Session W14: Focus Session: Spins in Semiconductors - Ferromagnetism and Spin Dynamics in Semiconductors

Spin Dynamics in Bi2Se3 / GaAs Heterostructures

The narrow band gap semiconductor Bi$_2$Se$_3$ has been characterized as a topological insulator (TI), wherein strong spin-orbit coupling and time-reversal symmetry give rise to spin-polarized surface conduction states. Molecular beam epitaxy (MBE) of Bi$_2$Se$_3$ thin films onto conventional semiconductors such as GaAs [1] provides an attractive pathway for the creation of hybrid devices, coupling the exotic spin physics of TIs with the well-understood properties of spin coherence in semiconductors. We employ spatio-temporally resolved optoelectronic techniques to probe the carrier spin dynamics at the heterointerface between a TI and GaAs. Results are compared with interface band structure calculations.\\[4pt] This work is supported by ONR and NSF. \\[4pt] [1] A.~Richardella, D.~M.~Zhang, J.~S.~Lee, A.~Koser, D.~W.~Rench, A.~L.~Yeats, B.~B.~Buckley, D.~D.~Awschalom and N.~Samarth, {\it Appl.~Phys.~Lett.} {\bf 97}, 262104 (2010).   

Optical Aharonov-Bohm Effect in Al$_{0.08}$Ga$_{0.92}$As/Al$_{.25}$Ga$_{0.75}$As Quantum Wells

The photoluminescence (PL) from Al$_{0.08}$Ga$_{0.92}$As/Al$_{.25}$Ga$_{0.75}$As quantum wells (QW) was studied as function of magnetic field applied along the normal to the QW planes. The PL intensity exhibits two maxima at 2.3 and 4.9 tesla. The time-resolved PL from the same sample has a decay time which is one order of magnitude longer than the PL from a GaAs/AlGaAs QW, indicating that the recombination in the AlGaAs QW is spatially indirect. The PL intensity oscillations are attributed to the optical Aharonov-Bohm effect associated with spatially quasi-indirect excitons, which are located in the vicinity of islands with lower Al composition. The holes are localized by the islands, while the electrons move around them in a ring-like geometry. This model is in agreement with the interpretation of earlier results from Al$_{x}$Ga$_{1-x}$As/Al$_{y}$Ga$_{1-y}$As Fe spin-LEDs.   

Optical response of relativistic electrons in the polar BiTeI semiconductor

The transitions between the spin-split bands by spin-orbit interaction are relevant to many novel phenomena such as the resonant dynamical magneto-electric effect and spin Hall effect. Here, we present a combined experimental and theoretical study of the dynamics of relativistic electrons in the recently discovered giant bulk Rashba spin splitting system BiTeI. Several novel features are observed in the optical spectra including sharp edge singularity due to the reduced-dimensionality of joint density of states and a systematic doping dependence of the intraband transitions between the Rashba-split branches. These confirm the bulk nature of the Rashba-type splitting in BiTeI and manifest relativistic nature of the electron dynamics in a solid.   

Room temperature ferromagnetism in transparent conducting Fe-doped In$_{2}$O$_{3}$ films

Oxide semiconductors have been widely studied as a host compound for spintronic devices since they can be doped with transition metals to realize a higher Curie temperature and can produce high n-type carriers by either doping with Group IV elements or introducing oxygen vacancies. Among various oxide semiconductors, Fe-doped In$_{2}$O$_{3}$ is a promising ferromagnetic semiconductor due to the high solubility of Fe-ions into the In$_{2}$O$_{3}$ lattice. Recently, at NRL, In$_{2-x}$Fe$_{x}$O$_{3}$ thin films have been deposited on MgO, sapphire, and YSZ substrates by pulsed laser deposition. The lattice constant decreases linearly with increasing Fe-doping concentration suggesting the incorporation of Fe ions into the In$_{2}$O$_{3}$ lattice matrix. Magneto-transport characteristics including anomalous Hall effect along with structural analysis demonstrate that an intrinsic ferromagnetism is observed for some films grown under optimized conditions. In this presentation, we will discuss our work to date on the growth of In$_{2-x}$Fe$_{x}$O$_{3}$ thin films grown by pulsed laser deposition with various deposition conditions and present the structural, optical, magnetic, and transport properties along with spin-polarization measurements.   

Selective optical excitation of in-plane and out-of-plane spin polarizations with linearly polarized light in InGaAs

Excitation with circularly polarized light is a standard technique for optical spin orientation in semiconductors. This method is based on the transfer of angular momentum from the photons to the electrons and yields a polar spin polarization directed along the propagation direction of the exciting laser beam. Here we present linearly polarized all-optical pump-probe experiments to excite and detect coherent electron spins in InGaAs [1]. We find the magnitude and the orientation of the spin polarization strongly depend on the polarization axes of the exciting light. While in general the excited spin ensemble is composed of both polar and transverse spin components, the polarization axis of the exciting light can be chosen such that polar and transverse spin components can be excited separately. Thus, selective excitation of in-plane and out-of-plane spin polarizations is feasible with linearly polarized light.\\[4pt] [1] K. Schmalbuch \textit{et al.}, Phys. Rev. Lett. 105, 246603 (2010)   

