Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S31: Focus Session: Spin-Dependent Phenomena in Semiconductors: Magnetic Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Igor Zutic, University at Buffalo, The State University of New York Room: 207A |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S31.00001: A new pathway for the manipulation of magnetism in the dilute ferromagnetic semiconductor (Ga,Mn)As by organic molecules Xiaolei Wang, Hailong Wang, Dong Pan, Lixia Li, Xuezhe Yu, Jun Lu, Jianhua Zhao, Timothy Keiper, Eric Lochner, Stephan von Moln\'ar, Peng Xiong Holes from Mn doping are known to mediate the ferromagnetic interaction in III-V dilute magnetic semiconductors. We investigated the effects of electron and hole donor molecule species on the surface of (Ga,Mn)As thin films with a focus on elucidating how the molecular coverage could modify the magnetism of the dilute ferromagnetic semiconductor thin films. Mn-doped GaAs thin films with various thicknesses were grown by molecular-beam epitaxy, and the organic molecules were deposited by solution-based self-assembly or vacuum thermal evaporation. Charge-transfer molecules on the surface of the films led to large carrier density modulation, resulting in significant changes in Curie temperature and magnetization. Through proper preparation of the (Ga,Mn)As surface, self-assembled monolayer patterns of organic molecules with sub-75 nanometer linewidth were successfully created by dip-pen nanolithography. This could open a new pathway to controlled nanoscale manipulation of magnetism in dilute magnetic semiconductors with potential applications in hybrid molecular nano-spintronics. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S31.00002: Different symmetry of the magnetization-direction dependence between the impurity band and valence band in GaMnAs Iriya Muneta, Toshiki Kanaki, Shinobu Ohya, Masaaki Tanaka In semiconductors with heavily doped with nonmagnetic shallow acceptors, an impurity band (IB) is formed around the valence band (VB) top and merged with VB. As a result, the parabolic VB top is strongly deformed in a non-parabolic dispersion. In GaMnAs, however, the VB top keeps the parabolic dispersion though there is energy overlap between VB and IB [1--3], which is completely different from the conventional nonmagnetic semiconductors. Here, we measure tunneling anisotropic magnetoresistance on GaMnAs tunnel devices in a spectroscopic way [4--6], analyze the magnetization-direction and energy dependence of the density of states (DOS), and investigate the different symmetry between VB and IB to clarify the mysterious overlap between the two bands. We find that the magnetization-direction dependence of VB DOS is mainly four-fold symmetry along [100] which is the same as the crystal symmetry, while that of IB DOS is mainly two-fold symmetry along [110] unlike the crystal symmetry. These results reveal the unique band structures of Mn-doped III-V ferromagnetic semiconductors. [1] S. Ohya et al., Nat. Phys. (2011). [2] I. Muneta et al., APL (2013). [3] M. Kobayahsi et al., PRB (2014). [4] C. Gould et al., PRL (2004). [5] H. Saito et al., PRL (2005). [6] L. Gao et al., PRL (2007). [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S31.00003: Anisotropic magnetic dynamics in (Ga, Mn)As film Xiang Li, Sining Dong, Taehee Yoo, Sergio Mello, Xinyu Liu, Jacek Furdyna, Margaret Dobrowolska (Ga,Mn)As shows excellent magnetic properties which are usually described by a single-domain model. In this study, we perform a systematic investigation of ac magnetic susceptibility in (Ga,Mn)As films as a function of temperature and field carried out in parallel with dc magnetization measurements. A single ac susceptibility peak is observed close to T$_{C}$ for the field along [1-10] orientation; a single peak is seen close to 22 K along [110]; and both peaks are observed along [100]. Detailed analysis indicates that the peak near T$_{C}$ is related to the para-ferromagnetic transition. And the ferromagnetic domains nucleate with their easy axis aligned with [1-10] direction, involving 180$^{\circ}$ magnetization flips along the easy axis. The peak near 22 K, on the other hand, originates from magnetization switching between two biaxial easy axes separated by a small angle, which is induced by the competition between uniaxial and cubic anisotropy. Dynamic properties emerging from the distinct frequency dependences of the ac susceptibility in these two temperature regions, such as magnetic relaxation times, have been analyzed using various models. Investigations in patterned films will be carried out as well. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 9:12AM |
