Session R18: Spintransport Phenomena II

Magnetic and electron-transport properties of spin-gapless semiconducting CoFeCrAl films.

Recently, spin-gapless semiconductors (SGS) with a semiconducting or insulating gap in one spin channel and zero gap in the other at the Fermi level have attracted much attention due to their new functionalities such as voltage-tunable spin polarization, the ability to switch between spin-polarized n-type and p-type conduction, high spin polarization and carrier mobility. For the development of spintronic devices utilizing SGS, it is necessary to have a better understanding of the magnetic and transport properties of the thin films of these materials. In this study, the structural, magnetic, and electron-transport properties of a SGS material CoFeCrAl in the thin film geometry have been investigated. CoFeCrAl films were grown on atomically flat SiO2 substrates using magnetron sputtering. The Curie temperature was measured to be 550 K very close to the value reported for bulk CoFeCrAl. Electron-transport measurements on the oriented films revealed a negative temperature coefficient of resistivity, small anomalous Hall conductivity and linear field dependence of magnetoresistance, which are transport signatures of SGS. The effect of elemental compositions and structural ordering on the SGS properties of the CoFeCrAl films will be discussed.   

Theory of spin relaxation at metallic interfaces

Spin-flip scattering at metallic interfaces affects transport phenomena in nanostructures, such as magnetoresistance, spin injection, spin pumping, and spin torques. It has been characterized for many material combinations by an empirical parameter $\delta$, which is obtained by matching magnetoresistance data for multilayers to the Valet-Fert model [J. Bass and W. P. Pratt, J. Phys.: Condens. Matter {\bf 19}, 183201 (2007)]. However, the relation of the parameter $\delta$ to the scattering properties of the interface remains unclear. Here we establish this relation using the scattering theory approach and confirm it using a generalization of the magnetoelectronic circuit theory, which includes interfacial spin relaxation. The results of first-principles calculations of spin-flip scattering at the Cu/Pd and Cu/Pt interfaces are found to be in reasonable agreement with experimental data.   

Spin relaxation through Kondo scattering in Cu/Py lateral spin valves.

Within non-magnetic metals it is reasonable to expect the Elliot-Yafet mechanism to govern spin-relaxation and thus the temperature dependence of the spin diffusion length might be inversely proportional to resistivity. However, in lateral spin valves, measurements have found that at low temperatures the spin diffusion length unexpectedly decreases. We have fabricated lateral spin valves from Cu with different concentrations of magnetic impurities. Through temperature dependent charge and spin transport measurements we present clear evidence linking the presence of the Kondo effect within Cu to the suppression of the spin diffusion length below 30 K. We have calculated the spin-relaxation rate and isolated the contribution from magnetic impurities. At very low temperatures electron-electron interactions play a more prominent role in the Kondo effect. Well below the Kondo temperature a strong-coupling regime exists, where the moments become screened and the magnetic dephasing rate is reduced. We also investigate the effect of this low temperature regime ($>$1 K) on a pure spin current. This work shows the dominant role of Kondo scattering, even in low concentrations of order 1 ppm, within pure spin transport.   

\textbf{Electronic structure and magnetocrystalline anisotropy of the Bi}$_{\mathrm{\mathbf{2}}}$\textbf{Se}$_{\mathrm{\mathbf{3\thinspace }}}$\textbf{topological insulator/ferromagnet interface}

Interesting spin-dependent phenomena are expected to emerge when a topological insulator is interfaced with a magnetic material. In this work the magnetic properties of the interface between a topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and ferromagnetic metals (FM) fcc (111) Ni and Co are investigated by first-principles calculations. Different interface terminations are considered, and the most stable interface termination is identified to be an interface Ni (Co) atom located atop the hollow site of the interfacial Se monolayer. We find that the proximity effect induces a small magnetic moment on the interface Se atom (0.028 $\mu _{\mathrm{B}}$ for Ni and 0.023 $\mu_{\mathrm{B}}$ for Co). The surface state in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ disappears due to the strong interface hybridization between FM and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and metal induced gap states appear in the bandgap region of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. We find that both the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Ni(111) and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Co(111) interfaces exhibit an in-plane easy axis with the magnetic anisotropy energy of around 2 erg/cm$^{\mathrm{2\thinspace }}$per interface. An interesting feature resulting from our calculations is a non-collinear k-dependent spin texture at the interface which may have important consequences for the spin-dependent transport properties, such as the spin transfer torque.   

Observation of thermal spin transfer torque via ferromagnetic resonance in magnetic tunnel junctions

The temperature gradient driven spin-transfer torque, called the thermal spin-transfer torque (TSTT) attracts people's attention since it has potential in magnetization switching by utilizing wasted heat as well as in the study of spin transportation. We observed the effects of TSTT on magnetic tunnel junction (MTJ) via analysis of the ferromagnetic resonance (FMR) spectra. We used an external laser beam to heat the MTJ in order to establish a temperature gradient effectively. A TSTT was driven by the temperature gradient and applied to the magnetization of the free FM layer of the MTJ. By measuring and analyzing the FMR spectra, after excluding the effects caused by the temperature rise, we conclude that the FMR line-shape change is a result of the TSTT generated by a temperature gradient via laser heating. The most interesting result is that the angular dependence of the TSTT and DC-bias spin-transfer torque are very different. A modified or new theory may be needed to explain this in the future.   

