Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L12: Focus Session: Magnetic Semiconducting Oxides |
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Sponsoring Units: GMAG DMP FIAP Chair: Darrell Schlomm, Pennsylvania State University Room: Colorado Convention Center Korbel 3C |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L12.00001: Challenges in the Synthesis of Diluted Magnetic Semiconducting Oxides Invited Speaker: In the rapidly advancing field of Spintronics, the quest for an above room temperature diluted magnetic semiconductor (DMS) has been thwarted by the lack of a conventional semiconductor with above room temperature ferromagnetism (FM). The recent observation of high temperature FM in numerous oxides has created a flurry of publications with controversial results. I will address the origin of this controversy and trace it to the fact that unlike their predecessors, the manganites, which had significantly large magnetization signals, we are now dealing with samples of weaker magnetization and the measurements are vulnerable to extraneous effects. With the example of Co doped TiO2, I will show that the substitutional incorporation of the magnetic ion in the host lattice is a process dependent phenomenon and Co incorporation and the lattice crystallization have opposing behavior with temperature. The role of TEM-EELS and XAS in distinguishing between intrinsic vs. extrinsic effects will be delineated. There are process regimes in which a homogeneously doped TiO2 DMS system does exist while in the rest of the region one obtains a super paramagnetic system with Co clusters embedded in a TiO2 host. Results from anomalous Hall and field effect studies will be discussed and other magnetically doped oxide host systems will also be covered. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L12.00002: Room temperature ferromagnetism in Fe-doped TiO$_{2}$ films is unrelated to magnetic ordering of iron. M Venkatesan, R.D. Gunning, J.M.D. Coey TiO$_{2-\delta }$ films doped with x = 1 -- 5 at.{\%} $^{57}$Fe produced in oxidizing or reducing conditions by pulsed laser deposition on sapphire substrates exhibit ferromagnetic behaviour at room temperature. Conversion electron M\"{o}ssbauer spectra show that most of the iron in the oxidized film (10$^{2}$ Pa) is present as paramagnetic Fe$^{3+}$ or Fe$^{2+ }$whereas in the reduced films (10$^{-3}$ Pa) most of the iron is present as well-crystallized iron metal. The moment per iron atom for 1{\%} and 5{\%} films significantly exceeds the value of 2.2 $\mu _{B}$ for iron metal and in the 1{\%} oxidized film it is as much as 6.9 $\mu _{B}$ per iron atom. Films where the dopant ions are not magnetically ordered possess some of the largest ferromagnetic moments. Saturation moments are in the range 2-20.10$^{-8}$ Am$^{2}$ or 90-900 $\mu _{B}$ nm$^{-2}$ of substrate area. The iron in these oxidized films is in Fe$^{3+}$ or Fe$^{2+}$ state, but it is \textit{not magnetically ordered} and cannot therefore contribute to the observed ferromagnetic moments. The isomer shifts indicate octahedral oxygen co-ordination for both ions. We conclude that Fe-doped TiO$_{2}$ cannot be regarded as dilute magnetic semiconductor. The magnetism of this, and may be that of many other dilute magnetic oxides might be explained in terms of oxygen-based electronic defects, or orbital currents which do not involve the dopant directly. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L12.00003: Room temperature ferromagnetism in Mn and Fe-doped indium-tin oxide films J.M.D. Coey, R.D. Gunning, M Venkatesan Following the reports of high temperature ferromagnetism in n-type Mn-doped Indium tin oxide (ITO) thin films, we have undertaken a systematic investigation of the magnetic and transport properties of ITO thin films doped with all the 3d transition metal ions. ITO films were grown on c-cut sapphire substrate by pulsed laser deposition. The X-ray diffraction patterns reveal that they are oriented mainly in (111) direction of cubic bixbyite structure. Undoped ITO films are diamagnetic. Room temperature ferromagnetism is observed in Fe and Mn-doped thin films