Session W12: Focus Session: Spin Dependent Tunneling II

Spin polarization of electrons in 2D structures due to reflection from a barrier

In two-dimensional semiconductor structures Rashba spin-orbit coupling can orient the electron's spin in two opposite directions perpendicular to the direction of motion. We analyze here the possibility to change the spin polarization of an electron beam which is reflected from a barrier in the 2D plane. In general, an unpolarized incident beam gives rise to three reflected beams with different polarizations [1]. We give the orbital and spin parts of the current densities inside and outside of the interference zones. Also we estimate for an initially unpolarized (or partially polarized) electron beam the change of the degree of polarization due to multiple reflections between two parallel barriers in a ballistic regime using realistic material parameters. [1] A.~O.\ Govorov et al., Phys.\ Rev.\ B 70, 245310 (2004)   

Spin-dependent properties of Fe/MgO/GaAs heterostructures

Developing efficient spin injectors and spin detectors is an important goal for semiconductor-based spintronics. Recently Jiang et. al.'s work using CoFe/MgO tunnel spin injectors showed significantly enhanced spin injection efficiency into GaAs due to a spin filtering effect of the MgO layer [a]. Using molecular beam epitaxy (MBE) deposition, we have successfully grown atomically flat MgO films on GaAs(001) epilayers. Below 2 nm thickness, the MgO films are found to be single crystalline. The spin-dependent properties of a Fe/MgO/GaAs heterostructure are investigated by time-resolved Faraday rotation (TRFR) to measure ferromagnetic proximity polarization (FPP) across MgO [b]. It is seen that a very small amount of MgO (less than 0.5 nm thickness) enhances the FPP significantly. We are investigating the FPP dependence on MgO thickness by scanning the optical beams across an MgO wedge. A systematic study on MgO thickness dependence will be presented and the mechanism of indirect FPP across MgO will be discussed. Supported by NSF, ONR, and CNID. (a) X. Jiang, et al., Phys. Rev. Lett. \textbf{94, }056601 (2005). (b) R. J. Epstein, et al., Phys. Rev. B \textbf{65, }121202 (2002)   

Magnetic field dependence of a resonant tunneling diode based in the GaMnAs/AlGaAs material system.

A resonant tunneling diode was fabricated with magnetic GaMnAs emitter and quantum well regions and a nonmagnetic p-GaAs collector.~ At 4K, below the Curie temperature for GaMnAs, negative differential resistance (NDR) associated with resonant tunneling of holes was observed.~ Both the magnitude of NDR as well as its associated bias were found to be dependent on magnetic field.~ If the device bias is held constant and the magnetic field is swept, our device exhibits either positive or negative tunneling magetoresistance (TMR) up to several tens of percent, depending on the device bias.   

Electron Tunneling across EuS / InAs Heterojunctions

The tunneling properties of the heterojunction formed between the ferromagnetic semiconductor EuS and the non-magnetic semiconductor InAs are investigated to explore the feasibility of injecting spin polarized electrons into a two dimensional electron gas. Below the ferromagnetic transition temperature, T$_{c}$, of EuS the barrier height of the heterojunction follows a Brillouin function with S=7/2, demonstrating that the transport is dominated by the large, $\sim$0.5 eV, Zeeman splitting of the conduction band in EuS.\footnote{J. Trbovic et al., Applied Physics Letters, 87, 82101 (2005).} At temperatures above T$_{c}$ the zero-bias conductance of EuS / InAs heterojunctions show two separate regimes, each having an exponential temperature dependence, indicating that other scattering mechanisms are present in the barrier in addition to magnetic fluctuation effects seen in Schottky barriers formed between EuS and metals.\footnote{W.A. Thompson et al., Physical Review Letters, 26, 1308 (1971).}   

