Session D33: Focus Session: Spin Dependent Phenomena in Semiconductors: III

Theory of Current-Induced Domain Wall Creep in (Ga,Mn)As.

A magnetic domain wall (DW) can interact with electrical current and as a result its displacement is induced by the application of the current, showing the possible electrical control of the magnetization direction. Thus electrical current in this non-uniform spin texture is now drawing much attention from the technological point of view, in addition to the longstanding interests from the fundamental physics points of view. Recently, systematic experimental data of the dependence of DW velocity spanning five decades on current density have been obtained in a microstructure made from a ferromagnetic semiconductor (Ga,Mn)As, providing deep insight to outstanding physics of DW dynamics. The quantitative analyses showed that the current-driven motion in (Ga,Mn)As can be explained by spin-transfer mechanism under currents beyond a threshold value. The linear mobility tempts ones to expect there is always an equivalent magnetic field which has the same effect upon DWs as the current does. Here we make a detailed comparison between these two sources of drive but in the subthreshold, ``creep,'' regime, where the velocity obeys an Arrhenius scaling law. The observed scaling law for the two drives is incompatibly different from each other, i.e., the effect of a driving current and field are not equivalent. We offer theory which explains the important features of experiment. When described by an Arrhenius law it is found that the barriers diverge as the drive approaches zero, manifesting the system is in a ``glassy" state. While the field driven case is compatible with the random field universality class, the case of current induced creep is not to fit any known such class. The work reported is the result of collaborations with M. Yamanouchi, F. Matsukura, S. E. Barnes, S. Maekawa, and H. Ohno. [Ref. Science 317, 1726-1729 (2007).]   

Analyzing the influence of magnetic domain walls on longitudinal and transverse magnetoresistance in tensile strained (Ga,Mn)As

We present a theoretical analysis of magnetoresistance in (Ga,Mn)As epilayers with perpendicular magnetic anisotropy [Phys. Rev. B {\bf 76}, 054440 (2007)]. The model reproduces the field-antisymmetric anomalies observed in experimental measurements [Phys. Rev. B {\bf 71}, 241307(R) (2005)] of the longitudinal magnetoresistance in the planar geometry (magnetic field in the epilayer plane and parallel to the current density), as well as the unusual shape of the accompanying transverse magnetoresistance. As in the case of metallic ferromagnets with perpendicular anisotropy [Phys. Rev. Lett. {\bf 94}, 017203 (2005), the magnetoresistance characteristics are attributed to circulating currents created by the presence of magnetic domain walls.   

Manipulation of Magnetic Domain Walls in Patterned (Ga,Mn)As Devices

Ferromagnetic semiconductors such as (Ga,Mn)As provide new opportunities for the electrical manipulation of magnetic domain walls in a different regime of parameter space compared with ferromagnetic metals [Chiba et al, PRL 96, 096602 (2006)]. Here, we discuss different approaches to pinning and controlling magnetic domain walls in laterally patterned (Ga,Mn)As microdevices with perpendicular magnetic anisotropy. The pinning/depinning of domain walls is monitored using measurements of the magnetoresistance, the anomalous Hall effect and high speed Kerr microscopy. The domain wall pinning potential is engineered using a variety of schemes, including lateral shape engineering and lithographically integrated micromagnets. We find that even simple schemes (such as lateral notches) can significantly enhance domain wall pinning in relatively large (micron scale) devices. Supported by ONR MURI.   

Spin valve effect in self-exchange biased ferromagnetic metal/semiconductor heterostructures

The systematic engineering of exchange biased ferromagnetic semiconductor spin valve devices is important for developing proof-of-concept semiconductor spintronics devices (such as spin torque oscillators). Here, we report magnetization and current-perpendicular-to-the-plane (CPP) magnetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures built from MnAs and (Ga,Mn)As [APL 91, 192503 (2007)]. We observe an exchange biased CPP spin valve effect in MnAs/(Ga,Mn)As bilayers, and discuss the dependence of the exchange field and the spin valve effect on (Ga,Mn)As layer thickness. We also demonstrate the CPP spin valve effect and exchange biasing in MnAs/ p-GaAs/ (Ga,Mn)As trilayers, and discuss the dependence of both phenomena on the doping and thickness of the non-magnetic spacer layer.   

