Session H12: Focus Session: Spin Transport

Dephasing in (Ga,Mn)As Nanowires {\&} Rings

Quantum correction to the conductivity of ferromagnetic semiconductors are thus far largely unexplored. But to understand quantum mechanical transport the knowledge of basic material properties like phase coherence length and corresponding dephasing mechanism are indispensable ingredients. The lack of observable quantum phenomena prevented experimental access to these quantities so far. Here we report on the observation of universal conductance fluctuations in ferromagnetic (Ga,Mn)As. The analysis of the length and temperature dependence of the fluctuations in one-dimensional wires reveals a 1/T dependence of the dephasing time. The measurement of the Aharonov-Bohm effect in nanorings as well as a weak localization correction to the conductivity, observed in arrays of wires, are in good agreement with the results obtained from the conductance fluctuations.   

Low frequency 1/f and random telegraph noise in (Ga,Mn)As

Resistance noise measurements can provide insights into the interplay between charge transport and magnetism in complex physical systems [B. Raquet {\it et al.}, Phys. Rev. Lett. {\bf 84}, 4485 (2000)]. We report the temperature- and magnetic field-dependence of the low frequency electrical noise in (Ga,Mn)As epilayers with different Mn concentrations (and different conductivity). Surprisingly, we do not observe any anomalies in the noise spectra across the Curie temperature. However, we find an enhancement in the integrated noise (over the frequency span 125mHz-11Hz) at temperatures below $\sim 10$ K where the resistivity shows a minimum. For more metallic samples, the normalized power spectrum density is $1/f$-like over the entire temperature range studied, while more insulating samples show Lorentzian spectra accompanied by random telegraph noise (RTN) at low temperatures. The magnetic field dependence of the integrated noise shows distinct correlations with magnetization switching, suggesting changes in scattering during domain wall nucleation/propagation. From the magnetic field driven suppression of the RTN, we infer the existence of nanoscale magnetic clusters that fluctuate between two states separated by a field-tunable barrier.   

Investigation of Planar Hall Effect in (Ga,Mn)As/GaAs/(Ga,Mn)As Structures

We present a study of the planar Hall effect in the multilayer structures (Ga,Mn)As/GaAs/(Ga,Mn)As. The planar Hall effect (PHE) in a single (Ga,Mn)As layer yields two electric states (high and low), ideal as a basis for device design. The present paper is motivated by the speculation that a coupled (Ga,Mn) As/GaAs/(Ga,Mn)As system provides the possibility of combining PHE with tunneling magnetoresistance, thus leading to complex multiplets of electric states. Our PHE studies were carried out on coupled structures in which the two (Ga,Mn)As layers were made different by either modulation doping or by low temperature annealing. A series of specimens were prepared with different thicknesses of the GaAs spacer (3nm or 6nm). Experimental results show that for samples with 3-nm spacers the magnetic coupling between the two (Ga,Mn)As layers is so strong that their magnetizations reverse together. The PHE then behaves similar to that of a single (Ga,Mn)As layer, except that in the multilayers the PHE voltage switchings are less abrupt. In samples with 6-nm-thick spacer, however, we see the emergence of switchings with multiple values of the PHE voltage. Such multiple electric states can be qualitatively explained by modeling the coupled structures as a network of resistors.   

Quantum corrections to the longitudinal and anomalous Hall conductivity in (Ga,Mn)As.

Although the canonical ferromagnetic semiconductor (Ga,Mn)As has now been studied extensively for over a decade, the fundamental understanding of the temperature-dependent conductivity and the origins of the anomalous Hall effect still remain open questions. Here, we report measurements of the longitudinal and transverse conductivity in (Ga,Mn)As samples in the regime of ``dirty" diffusive transport ($1 \leq k_F l_e \leq 3$ as calculated from the Drude formula) over a wide temperature range ($50$ mK $\leq T \leq 5$ K). Although we observe a power law temperature dependence of the conductivity, the scaling is inconsistent with standard expectations based upon known quantum corrections to the conductivity. We also examine the scaling of the anomalous Hall conductivity with longitudinal conductivity and compare our observations with theories of the anomalous Hall effect.   

