Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J33: Focus Session: Mostly Spins in Group IV Semiconductors and Organics |
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Sponsoring Units: GMAG FIAP DMP Chair: Jiwei Lu, University of Virginia Room: Morial Convention Center 224 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J33.00001: Ferromagnetism in Mn-implanted Ge and epitaxial GeC Samaresh Guchhait, John Markert, Mustafa Jamil, Sanjay Banerjee 20 keV energy Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer. The GeC thin film was grown by UHV chemical vapor deposition using a mixture of germane (GeH$_4$) and methylgermane (CH$_3$GeH$_3$) gases and contains less than 1\% carbon. X-ray diffraction data shows a single crystal phase for the GeC film, and the surface rms roughness is about 0.3 nm, measured with AFM. The Mn implant dose was $1.1\times10^{16}$/cm$^2$ at a temperature of 300$^{\circ}$C for both samples. For this relatively low energy Mn ion implant, the range is about 17 nm and the straggle is about 9 nm. A SQUID magnetometer study shows ferromagnetism in both samples. While the Curie temperature for both samples is about 180 K, the in-plane saturated magnetic moment per unit area for the first sample is about $2.2\times10^{-5}$emu/cm$^2$ and that for the second sample is about $3.0\times10^{-5}$emu/cm$^2$. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of carbon. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J33.00002: Influence of carrier type on the ferromagnetism in Ge$_{1-x}$Mn$_{x}$ thin films Wenjing Yin, Jiwei Lu, Li He, Jiani Yu, Wenbin Fan, Robert Hull, Stuart Wolf We have been studying the magnetic and transport properties of Mn doped Group IV semiconductors. Mn ions have been implanted into both boron doped P type and phosphorus doped N type Ge thin films respectively. A ferromagnetic hysteresis loop has been observed in P type samples at low temperatures and supermagnetism remains strong at 300 K. The P type samples show much stronger ferromagnetism than N type samples. At 5K, the ferromagnetic saturation moment (Ms) of 5{\%} Mn doped p-Ge sample is $\sim $0.65 Bohr magneton per Mn, which is almost twice as much as that of the 5{\%} Mn doped n-Ge. Rapid thermal annealing has been used to reduce the ion implantation damage as well as to help Mn ions to incorporate into Ge lattice. In this talk we will present magnetic, transport and electron microscopy characterization of these samples. We are in the process of trying to understand the nature of the ferromagnetism in these films and its correlation to carrier type. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J33.00003: Epitaxial Growth and Properties of Multilayers Containing (CoMn)$_{0.1}$Ge$_{0.9 }$and Ge (001) Liang He, Charles Malmberg, Brian Collins, Frank Tsui Epitaxial growth and magnetic and magnetotransport properties of superlattices containing Co and Mn codoped Ge magnetic semiconductor layers with Ge (001) interlayers have been studied. Layer-by-layer epitaxial growth has been observed during the deposition of the doped magnetic semiconductor layers, i.e. (CoMn)$_{0.1}$Ge$_{0.9}$, and that of the undoped Ge interlayers, as indicated by persistent oscillations in the intensity and width of reflection high energy electron diffraction. The superlattices exhibit a ferromagnetic transition near 100 K and superparamagnetism at higher temperatures, as determined by temperature and field dependent magnetic measurements using SQUID magnetometry. Magnetotransport properties indicate that the superlattices are p-type semiconductors with very large positive magnetoresistance (MR) and anomalous Hall effect (AHE). Conduction at low temperatures ($<$10 K) is dominated by variable range hopping in the impurity band. Below 100 K, the MR is found to scale with square of the magnetization, whereas the AHE exhibits a linear dependence on magnetization. Above 100 K, in the superparamagnetic regime the magnetotransport parameters scale with magnetic field. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J33.00004: Different magnetic moment in Mn-doped amorphous group-IV semiconductors: a comparison study between Si and Ge matrices. Li Zeng, Erik Helgren, Cinthia Piamonteze, Elke Arenholz, Addison Huegel, Frances Hellman Mn-doped amorphous Si ($a-$Si) and Ge ($a-$Ge) are prepared by $e$-beam co-evaporation for a wide range of concentrations (0.5-18 at.