Session L14: Focus Session: Spins in Semiconductors - Magnetic Semiconductors II

The intra-gap electromagnetic response of gated GaMnAs in an electric double-layer field effect

We have fabricated field effect transistors (FETs) utilizing an electrolyte oxide gate insulator for monitoring electrostatic doping in GaMnAs via infrared (IR) spectroscopy. Previous studies of gated GaMnAs have been confined primarily to transport measurements. IR experiments are able probe the electric field induced changes to the optical conductivity spectrum, providing direct insight into electronic structure of GaMnAs. The IR spectra show an enhancement of the Drude response and mid-IR resonance of GaMnAs upon hole accumulation, with symmetric decrease in these features in the depletion mode. A sum rule analysis of the IR spectra, combined with charge accumulation/depletion information from transport measurements for the same gated structures, enables accurate evaluation of the effective masses of mobile holes without any underlying assumptions about the level of compensation and/or disorder in the films. The low bound for the effective mass is several $m_{e}$, where $m_{e}$ is the free electron mass. We therefore conclude that electronic states in the vicinity of the Fermi energy retain significant impurity band character.   

Effect of As vacancies on the binding energy and exchange splitting of Mn impurities on a GaAs surface

State-of-the-art STM spectroscopy is nowadays able to manipulate and probe the magnetic properties of individual magnetic impurities located near the surface of a semiconductor. A recent advance of these technique employs the electric field generated by a As vacancy in GaAs to affect the environment surrounding substitutional Mn impurities in the host material [1]. Here we calculate the binding energy of a single Mn dopant in the presence of nearby As vacancies, by using a recently-introduced tight-binding method [2] that is able to capture the salient features of Mn impurities near the (110) GaAs surface. The As vacancies, modeled by the repulsive potential they produce, are expected to decrease the acceptor binding energy in agreement with experiment [1]. Within this theoretical model, we investigate the possible enhancement of the exchange splitting for a pair of ferromagnetically ordered Mn impurities, observed experimentally when As vacancies are present [3]. We also calculate the response of the Mn-impurity---As-vacancy complex to an external magnetic field. \\[4pt] [1] H. Lee and J. A. Gupta, Science, 1807-1810, (2010). \\[0pt] [2] T. O. Strandberg, C. M. Canali, A. H. MacDonald, Phys. Rev. B 80, 024425, (2009). \\[0pt] [3] J.A. Gupta, private communication.   

Observation of a Sharp Band Edge Response in GaMnAs Using Nonlinear Spectroscopy

Diluted magnetic semiconductors (DMS) have gained considerable interest over the last decade owing to their potential use in magneto-sensitive electrical and optical devices, in which the carrier-mediated nature of the ferromagnetism allows the magnetic properties to be controlled using optical or electrical gating techniques. The position of the Fermi level, which may lie within the valence band or within an impurity band, plays a critical role in current theories of ferromagnetism in DMS and has been the subject of intense controversy in recent years. Linear spectroscopy techniques are unable to address this issue due to strong band tailing in the vicinity of the fundamental band gap. Here we present results of time- and spectrally-resolved differential reflection measurements on GaMnAs, as well as low-temperature-grown GaAs and semi-insulating GaAs. Our results indicate a sharp band edge response due to the diminished contributions of band tail states in the nonlinear optical regime. We observe a blue shift of the band gap in GaMnAs, supporting a valence band model of ferromagnetism in DMS. Numerical simulations of the measured nonlinear response using an effective mass model support our conclusions.   

Tuning the magnetic interaction between Mn dopants in GaAs

Manganese can be used as a dopant in gallium arsenide to create a ferromagnetic semiconductor. We use low-temperature scanning tunneling microscopy to study these magnetic properties. The magnetic coupling between Mn dopants in GaAs(110) changes between ferromagnetic and antiferromagnetic depending on the orientation of the acceptors due to the zincblende crystal structure of the surface [Kitchen et al, Nature, 2006]. We have recently reported tuning of the resonance energy for a single Mn acceptor by moving charged atomic point defects [Lee and Gupta, Science, 2010]. Here, we tune the magnetic interaction between surface-layer Mn atoms in the same way. Funding for this research was provided by the Center for Emergent Materials at the Ohio State University, an NSF MRSEC (Award Number DMR-0820414). http://www.physics.ohio-state.edu/$\sim$jgupta/   