Digital magneto resistance in magnetic MOBILEs

Resonant tunneling structures comprising magnetic semiconductor layers are promising for realizing efficient spin filters and detectors [1]. Recently [2], we showed that a paramagnetic MOBILE (Monostable-Bistable Transition Logic Element), which consists of two serial connected resonant tunneling diodes (RTDs), the nonmagnetic load and the driver with a paramagnetic quantum well (QW), exhibits digital magneto resistance (DMR): a continuous change of the external magnetic field above a threshold value leads to a discrete jump of the output voltage from low to high. We have also proposed a nonvolatile ferromagnetic MOBILE, where the driver-RTD comprises a ferromagnetic emitter and QW. We show that DMR is realized by changing the relative orientation of the magnetizations above a threshold angle. In the low voltage regime the driver IV can be changed from ohmic to negative differential resistance behavior. Since conventional MOBILEs have been demonstrated to work up to 100 GHz the proposed device might be useful for performing very fast detections of magnetic signals. [1] I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] C. Ertler and J. Fabian, Appl. Phys. Lett. 89, 193507 (2006).   

Reversing the sign of the spin-polarized current across a Fe/GaAs tunnel barrier at finite voltage bias

As a function of the voltage bias across a Fe/GaAs Schottky tunnel barrier, we measure the sign and magnitude of the electrically-injected electron spin polarization in the semiconductor, $P_{GaAs}$, using magneto-optical Kerr rotation at 10 K. Both images and Hanle depolarization curves reveal that the sign of $P_{GaAs}$ inverts when sweeping from small reverse bias (electrons flowing into GaAs) to small forward bias (electrons flowing into Fe), as expected from linear response. More strikingly, $P_{GaAs}$ inverts sign again at higher bias across the Fe/GaAs barrier. This crossover bias ($|V_{cross}| < 0.1$ V in the structures studied) is sample-dependent, and can occur under either forward- or reverse-bias conditions, depending on sample. These data concur with all-electrical measurements of $P_ {GaAs}$ in lateral spin transport devices having a source, drain, and a third `non-local' detection electrode. Models to describe these data will be discussed. We further show that, when Fe/GaAs tunnel barriers are employed as electrical spin {\it detectors}, both the sign and magnitude of the detection sensitivity can be tuned with applied bias on the detector. This work is supported by the Los Alamos LDRD and NSF MRSEC programs, and ONR.   

Spin extraction theory and its spintronics applications

Extraction of electrons from a semiconductor to a ferromagnet as well as the case of injection in the reverse direction may be formulated as a scattering theory. However, the presence of bound states at the interface arising out of an inhomogeneous doping on the semiconductor side must be taken into account in the scattering theory. Inclusion of the interface states yields an explanation of a recent result of spin imaging measurement which contradicts the current understanding of spin extraction (S. A. Crooker \textit{et al.}, Science \textbf{309}, 2191 (2005)). A particular consequence of our theory is a proposed electrically controlled spin-switch in which a non-magnetic back-gate monitors the spin polarization in a semiconductor. The switch also utilizes a ferromagnet to filter either of the spin species depending on the gate bias. Based on these ideas (and if time allows), we will also present a semiconductor spintronics prototype of a reprogrammable, universal \textbf {logic} gate which does not require magnetic fields throughout its operation. (See also, cond-mat/0609045)   

Spin-dependent tunneling properties in GaMnAs-based magnetic tunnel transistors

III-V-based ferromagnetic-semiconductor heterostructures comprising GaMnAs are hopeful candidates for future spintronic devices. Thus far, only two-terminal devices have mainly been studied. Meanwhile, GaMnAs-based `three-terminal' magnetic tunnel transistors (MTTs) have a potential to add novel functions to integrated circuits. We prepared MTT structures composed of GaMnAs (30 nm)/ AlAs (2 nm)/ GaMnAs (30 nm)/ GaAs:Be (30 nm; 1*10$^{17}$cm$^{-3})$ on $p$-GaAs(001) substrates using molecular-beam epitaxy (MBE). The $V_{EB}$ dependence of $I_{C}$, $I_{E}$, and $I_{B}$ shows that the current transfer ratio \textit{$\alpha $} (= $I_{C }$/$ I_{E})$ is 0.8-0.95; this is much higher than 0.03, the maximum value reported in metal-based MTTs. The current gain \textit{$\beta $} (= $I_{C }$/$ I_{B})$ is of the order of 10, which means that GaMnAs-based MTTs have current amplifiability. The $V_{EC}$ dependence of the tunneling magnetoresistance (TMR) ratio differed significantly from that observed in single-barrier magnetic tunnel junctions (MTJs). This work was partly supported by PRESTO / SORST of JST, Grant-in-Aids for Scientific Research, IT-RR2002 of MEXT, and Kurata-Memorial Hitachi Sci. {\&} Tech. Foundation.   

