Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y33: Focus Session: Diluted Magnetic Oxides |
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Sponsoring Units: GMAG FIAP DMP Chair: Stuf Wolf, University of Virginia Room: Morial Convention Center 224 |
Friday, March 14, 2008 11:15AM - 11:51AM |
Y33.00001: Room-Temperature Electron Spin Dynamics in Free-Standing ZnO Quantum Dots Invited Speaker: |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y33.00002: Insulator-ferromagnetic metal transition in anatase Fe:TiO$_2$ Enju Sakai, Yasushi Hirose, Taro Hitosugi, Toshihiro Shimada, Tetsuya Hasegawa Local Fe valence states in Fe-doped anatase TiO$_2$ (Fe:TiO$_2 $) were investigated in relation with transport and magnetic properties. Anatase Fe:TiO$_2$ films were deposited on LaAlO$_3 $ (100) substrates by pulsed laser deposition technique. Amounts of oxygen vacancies in the films were controlled by varying partial oxygen pressure during deposition ($P_{O_2}$). Magnetic and transport properties of the synthesized Fe:TiO$_2$ films measured by SQUID and conventional fourt probe measurements. An insulator-ferromagnetic metal transition was clearly observed between $P_{O_2}$= 1$\times 10^{-6}$ and $3 \times 10^{-6}$ Torr. X-ray photoemission spectroscopy (XPS) measurements have revealed that the local Fe valence state changes from 3+ to 2+, accompanied with the insulator- ferromagnetic metal transition. These results strongly suggest that carriers bound to Fe-oxygen vacancy pairs form magnetic polarons, and that mutual overlap of magnetic polarons triggers the insulator to ferromagnetic metal transition in Fe:TiO$_2$. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y33.00003: Insights into magnetically doped semiconductors from soft x-rays D.J. Keavney, D.B. Buchholz, Q. Ma, R. Chang, T.C. Droubay, T.C. Kaspar, S. Chambers Soft x-ray absorption and dichroism provide a crucial test of the intrinsic nature of magnetism in doped oxide semiconductors. Experiments on Mn, Co, and Cu- doped ZnO reveal that the magnetic dopants have primarily paramagnetic field and temperature dependence regardless of whether the bulk behavior is ferromagnetic. In PLD-grown ferromagnetic Cu:ZnO, no zero-field dichroism is detected at the Cu L, O K, or Zn L edges. In MOVCD-grown Co:ZnO, we find a small remanent signal that is consistent with bulk magnetization measurements of $\sim $0.04$\mu _{B}$/Co ion, however the signal is insufficient to rule out metallic Co as its origin. These results suggest that the origins of ferromagnetism in doped oxides may be unrelated to the presence of magnetic dopants. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y33.00004: Structural, Magnetic and Transport Properties of a New Class of Ferromagnetic Semiconductors/Metals: (Ba, Sr)M$_{2\pm x}$Ru$_{4-x}$O$_{11}$ (M = Fe, Co) Larysa Shlyk, Lance De Long, Sergiy Kryukov, Barbara Sch\"upp-Niewa, Rainer Niewa Single crystals (mm size) of (Ba, Sr)Fe$_{2+x}$Ru$_{4-x}$O$_{11}$ and (Ba, Sr)Co$_{2+x}$Ru$_{4-x}$O$_{11}$ were grown for the first time. X-ray refinements confirmed a hexagonal space group (P6$_{3}$/mmc, No. 194) with two crystallographic sites having mixed Ru and Fe/Co occupation, and one site occupied exclusively by the 3d species. Structural parameters and charge balance suggest oxidation states Co$^{2+}$ and mixed Ru$^{3+}$/Ru$^{5+}$ in the Co compound, and mixed Fe$^{2+}$/Fe$^{3+}$ and Ru$^{3+}$/Ru$^{5+}$ in the Fe compound. The physical properties of these single crystals are sensitive to site disorder among the transition metal ions. Magnetic and transport measurements show the Co-bearing barium ferrite is a ferromagnetic metal below 105 K. In contrast, Fe-bearing barium and strontium compounds exhibit long-range ferromagnetic order at temperatures above 400 K, and narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. These characteristics make the Fe-bearing materials promising new candidates for spintronic applications. