Session E20: II-VI Magnetic Semiconductors

Dynamics of the matrix in DMS Type-II quantum dot systems

Magnetic field, temperature, and polarization dependent continuous wave photoluminescence spectroscopy (PL) is used to study two related Type-II quantum dots (QDs). These techniques were used to study how the location of magnetic impurities affects the formation of magnetic polarons in these two (related) systems. The ZnMnTe/ZnSe system has Mn impurities located within the QDs, with (ideally) no Mn in the surrounding ZnSe matrix. The ZnTe/ZnMnSe QDs have Mn impurities grown within the matrix, which ideally is excluded from the QDs. For both these systems, the holes are confined within the dots, while the electrons are located in the surrounding matrix. The location of the Mn and its coupling with the spin of the corresponding carrier leads to distinct characteristics for each system. Due to difficulties growing these systems, some diffusion of Mn during the growth of these samples is suspected, leading to a percentage of magnetic impurities unintentionally located in the non-magnetic region for both samples. The emission from the matrix in particular was studied to determine the effect/composition of Mn in this region and its contribution to the characteristics of the QDs.   

Mn$^{\mathrm{2+}}$-Doped CdSe/CdS Core/Multishell Colloidal Quantum Wells Enabling Tunable Carrier-Dopant Exchange Interactions

We report the manifestations of carrier-dopant exchange interactions in colloidal Mn$^{\mathrm{2+}}$-doped CdSe/CdS core/multishell quantum wells. In our solution-processed quantum well heterostructures, Mn$^{\mathrm{2+}}$ was incorporated by growing a Cd$_{\mathrm{0.985}}$Mn$_{\mathrm{0:015}}$S monolayer shell on undoped CdSe nanoplatelets using the colloidal atomic layer deposition technique. The carrier-magnetic ion exchange interaction effects are tunable through wave function engineering. This is realized by controlling the spatial overlap between the carrier wave functions with the manganese ions through adjusting the location, composition, and number of the CdSe, Cd$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$S, and CdS layers. Our colloidal quantum wells, which exhibit magneto-optical properties analogous to those of epitaxially grown quantum wells, offer new opportunities for solution-processed spin-based semiconductor devices.   

Time resolved photoluminescence study of CdSe/CdMnS/CdS core/shell/shell nanoplatelets heterostructures

We have recorded the time evolution of the photoluminescence (PL) for CdSe/CdMnS/CdS core/shell/shell solution-processed nanoplatelets (NP) using ultrafast pulses at 400 nm and 514 nm. Our NPs consist of a core with 5 monolayers (1.5 nm) of CdSe and average lateral dimensions of 55 x 10 nm$^{\mathrm{2}}$. Using 400 nm pulses we excite electron-hole pairs above the CdS shell bandgap; with 514 nm pulses we excite only in the CdSe core. The holes are primarily localized in the CdSe core, while the electrons are delocalized. Our measurements show that at $\Delta t=$0, the peak PL energy for both kind of excitations is the same. ~As a function of time, both types of excitations result in a red-shift. The red shift of with 400 nm excitation is 60 meV and is described by two time scales: ~$\tau _{\mathrm{1}}=$270 ps and $\tau_{\mathrm{2}}=$2.5 ns. ~The red shift with the 514 nm excitation is 30 meV and is described by a single time scale: ~$\tau_{\mathrm{2}}^{\mathrm{\mbox{'}}}=$2.5 ns. ~These results are discussed in terms of dipole layer formation[1]. ~[1] Gu, Y et. al. Phys. Rev. B 71 045340 (2005).   

Spin-orbit twisted spin-flip waves in CdMnTe quantum wells

We present a numerical study of spin-flip wave dispersions in a spin-polarized electron gas in a dilute magnetic semiconductor heterostructure, using time-dependent density-functional response theory. The system under study is an n-doped CdMnTe quantum well with an in-plane magnetic field. Rashba and Dresselhaus spin-orbit coupling induces a wavevector-dependent spin splitting in the conduction bands. The spin waves hence travel through a spin-orbit twisted medium. We calculate the spin-wave dispersion to second order in spin-orbit coupling, including impurity scattering effects. Our results are compared with recent inelastic light scattering experiments.   

