Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P10: Focus Session: Magnetic Semiconductors: Oxides |
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Sponsoring Units: DMP GMAG Chair: Christoph Adelmann, University of Minnesota Room: LACC 153B |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P10.00001: Oxygen vacancies and ferromagnetism in Co-doped anatase John Jaffe, Timothy Droubay, Scott Chambers Cobalt-doped titanium dioxide, or CTO, has emerged in the past few years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in anatase-structure CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40{\%} of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the Generalized Gradient Approximation (GGA) to density functional theory (DFT) within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system's magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the Co atoms in the +2 charge state, as they indeed appear to be experimentally, so that the conventional DMS mechanism could only apply via n-type carriers. We find a small but not negligible spin density associated with Ti atoms near the vacancy sites, suggesting an F-center-mediated interaction between the much larger Co moments. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P10.00002: Magnetic circular dichroism (MCD) measurements of cobalt-doped titanium dioxide films J.R. Simpson, H.D. Drew, S.R. Shinde, S.B. Ogale, T. Venkatesan Cobalt-doped TiO$_2$ has generated interest as a dilute magnetic oxide displaying room-temperature ferromagnetism with $T_c \ge 650\,$K for low-doped materials. However, controversy surrounding the mechanism for such a high $T_c$ and the observation of Co clusters cast doubts on this system as an intrinsic ferromagnetic oxide. A recent study\footnote{S. R. Shinde \textit{et al.}, Phys. Rev. B \textbf{67}, 115211 (2003).} reporting the importance of growth conditions on Co solubility confirms the existence of ferromagnetism in films showing no direct evidence of clustering. MCD offers promise as a technique to characterize the intrinsic nature of magnetism and probe the band structure. A sensitive heterodyne technique using a photoelastic modulator measures MCD in the visible frequency range at magnetic fields up to 1.5\,T. We report a comparison of MCD measurements on thin films of well-oxygenated anatase Ti$_{1-x}$Co$_x$O$_{2-\delta}$ ($x\le 0.07$) exhibiting no clustering with those on clustered films. Additionally, we compare MCD results to optical absorption measurements,\footnote{J. R. Simpson \textit{et al.}, Phys. Rev. B \textbf{69}, 193205 (2004).} which reveal a shift of the band edge upon cobalt doping and an absence of mid gap cobalt impurity states. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P10.00003: Origin of ferromagnetism in transition metal doped TiO$_2$ Rebecca Janisch, Nicola A. Spaldin Reports of robust room-temperature ferromagnetism in oxide-based dilute magnetic semiconductors, such as Co-doped TiO$_2$ and ZnO, make these materials promising candidates for device applications. However, the origin of this robust ferromagnetism and the factors that influence its strength have not yet been fully understood. One model, proposed by Venkatesan and Coey, suggests that the observed ferromagnetism in doped ZnO and SnO$_2$ is mediatied via an electron trapped in a bridging oxygen vacancy [1,2]. On the other hand there is also evidence, e.g. in the case of Co-doped TiO$_2$, that free carriers play a role in mediating the magnetic interaction (e.g.