Resonant tunneling effect and tunneling magnetoresistance in ferromagnetic-semiconductor quantum heterostructures

Ferromagnetic-semiconductor quantum heterostructures are expected to realize novel functions by combining the resonant tunneling effect and the tunneling magnetoresistance (TMR). However, there are no reports on the clear observation of the resonant tunneling effect and TMR associated with it in these structures. We fabricated the GaMnAs quantum-well (QW) double-barrier heterostructures composed of GaMnAs(20 nm)/AlGaAs(4nm)/GaMnAs(d=3.8-20 nm)/ AlAs(4nm)/GaAs:Be on p-GaAs (001) substrates using molecular-beam epitaxy (MBE). The dI/dV-V characteristics and bias dependence of TMR measured at 2.6 K clearly show oscillatory behaviors in the negative bias region where holes are injected from the GaAs:Be layer to the GaMnAs QW. With increasing d, the peaks of these oscillations shift to smaller voltages and the period becomes short, which indicates that they are induced by the resonant tunneling effect. This work was partly supported by PRESTO/SORST of JST, Grant-in-Aids for Scientific Research, IT Program of RR2002 of MEXT, and Kurata-Memorial Hitachi Science {\&} Technology Foundation.   

Exchange Splitting and 100\% Spin Polarization in Monolayer level EuO Films

The exchange splitting of the conduction band in an ultrathin film of ferromagnetic EuO just 2.5 nm thick has been determined for the first time using tunneling techniques. In a Al/EuO/Y tunnel junction, a huge drop in junction resistance versus temperature was observed below the EuO Tc=69K, resulting from an exchange splitting of 0.3 eV, which corresponds to a spin filter efficiency of 98\% ! Furthermore, substantial tunnel magnetoresistance = 280\% has been observed in Cu/EuO/Gd quasi-magnetic tunnel junctions. From these observations, it appears that EuO is approaching its theoretical spin polarization P of 100\%. Whereas previously, a value of only 30\% was obtained using the Meservey-Tedrow technique of directly measuring P. This drastic improvement occurred after examining the chemical and magnetic properties of EuO at the monolayer level and its interfacial properties with metals, using SQUID magnetometry, XAS, XMCD and XRS. With the right combination of interface materials and deposition parameters, one can have a 1nm EuO film with a high moment of $>$7 $\mu_{B}$. With this high spin filter efficiency and its compatibility with Si, the EuO spin filter shows promise for injecting highly-polarized spins into Si-based semiconductors.   

Room Temperature Tunneling Characteristics through SDT Nanoscaled Lines into n-Doped Si

Nanoscaled spin-dependent tunneling (SDT) lines were patterned on n-doped Si layer and studied for tunneling characteristics from ferromagnetic nano-lines through an AlO$_{2}$ insulating barrier into the semiconductor. The functional magnetic layering was deposited on doped Si with phosphorus (n-type) having resistivity of 0.006-0.02 Ohm-cm. The configuration of the SDT film is 1.5 nm AlO$_{x}$ / 4 nm NiFeCo / 1 nm FeCo / 15 nm Cu / 15 nm CrSi / 10 nm Si$_{3}$N$_{4}$ as spin injection contact. The patterned lines with line width and separation of 100 nm were produced using e-beam lithography. The tunneling characteristics versus temperature (80 to 300 K) were measured by wire bonding and with assistance of ohmic contacts of heavily doped regions. The tunneling studied through the barrier between layered-magnetic metals and semiconductor clearly showed the electronic transport as ballistic tunneling, showing weakly dependence on the temperature. This is qualitatively different similarly scaled-up SDT line-structures with 2 micron gap distance. In the later configuration, the electronic transport was observed to be mainly thermal emission dominant process at elevated temperatures, with characteristic activation energy in agreement with the impurity level.   