Spin-Seebeck Effect in III-V Based Semiconductors

The spin-Seebeck effect has now been observed in metals$^{1}$ (NiFe), semiconductors$^{2}$ (GaMnAs), and insulators$^{3}$ (YIG). It consists of a thermally generated spin distribution that is phonon driven. Here we extend our measurements of the spin-Seebeck effect to other group III-V based magnetic semiconductors and present measurements of conventional thermomagnetic and galvanomagnetic properties as well as the spin-Seebeck effect. Work supported by the National Science Foundation, NSF-CBET-1133589 1. K. Uchida, et al., Nature \textbf{455} 778 (2008) 2. C.M. Jaworski et al., Nature Materials \textbf{8} 898 (2010), Phys. Rev. Lett. \textbf{106} 186601 (2011) 3. K. Uchida, et al., Nature Materials \textbf{8} 893 (2010)   

High Resolution Magneto-Optic Measurements in GaAs using a Sagnac Interferometer

The Sagnac Interferometer is a tool which measures the Polar Kerr effect--a direct indicator of magnetism. ~Using 820 nm light from a superluminescent diode, we probe GaAs structures and measure the Kerr angle with sub-microradian resolution. ~By utilizing diffraction limited optics and a piezoelectric scanner, we also achieve high spatial resolution. ~Our measurements are performed at cryogenic temperatures and offer a way to measure the Spin Hall Effect in the DC regime along with other forms of magnetic order.   

All-optical four-state magnetization reversal in (Ga,Mn)As ferromagnetic semiconductors

The emerging field of femtomagnetism has revealed the central role of non-equilibrium interactions and transient optical coherence in determining photoinduced spin dynamics. However the many-body theory of such effects remains controversial. A microscopic theory that engages the elements of coherence, correlation and nonlinearity on an equal footing is needed. We propose here such a theory, based on density matrix equations of motion and a tight-binding band calculation. We prepare the system within 100fs, via coherent nonlinear photoexcitation close to the strong peak of the density of states for interband transitions along the eight equivalent directions {\{}111{\}} of the GaAs BZ in the vicinity of 3eV. It then selectively relaxes to one of the four local minima of the magnetic free energy with biaxial anisotropy. We thus propose a non--thermal mechanism for all-optical switching between four metastable magnetic states, initiated non-thermally within 100fs and completed within 100ps. Our predicted switching comes from magnetic nonlinearities triggered by a femtosecond magnetization tilt that is sensitive to un-adiabatic light-induced spin interactions and controlled via the optical and the external magnetic field.   

First-Principle Calculation of The Effective Hamiltonian for (Ga,Mn)As and (Ga,Mn)N

Most of the models used to study (Ga,Mn)As have failed to explain the experimental results of (Ga,Mn)N especially its ferromagnetic critical temperature $T_c$. The need for a consistent and comprehensive model for the dilute magnetic semiconductors (DMS) motivates our study. We obtain the effective Hamiltonian for (Ga,Mn)As and (Ga,Mn)N using a Wannier function based first-principles method. We use density functional theory to calculate the band structure of a range of disordered supercell configurations of (Ga,Mn)As and (Ga,Mn)N and Wannier functions to obtain downfolded Hamiltonians. Those are then disorder averaged to get an effective Hamiltonian. We solved this effective model using the dynamical mean field approximation.   

Polaronic trapping in magnetic semiconductors

GaN doped with iron is an interesting candidate material for magnetic semiconductors, as p-d coupling between the localized Fe-d and extended N-p hole states is expected to facilitate long-range ferromagnetic alignment of the Fe spins [1]. This picture of extended states in GaN:Fe, however, falls apart due to a polaronic localization of the hole carriers nearby the Fe impurities. To elucidate the carrier localization in GaN:Fe and related iron doped III-V semiconductors, I present a systematic study using self-interaction corrected density-functional calculations [2]. These calculations predict three distinct scenarios. (i) Some systems do sustain extended host-like hole states, (ii) some exhibit polaronic trapping, (iii) and some exhibit carrier trapping at Fe-d orbitals. These behaviors are described in detail to give an insight as to how to distinguish them experimentally. I thank T. Fujita, C. Echeverria-Arrondo, and A. Ayuela for their collaboration.\\[4pt] [1] T. Dietl et al, Science, 287, 1019 (2000).\\[0pt] [2] S. Lany and A. Zunger, Phys. Rev. B, 80, 085202 (2009).   