S31.00004: Theory of the Novel Mn-doped II-II-V Dilute Magnetic Semiconductors Invited Speaker: James Glasbrenner A recently discovered magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_2$As$_{2}$, with its decoupled spin and charge doping, provides a unique opportunity to elucidate the microscopic origin of the magnetic interaction and ordering in dilute magnetic semiconductors (DMS). We show that (i) the conventional density functional theory (DFT) accurately describes this material, and (ii) the magnetic interaction emerges from the competition of the short-range superexchange and a longer-range interaction mediated by the itinerant As holes, coupled to Mn \emph{via} the Schrieffer-Wolff $p-d$ interaction representing an effective Hund's rule coupling, $J_{H}^{\mathrm{eff}}$. The key difference between the classical double exchange and the actual interaction in DMS is that an effective $J_{H}^{\mathrm{eff}}$, as opposed to the standard Hund's coupling $J_{H}$, depends on the Mn $d-$band position with respect to the Fermi level, and thus allows tuning of the magnetic interactions. The physical picture revealed for this transparent system may also be applicable to more complicated DMS systems. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S31.00005: Annealing studies of ion-beam irradiated GaMnAs thin films Segio L.A. Mello, M.M. Sant'Anna, C.F.S. Code\c{c}o, S. Dong, T. Yoo, X. Li, X. Liu, J.K. Furdyna We have studied the effect of ion-beam irradiation on GaMnAs films by means of systematic transport and magnetization measurements. Both the magnetization and the conductivity of such samples decrease as a result of increase of disorder and defects in the system caused by the irradiation process. In this study we investigate the recovery of such irradiated samples, by measuring their magnetization and transport properties before and after annealing. Our preliminary transport measurements on annealed-irradiated samples show significant enhancement of the transport properties of the films (revealed by the increase of conductivity and of the critical temperature). Samples comparison of resistivity curves $\rho$(T) of annealed-irradiated and annealed-non-irradiated samples indicates that most defects created by low fluences of ion beams are similar to those created while growing the samples. This is evidenced by the fact that low ion-fluence irradiation $\rho$(T) curves of annealed-irradiated and annealed-non-irradiated samples nearly match. For high ion-fluence, however, the sample properties cannot be fully recovered by annealing. This suggests that the nature of a minor fraction of irradiation-created defects is different from those created during sample growth. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S31.00006: Spin relaxation time dependence on optical pumping in GaAs:Mn Veronika Burobina, Christian Binek We analyze the dependence of electron spin relaxation time on optical pumping in a partially-compensated acceptor semiconductor GaAs:Mn using analytic solutions for the kinetic equations of the charge carrier concentrations [1]. Our results are applied to previous experimental data of spin-relaxation time vs. excitation power for magnetic concentrations of approximately 10$^{17}$cm$^{-3}$ [2]. The agreement of our analytic solutions with the experimental data supports the mechanism of the earlier-reported atypically long electron-spin relaxation time in the magnetic semiconductor.\\[4pt] [1] V. Burobina and Ch. Binek, J. Appl. Phys. 115, 163909 (2014).\\[0pt] [2] G. V. Astakhov et al., Phys. Rev. Lett. 101, 076602 (2008). [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S31.00007: Visible-light electroluminescence in Mn-doped GaAs light-emitting diodes Daiki Maruo, Pham Nam Hai, Masaaki Tanaka We demonstrate visible-light electroluminescence (EL) due to $d$-$d$ transitions in GaAs:Mn based LEDs. We design p$^{+}$-n junctions with a p$^{+}$ GaAs:Mn layer, in which at a reverse bias voltage (-3 to -6 V), an intense electric field builds up in the depletion layers of the p$^{+}$-n junctions. Holes are injected to the depletion layer by Zener tunneling from the conduction band or by diffusion of minority holes from the valence band of the n-type layer. These holes are accelerated by the intense electric field in the depletion layer, and excite the $d$ electrons of Mn in the p$^{+}$ GaAs:Mn layer by impact excitations. We observe visible-light emission at $E_{1}=$ 1.89 eV and $E_{2}=$ 2.16 eV, which are exactly the same as the $^{4}T_{1}\to^{6}A_{1}$ and $^{4}A_{2}\to^{4}T_{1}$ transition energy of Mn. The threshold voltage for observation of visible-light EL is -4 V, corresponding to -($E_{1}+E_{2})$/$e$. This indicates that the impact excitation is most effective for the one step excitation from the ground state $^{6}A_{1}$ to the highest excited state $^{4}A_{2}$. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S31.00008: Magnetic Circular Dichroism (MCD) Studies on MOVPE Grown InMnSb and InMnAs M.A. Meeker, B.A. Magill, G.A. Khodaparast, D. Saha, C.J. Stanton, S. McGill, B.W. Wessels Carrier-induced ferromagnetism in magnetic III-V semiconductors has opened up several opportunities for device applications as well as for fundamental studies of a material system in which itinerant carriers interact with the localized spins of magnetic impurities. The origin of the carrier-induced ferromagnetism is still an open and exciting question. In order to understand the hole mediated ferromagnetism, probing the band structure in these material systems is crucial. Here we present magnetic circular dichroism (MCD) studies on MOVPE grown InMnSb and InMnAs, both with Curie temperatures above 300K. The measurements were performed on samples with different Mn contents, with the laser excitation energy tuned from 0.92-1.45eV, and external magnetic fields ranging up to 31 Tesla. The measurements are compared with MCD calculations based on an 8 band Pidgeon-Brown model. Comparison of the experimental results with the theoretical calculations provides a direct method to estimate the sp-d Mn- electron/hole coupling constants. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S31.00009: Charge transport in diluted magnetic semiconductor p-n heterojunctions Jindong Liu, J. Archibald Peters, Bruce Wessels In recent years, semiconductor spintronic devices have been proposed as promising candidates for memory, sensor, logic, and multifunctional devices. The p-n-p bipolar magnetic junction transistor (MJT) was recently demonstrated using the diluted magnetic semiconductor (DMS) InMnAs as the collector [1]. A current gain $\beta $dc as high as 20 of the transistor was observed at 300K. A negative magneto-amplification of -150{\%} is obtained when the applied magnetic field is 8T. In order to assess the theoretical gain for such transistors, we measured the minority carrier lifetime in a p-n InMnAs/InAs heterojunction diode. Using the reverse recovery transient technique, a minority carrier lifetime of 320ns was obtained at room temperature under low-injection conditions. From the measured lifetime and calculated base transport factor, a transistor current gain as high as 5000 is predicted. This is more than two orders of magnitude larger than the gain previously measured for heteroepitaxial InMnAs p-n-p transistors. The large gain should enable realization of highly sensitive magnetic sensors and new spin logic circuits. \\[4pt] [1] N. Rangaraju et al, PRL 105, 117202 (2010) [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S31.00010: Ge doping of FeGa$_3$ J.C. Alvarez-Quiceno, M. Cabrera-Baez, J. Mun\'evar, H. Micklitz, E.M. Bittar, E. Baggio-Saitovitch, R.A. Ribeiro, M.A. Avila, G.M. Dalpian, J.M. Osorio-Guill\'en The intermetallic narrow-gap semiconductor FeGa$_3$ is one of the few Fe-based diamagnetic materials. Experimentally, Ge doping induces a ferromagnetic (FM) state. The mechanism responsible for this FM response is still unestablished, but there are proposals of itinerant magnetism to explain this behavior. Our DFT simulations show that inserting holes induces a delocalized FM response, while inserting electrons induces a localized FM response around some Fe atoms. We also modeled different distributions of Ge substitution and observe that the FM response depends on the Ge concentration and also on the Ge distribution on the Ga sites. We observed that the extra electrons become localized in some specific Fe atoms, rather than delocalized over the entire crystal lattice, as expected from an itinerant model. For experimental probing of this scenario, we have performed $^{57}$Fe M\"ossbauer spectroscopy on flux-grown singlecrystalline samples. The resulting resonance peak shape supports a localized model for ferromagnetism, since it is possible to resolve the presence of two distinct Fe isomer shifts (despite a single crystallographic site), expected to correspond to Fe atoms with high and low magnetic moments. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S31.00011: Ge:Mn Dilute Magnetic Semiconductor Laila Obied, Sjored Roorda, David Crandles This work aims to develop Ge:Mn dilute magnetic semiconductor and study the fundamental origin of ferromagnetism in this system. Using ion implantation at 77 K, a single crystal Ge wafer was doped with magnetic Mn ions. The implantation was done at ion energy of 4.76 MeV with a fluence of 2 x 10$^{16}$ ion/cm$^{2}$. X-ray diffraction (XRD) of the as-implanted sample showed that the implanted layer was amorphous. Therefore, different samples were annealed at 200 $^{\circ}$C, 330 $^{\circ}$C and 600 $^{\circ}$C in a tube furnace to achieve a solid phase epitaxial regrowth of the implanted layer. XRD of the sample annealed at 330 $^{\circ}$C for 33 hours showed a polycrystalline layer. The depth profile of Mn in the as-implanted sample and the post-annealed sample at 330 $^{\circ}$C was determined using secondary ion mass spectroscopy (SIMS) and it was found that some Mn diffused to the surface during the annealing. XRD of the sample annealed at 600 $^{\circ}$C for 35 minute showed peaks corresponding to an unknown phase in addition to peaks from amorphous and polycrystalline Ge. The sample annealed at 200 $^{\circ}$C for 168 hour showed no evidence of solid phase epitaxy. A SQUID was used to measure the magnetic properties of all samples. At low temperature, the as-implanted sample showed a paramagnetic behaviour. A magnetic hysteresis at 5K and up to 200K was observed for the samples annealed at 330 $^{\circ}$C and 200 $^{\circ}$C. The 600 $^{\circ}$C annealed sample showed no ferromagnetic response and a significant reduction in the paramagnetic response at low temperature compared to the as-implanted sample. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S31.00012: Room temperature reddish-yellow electroluminescence in manganese-doped silicon light-emitting diodes Pham Nam Hai, Daiki Maruo, Le Duc Anh, Masaaki Tanaka Silicon (Si) is an indirect band-gap semiconductor that does not efficiently emit light. Here, by utilizing optical transitions between the $p$-$d$ hybridized orbitals of manganese (Mn) atoms doped in Si, we demonstrate Si-based light-emitting diodes (LEDs) that continuously emit reddish-yellow visible light at room temperature. The Mn $p$-$d$ hybrid states are excited by hot holes that are accelerated in the depletion layers of reverse biased Si p-n junctions. Above a threshold reverse bias voltage of about -4 V, our LEDs show strong visible light emission with two peaks at $E_{\mathrm{1}}=$ 1.75 eV and $E_{\mathrm{2}}=$ 2.30 eV, corresponding to optical transitions from the $t_{\mathrm{-}}^{\mathrm{a}}$ (spin-down anti-bonding) states to the $e_{\mathrm{-}}$ (spin-down non-bonding) states, and from the $e_{\mathrm{-}}$ to the $t_{\mathrm{+}}^{\mathrm{a}}$ (spin-up anti-bonding) states as predicted by ab initio calculations. The internal quantum efficiency of the $E_{\mathrm{1}}$ and $E_{\mathrm{2}}$ transitions is 3 - 4 orders of magnitude higher than that of the indirect band-gap transition. We also demonstrate direct amplitude modulation of our LEDs at 1 Mbps. Our results open a way to utilize the 3d orbitals of transition metals in Si-based photonic devices. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S31.00013: Silicon-based current-controlled reconfigurable magnetoresistance logic combined with non-volatile memory Xiaozhong Zhang, Zhaochu Luo Silicon-based complementary metal-oxide-semiconductor (CMOS) transistors have achieved great success. However, the traditional development pathway is approaching its fundamental limits. Magnetoelectronics logic, especially magnetic-field-based logic, shows promise for surpassing the development limits of CMOS logic. Existing proposals of magnetic-field-based logic are based on exotic semiconductors and difficult for further technological implementation. We proposed a kind of diode-assisted geometry-enhanced low-magnetic-field magnetoresistance (MR) mechanism. It couples p-n junction's nonlinear transport characteristic and Lorentz force by geometry, and shows extremely large low-magnetic-field MR (\textgreater 120{\%} at 0.15 T) Further, it is applied to experimentally demonstrate current-controlled reconfigurable MR logic on the silicon platform at room temperature [1]. This logic device could perform Boolean logic AND, OR, NAND and NOR in one device. Combined with non-volatile magnetic memory, this logic architecture has the advantages of current-controlled reconfiguration, zero refresh consumption, instant-on performance and would bridge the processor-memory gap. [1] \textit{Z.C Luo, X.Z. Zhang, et al., Adv Func Mater, DOI:~10.1002/adfm.201402955} [Preview Abstract] |
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