Carrier Dynamics in Narrow Gap Ferromagnetic Semiconductors

Narrow gap ferromagnetic semiconductors are promising materials for spin photonic and spin transport devices because of their small effective masses, small energy gap, and high carrier mobility. We use time resolved differential transmission (TRDT) experiments to study carrier dynamics in ferromagnetic InMnAs and InMnSb. Electronic structure for InMnAs and InMnSb is calculated using an 8-band Pidgeon-Brown model generalized to include the effects of an external magnetic field. Our model includes the effects of the ferromagnetic Mn ions and their coupling to electrons and holes with or without an external magnetic field. Optical transitions are calculated from Fermi's Golden rule and interband transitions at a given pump or probe laser energy are identified. This allows us to understand a sign change seen in the TRDT. Our results show that 1) Phase-Space Filling, 2) Band Gap Renormalization and 3) Free Carrier Absorption all contribute to the TRDT and that the relative importance of these effects depends on the laser probe energy.   

Ferromagnetism in cobalt-doped SrTiO3 on Si grown by molecular beam epitaxy

We report the epitaxial growth of ferromagnetic cobalt-doped SrTiO$_{3}$ directly on silicon without the use of any buffer by molecular beam epitaxy (MBE). Magnetization as a function of magnetic field was performed for samples with varying doping concentration at room temperature and at 10 K. Room-temperature ferromagnetism is confirmed in single phase samples with composition 20-30\% cobalt. We also performed x-ray photoelectron spectroscopy of the Co and Ti 2p levels to determine stoichiometry and cobalt oxidation state. In order to elucidate the origin of ferromagnetism, we also performed first-principles calculations of cobalt-doped SrTiO$_{3}$ with different doping concentrations and dopant configurations. The calculations show that intrinsic ferromagnetism can be stabilized beyond a critical concentration in SrTiO$_{3}$ under particular conditions. The ability to directly integrate a ferromagnet on silicon in epitaxial form may potentially overcome the problems of impedance mismatch and interface losses in applications involving spin injection in silicon.   

Two-dimensional MnGaN Layer formed by Nitridation of Mn $\surd $3 $\times $ $\surd $3-R30 structure on Wurtzite GaN (000\underline {1})

There has been much interest in dilute Mn-doped GaN as a spintronic material. Recently, it has become of interest to consider the possible advantages of delta-doped magnetic layers, rather than a bulk alloy. Here we investigate experimentally the growth of single Mn-containing layers on top of wurtzite GaN as well as the overgrowth of GaN onto the Mn-containing layer, using a combination of N-plasma molecular beam epitaxy and reflection high energy electron diffraction. Sub-monolayer Mn deposition on GaN(000\underline {1}) results in a novel $\surd $3 $\times \quad \surd $3-R30 structure [1]. Upon nitrogen plasma exposure, this periodicity is removed; whereas, Mn is found remaining on the surface as measured by Auger electron spectroscopy. Unexpectedly, and in dramatic comparison to tiny lattice constant changes seen for bulk films, we find a huge -3.3 percent surface lattice contraction in both [10\underline {1}0] and [11\underline {2}0] azimuthal directions. This suggests the possible formation of a Mn0.25Ga0.75N 2-D alloy. Furthermore, subsequent overgrowth of GaN layers does not show significant lattice constant change compared to the nitridation process, up to a thickness of 20 ML or more.\\[4pt] [1] Chinchore et al., Applied Physics Letters \textbf{93(18)}, 181908 (2008).   

Structural transition and giant spintronic response of two dimensional Manganese-Gallium $\surd $3$\times \surd $3 R30\r{ } surface structure

In recent experiments, we have found that gallium nitride surface when exposed to transition metal atoms, results in novel well-ordered two dimensional spintronic structure, with tunable spintronic properties. A 2000 {\AA} N-polar $w$-gallium nitride (000\underline {1}) layer is grown on a sapphire substrate, by molecular beam epitaxy. The growth is monitored using reflection high energy electron diffraction system. Post growth, the standard 1$\times $1 gallium nitride surface, is exposed to sub monolayer doses of manganese. At low deposition temperature the diffraction patterns show manganese atoms forming a metastable 3$\times $3 structure, on supplying little heat to the manganese 3$\times $3 structure, it undergoes an irreversible transition to form a stable $\surd $3$\times \surd $3R30\r{ } structure. Both the structures are studied using a scanning tunneling microscope. The $\surd $3$\times \surd $3R30\r{ } structure shows a giant change in spectroscopic response on application of a very small out-of-plane magnetic field. The new findings suggest that the two dimensional magnetic nitride systems have excellent potential for both fundamental investigations and for use in future spintronic devices.   