{\%}) to explore the Mn local moment in group-IV semiconductors. We find that Mn behaves quite differently in these two matrices: in $a$-Si, the Mn local moment is quenched, even for the lowest doping (0.5 at.{\%}), while in $a-$Ge, a large Mn moment is observed, with a spin-glass ground state. X-ray absorption spectra (XAS) of $a-$Mn$_{x}$Si$_{1-x}$ have very broad $L$-edge absorption peaks which correlate with the quenched magnetic state. The quenched Mn moment in $a$-Si is unexpected and can be understood by the formation of Anderson-localized itinerant states even on the insulating side of the metal-insulator transition. By contrast, XAS of $a-$Mn$_{x}$Ge$_{1-x}$ show atomic multiplets. $a$-Mn$_{x}$Si$_{1-x}$ has positive magnetoresistance (MR) like typical non-magnetic disordered electronic systems, while $a-$Mn$_{x}$Ge$_{1-x}$ has negative MR, consistent with magnetization data. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J33.00005: Magnetic Properties of Ge$_{1-x}$Mn$_{x}$Te Thin Films James R. Anderson, W. Knoff, Malgorzata Gorska, T. Story, Costel R. Rotundu We have measured the magnetization of Ge$_{1-x}$Mn$_{x}$Te thin films with 0.08 $<$ x $<$ 0.19 at magnetic fields up to 7 T at temperatures from 2 to 385 K. The monocrystalline epitaxial layers of Ge$_{1-x}$Mn$_{x}$Te were grown on (111)-oriented BaF$_{2}$ crystalline substrates in a home-built MBE system. The layer structure was rhombohedral, thickness in the range 0.5 -- 1 micron, and hole carrier concentration of the order 10$^{21}$ cm$^{-3}$. Magnetization measurements were made using a Quantum Design MPMS system. At low temperatures the samples were ferromagnetic. The ferromagnetic -- paramagnetic transition was observed in various samples in a broad temperature range from 20 -- 100 K. In some samples we have seen two peaks in the temperature dependence of the low-field magnetization. These peaks may be evidence of two chemical phases or of an electronic phase separation. The origin of this effect is under investigation at the present time. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J33.00006: Spin Engineering with Ion Implantation of Diamond Nitrogen-Vacancy Centers G.D. Fuchs, F.J. Heremans, D.D. Awschalom, R. Hanson, A. Batra, T. Schenkel, S. Shirvastava, T. Mugato, E. Sideras-Haddad Nitrogen-vacancy (NV) defect centers in diamond exhibit long coherence times of spin states at room temperature. Individual NV centers can be optically initialized and read-out, making them attractive candidates for quantum information. By exploiting the interactions with naturally occurring, nearby spins, NV centers have been incorporated into two quantum bit (qubit) systems. Despite these successes, scaling NV qubit systems with naturally occurring spins is a challenge. Here we present an alternative approach where we deliberately place NV centers using spatial and energy selective ion-implantation of nitrogen into synthetic diamond samples with low nitrogen content. Since we use isotopically pure N-15 for implantation, we can distinguish the implanted NV centers from the naturally occurring N-14 centers by measuring the hyperfine splitting of the electron spin resonance peaks of individual NV centers. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J33.00007: Electronic transport in nitrogen-rich diamond F.J. Heremans, G.D. Fuchs, C.F. Wang, D.D. Awschalom, R. Hanson Electronic transport in carbon-based materials, including carbon nanotubes, graphene, and diamond, have been receiving significant attention as potential alternatives to silicon-based electronics. In particular, diamond`s excellent thermal properties provide a promising alternative in power-sensitive applications. Here we present studies of the photo-excited electronic transport in nitrogen-rich type IB diamonds. In addition to the study of the carrier dynamics within this system, we discuss a charge storage effect that may find potential application in charging-based memories.~ We find that the discharge curves follow a ``stretched-exponential'' form [1] with a fixed exponent, which does not depend on electrode spacing, voltage, and illumination intensity.