Magnetotransport and magneto-optical properties of GaMnAs thin films with high Mn concentrations

III-V-based ferromagnetic-semiconductor (FMS) GaMnAs is a good model system for future semiconductor-spintronics devices. For practical applications, it is important to increase the Curie temperature ($T_{C})$ of GaMnAs (the current record is 173 K) to room temperature. The mean field theory predicts that $T_{C}$ of GaMnAs increases in proportion to its Mn concentration $x$. However, it is difficult to grow GaMnAs with $x \quad >$ 10{\%}, because MnAs clusters and Mn interstitial defects are easily formed in such a high $x$ region. Here, we have successfully grown GaMnAs films with $x$ of 12 - 21{\%} by decreasing the growth temperature to 150-200$^{o}$C and by reducing the film thickness to 10 nm. The magnetic circular dichroism and the anomalous Hall effect measurements indicated that these GaMnAs films have the intrinsic FMS features. A high $T_{C}$ value of 170 K was obtained when $x$ = 12{\%}. This work was partly supported by PRESTO/SORST of JST, Grant-in-aid for Scientific Research, IT Program of RR2002 of MEXT.   

Electronic transport in diluted magnetic semiconductors: application of the memory function formalism for spin and charge disordered media.

To get an expression for electrical conductivity in diluted magnetic semiconductors (DMSs) we employ the memory function formalism and derive a general expression for the current relaxation kernel in spin and charge disordered systems. To illustrate the model we performed simplified calculations of spin and charge scattering rates in the weak-disorder limit for some special cases of interest: (i) In a system with positional correlation of the scattering centers we found a significant enhancement of the charge scattering. The enhancement is sensitive to cluster parameters and may be influenced through post-growth annealing. (ii) In the magnetically ordered system we show that the suppression of localized spins fluctuations results in the reduction of the spin scattering that substantially contributes to the experimentally observed resistivity drop below $T_c$. (iii) Memory function formalism gives the possibility to include electronic many-body effects in a consistent and systematic manner through time-dependent density functional theory. We use this approach to study the combined effect of disorder and electron-electron interaction on the transport properties of DMSs.   

Spin transport through individual single-walled carbon nanotubes

We have investigated spin transport through individual single-walled carbon nanotubes contacted with ferromagnetic permalloy electrodes. At low temperatures, hysteretic magnetoresistance (MR) is observed in both the Fabry-Perot interference regime and the Kondo regime. Both the sign and magnitude of the MR oscillate as a function of gate and bias voltages. The behavior in the interference regime can be explained well using non-interacting ballistic model incorporating the effect of spin-dependent interfacial phase shift. In the strongly interacting Kondo regime, however, the behavior of the MR is qualitatively different. We will present possible theoretical models and numerical fittings to elucidate the MR features in our data.   

Spin Transport through Multilayer Graphene

We have demonstrated spin valve behavior in structures in which crystals containing multiple graphene layers were positioned between two ferromagnetic contacts. Graphene is a promising candidate for the spacers of spin valves, because of its small spin-orbit interaction and high mobility. We used a 3nm thick crystal which contained several layers of graphene. Cobalt electrodes with a 100nm gap were fabricated on the crystal using electron beam lithography. The device showed $\sim $0.2{\%} magnetoresistance at 10K using an in-plane magnetic field. The effect was found at temperatures as high as 150K. The observed behavior could be explained by the switching of the magnetizations of the Co electrodes, which was inferred from measurements of their anisotropic magnetoresistance.   

Charge and spin transport in graphene nanostructures

We have studied spin injection from ferromagnetic (permalloy) electrodes into graphene devices using a non-local four-probe geometry. We observe sign reversal of the non-local resistance upon switching of the magnetization direction of the electrodes, indicating injection and detection of a spin current. We report the temperature and carrier density dependence of the spin valve signal. We observe an unusual reversal of the sign of the spin valve signal at some carrier densities. We have also examined the magnetotranport in the low field and quantum Hall regimes in devices with mobilities differing by an order of magnitude. The results will be discussed in terms of the physics of conduction at the Dirac point. Support provided by the Office of Naval Research and the UMD-MRSEC Shared Equipment Facilities.   