Magnetic properties of nano-patterned GaMnAs films grown on ZnCdSe buffer layers

Magnetic semiconductor nanostructures are attracting intense attention, both because of their fundamental physical properties, and because of the promise which they hold for building smaller, faster and more energy-efficient devices. In this study we report successful MBE growth of GaMnAs films on the GaAs (100) substrates with ZnCdSe buffer layers, which results in perpendicular magnetic easy axis in the GaMnAs films. The GaMnAs/ZnCdSe films have been etched into nano-stripe shapes with various widths below 200nm by e-beam lithography, which resulted in a new geometry of interest for perpendicular magnetic recording. Magnetic anisotropy of as-grown GaMnAs films and nano-stripes was then studied by SQUID magnetometry. The results indicate that the GaMnAs films consist of magnetic domains with magnetization normal to the film plane, having rather high coercivety, which survives after nanofabrication. This is also confirmed by the dynamics of the domain motion as shown by AC susceptibility measurements. These findings are of interest for understanding the magnetic anisotropy mechanisms in GaMnAs and its domain structures, as well as for designing of nano-sized spintronic devices which require hard ferromagnetic behavior with perpendicular easy axes.   

Gate tunable spin exchange interaction and inversion of magnetoresistance in ferromagnetic ZnO nanowire

Tuning magnetism in diluted magnetic semiconductor (DMS) is one of the central issue to the development of future spintronic device applications. Particularly, realizing such control in nanostructure has received growing attention. Here, we report the dramatic change of MR in ferromagnetic ZnO nanowire with varied gate voltages ($+$50 V to -40 V) at different temperatures (2 K to 50 K). The MR signal was greatly influenced by the gate voltage induced carrier concentrations which results the inversion of MR from positive to negative sign while pertaining the coexistence of both parts before inversion in the range of -2T to 2T. The origin of negative MR is mainly due to spin scattering while the positive one is due to a field induced change in relative populations of conduction bands with different conductivities. The extracted spin exchange related parameter was well tuned with the varied gate voltages at different temperatures. More importantly this type of gate tuning of spin exchange interactions in ferromagnetic single ZnO nanowire is well suitable for future spintronic device applications.   

The State of the Art in (Cd,Mn)Te Heterostructures: Fundamentals and Applications

In my talk I will review recent progress in the MBE technology of (Cd,Mn)Te nanostructures containing two dimensional electron gas (2DEG) that led to the first ever observation of fractional quantum Hall effect in magnetic system [1]. This opens new directions in spintronics. I will first discuss already demonstrated applications of such high mobility magnetic-2DEG system for: a) THz and microwave radiation induced zero-bias generation of pure spin currents and very efficient magnetic field induced conversion of them into spin polarized electric current [2]; b) clear demonstration of THz radiation from spin-waves excited via efficient Raman generation process [3]; c) experimental demonstration of working principles of a new type of spin transistor based on controlling the spin transmission via tunable Landau-Zener transitions in spatially modulated spin-split bands [4]. I will also explain the possibility to use magnetic-2DEG for developing of a new system where non-Abelian excitations can not only be created, but also manipulated in a two-dimensional plane. The system is based on high mobility CdTe quantum wells with engineered placement of Mn atoms, where sign of the Lande g-factor can be locally controlled by electrostatic gates at high magnetic fields. Such a system may allow for building a new platform for topologically protected quantum information processing. I will also present results demonstrating electrostatic control of 2D gas polarization in a quantum Hall regime [5]. \\[4pt] [1] C. Betthausen, {\it et al.}, Phys. Rev. B {\bf 86}, 085310 (2014).\\[0pt] [2] P. Olbrich, {\it et al.}, Phys. Rev. B {\bf 86}, 085310 (2012).\\[0pt][3] R. Rungsawang, {\it et al.}, Phys. Rev. Lett. {\bf 110}, 177203 (20130.\\[0pt] [4] C. Betthausen, {\it et al.}, Science {\bf 337}, 324 (2012).\\[0pt] [5] A. Kazakov, {\it et al.}, APS March Meeting 2015, http://meetings.aps.org/link/BAPS.2015.MAR.A7.13.   

Structural and Optical properties of Er doped ZnO diluted magnetic semiconductor nano thin films produced by sol gel method.

Undoped and Er doped ZnO (Zn$_{1-x}$Er$_{x}$O) transparent semiconductor thin films were coated using sol-gel method on non-alkali glass. Erbium was doped 1{\%}, 2{\%}, 3{\%}, 4{\%} and 5{\%} ratio. Methanol and monoethanolamine were used as solvent and stabilizer. In this study, the effect of Er doping was examined on the structural and optical properties of ZnO DMS thin films. XRD, SEM and UV-VIS-NIR spectrometer measurements were performed for the structural and optical characterization. XRD results showed that, all of Er doped ZnO thin films have a hexagonal structure. The optical transmittance of rare earth element (Er) doped ZnO thin films were increased. The Er doped ZnO thin films showed high transparency (\textgreater 84) in the visible region (400-700 nm).   