Spinglass Dynamics of Amorphous Ferromagnetic Ge:Mn

Ge$_{0.84}$Mn$_{0.16}$ is an amorphous ferromagnetic semiconductor with a Curie temperature of about 160 K (measured at 1000 Oe). Magnetic field-cooled and zero-field-cooled experiments show existence of a spinglass phase well below the Curie temperature. The spinglass temperature is about 23 K at a 50 Oe field. The spinglass temperature scales monotonically with applied magnetic field. Magnetic hysteresis experiments show a non-zero coercive field below the spinglass temperature, and a very small coercive field or a superparamagnetic phase above it. Under a rapid quench, thermoremanent magnetization (TRM) decay experiments (at 14 K) exhibit a very large effective waiting time. This is associated with a slow rate of cooling, allowing communication between phase space states before the measuring temperature is reached, leading to an effective reduction in the attempt frequency. This is consistent with experiments on other more typical spin glass systems.   

Magnetism in Iron-Titanium Oxide Nanostructures

Modification of the titania nanotubes with magnetic transition metal additions is anticipated to provide the opportunity of creating a novel multifunctional nanostructured material with magnetic, semiconducting and catalytic properties. Incorporation of Fe into titania to produce ordered arrays of amorphous (Fe+Ti)O$_{2}$ nanotubes was achieved by electrochemical anodization. As-made nanotubes were subjected to a systematic thermal treatment in a variety of atmospheres for crystallization; their associated morphological, structural and magnetic character was examined. Preliminary results indicate that the Fe-modified nanotubes possess a unit cell volume that is slightly larger than that of titania (137 \textit{vs} 136 {\AA}$^3$) confirming Fe incorporation into the lattice. The temperature-dependent magnetic susceptibility data obtained from the samples may be decomposed into a Curie-Weiss component that represents the localized magnetic character of titania nanotubes and a Pauli paramagnetic component that represents the semiconducting behavior of the nanotubes. It is noted that Fe incorporation causes an increase in both components of the magnetic signal, suggesting modification of the electronic structure of the crystalline titania phase with iron incorporation.   

Magnetoluminescence Studies of Mn-doped PbS Quantum Dots

Diluted magnetic semiconductor quantum dots are interesting model systems for the investigation of carrier dopant exchange interactions. In this work, we report the carrier spin polarization studies in narrow band gap Mn-doped PbS quantum dots, a much less studied system compared to their II-VI counterparts. The PbMnS quantum dots were synthesized by hot colloidal solution technique. They are single crystalline with cubic structure. The doping concentration is 3-4{\%} as measured by energy dispersive X-ray spectroscopy. The system is paramagnetic down to 2 K, as measured by VSM. We have recorded the PL spectra in the Faraday geometry for magnetic fields of up to 7 tesla in the 0-50 K temperature range. The PL was excited at 780 nm and the emission is centered at 940 meV with a FWHM of 100 meV. In the presence of a magnetic field the emission becomes strongly $\sigma $+ polarized (P = 35{\%} at 4 tesla), suggesting carrier spin polarization. The polarization is temperature sensitive and decreasing sharply with increasing temperature. The polarization vanishes at around T = 40 K. The degree of spin polarization can be tuned by quantum confinement.   

All-Optical observation of Nuclear Magnetic Resonance in a 2D electron system

Electron-nuclear spin interaction may be utilized to manipulate nuclear states coherently in quantum computation. Here we report on an all-optical observation of nuclear magnetic resonance (NMR) in a 2D electron system embedded in a GaAs/AlGaAs heterostructure. In analogy to radio-frequency fields used in traditional NMR, circularly polarized light creates electron spins in semiconductors whose hyperfine coupling with nucleis could tip nuclear moments. At a fixed time-delay $\sim $12.5ns, time-resolved Kerr-rotation (TRKR) signals were measured as a function of the modulation frequency (from 1 KHz to 100 kHz) of the pump laser. Spin-polarized carriers generated by the pump laser pulse train acts on the nuclear spins as an effective rf magnetic field synchronized at the pulse repetition frequency, which resonates nuclear spins at suitable frequencies in an external magnetic field in Voigt geometry. Several dips were observed in a TRKR trace at, which shift linearly with increasing magnetic field. They are ascribed to be NMR signals of elements As-75, Ga-69, Al-27 and Ga-71.   

Confinement and Diffusion Effects in Dynamical Nuclear Polarization in Low Dimensional Nanostructures

We investigate the dynamic nuclear polarization as it results from the hyperfine coupling between nonequilibrium electronic spins and nuclear spins in semiconductor nanostructures. The natural confinement provided by low dimensional nanostructures is responsible for an efficient nuclear spin - electron spin hyperfine coupling [1] and for a reduced value of the nuclear spin diffusion constant [2]. In the case of optical pumping, the induced nuclear spin polarization is position dependent even in the presence of nuclear spin diffusion. This effect should be measurable via optically induced nuclear magnetic resonance or time-resolved Faraday rotation experiments. We discuss the implications of our calculations for the case of GaAs quantum well structures.\\[4pt] [1] I. Tifrea and M. E. Flatt\'{e}, Phys. Rev. B 84, 155319 (2011).\\[0pt] [2] A. Malinowski and R. T. Harley, Solid State Commun. 114, 419 (2000).