Magnetoresistance of Gd doped Carbon films

The rare earth dopant Gd was introduced into amorphous carbon ($a-$C) by two quite different techniques; mass selected ion beam deposition (MSIBD) of tetrahedral amorphous carbon (\textit{ta-}C) followed by Gd implantation and magnetron co-sputtering of Gd and C targets. Raman, RBS and TEM characterization indicate the films are metastable. Films prepared by sputtering ($a-$Gd$_{x}$C$_{1-x}$ x=4.2 $\sim $15.6 at.{\%}) have a spin-glass freezing with a temperature which (2-6K) scales with Gd concentration. Films prepared by MSIBD followed by ion implantation (\textit{taC}:Gd$_{x}$, x=4, 7, 13 at.{\%}) show no freezing and a paramagnetic Curie-Weiss law down to 1.9K. Transport measurements show typical doped amorphous semiconductor behavior with very large negative magnetoresistance (MR). The MR of the two types of films are similar, which indicates a universal magnetic moment-carrier interaction in these Gd doped amorphous semiconductor systems. A comparison of these films' MR with other Gd doped semiconductors such as $a-$Gd$_{x}$Si and $a-$Gd$_{x}$Ge will be discussed. The MR properties of this type of thin film material indicate the importance of the local materials structure and the consequence of the electron screening effects.   

Anti-Weak Localization Measurements in the Ballistic Regime

Anti-weak localization dominates at low fields in systems in which spin-orbit coupling is strong. The experimental results are well described by theory [1] in low mobility systems in which the magnetic length ($l_{B})$ is greater than the mean free path; however high mobility systems with strong spin-orbit interactions, such the InSb based two dimensional systems (2DESs) examined here, are not in this diffusive regime. A recently developed theory [2] addresses both the diffusive and ballistic regimes taking into account both the backscattered and non-backscattered contributions to the conductivity. We will discuss the agreement of the new theory to measurements of InSb 2DESs prepared with both strong Dresselhaus and Rashba effects. [1] S.V. Iordanskii, Yu B. Lyanda-Geller, and G.E. Pikus, \textit{JETP Lett.} \textbf{60}, 206 (1994). [2] L.E. Golub, \textit{Phys. Rev. B}. \textbf{71}, 235310 (2005).   

Electrical measurement of pure spin currents in a two dimensional electron gas

We present an electrical measurement of pure spin currents in a $AlGaAs/GaAs$ two dimensional electron gas. Spin polarized electrons are injected into the centre of a $90\mu$m long channel through a quantum point contact (QPC) in a large in-plane magnetic field. The charge current flows to one end of the channel. A pure spin current flows to the opposite end, driven by a chemical potential difference between the two spin populations. This difference is recorded using spin polarized QPCs along the channel in regions free of charge current.   

$^{77}$Se NMR investigation of the paramagnetic metal phase of $\lambda $-(BETS)$_{2}$FeCl$_{4}$

We report $^{77}$Se NMR measurements of the spectrum and the spin-lattice relaxation rate (1/$T_{1})$ in a 7 $\mu$g single crystal of $\lambda $­(BETS)$_{2}$FeCl$_{4}$ over the temperature ($T)$ range 2.5-10 K in an applied field of 10.9 T parallel to the $a$-axis (paramagnetic metal phase). A behavior close to 1/$T_{1}T$ = constant is observed. It indicates that for these conditions, 1/$T_{1}$ is dominated by the hyperfine interaction between the $^{77}$Se spins and the conduction electrons, in contrast to 1/$T_{1}$ for the protons, which is driven by the magnetic fluctuations of the Fe$^{3+}$ spins [W.G. Clark et al., Appl. Mag. Res. \textbf{27}, 279 (2004)]. From these proton measurements, we estimate that the contribution of the Fe$^{3+}$ fluctuations to 1/$T_{1}$ of $^{77}$Se is negligible. Work at UCLA was supported by NSF Grants DMR-0334869 (WGC) and DMR-0520552 (SEB).