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y33.00005: Spin coherence of electrons in singly-charged quantum dots Invited Speaker: We present experimental and theoretical study on electron spin coherence in ensemble of n-type doped InGaAs/GaAs quantum dots containing one electron per dot. A pump-probe time-resolved Faraday rotation technique is exploited. Long-lived spin precession of resident electrons in external magnetic fields is observed with the dephasing time of spin ensemble exceeding 6 ns. Rabi oscillations of the Faraday rotation amplitude has been detected confirming the suggested model of generation electron coherent via excitation of coherent superposition of the trion state and the resident electron [1]. We show that the ensemble dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins. This mode-locking leads to constructive interference of contributions to Faraday rotation, and presents potential applications based on robust quantum coherency within an ensemble of dots [2, 3]. Under these experimental conditions spins of the dots nuclei are aligned in a way that all dots in the ensemble contribute to the coherent signal with a potential to focus the electron Larmor frequencies in the ensemble to a single mode [4]. The used optical technique allows to recover the coherence time of a single quantum dot. The measured spin coherence time is 3 microseconds, which is four orders of magnitude longer than the ensemble dephasing time of 400 picoseconds. This work is done in collaboration with A. Greilich, I. A. Yugova, R. Oulton, M. Bayer, A. Shabaev, Al. L. Efros, D. Reuter and A. D. Wieck. \newline \newline [1] A. Greilich, et al., Phys. Rev. Lett. 96, 227401 (2006). \newline [2] A. Greilich, et al., Science 313, 331 (2006). \newline [3] A. Greilich, et al., Phys. Rev. B 75, 233301 (2007). \newline [4] A. Greilich, et al., Science 317, 1896 (2007). [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y33.00006: Impurity-Ion pair induced high-temperature ferromagnetism in Co-doped ZnO Stefano Sanvito, Chaitanya Das Pemmaraju, Ruairi Hanafin, Thomas Archer, Hans Benjamin Braun Magnetic 3d-ions doped into wide-gap oxides show signature of room temperature ferromagnetism, although their concentration is two orders of magnitude smaller than that of conventional magnets. The prototype of these exceptional materials is Co-doped ZnO, for which an explanation of the room temperature ferromagnetism is still elusive. Here we demonstrate that magnetism originates from Co2+ oxygen-vacancy pairs with a partially filled level close to the ZnO conduction band minimum. The magnetic interaction between these pairs is sufficiently long-ranged to cause percolation at moderate concentrations. However, magnetically correlated clusters large enough to show hysteresis at room temperature already form below the percolation threshold and explain the current experimental findings. Our work demonstrates that the magnetism in ZnO:Co is entirely governed by intrinsic defects and a phase diagram is presented. This suggests a recipe for tailoring the magnetic properties of spintronics materials by controlling their intrinsic defects. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y33.00007: Magnetic 3$d$ interactions in ZnO and In$_{2}$O$_{3}$ in a band-gap corrected approach Stephan Lany, H. Raebiger, A. Zunger The electronic and magnetic configuration of 3$d$ transition metal (TM) impurities in wide-gap oxides like ZnO and In$_{2}$O$_{3}$ is misrepresented in the standard LDA and GGA approximations to density functional theory: Because the conduction band minimum lies energetically much too low, the spin-polarized impurity states wrongly occur as resonances inside the conduction band rather than as gap states. Due to spurious ``charge spilling'' from the TM impurity state into the host conduction band, the magnetic moment and the occupancy of the TM impurity state is incorrect, and the TM state becomes partially occupied, which is prone to cause overestimated ferromagnetic interactions. These errors are not corrected by the LDA+U or GGA+U methods often applied to TM-$d$ states. In our band-gap corrected approach, we augment the GGA+U functional by empirical non-local external potentials (NLEP) for the $s$- and $p$-states of Oxygen and the cations. In this approach the correct spin and orbital configuration of the TM impurity-states is recovered. In the absence of additional doping, we find generally short-ranged magnetic interactions, and pronounced Jahn-Teller effects in case of partially occupied gap states. Additional electron-doping can lead to more long-range ferromagnetic interactions for those TM-dopants that have unoccupied $d$-states which hybridize strongly with the conduction band. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y33.00008: Study of the anomalous Hall effect, magnetoresistance, and magnetic anisotropy in ZnO:Co and ZnO:Mn thin films Z. Yang, Z. Zuo, Y. Pu, M. Biasini, W. Beyermann, J. Shi, J. Liu ZnO-based diluted magnetic semiconductor (DMS) materials have attracted much attention in these years due to the theoretical prediction of above room-temperature ferromagnetism. So far, most of the experiments were focused to the study of the magnetization of the ZnO DMS materials. However, the magnetization study, by only providing a global information on the moment of the material, cannot distinguish the intrinsic magnetic properties from the extrinsic contributions. The ZnO thin films were grown on sapphire substrates by molecular-beam epitaxy. The Co- and Mn-implantations were performed on the as-grown ZnO samples with different free carrier concentrations. Anomalous Hall effect (AHE) and magnetoresistance measurements were performed on the ZnO:Co and ZnO:Mn thin films. AHE hysteresis loops were observed in both ZnO:Co and ZnO:Mn thin films, which confirm the intrinsic ferromagnetism in both films. However, the AHE hysteresis loops are distinctly weaker than the magnetization hysteresis loops ($M-H)$ measured by SQUID. Therefore we conclude that both intrinsic and extrinsic ferromagnetism co-exist in the samples. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y33.00009: Influence of oxygen partial pressure on structural, transport and magnetic properties of Co doped TiO2 films Bakhtyar Ali, Abdul Rumaiz, Arif Ozbay, S. Ismat Shah, Edmund Nowak Crystal structure, transport and magnetic properties of Co dopedTiO2 laser ablated thin films are investigated and are found to have a strong dependence on the oxygen partial pressure. X-ray diffraction reveals the presence of mixed phase material containing anatase and rutile. However, these phases intertransform with the change in the oxygen partial pressure in the chamber during the growth of the films under the same temperature and other growth conditions. Electrical conductivity enhances as more oxygen vacancies are created. Concomitantly, the magnetization increases with increased vacancy concentration. The electrical transport data is suggesting that the conduction is dominated by polarons. The activation energies obtained are in the range from 100 to 150meV, typical for semiconducting oxides. APS Membership: Pending [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y33.00010: Spin-polarized tunneling study of the room temperature spin filter CoFe2O4 Ana Ramos, Jean-Baptiste Moussy, Richard Mattana, Frederic Petroff, Tiffany Santos, Guo-Xing Miao, Jagadeesh Moodera The spin filter effect has the potential of generating highly spin-polarized electron currents by the spin selective transport of electrons across a ferromagnetic tunnel barrier. In this work, we investigate the spin-polarized tunneling characteristics of cobalt ferrite (CoFe2O4), which we show is a room temperature spin filter. Tunnel junctions containing epitaxial CoFe2O4(111) tunnel barriers have been grown by oxygen plasma-assisted molecular beam epitaxy. Their structural, chemical and magnetic properties having previously been optimized by a number of in situ and ex situ methods, we focus on the spin-polarized tunneling in the CoFe2O4-based systems using different measurement techniques. Following the demonstration of spin filtering by TMR measurements, both at low temperature and at room temperature, we further investigate the spin filter characteristics of CoFe2O4 in detail. In particular, we pay special attention to the influence of defects on the spin polarization, as well as the role of different spin-detecting electrodes. [Preview Abstract] |
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