[3]). To test the validity of the different models for the TiO$_2$-based dilute magnetic semiconductors, and to modify them where necessary, we study transition metal doped TiO$_2$ in the anatase structure by means of ab-initio density-functional band-structure calculations. We analyse the interaction of the transition metal dopants with the electronic states of the host as a function of the distribution and concentration of the impurity atoms. Finally we determine the influence of point and extended defects on the magnetic interactions. \newline [1] D.M.C. Coey et al., J. Appl. Phys. Lett. {\bf 84}, 1332 (2004). [2] M. Venkatesan et al., cond-mat/0406719. [3] H. Toyosaki et al., Nature {\bf 3}, 221 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P10.00004: Structure and magnetic properties of Co doped anatase TiO2 particles Hao Zhu, Lubna Shah, Yuwen Zhao, John Q. Xiao, Mohamed A. Barakat, G. Hayes, Ismat Shah Co$_{0.028}$Ti$_{0.972}$O$_{2-\delta }$was synthesized via sol-gel method. It's annealed at 600$^{\circ}$C for one hour in air. The X-Rays diffraction study confirmed that samples are of anatase structure and no detectable cobalt clusters or other impurities are observed. The magnetic properties are characterized by SQUID in a broad temperature range 5K $\sim $ 300K, and no ferromagnetic property was observed. The magnetization verse temperature curve fits well with the Curie-Weiss law and the extracted atomic effective moment suggests that the Co(III) is in high spin state. These paramagnetic particles can be turned into ferromagnetic phase with a Curie temperature above 300K after heat treatment in a mixture of H$_{2}$ and Ar gases at 600$^{\circ}$C for one hour. The origin of ferromagnetism has been studied by XPS, SQUID, XRD and TEM. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P10.00005: Characterization of transition metal doped CVD-grown ZnO films and nanostructures D. Hill, R.A. Bartynski, L. Wielunski, P. Wu, Y. Lu, V. Poltavets, M. Greenblatt Diluted magnetic semiconductors (DMS) are intriguing materials that offer the possibility of studying magnetic phenomena in crystals with a simple band structure and excellent magneto-optical and transport properties. Theoretical and experimental studies indicate that ZnO is a promising DMS candidate for room temperature spintronics applications. We have characterized the chemical, compositional, and magnetic properties of TM-doped ZnO films grown by MOCVD and sputter deposition on a variety of substrates. Doping with Mn, and Fe by either diffusion, co-sputtering, or ion implantation has been investigated, and each doping method results in very different dopant depth profiles as revealed by Rutherford backscattering spectrometry. Soft x-ray absorption spectroscopy (SXAS) indicates that the TM dopant may be in either the 2+ or 3+ oxidation state and depends upon doping method and/or sample processing. Furthermore, the XAS results are consistent with the TM ions being substitutional for Zn. Squid magnetometry shows that some doping methods yield films exhibiting ferromagnetic behavior, with some Fe-doped films having the Curie temperatures above room temperature. Finally, we discuss the properties of MOCVD-grown ZnO nanotips that have been doped by TM ion implantation. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P10.00006: Magnetism in transition doped ZnO Priya Gopal, Nicola Spaldin We present results of our detailed density functional investigation of ZnO doped with a series of \textit{3d} transition metal ions (Cr, Mn, Fe, Co, Ni and Cu). We have calculated the strength of the magnetic interactions when a single atom type is used as a dopant as well as the effects of simultaneous doping with two different transition metal ions. In addition, we have also done simulated \textit{p}-type doping in ZnO by substituting one of the Zn atoms by the monovalent ions , Li$^{+}$ and Cu$^{+}$ and determined the influence on the magnetism. We have also introduced defects in the form of O and Zn vacancies and its effect on magnetism. We find that our results are highly sensitive to the details of the calculations, including energy and k-point convergence, structural optimization and choice of exchange-correlation functional. However, our highly converged results suggest that above room-temperature ferromagnetism is not possible in transition metal doped ZnO without additional carriers and the experimental reports of high temperature ferromagnetism in this system could be due to the presence of secondary phases. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P10.00007: Ferromagnetism in Ti-Doped ZnO Nanoclusters Jiji Antony, Sweta Pendyala, Amit Sharma, Daniel Meyer , You Qiang Ferromagnetic behavior at room temperature is observed when a small percentage of non-magnetic titanium is combined with ZnO to form nanoclusters in the presence of oxygen atmospheres. The 5{\%} Ti- doped ZnO nanocluster film is prepared at room temperature using a technique that is a combination of high pressure sputtering and aggregation. A SQUID magnetometer measures the magnetic properties of this cluster film at various temperatures. The coercivity of the samples decreases exponentially with the increase of temperature. The maximum value of coercivity is 204.76 Oe obtained at 5K. The remanent magnetization increases at low temperatures up to 30K and decreases after wards. A distorted hysteresis curve is observed at 45K, 50K, 55K and 300K, where as at 5K, 30K and 90K the hysteresis curve showed normal ferromagnetic behavior. The field cooling (FC) and zero-field cooling (ZFC) measurements reveal a phase transition mechanism related to the spin ordering/disordering, which depends on the temperature. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P10.00008: Carrier Mediated Ferromagnetism above 300 K in ZnO:Mn Nikoleta Theodoropoulou, Vinith Misra, Jagadeesh Moodera, Biswarup Satpati We will present evidence that Zn$_{1-x}$Mn$_{x}$O thin films, grown by reactive magnetron sputtering, are ferromagnetic at temperatures significantly above 300 K. The onset of the ferromagnetic behavior is sensitive to the exact growth conditions - in addition to the Mn concentration, the magnetic properties strongly depended on the substrate type, film growth temperature and Oxygen partial pressure. Anomalous~Hall Effect shows that the charge carriers are spin-polarized electrons, participating in the observed ferromagnetic behavior. Specifically, Zn$_{1-x}$Mn$_{x}$O on Al$_{2}$O$_{3}$(0001) substrates are single-phase, as characterized by XRD and TEM and the magnetic moment for a Mn concentration of x=0.03 is 4.8$\mu _{B}$/Mn at 350 K, one of highest moments yet reported for any Mn doped magnetic semiconductor. Growth of Zn$_{1-x}$Mn$_{x}$O films on Si/SiO$_{2}$ substrates leads to the formation of secondary phases and no ferromagnetism is observed in these cases. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P10.00009: Metastable defect ferromagnetic phases by low temperature interface reactions between transition metal oxides Darshan C. Kundaliya, S.B. Ogale, S.R. Shinde, M.S.R. Rao, T. Venkatesan, S.E. Lofland Recently we showed [Nature Materials 3, 709 (2004)] that the room temperature ferromagnetism observed in low temperature (500 $^{\circ}$C) processed mixtures of 2 at{\%} MnO$_{2}$ with ZnO [Nature Materials 2, 673 (2003)] is caused by an interface phase suggested to be of the form Mn$_{2-x}$Zn$_{x}$O$_{3-\delta , }$wherein Zn is incorporated into Mn$_{2}$O$_{3}$. In order to establish the anticipated generic nature of the process, experiments were performed on low temperature sintering of 2 at{\%} MnO$_{2}$ with other transition metal oxides such as NiO, TiO$_{2}$ or CuO. Room temperature ferromagnetism was observed in all the three cases. Our results suggest that ferromagnetism in these new cases also resides at the interface. The same mixtures when sintered at 800 $^{\circ}$C resulted in compound phases (e.g. NiMn$_{2}$O$_{4}$ in the Ni-Mn-O case) that are nonmagnetic at room temperature. We suggest a new mechanism of ferromagnetism based on the valence and spin controlled defect state in the interface phases. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P10.00010: Paramagnetic properties of Mn$^{2+}$ in Mn-added ZnO S. H. Choh, I.