~ These findings are discussed in the context of a transport mechanism in this nitrogen-rich diamond substrate. \newline \newline [1] C.G. Van de Walle Phys. Rev. B, \textbf{53}, 11292 (1996) [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J33.00008: Polarization-selective excitation of nitrogen vacancy centers in diamond T. P. Mayer Alegre, C. Santori, G. Medeiros-Ribeiro, R. G. Beausoleil The nitrogen-vacancy (N-V) center in diamond is promising as an electron spin qubit due to its long-lived coherence and optical addressability. Nevertheless, some work remains in determining the detailed energy level structure. In particular, little emphasis has been put on the microwave polarization selection rules. In typical optically-detected magnetic resonance (ODMR) experiments, the microwave transitions are driven by linearly polarized fields. Thus the individual transitions can be selected only by applying a constant magnetic field to lift the degeneracy. Typically the magnetic field is applied along the quantization axis of the NV center to avoid mixing of the spin states. This applied magnetic field is in principle unnecessary with circularly polarized microwave excitation. Using a resonator designed to produce circularly polarized microwaves, we have investigated the polarization selection rules of the N- V center. We first apply this technique to N-V ensembles in [100] and [111]-oriented samples. Next, we demonstrate an imaging technique, based on optical polarization dependence, that allows rapid identification of the orientations of many single N-V centers. Finally, we test the microwave polarization selection rules of individual N-V centers of known orientation. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J33.00009: Spin transport studies in high mobility organic semiconductor, Pentacene (C$_{22}$H$_{14}$) J.H. Shim, V. Karthik, J.S. Moodera Spin transport in organic semiconductors (OSs) is currently an attractive research area because these materials in general can have long spin coherence length due to weak spin-orbit scattering and hyperfine interaction. Charge and spin transport in OSs strongly depend on the intermolecular overlap of electronic wave functions and defects in OS which can be influenced by the thin film growth conditions. Here we chose OS, pentacene (C$_{22}$H$_{14})$, which is among the most promising materials due to its high mobility. From the measured temperature dependence of the conductance in Al/Pentacene/Co thin film tunnel junctions, different transport mechanisms were observed for pentacene barriers of various thicknesses. Thin, amorphous pentacene films were deposited with and without Al$_{2}$O$_{3}$ seed layer at room temperature. Significant tunnel magnetoresistance (TMR) was observed at room temperature in junctions with Co and Fe electrodes and pentacene tunnel barrier. These results will be presented and discussed. Research supported by ONR and KIST-MIT programs. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J33.00010: Is there really spin transport in Alq$_{3}$ spin-valves? J. Samuel Jiang, J.E. Pearson, S.D. Bader There have been reports of GMR and extremely long spin relaxation in Alq$_{3}$- based spin valves.\footnote{Z. H. Xiong et al. Nature, 427, 821 (2004); S. Pramanik et al. Nature Nanotech. 2, 216 (2007).} However, it has also been suggested that direct tunneling through locally-thin regions of the Alq$_{3}$ layer could be the magnetoresistance (MR) mechanism, i.e. the reported MR may be due to artifacts rather than spin transport via the molecular levels in Alq$_{3}$.\footnote{W. Xu, et al. Appl. Phys. Lett. 90, 072506 (2007).} We present transport measurements on Alq$_{3} $-based spin valves and unipolar devices where the Alq$_{3}$ thickness is beyond the tunneling limit. The I-V characteristic is highly asymmetric and strongly temperature- dependent, different from the behaviors of devices where GMR has been reported. The charge transport in the Co/Alq$_{3}$/Fe spin valves is by holes only and is injection- limited. More importantly, we observe no measurable MR in our non-tunneling Co/Alq $_{3}$/Fe spin valves, or in Co/AlO$_{x}$/Alq$_{3}$/Fe structures where spins can be injected via the AlO$_{x}$ barrier. These results indicate that spin transport in Alq$_ {3}$ is unlikely. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J33.00011: Inelastic tunneling spectroscopy study on organic semiconductor tunnel barriers with magnetic electrodes K.V. Raman, J.H. Shim, J.S. Moodera Spin injection and transport through organic semiconductor (OS) is recently being researched extensively. Exploring the interfacial structural and chemical modifications in FM/OS/FM tunnel junctions can lead to a better understanding of spin injection and transport in OS. Inelastic tunneling spectroscopy (IETS), a high sensitivity technique, measures the vibrational and excitational modes of the molecules within a tunnel barrier, which are greatly influenced by any distortions in the molecules. These measurements are performed on thin films of OS, rubrene and pentacene, using Co/seed/OS/Py and Al/seed/OS/Al junctions, all grown in-situ, for two different seed layers viz. Al$_{2}$O$_{3}$ and LiF. The IETS spectra matches well with the reported Raman and IR spectroscopy measurements performed for powder and bulk single crystal samples. In addition, the IETS spectra show weak signatures of the molecular distortions through modifications to certain phonon peaks. Due to the amorphous nature of the films certain electronic states are also observed at higher bias voltages. The effect of vibrational modes on the spin conserved tunneling and the effect of different electrodes on the IETS spectra will also be presented and discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J33.00012: Spin transport studies through spin filter and organic semiconductor hybrid tunnel barriers DeungJang Choi, Tiffany Santos, Tae Hee Kim, Jagadeesh Moodera Spin polarized tunneling through a hybrid tunnel barrier of spin filter EuO magnetic semiconductor and an organic semiconductor Rubrene was investigated. With magnetic tunnel junction structures such as Co/Rubrene/EuO/Al or Cu we observed a magnetoresistance (MR) of up to 8.5{\%}, whereas from the junction resistance versus temperature data, we deduced the capability to produce spin polarization (P) of up to 99{\%}. Thus the observed low MR has been attributed to spin scattering by defects at various EuO and Rubrene interfaces and possible nonstoichiometry in EuO. With further optimization it should be possible to reach towards the expected large MR in such systems. Moreover, Rubrene which has a low barrier height also serves to magnetically decouple the ferromagnetic electrode from EuO, necessary for independent magnetic switching and reaching high MR. This study demonstrates the possibility of combining organic and spin filter materials as tunnel barriers. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J33.00013: Quantitative analysis of molecular spintronics Zhanyu Ning, Yu Zhu, Jian Wang, Hong Guo We report quantitative analysis of nonequilibrium spin injection from Ni contacts to octanethiol molecular spintronic system. Our calculation is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. The first principles results allow us to establish a clear physical picture on how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magneto-resistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent {\it ab initio} transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of Ni surface is crucial for accurate results. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J33.00014: Hall conductivity and colossal magnetoresistance in FeSb2 Rongwei Hu, Vesna Mitrovic, Cedomir Petrovic We report the anisotropic Hall conductivity and discuss mechanism of colossal magnetoresistance in Co doped FeSb2. The Hall conductivity is described in a mutil-carrier picture, in which large carrier mobility was found in Co doped FeSb2. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J33.00015: Electric field induced metal-insulator transition and colossal magnetoresistance in CdCr$_{2}$S$_{4}$ C.P. Sun, C.C. Lin, J.L. Her, S. Taran, C.C. Chou, C.L. Chan, C.L. Huang, H. Berger, H.D. Yang Multiferroic ordering existing in a single material is a recent hot topic in the field of condensed matter physics due to its potential application in device control. The chromium chalcogenide spinel CdCr$_{2}$S$_{4}$ is one of the attractive materials investigated by Hemberger \textit{et al.} recently.[1] Based on the electrical measurement, there is no discontinuity through the ferromagnetic ordering at $T_{C} \quad \sim $ 85K.[2] We measure the temperature dependent resistance under various electric fields to investigate the electrical properties of the present material. To our knowledge, we first observe the electric field induced metal-insulator transition in this material around $T_{C}$. Moreover, a colossal magnetoresistance (CMR), which is comparable to that of manganese-based CMR material, is also observed near $T_{C}$. The origin for these properties is discussed. [1] J. Hemberger, P. Lunkenheimer, R. Fichtl, H.-A. Krug von Nidda, V. Tsurkan, A. Loidl, Nature 434, 364 (2006). [2] P. K. Baltzer, H. W. Lehmann, and M. Robbins, Phys. Rev. Lett. 15, 493 (1965). [Preview Abstract] |
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