The Influence of the Doping Profile on Spin Transport in Fe/GaAs Schottky Tunnel Barrier Heterostructures

A strong non-monotonic dependence of the spin polarization on the bias across the injector has been observed in recent studies of spin transport in Fe/GaAs heterostructures. We have conducted a study of spin transport in non-local Fe/GaAs spin valves in which the doping profile of the Schottky barrier has been systematically modified. The samples were $\mbox{Fe}/n^ {+}/n\mbox{-GaAs}$ heterostructures in which the thickness $d$ of the $n^{+}$ layer ($n^{+}$ fixed at $5\times10^{18}$ cm$^{-3}$) was varied from 5 nm to 50 nm while $n \approx 5\times10^{16}$ cm$^{-3}$ in the 2.5 $\mu$m thick channel. We performed non- local spin valve measurements at 15 K for unannealed samples and after annealing at 200$^{\circ}$C and 250$^{\circ}$C. For $d$ less than 10 nm, no spin accumulation is observed under either forward or reverse bias. For $d \approx $ 15 nm, spin accumulation is observed under forward bias only. Spin accumulation is observed for both bias polarities at larger thicknesses, with an optimal $d \approx $ 20-25 nm. Although this overall trend with $d$ is observed in both unannealed and annealed samples, the sign and magnitude of the non-local signal can change upon annealing. These results suggest that spin accumulation is sensitive to both the tunnel barrier profile and interfacial conditions. This work was supported by ONR and the NSF MRSEC, IGERT, and NNIN programs.   

Single domain switching by spin-polarized current in GaMnAs nanodevice

Dilute magnetic semiconductors (DMS) have a potential to bring electrostatic control into magnetic domain and bridge the gap in control efficiency between conventional ferromagnetic materials and semiconductors. A significant progress has been demonstrated in current-induced magnetization reversal, where DMS materials show a few orders of magnitude current reduction compared to the conventional ferromagnets. In this work we demonstrate and investigate in-plane single domain magnetization rotation and reversal in GaMnAs nanodevices by spin-polarized dc electric current. Single domain is defined lithographically, which eliminates unambiguity associated with previously investigated multi-domain switching. The magnetization orientation can be controllably switched between two [100] and [010] easy axes or reversed. Current alone is not sufficient to switch the magnetization and have been aided by small in-plane magnetic field ($\sim $10mT). We observe linear dependence of critical currents with respect to magnetic field and analyze it in terms of current induced torque on the domain walls. The critical current densities are of the same order as for out-of-plane magnetization switching reported by Ohno, et. al.   

Magnetoresistance Enhancement through a Resonant Tunneling Diode based in the GaMnAs/AlGaAs Materials System

A resonant tunneling diode was fabricated with ferromagnetic GaMnAs emitter and quantum well regions and a nonmagnetic p- GaAs collector. Negative differential resistance (NDR) associated with resonant tunneling of holes was observed at 4K, which is below the Curie temperature for GaMnAs. 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 30\%, depending on device bias. Current-voltage sweeps reveal the source of the magnetoresistance as a shift in the NDR features to higher bias when the magnetizations of the GaMnAs films become antiparallel. We attribute this bias shift to an increase in tunneling conductivity from the emitter to quantum well for antiparallel GaMnAs magnetization alignment.   

Dissipationless anomalous transport properties and Mott relation in Ga$_{1-x}$Mn$_{x}$As

We have found an anomalously large Nernst effect (ANE) accompanying the anomalous Hall effect (AHE) in~a series of Ga$_{1-x}$Mn$_{x}$As (x=0.04-0.07) ferromagnetic semiconductor samples with perpendicular anisotropy. Without applying a magnetic field, none-zero ANE and AHE are observed, and both effects are very well scaled with the sample magnetization. We have developed a method, which dose not depend on the accuracy of magnetization measurement, to study the anomalous transport effects. By measuring AHE and ANE under zero magnetic field and over a wide range of temperatures, we have demonstrated the dissipationless origin of the anomalous electrical and thermoelectric transport properties in these samples.~ Furthermore, we have successfully verified the Mott relation for the off-diagonal transport coefficients in the regime of dissipationless transport that may not depend on scattering.   