-W. Park, Y. M. Kim, C. H. Son, J. H. Lyou, Y. J. Park, U. Chon Spin injected semiconductors, in which spins are introduced into the lattice, have been intensively studied due to their wide potential applications. Ferromagnetic ordering above room temperature in some of Mn-added ZnO was theoretically predicted by Dietl et al [1] and experimentally observed in Mn-doped ZnO [2]. In this work, we report the electron magnetic resonance (EMR) studies as well as physical properties of Zn$_{1-x}$Mn$_{x}$O ( x = 0.005 - 0.20 ) powders and a thermally Mn diffused ZnO crystal at 1000 $^{\circ}$C. The crystal structure of all samples showed a hexagonal wurtzite. However, even for the lowest Mn content (x=0.005) the samples turn out to contain a secondary phase [3], ZnMn$_{2}$O$_{4}$, from the XRD pattern. As the Mn content in the samples increases, so does the concentration of the secondary phase. In addition the electron magnetic resonance signal intensity of the paramagnetic Mn$^{2+}$, successfully incorporated into ZnO powder, decreases as the incorporated Mn content increases. This means that the Mn-rich secondary phase can more easily be formed than the Mn incorporated ZnO powder at 700 $^{\circ}$C. Paramagnetic Mn$^{2+}$ ions in a Mn-diffused ZnO crystal are turned out to sit on the Zn site from the EMR spectra. [1] T. Dietl et al, Science 287, 1019 (2000). [2] P. Sharma et al, Nature Materials 2, 673 (2003). [3] Y. M. Kim et al, Solid State Comm., 129, 175 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P10.00011: Magnetic studies of ion-implanted p-GaN,\\Al$_{0.35}$Ga$_{0.65}$N, and ZnO with tran\-sition metals Jeremy Raley, Yung Kee Yeo, Mee-Yi Ryu, Robert Hengehold, Todd Steiner, Pan Wu, Yicheng Lu Examination of the viability of ion-implantation for creating dilute magnetic semiconductors with ferromagnetic properties persisting to room temperature has been undertaken. Samples of $p$-GaN, Al$_{0.35}$Ga$_{0.65}$N and ZnO (film and nanotips) have been implanted with Fe, Mn and Cr at doses of 5$\times $10$^{16}$ cm$^{-2}$ and Ni at 3$\times $10$^{16}$ cm$^{-2}$. The samples were annealed at temperatures ranging from 600 to 800$^{\circ}$ C. The GaN and AlGaN samples were annealed in flowing N$_{2}$ for 5 min, and the ZnO samples in flowing O$_{2}$ for 10 min to determine the effect of annealing temperature. Using a superconducting quantum interference device (SQUID) magnetometer, we quantify ferromagnetism by the magnitude of coercive fields and show that an optimum annealing temperature is reached and passed within the range tested for a majority of the material/dopant combinations. Finally, we measure field-cooled and zero-field-cooled magnetization versus temperature. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P10.00012: Preparation and Magnetic Properties of Transition-metal-doped SnO$_2$ Jun Zhang , Xingzhong Li, David Sellmyer, Kiran Mendu, Jing Shi, Yongfeng Lu There is strong interest in the ferromagnetism (FM) at room temperature in doped oxides, e.g., Co or Fe doped SnO$_{2 }$[1, 2]. We report the preparation and magnetic properties of transition-metal-doped SnO$_{2}$ (Sn$_{1-x}$TM$_{x}$O$_{2}$, TM=V and Mn, x=0--0.05). Bulk and thin-film samples were prepared by solid-state reaction and pulsed-laser deposition, respectively, and characterized by X-ray diffraction, electron microscopy and SQUID magnetometry. The magnetic properties strongly depend on the sample-processing temperature. Room-temperature FM has been observed in the bulk samples sintered at a low temperature of 500 $^{\circ}$C, but not in those sintered or annealed at higher temperatures (650 $^{\circ}$C and 900 $^{\circ}$C for TM=V and Mn, respectively). Additional Sb-doping has no strong effect on the FM, while vacuum annealing enhances the FM. Effects of sample-processing conditions and additional Sb-doping on the magnetic properties will be discussed. This research is supported by ONR, NSF-MRSEC, W. M. Keck Foundation and CMRA. [1]. S. B. Ogale, et al., Phys. Rev. Lett. 91, 077205 (2003). [2]. J. M. D. Coey, et al., Appl. Phys. Lett. 84, 1332 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P10.00013: Variable range hopping behavior in the magnetic semiconductor SnO$_2$:Co P.A. Stampe, R.J. Kennedy, Yan Xin, E. Hu, Peng Xiong, S. von Moln\'{a}r To explore the origin of ferromagnetism in doped oxide systems we have grown SnO$_2$: Co thin films on R-plane Al$_2$O$_3$ substrates by laser ablation. We