Measuring Spin Dependent Hot Electron Transport in Fe/Si(001) Schottky Diodes

Devices that utilize the spin degree of freedom rely on transport of electron spin through materials and material interfaces.~ Further\textbf{ }knowledge of spin-polarized electron transport can aid in the development of spintronic devices.~ To this end, we developed a novel technique; spin polarized ballistic electron emission microscopy (SP-BEEM). This technique has been utilized to study the spin dependent transport properties in Fe/Si(001) Schottky diodes.~ The energetic dependence of the spin dependent attenuation lengths was measured.~ Most interestingly, it was found that the interface band structure played a prominent role in this dependence.~~   

Spin-Valve Photo-Transistor

The Spin-Valve Photo-Transistor is a semiconductor-ferromagnetic metal multilayer-semiconductor transistor operated by photo- exciting hot electrons in the emitter semiconductor into a Schottky collector. We have realized this device using a vacuum- bonded float-zone Si/multilayer/n-InP structure. To distinguish the emitter interband-excited component of collector current from base/collector internal photoemission, we use a lockin spectroscopy sensitive only to the magnetocurrent. Our experimental results indicate a pathway to improve the magnetocurrent of a related device, the Spin- Valve Photo-Diode, by increasing the fraction of hot electron current that travels through both layers of the ferromagnetic spin-valve.   

Influence of Spin-Orbit Interactions on Point Contact Andreev Reflection

In PCAR (point contact Andreev reflection) the I(V) characteristics of an interface between a singlet superconductor and a ferromagnetic metal is used (1)(2) to probe the degree of spin-polarization near the Fermi energy of the ferromagnet. Motivated by recent PCAR studies (3)(4) of (III,Mn)V ferromagnetic semiconductors, in which the spin-orbit interaction scale is comparable to the exchange energy scale, we report on a theoretical study the effect of spin-orbit interactions on the quasiparticle current through a ferromagnet-superconductor interface. Our theoretical analysis generalizes the Blonder-Tinkham-Klapwijk model results commonly used to interpret PCAR experiments. We find that PCAR provides a good qualitative measure of Fermi energy spin-polarization, even when the quasiparticle bands are strongly spin-orbit coupled.\\ (1) R.J. Soulen \emph{et al.}, Science \textbf{282},85 (1998)\\ (2) S.K. Upadhyay \emph{et al.}, Phys. Rev Lett. \textbf{81}, 3247 (1998)\\ (3) J.G. Braden \emph{et al.}, Phys. Rev. Lett. \textbf{91},56602 (2003)\\ (4) R.P. Pangulury \emph{et al.}, Appl. Phys. Lett. \textbf{84},4947 (2004)   

Zero-Bias Conductance Peak in Al/AlOx/Sc Tunnel Junctions

We have fabricated a series of Al/AlO$_{x}$/Sc tunnel junctions and measured the differential conductances at low temperatures. 25-nm thick Al (99.999{\%}) stripes were first thermally evaporated onto a glass substrate, followed by glow discharge under an O$_{2}$ atmosphere, to form a thin insulating AlO$_{x}$ layer. Subsequently, a 60-nm thick Sc (99.99{\%}) film was thermally evaporated across the oxidized Al stripes to form tunnel junctions of 1 mm $\times $ 1 mm. Lock-in techniques were used to measure the differential conductances dI/dV(G) of the junctions. Zero-bias conductance peaks were found in all the tunnel junctions. In particular, the magnitudes of the zero-bias conductance peaks reveal a -ln$T$ dependence below about 30 K, which could be attributed to the electron-magnetic impurities interactions according to the theory of Appelbaum. However, the magnetic field has only a small effect on the conductance peaks. An asymmetric term in G(V) was observed, which is strongly temperature dependent and magnetic-field insensitive. Possible explanations will be discussed.