Localization effects in ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb random alloys

We have investigated temperature and magnetic field dependence of resistance (R$_{xx})$ in MBE-grown ferromagnetic Ga$_{1-x}$Mn$_{x}$Sb films in which the ferromagnetism is mediated by holes [1]. Samples with higher carrier densities (6.7 x 10$^{19 }$cm$^{-3}$ and 1.3 x 10$^{20 }$cm$^{-3}$, with Curie temperatures, T$_{c}$, of 13 K and 24 K respectively) show metal-like behavior in the temperature dependence of R$_{xx}$. These samples exhibit small positive magnetoresistance (MR) up to 0.5 T between 1.6 K and 20 K, followed by negative MR up to 10 and 11 T (at 2.4 K), respectively. Samples with lower carrier densities (2.9 x 10$^{19 }$cm$^{-3}$ and 3.9 x 10$^{19 }$cm$^{-3}$, with T$_{c}$'s of 13 K and 24 K, respectively) show ``weak'' thermally activated behavior and negative MR between 1.6 K and 50K (but no positive MR at low fields). The latter samples exhibit low field positive MR between 35 mK and $\sim $ 400 mK, followed by negative MR up to 8 T. R$_{xx}$ increases with decreasing temperature for both samples at zero field, and the magnitude of the change increases with applied magnetic field. These results will be discussed in terms of localization behavior in this system. [1] Eginligil et al.,\textit{ in press} PhysicaE (2007)   

Huge tunneling magnetoresistance ($>$18300{\%}) in semiconductor based magnetic tunnel junctions with zinc-blende MnAs nanoparticles

Zinc-blende (ZB) MnAs nanocrystallite is a new prospective material for semiconductor spintronics, since it is expected to be haft-metallic. However, there is no report on the magneto-transport characteristics of ZB MnAs nanoparticle system. In this paper, we report on the huge tunneling magnetoresistance (TMR) effect in MBE-grown magnetic tunnel junctions (MTJs), whose structure is (from the top to the bottom) hexagonal MnAs film (20 nm) / GaAs (1 nm) / AlAs (2.1 nm) / GaAs:MnAs (10 nm), revealing the haft-metallicity of ZB MnAs nanocrystallite. Here, the GaAs:MnAs layer contains ZB MnAs nanoparticles embedded in a GaAs matrix. The tunnel resistance decreases sharply with increasing the magnetic field, resulting in a huge TMR ratio = ($R_{max}-R_{min})$/$R_{min }>$ 18300{\%}. The TMR ratio decreases quickly with increasing the bias voltage and temperature, but survives up to 100 K. Such a huge TMR effect can be explained by an unique combination of Coulomb Blockade effect and large Zeeman splitting in half-metallic ZB MnAs nanoparticles. A magnetic-field dependent electromotive force emerged from those MTJs was also observed.   

Ultrafast Photoinduced Demagnetization in (III,Mn)V Ferromagnetic Semiconductors

Ultrafast light-induced demagnetization, in which photoexcitation leads to a decay of magnetization in less than a picosecond, has been recently observed in (III,Mn)V materials [1]. To explain these measurements, we have proposed a theory of ultrafast magnetization dynamics within the sp-d model [2]. We have calculated the spin-flip scattering between the localized spins and the carriers strongly excited by the laser pulse. In this process the energy is pumped into the localized spin system, while the angular momentum is transferred to the carriers, leading to their dynamical spin polarization. For significant ultrafast demagnetization, this polarization has to be efficiently relaxed by spin-orbit assisted scattering of carriers - otherwise a ``spin bottleneck'' can occur, in which the carriers' spin polarization quickly becomes large enough to suppress further spin-flip scattering. Because of that, and also due to their larger exchange coupling to Mn spins, the holes (having a very short spin relaxation time) are much more important than photoelectrons for demagnetization. Since the spin-flip transition rate is proportional to the carrier temperature, the time-scale for this two-step process of demagnetization is given by the energy relaxation time of very hot holes. I will discuss in detail the application of this theory to (III,Mn)V semiconductors taking into account their valence band structure, and the fact that their optical properties are strongly affected by disorder inherent to these materials. \newline 1. J. Wang, C. Sun, J. Kono, A. Oiwa, H. Munekata, L. Cywinski, and L.J. Sham, Phys. Rev. Lett. {\bf 95}, 167401 (2005) 2. L. Cywinski and L.J. Sham, Phys. Rev. B, {\bf 76}, 045205 (2007)