present detailed structural (x-ray diffraction and TEM), transport and magnetic measurements on films grown at different temperatures, growth rates and oxygen pressures. TEM studies show no evidence of cobalt clustering in any of the SnO$_2$:Co films investigated. Variations in growth conditions have been found to affect the physical properties of the material. For example, films with high crystallinity, grown at low growth rates, show transport consistent with Mott variable range hopping, with $\log(\rho)\propto T^{-1/4}$. This is in contrast to the $\log(\rho)\propto T^{-1/2}$ dependence found previously in TiO$_2$:Co containing metallic cobalt clusters$[1] $. We will discuss implications of these findings on the ferromagnetism observed in SnO$_2$:Co.\\ This work was supported by the DARPA SPINS program. The TEM work was supported by NSF DMR-0084173.\\ $[1]$ R.J. Kennedy, et al, Appl. Phys. Lett. 84 2832 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P10.00014: Intrinsic Defect Ferromagnetism: The case of Hafnium Oxide Chaitanya Das Pemmaraju, Stefano Sanvito In view of the recent experimental reports of intrinsic ferromagnetism in Hafnium Oxide (HfO$_{2}$) thin film systems \footnote{M. Venkatesan, C. B. Fitzgerald, J. M. D. Coey Nature {\bf 430}, 630 (2004) Brief Communications}, we carried out first principles investigations to look for magnetic structure in HfO$_{2}$ possibly brought about by the presence of small concentrations of intrinsic point defects. {\it Ab initio} electronic structure calculations using Density Functional Theory (DFT) show that isolated \textbf{cation} vacancy sites in HfO$_{2}$ lead to the formation of high spin defect states which couple ferromagnetically to each other. Interestingly, these high spin states are observed in the low symmetry monoclinic and tetragonal phases while the highly symmetric cubic flourite phase exhibits a non-magnetic ground state. Detailed studies of the electronic structure of cation vacancies in the three crystalline phases of Hafnia show that symmetry leading to orbitally degenerate defect levels is not a pre-requsite for ferromagnetism and that the interplay between Kinetic, Coulomb and Exchange energy together with favourable coupling to the Crystalline environment can lead to high spin ferromagnetic ground states even in extreme low symmetry systems like monoclinic HfO$_{2}$. These findings open up a much wider class of systems to the possibility of intrinsic defect ferromagnetism. [Preview Abstract] |
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P10.00015: Electric Field Modulation of Ferromagnetism in Diluted Magnetic Insulating Co:TiO$_2$ T. Zhao, S.R. Shinde, S.B. Ogale, H. Zheng, T. Venkatesan, S. Das Sarma, J. Misewich, R. Ramesh In this work we report the first successful implementation of an external electric field modulation of ferromagnetism in an oxide-based diluted magnetic system. An anatase TiO$_{2 }$layer with 7{\%} Co doping and a ferroelectric PbZr$_{0.2}$Ti$_{0.8}$O$_{3 }$(PZT) layer were epitaxially grown on a conducting SrRuO$_{3 }$buffered LaAlO$_{3}$ substrate by pulsed laser deposition. The Co:TiO$_{2}$ channel grown in this case at a high temperature of 875 $^{0}$C is insulating in nature. The magnetic hysteresis loops of the Co:TiO$_{2}$ were measured by SQUID after positive or negative electric poling on PZT. A 15{\%} difference in the room temperature saturated magnetic moment as well as the coercive filed of Co:TiO$_{2}$ is observed according to the two polarization states of PZT, which can be modulated over several cycles. This first demonstration of electric field effect in an oxide based diluted ferromagnetic insulator system provides evidence of its intrinsic nature. Possible mechanisms for insulating ferromagnetism and its modulation by an electric field are discussed. This work was supported by DARPA SpinS program (through US-ONR) and the NSF-MRSEC (DMR 00-80008) at Maryland and by a grant from Brookhaven National Laboratory. [Preview Abstract] |
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