Intrinsic Gilbert Damping in Metallic Ferromagnets in Ballistic Regime and the Effect of Inelastic Electron Scattering from Magnetic Moments: A Time Dependent Keldysh Green Function Approach

Gilbert damping in metallic ferromagnets is mainly governed by the exchange coupling between the electrons and the magnetic degree of freedom, where the time dependent evolution of the magnetization leads to the excitation of electrons and loss of energy as a result of flow of spin and charge currents. However, it turns out that when the magnetization evolves slowly in time, in the presence of spin-orbit interaction (SOI), the resonant electronic excitations has a major contribution to the damping which leads to infinite result in ballistic regime. In this work we consider the inelastic spin-flip scattering of electrons from the magnetic moments and show that in the presence of SOI it leads to the relaxation of the excited electrons. We show that in the case of clean crystal systems such scattering leads to a linear dependence of the Gilbert on the SOI strength and in the limit of diffusive systems we get the Gilbert damping expression obtained from Kambersky's Fermi breathing approach. This research was supported by NSF-PREM Grant No. DMR-1205734   

\textbf{Carbon Tetragons as Definitive Spin Switches in Narrow Zigzag Graphene Nanoribbons}

Precise spatial control of the spin propagation channels is of fundamental and practical importance in future graphene-based spintronic devices. Here we use first-principles calculations to show that when narrow zigzag graphene nanoribbons are connected to form junctions or superlattices, properly placed square-shaped carbon tetragons not only serve as effective bundles of the two incoming spin edge channels, but also act as definitive topological spin switches for the two outgoing channels. The nanoribbon segments are largely drawn from different acene molecules. We further show that such spin switches can lift the degeneracy between the two spin propagation channels, which enables tunability of different magnetic states upon charge doping. Preliminary experimental supports for the realization of such tetragons connecting nanoribbon segments are also presented.   

Spin Transport and Giant Electroresistance in Ferromagnetic Graphene Vertical Heterostructures

We investigate spin transport through ferromagnetic graphene vertical heterostructures where a sandwiched tunneling layer is either a normal or ferroelectric insulator. We show that the spin-polarization of the tunneling current is electrically controlled via gate voltages. We also demonstrate that the tunneling current of Dirac fermions can be prohibited when the spin configuration of ferromagnetic graphene sheets is opposite. Giant electroresistance can thus be developed by using the proposed heterostructure in this study. The effects of temperature on spin transport and the giant electroresistance ratio are also investigated. Our findings discover the prospect of manipulating the spin transport properties in vertical heterostructures through electric fields via gate and bias electrodes.   

\textbf{Spin relaxation mechanism in graphene spin valves with Al}$_{\mathrm{\mathbf{2}}}$\textbf{O}$_{\mathrm{\mathbf{3}}}$\textbf{ and MgO tunnel barriers}

Contact induced spin relaxation in graphene lateral spin valves is one of major limiting factors for obtaining long spin lifetimes in graphene. There are various spin relaxation mechanisms, including spin absorption, interfacial spin scattering, and fringe field effects, which may account for the observed short spin lifetimes. One possible solution is to introduce a tunnel barrier between graphene and the ferromagnetic electrode, which should reduce contact induced spin relaxation and allow for longer spin lifetimes. We study the spin relaxation mechanisms in our graphene spin valves with two different types of tunnel barriers, aluminum oxide and MgO/TiO$_{\mathrm{2}}$ using the standard non-local measurement geometry. To extract the spin lifetime from Hanle spin precession data, we perform fits based on Bloch equation models that include the effects of spin absorption into the magnetic contacts. We observe a strong dependence of the extracted spin lifetime on the resistance-area (RA) product of the contacts. To understand the role of spin absorption, we compare these results to fits obtained using Hanle models that do not take spin absorption into account. Analysis shows that spin absorption might not be the dominant source of contact induced spin relaxation for graphene spin valves with sputtered Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ and MgO/TiO$_{\mathrm{2}}$ barriers. Interfacial spin-flip scattering or spin dephasing resulting from local magnetostatic fields due to contact roughness are likely to be more important.   

Spin fluctuations in 3d paramagnetic metals

Spin fluctuations (SFs) in 3d paramagnetic metals were investigated using the linear response formalism within the time dependent density functional theory. An efficient scheme of frequency integration using the Matsubara technique has been implemented and tested. The SFs spectrum in 3d paramagnets is analyzed in real and reciprocal spaces as a function of frequency and temperature. For all materials the SFs are characterized by the coexistence of low and high energy branches which originate from different regions of the Brillouin zone. The low-energy ones can be measured by neutron scattering experiments while the high-energy SFs appear to be more localized. Further, we studied the nature of square of fluctuating magnetic moment in these materials. This work was supported, in part, by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), and by the Office of Basic Energy Science, Division of Materials Science and Engineering. The research was performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract {\#} DE-AC02-07CH11358.