of varying concentrations, when deposition temperatures are greater than 600\r{ }C. The largest magnetic moments were found in 2.5{\%} Mn-doped and 7.5{\%} Fe-doped ITO films. The Mn-doped films are anhysteretic, while the Fe-doped films exhibit a hysteresis with a coercivity of 30 mT and a moment which increases with concentration. However, in the Mn-doped samples, we see a higher moment for the lower concentrations. None of the other doped ITO films were found to be magnetic, ruling out the possibility of cluster based magnetism. Conversion electron M\"{o}ssbauer spectra of the ferromagnetic iron-doped films show the presence of magnetite in quantities sufficient to explain the magnetization. No such secondary phase is found for Mn. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L12.00004: Room Temperature Ferromagnetism in Mn-implanted CVD-Grown ZnO Films and Nanostructures D. Hill, R. Gateau, R.A. Bartynski, P. Wu, Y. Lu, L. Wielunski, V. Poltavets, M. Greenblatt, D.A. Arena, J. Dvorak, A. Moodenbaugh, S. Calvin We have characterized the chemical, compositional, and magnetic properties of Mn-ion implanted epitaxial ZnO films and single crystal nanostructures grown by MOCVD as candidate room temperature diluted magnetic semiconductors. X-ray absorption spectroscopy (SXAS) and EXAFS show that the as Mn-implanted films contain isolated Mn$^{2+}$ ions substitutional for Zn. Upon annealing the distribution of Mn ions changes becomes locally enriched with a substantial fraction of the nearest cation neighbors being Mn. SQUID magnetometry shows that as-implanted films are ferromagnetic at 5K with a saturation magnetization of $\sim $ 0.2 \textit{$\mu $}$_{B}$/Mn-ion. The annealed films have an Ms that is $\sim $ 1 \textit{$\mu $}$_{B}$/Mn-ion and are ferromagnetic at room temperature. Elemental analysis of the nanorods in the transmission electron microscope shows that the Mn concentration is relatively uniform perpendicular to the axis of the structure, but has a higher concentration near the tip than at the base. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L12.00005: A comparative RIXS study on Co2+ systems Dieter Schmeisser, Jonathan Denlinger We use RIXS at the ALS BL8 to investigate systems in which Co is preferentially in the Co2+ state. The systems include Co:ZnO, Co2O3, Co doped in polypyrrole, Co-phthalocyanine films, and CoO. For all these systems we report on the XAS and RIXS data at the Co2p edge. We separate the inelastic Raman losses due to d-d excitations from valence band induced excitations. We identify and quantify the relative contributions of the d7 HS and LS states and d8L charge transfer states. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L12.00006: V, Nb and Ta doping of anatase TiO$_{2}$: from a dilute magnetic semiconductor to a transparent conducting oxide Jorge Osorio-Guill\'en, Stephan Lany, Alex Zunger We have investigated by means of first-principle supercell calculations the effects of doping anatase TiO$_{2}$ by V, Nb and Ta. We find: ({\em i}) V doping makes TiO$_{2}$:V ferromagnetic. A single V impurity has a magnetic moment of 1.0 $\mu_{B}$/V atom with an electronic configuration $a_1^2\, t_{1+}^3\, t_{1-}^3\, t_{2+}^1\, t_{2-}^0\, e_+^{0}\, e_-^0$. The ferromagnetic interaction between two V impurities is found to extend to more than fifth neighbors, with calculated ferromagnetic stabilization energy raging from 124 meV at the first neighbor to 27 meV at the fifth neighbor. ({\em ii}) Nb and Ta doping of TiO$_{2}$ makes the system conductive, but not magnetic. The calculated equilibrium free-electron concentration ($n_{e}$) at $T = 1000$K for Ti-rich--O-poor growth conditions is $2.7 \times10^{21}$ and $5.9 \times10^{21}$ cm$^{-3}$ for Nb and Ta doping respectively, whereas pure TiO$_{2}$ is calculated to have a electron density of only $1.8 \times10^{18}$ cm$^{-3}$ due to intrinsic defects. Thus, Nb and Ta doping of TiO$_{2}$ enhance dramatically the electron concentration and hence are good transparent conductor oxides. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L12.00007: Ferromagnetism and n-type conductivity in Zn$_{1-x}$Fe$_{x}$O S. Kolesnik, B. Dabrowski, O. Chmaissem, W. L. Lim, M. Pekala Room temperature ferromagnetism in Zn$_{1-x}$Fe$_{x}$O can be obtained by precipitation of ZnFe$_{2}$O$_{4}$ impurity phase (with the Curie temperature of 440 K) after low-oxygen-pressure synthesis.* This impurity can be controlled by changing the synthesis temperature, which makes this material promising for spintronic applications. We have studied this material by magnetic, transport and thermoelectric experiments. The electrical resistivity shows a semiconducting behavior with $\rho \sim $0.4 $\Omega $cm at 300 K, much lower than Mn- and Co-substituted ZnO. Hall effect measurements show n-type conductivity with mobility $\sim $1-10 cm$^{2}$/Vs. The n-type conductivity is independent of the presence of ferromagnetic impurities. A high negative Seebeck (-300 $\mu $V/K at 300 K) would make this material suitable for thermoelectric applications if its resistivity could be further reduced. *S. Kolesnik et al., J. Appl. Phys. \textbf{95}, 2582 (2004). Supported by NSF (DMR-0302617) and the U.S. Department of Education. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L12.00008: Anisotropy Lock-in model for the magnetism of (Zn,Co)O Stefano Sanvito An explanation for the magnetism of diluted magnetic oxides remains elusive to the present day. Super-exchange is short ranged and leads to anti-ferromagnetic interaction, while double exchange needs unrealistically large charge densities to give room temperature ferromagnetism. Moreover there is growing experimental evidence that free-charges alone are not sufficient for the magnetism, which in turn is driven by intrinsic defects. Supported by density functional theory and Monte Carlo simulations we will present a coherent and complete picture of the magnetism in (Zn,Co)O. We will argue that Co clustering is essential for the magnetism and that a wurtzite CoO phase, difficult to detect by X-ray, is responsible for most of the magnetic signal. In this picture, strongly compensated CoO clusters with random anisotropy fields mimic the hysteresis loops often observed experimentally and attributed to long-range ferromagnetic interaction. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L12.00009: Enhanced Magnetism in Fe-Doped TiO$_{2}$ Anatase Nanorods Yi Ding, L. H. Lewis, Wei-Qiang Han In addition to the applied interest concerning dilute magnetic semiconductors, ferromagnetism in $d^{0}$ oxides is of fundamental interest in understanding the interaction character between magnetic impurities in insulating systems. We report here simultaneous ferromagnetism and enhanced Pauli Paramagnetism in TiO$_{2}$ anatase nanorods doped with nominal 0.5 at{\%} Fe, synthesized by a hydrothermal route followed by low-temperature heating in air. The resultant nanorods are $\sim $ 20 nm in width and several hundreds nanometers in length. Transmission electron microscopy (TEM) reveals that the Fe concentration ranges from 0.3 at{\%} - 0.8 at{\%} within the nanorods. No evidence of pure iron nanoparticles in the sample is detected with TEM or with synchrotron diffraction. SQUID magnetometry performed in the temperature range 10 K $\le $ T $\le $ 800 K in fields up to 1 T show clear ferromagnetism at low fields that transitions to paramagnetic behavior at higher fields. Decomposition of demagnetization curves reveals that the nanorods possess Pauli paramagnetism that is over 100 times larger than that of pure bulk anatase TiO$_{2}$ as well as ferromagnetism that persists to temperatures slightly above 800 K. It is hypothesized that the Pauli paramagnetism originates from anatase regions with lower Fe doping, while the ferromagnetism originates from regions of higher Fe doping, suggesting a percolative mechanism for ferromagnetism. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L12.00010: Room temperature ferromagnetism in undoped TiO$_{2}$ films P. Kharel, C. Sudakar, J. Thakur, G. Lawes, R. Naik, V.M. Naik, R. Suryanarayanan We have prepared thin films of undoped TiO$_{2}$ having rutile and anatase structures, using both spin coating and sputter deposition techniques, on sapphire and quartz substrates. The structural characteristics of the films have been investigated using Raman spectroscopy and transmission electron microscopy (TEM). We found that the annealing condition strongly influences the magnetic properties of the films. When annealed in high vacuum, all films demonstrate room temperature ferromagnetism (FM) whereas air annealed samples show insignificant FM. The ferromagnetic moment in vacuum-annealed samples stored under ambient conditions was not stable, but decayed on a time scale of hours. The sample magnetization was found to depend on the film thickness; the saturation magnetic moment was observed to decrease with increasing film thickness. These results suggest that FM in TiO$_{2-\delta }$ films is mediated by surface oxygen defects. The details of Raman and TEM studies will be presented and the appearance of FM on vacuum annealing will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L12.00011: Preparation of Dilute Magnetic Oxide Thin Films by Reactive-Biased Target-Ion Beam-Sputter Deposition Kevin G. West, Jiwei Lu, Jiani Yu, Wei Chen, Yonghang Pei, Li He, Stuart A. Wolf We have used reactive-biased target-ion beam-sputter deposition to prepare Co$_{x}$Ti$_{1-x}$O$_{2 }$thin films on LaAlO$_{3}$ (100) and SrTiO$_{3 }$(100) substrates for \textit{0.005$<$x$<$0.06}. The influence of the growth parameters on the microstructure, magnetic and transport properties of Co$_{x}$Ti$_{1-x}$O$_{2 }$was systematically investigated. Both pure anatase phase and mixed anatase/rutile phases of TiO$_{2}$ films were obtained by varying the growth conditions and subsequently demonstrated different magnetic and transport properties. All samples show a curie temperature higher than 300 K. The pure anatase Co$_{x}$Ti$_{1-x}$O$_{2 }$thin films have saturated magnetic moments of 1$\sim $2 $\mu _{B}$/Co at 10 K. The presence of rutile phase seems to greatly enhance the moments at lower temperatures. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L12.00012: Observation of ferromagnetic behaviors in doped and undoped TiO$_{2}$ and ZnO P.V. Wadekar, Q.Y. Chen*, P.V. Chinta, O. Lozano, Z.H. Zhang, W.K. Chu, H.W. Seo, C.P. Sun, C.C. Chou, H.D. Yang, L.W. Tu, Y.L. Cheng, M.Z. Hsu Transition-metal doping has been widely used to produce ferromagnetic oxides such as TiO$_{2}$ {\&} ZnO for use as a diluted magnetic semiconductor. Recently, however, undoped samples were also found to be ferromagnetic. We have studied the ferromagnetic behaviors of ZnO and TiO$_{2}$ single crystals and powders annealed both in vacuum and in flowing oxygen at various temperatures. In order to understand the observed ferromagnetism, we have used X-ray photoemission and electron spin resonance spectrometry to characterize the possible valance states or chemical bonding variation to study the roles that oxygen vacancies may have played in the occurrence of ferromagnetism. Comparisons on the doped and pure samples will be discussed.* Also with NSYSU . [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L12.00013: Electronic structure of Gd-doped GaN: vacancy-stabilized ferromagnetism Paul Larson, Sashi Satpathy The study of dilute-doped magnetic semiconductors has attracted significant interest recently, especially that of Gd-doped GaN. While experimental evidence for a colossal moment/Gd have been found in the low doping regime, we will focus on the stabilization of ferromagnetism over antiferromagnetism with much better understood 7--8 $\mu_B$/Gd. Electronic structure calculations have been performed using the LAPW method within the WIEN2k code. Supercell calculations have shown that, in the absence of Ga or N vacancies, the large distance between Gd atoms leads to antiferromagnetism by the superexchange mechanism. Ga and N defects allows for extra electrons or holes to mediate the magnetism between the Gd atoms and stabilizes the ferromagnetic state. The degree of vacancies necessary to stabilize ferromagnetism has been approximated within a percolation model. This work is supported by the Air Force Office of Scientific Research. [Preview Abstract] |
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