Microstructural and Optical properties of transition metal (Cu) doped ZnO diluted magnetic semiconductor nano thin films fabricated by sol gel method.

Undoped and Cu (Copper) doped ZnO (Zn$_{1-x}$Cu$_{x}$O) semiconductor thin films were produced by using sol-gel method. Cu was doped 1{\%}, 2{\%}, 3{\%}, 4{\%} and 5{\%} ratio. Methanol and monoethanolamine (MEA) were used as solvent and stabilizer. In this study, the effect of Cu doping was investigated on microstructural and optical properties of ZnO DMS thin films. XRD, SEM, AFM and UV-VIS spectrometer measurements were performed for the microstructural and optical characterization. XRD, SEM and AFM results were showed that all of Cu doped ZnO based thin films have a hexagonal structure. The grain size of Cu doped ZnO thin films and morphology of surface were changed with increasing Cu doping. The optical transmittance of transition metal (Cu) doped ZnO thin films were decreased with doping. \textbf{Keywords: }Diluted Magnetic Semiconductor (DMS), Thin Film, Cu-doping, Bandgap Energy, ZnO.   

Oxygen vacancies induced Spin polarized current in Co-doped ZnO by Andreev reflection technique

Dilute magnetic semiconductor (DMO) is a semiconducting system with spin-polarized carriers and magnetic properties. However, since most studies had been focused on existence of FM, the proportion of spin-polarized current (SPC) in DMO is far from being determined. We used Point-contact Andreev reflection measurements on various Zn$_{\mathrm{0.95}}$Co$_{\mathrm{0.05}}$O thin films, with controlled oxygen vacancies by sputtering in various H$_{\mathrm{2\thinspace }}$partial pressure with Ar atmosphere. We found that conductance versus voltage (G-V) spectra suppresses as oxygen vacancy concentration increases. It indicates oxygen vacancies play significant role in inducing the SPC. To understand the origin of spin polarized current at the interface of the superconducting tip/CZO system, we use modified Blonder--Tinkham--Klapwijk (MBTK) model in ballistic and diffusive regime to interpret GV curve. The extracted SPC value were up to 70{\%} in ballistic regime and 65{\%} in diffusive regime. The results suggest tiny routes have been formed by oxygen vacancies which are extended throughout the whole films. This result confirmed that MBTK model in ballistic regime is more suitable for our GV spectra and this explains the observation of such a high SPC   

Study of the new diluted magnetic semiconductors based on the doping of iron-based superconductors

Diluted magnetic semiconductors(DMSs) have attracted increasing attention because of their potential applications in spintronics. Recently, a series of new bulk DMS materials $^{[1,2]}$were synthesized by doping in the 122 and 1111 phases of iron-based superconductors(Fe-SC), which sheds light on the DMS research$^{[3]}$. In this report, we have synthesized two systems of 1111 phases of DMSs based on Fe-SC materials (La$_{1\, -\, x}$Sr$_{x})$ (Ag$_{0.925}$ Mn$_{0.075})$SO(x$=$0, 0.025, 0.05, 0.075 and 0.1) and (Y$_{1\, -\, x}$Sr$_{x})$ (Cu$_{0.925}$ Mn$_{0.075})$ SO (x$=$0, 0.025, 0.05,0.075 and 0.1) by solid state method. The structure and electrical, magnetic and optical properties have been investigated by means of XRD, 4KCCS, MPMS, PL, UV-Vis and Raman technique, respectively. Some interesting phenomena are found (Such as the Curie temperature \textit{Tc} and band-gap energy \textit{Eg} change regularly with the dopants additon). The results are helpful to clarify the intrinsic mechanism of the DMSs, and will provide new insights on the fabrication and application of devices based on these materials. This work was supported by the National Science Foundation of China (Grant No 61376094). Li Zhang would like to acknowledge a scholarship granted by China Scholarship Council (CSC-201408330028) \textbf{References: } [1] K.Zhao,Z.Deng and X.C.Wang et al., Nature Communications 4, 1442 (2013). [2] X.J. Yang, Y.K. Li, C.Y. Shen et.al., Appl. Phys. Lett. 103, 022410 (2013). [3]T. Ditel and H.Ohno, Rev.Mod.Phys., 86,187(R)(2014).