Magnetoresistance in antiferromagnet-based spin tunnel junctions

To date spintronics research and applications of magnetically ordered systems have focused on ferromagnets (FMs). There are, however, fundamental physical limitations for FM materials which may make them impractical to realize the full potential of spintronics. Metal FMs offer high temperature operation but the large magnetic stray fields make them unfavorable for high-density integration and metals are unsuitable for transistor and information processing applications. FM semiconductors on the other hand do not allow for high-temperature operation. We present a concept in which these limitations are circumvented in spintronics based on antiferromagnets. The concept is based on relativistic magnetic and magneto-transport anisotropy effects in nanodevices whose common characteristics is that they are an even function of the microscopic magnetic moment vector, i.e., can be equally strong in AFMs as in FMs. As a demonstration we present our experimental observation of $>$100\% tunneling anisotropic magnetoresistance in a device with an IrMn AFM tunnel electrode [1]. We will also discuss candidate materials for high-temperature AFM semiconductor spintronics [2].\\[4pt] [1] B.~G. Park, J.Wunderlich, X.Marti, V.Holy, Y.Kurosaki, M.Yamada, H.Yamamoto, A.Nishide, J.Hayakawa, H.Takahashi, A.B.Shick, T.Jungwirth, Nature Mat. \textbf{10}, 347 (2011). \\[0pt] [2] T.Jungwirth, V.Nov\'{a}k, X.Marti, M.Cukr, F.M\'{a}ca, A.B. Shick, J.Ma\v{s}ek, P.Horodysk\'a, P.N\v{e}mec, V.Hol\'y, et~al., Phys. Rev. \textbf{B 83}, 035321 (2011).   

Numerical studies of non-Drude ac-conductivity and infrared magneto-optics in (Ga$_{1-x}$,Mn$_{x}$As

Optical absorption experiments on (III,Mn)V diluted magnetic semiconductors (DMS's) show that the ac-conductivity has non-Drude behavior at low frequency. The numerical simulation of this problem has been done previously using the effective Hamiltonian model with various treatments of the disorder effects. We are re-examining the previous works with a similar numerical method to establish the nature of the transitions in the low to the high doped regime. We use the effective Hamiltonian k.p model to describe the holes introduced by Mn impurities and treat the Mn impurities exactly using the envelope function approximation. With this technique we are able in principle to analyze spectrally the origin of the mid-infrared absorption peak, its trends, and the nature of the states near the Fermi energy as well as the excitation states. We will also report on numerical results of the magneto-optical response with this more accurate treatment of the effect of disorder.   

Electric field manipulation of room temperature ferromagnetism in anatase Ti$_{1-x}$Co$_{x}$O$_{2-\delta}$

Ferromagnetic semiconductor is one of the most attractive materials for semiconductor spintronics because of the controllability of both charge and spin degrees of freedom. Electric field effect of magnetism in the ferromagnetic semiconductors such as (Ga,Mn)As has been demonstrated only at low temperature due to their low Curie temperatures. In this study, we report the electric field manipulation of ferromagnetism in a ferromagnetic semiconductor Ti$_{1-x}$Co$_{x}$O$_{2-\delta }$ at room temperature [1]. Anatase Ti$_{1-x}$Co$_{x}$O$_{2-\delta }$ (001) epitaxial film was deposited on TiO$_{2}$ buffer 5 nm / LaAlO$_{3}$ (100) substrate in various oxygen pressures in order to vary an electron density by pulsed laser deposition method. An electric double layer transistor was fabricated on a paramagnetic film with an electron density of 1x10$^{19}$ cm$^{-3}$. With increasing gate voltage, the electron density was increased to 7x10$^{19}$ cm$^{-3}$. Ferromagnetic hysteresis loop was observed for $V_{G}$ above 3.0 V in an anomalous Hall resistivity, which is proportional to a magnetization of the film. This result represents that the ferromagnetism was induced at room temperature by an electrostatic charge accumulation, indicating that the ferromagnetism in this compound is mediated by the electron carriers. \\[4pt] [1] Y. Yamada et al., Science \textbf{332}, 1065 (2011).   

BaMn$_{2}$Sb$_{2}$: A New Semiconducting Ferromagnet

We have grown high-quality single crystals of BaMn$_{2}$Sb$_{2}$, which possesses the ThCr$_{2}$Si$_{2}$ structure as determined by X-ray powder diffraction technique. Magnetization measurements indicate that BaMn$_{2}$Fe$_{2}$ is ferromagnetic below $T_{C}$ = 580K. On the other hand, the temperature dependence of electrical resistivity shows semiconducting behavior, which can be described by thermally-activated resistivity formula with thermal activation energy about 0.25 eV . While the Hall coefficient has positive sign between 2 and 300 K, the Seebeck Coefficient undergoes sign change from positive at high temperatures to negative at low temperatures, reaching -260 $\mu $V/K at 70 K. The implication will be discussed.