Papers

AVS2004 Session MI-ThM: Magnetic Oxides and Half-Metallics

Thursday, November 18, 2004 8:20 AM in Room 304A

Thursday Morning

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8:20 AM		MI-ThM-1 Defect Mediated Ferromagnetic Coupling Through an Insulating Barrier Layer¹ P.A. Dowben (University of Nebraska); R.-H. Cheng (Argonne National Laboratory); B. Doudin (University of Nebraska) Interlayer exchange coupling between two ferromagnetic films, separated by a nonmagnetic non metallic spacer (semiconductor and insulator spacer materials) does occur. This coupling sometimes appears to be distinct from the very low temperature tunneling phenomena between two ferromagnets, through a dielectric spacer layer, as the coupling is sometimes oscillatory. The ferromagnetic coupling between Co and CrO₂, through an insulator (Cr₂O₃) may be rel ated to defect states in the insulating barrier layer.² In the native Cr₂O₃ surface layer, it appears that at low temperature the conduction band edge electrons are trapped or immobile, and at high temperature there is greater mobili ty. Combined photoemission and inverse photoemission temperature dependent studies confirm the occurrence of a defect mediated blockade energy. It may well be that many defect states are spin polarized, possibly by proximity to the ferromagnetic interface. Other complications exist. The interpretation of junction magneto-resistance results must now assume that ferromagnetic metals will NOT generally form abrupt interfaces with transition metal oxide dielectric barriers. It must be recognized that many met al to metal oxide interfaces involve further oxidation and reduction making such interfaces very heterogeneous, so that nominal CrO₂/Cr₂O₃/Co magnetic junctions are, in fact, more complex multilayers systems akin to a CrO₂/Cr₂O₃/CoO/Co system. ¹ The support of the Office of Naval Research, and the NSF MRSEC (DMR 0213808) are gratefully acknowledged. The authors would like to acknowledge a number of helpful conversations with E. Tsymbal. ² Ruihua Cheng, A.N. Caruso, L. Yuan, S.-H. Liou, and P.A. Dowben, Applied Physics Letters 82 (2003) 1443-1445 .
9:00 AM		MI-ThM-3 Measuring Spin Polarization at the Fermi Level in Potential Half-Metallic Ferro-magnets J.G. Tobin (Lawrence Livermore National Laboratory); T. Komesu, G.D. Waddill (University of Missouri-Rolla) We have utilized synchrotron-radiation-based techniques to investigate possibly half-metallic ferromagnetic (HMFM) materials. HMFM’Ts are, of course, potential sources of pure spin polarized electrons for spintronic and magnetic information storage devices. These investigations include the application of spin-resolved photoelectron spectroscopy (SPES) to systems such as Fe3O4¹ and X-ray Absorption Spectroscopy (XAS) to Zintl compounds such as Yb14MnSb11. ² In the case of Fe3O4, despite having performed the studies at the relatively high photon energy of 160 eV, significant problems with residual surface effects were observed. In order to circumvent or at least minimize these surface complications and to get a better measure of the true bulk spin polarization, we have moved our spin-resolving spectrometer³ to Beamline 4 at the Advanced Photon Source,⁴ where an Elliptically Polarizing Undulator (EPU) can provide high brightness radiation in the range of 500 eV to 3000 eV. Our first spin-resolved results of the Fe2p core levels⁵ have confirmed the feasibility of these experiments. Plans for the interrogation of potential half-metallic ferromagnets such as GaMnAs and related materials at these higher energies will be discussed. This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. ¹ S.A. Morton, G.D. Waddill, S. Kim, I.K. Schuller, S.A. Chambers, and J.G. Tobin, Surface Science Letters 513, L451 (2002). ² A.P. Holm, S.M. Kauzlarich, S.A. Morton, G.D. Waddill, W.E. Pickett, and J.G. Tobin, J. Amer. Chem. Soc. 124, 9894 (2002). ³ M. Hochstrasser, J.G. Tobin, E. Rotenberg and S.D. Kevan, "Spin-Resolved Photoemission of Surface States of W(110)-(1X1)H," Phys. Rev. Lett. 89, 216802 (2002). ⁴ J.W. Freeland et al, Rev. Sci. Instrum. 73, 1408 (2002). ⁵ http://www-cms.llnl.gov/s-t/aps.html
