and associated strains. We found that the effect of stress is to alter the martensite temperature. We also studied a theoretical thermodynamic constitutive model and Clausius-Duhem inequality, to determine the stress resulting from an applied strain for an isothermal system. In the absence of an applied magnetic field the free energy of the system consists of only the mechanical energy contribution which in turn is dependent upon the elastic moduli pertaining to the elastic and twinning strains. The paper describes the details of measurements and the model chosen, along with the discussions on the correspondence between the experimental observations and various theoretically determined quantities. [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L19.00012: Influence of Goldstone modes on the electronic properties of helical magnets Yan Sang, Dietrich Belitz, Kwan-yuet Ho, Ted Kirkpatrick We have investigated the influence of Goldstone modes on the electronic properties of helical magnets, such as MnSi, in which a Dzyaloshinsky-Moriya term in the action leads to helical order with a pitch wave number $q$ [1]. In the presence of a homogeneous external magnetic field $H$ the helix is superimposed by a homogeneous magnetization, which leads to a conical phase [2]. The Goldstone mode in this conical phase has the form $\Omega^2 \propto c_z k_z^2 + c_{\perp} k_{\perp}^4/q^2 + c_H k_{\perp}^2$, where $k_z$ and $k_{\perp}$ denote the components of the wave vector parallel and perpendicular to $H$, respectively. The elastic constants $c_z$ and $c_{\perp}$ are independent of $H$ for small $H$, whereas $c_H \propto H^2$. This Goldstone mode couples to the conduction electrons and leads to nonanalytic temperature dependences of various observables. In the conical phase, the strongest effect is on the thermal conductivity and the single-particle relaxation rate, which both are proportional to $T^{3/2}$. We also report results for the specific heat and the electrical resisitivity, both in the conical phase and in other phases of MnSi [3]. [1] P. Bak and M.H. Jensen, J. Phys. C 13, L881 (1980). [2] Y. Ishikawa, G. Shirane, J.A. Tarvin, and M. Kohgi, Phys. Rev. B 16, 4956 (1977). [3] Kwan-yuet Ho, T.R. Kirkpatrick, Yan Sang, D. Belitz, Phys. Rev. B 82, 134427 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 5:18PM - 5:30PM |
L19.00013: Is CrO$_2$ Fully Spin Polarized? Analysis of Andreev Spectra and Excess Current Tomas Lofwander, Roland Grein, Matthias Eschrig We report an extensive theoretical analysis of point-contact Andreev reflection data available in the literature on ferromagnetic CrO$_2$. We find that the spectra can be well understood within a model of fully spin-polarized bands in CrO$_2$ together with spin active scattering at the contact. This is in contrast to analyses of the data within extended Blonder-Tinkham-Klapwijk models, which lead to a spin polarization varying between 50\% and 100\% depending on the transparency of the interface. We propose to utilize both the temperature dependence of the spectra and the excess current at voltages above the gap to resolve the spin-polarization in CrO$_2$ in a new generation of experiments.\\[4pt] T. L\"ofwander, R. Grein, and M. Eschrig, Phys. Rev. Lett. {\bf 105}, 207001 (2010) [Preview Abstract] |
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