9:20 AM		MI-ThM-4 Transition Metal Oxides in Reduced Dimensions J. Shen (Oak Ridge National Laboratory) When the spatial dimension of a material becomes comparable or even smaller than the characteristic length scale of the relevant cooperative phenomena, it is expected that all related physical properties including phase transitions of this material will be dramatically changed. In this work, we focus on the discovery, understanding, and design of low-dimensional 3d transition metal oxides (TMO). We use both physical and chemical methods including laser MBE growth, hydrothermal synthesis, and nanoparticle-catalyzed processes to grow TMO thin films and nanowires. The electronic and magnetic properties of the TMO thin films have been investigated by in-situ scanning tunneling microscopy and ex-situ SQUID magnetometer. We have observed both large-scale (over a few tens nanometers) and nano-scale electronic phase separation (PS) in epitaxially grown thin films of (La5/8-0.3Pr0.3)Ca3/8MnO3. While the large PS domains are present only below the Curie temperature, the nano-scale PS clusters exist at temperatures both below and above Curie temperature. The latter implies that the small PS may originate from doping-related disorder. The TMO nanowires of doped manganites are single crystals with tunable diameters. SQUID magnetometer and e-beam lithography prepared four-point probes have been used to study their magnetic and transport properties.
10:00 AM		MI-ThM-6 Growth and Characterization of PLD Grown Co Doped TiO₂ S.T. Prisbrey (UC Davis, Lawrence Livermore National Laboratory); S.P. Vernon (Lawrence Livermore National Laboratory) 14-20 nm thick 3.7%Co doped TiO₂ and 5.4% Co doped TiO₂ have been grown by PLD on LaAlO₃ (100) substrates. θ-2θ x-ray diffraction scans suggest that the films are crystalline and oriented with the z-axis perpendicular to the film surface. AFM/MFM scans show the film to be smooth but with a high defect density (~10⁶/cm²). Defects on the surface of the film do have a magnetic response. X-ray absorption spectroscopy with CoTiO₃, CoO, and Co₃O₄ shows the cobalt to be in the Co⁺² formal oxidation state in the majority of the films grown but with no discernable correlation between known deposition parameters and the formal oxidation state of the Co. SQUID magnetic measurements and VSM measurements show that the films are magnetic at room temperature with T_c > 340°C. Surface ion mass spectrometry was employed to determine that the actual cobalt concentrations in the films are the same as those of the target. Samples were cooled in a field and B-H loops were measured to determine if the magnetic interaction is similar to that of a spin glass.
10:20 AM		MI-ThM-7 Room-Temperature Ferromagnetism in Cr-doped TiO₂ Anatase T.C. Droubay, S.M. Heald, S.V. Shutthanandan, S.T. Thevuthasan, S.A. Chambers (Pacific Northwest National Laboratory); J. Osterwalder (Physik-Institut der Universitat Zurich, Switzerland) Since the initial discovery in 2001, the possibility of ferromagnetism in doped oxide semiconductors has spawned a flurry of research activity around the world. Among the new materials that have been investigated, Co-doped TiO₂ anatase has garnered much attention due to its ferromagnetic response, which persists well above room temperature. Reports of doping TiO₂ anatase with other magnetic transition elements, however, are scarce. We have carried out a detailed study of the growth and properties of epitaxial Cr-doped TiO₂ anatase on LaAlO₃ (001) using oxygen-plasma assisted molecular beam epitaxy. These films are found to be single-phase and homogenous, with Cr uniformly substituting for Ti in the lattice. Cr K-shell x-ray absorption near-edge spectroscopy shows that the formal oxidation state of Cr is +3 throughout the films, with no evidence for either elemental Cr or half-metallic CrO₂. These films are insulating as grown, yet exhibit room temperature ferromagnetism aligned in-plane with a saturation magnetization of ~0.6 Bohr magnetons per Cr atom. Introduction of free electrons by incorporation of additional oxygen vacancies via post-growth annealing in UHV increases both the n-type conductivity and the saturation magnetization, without deleterious effects on the homogeneity or crystallinity of the films. None of the existing models of magnetism in semiconductors can explain ferromagnetism in the absence of free carriers for a dilute system. We will present a novel exchange mechanism to do so.
10:40 AM		MI-ThM-8 Synthesis of Room-Temperature Ferromagnetic Materials by Ion Implantation: Transition-Metals-Doped TiO₂ (110) Rutile V. Shutthanandan, S.T. Thevuthasan, S.M. Heald, T.C. Droubay, M.H. Engelhard, C.M. Wang, D.E. McCready, T.C. Kaspar, S.A. Chambers (Pacific Northwest National Laboratory); P. Nachimuthu, B.S. Mun (Lawrence Berkeley National Laboratory) There is growing interest in diluted magnetic semiconductors (DMS) in the emerging field of spintronics. It has recently been demonstrated that certain oxide semiconductors doped with magnetic transition metal elements show room-temperature ferromagnetism. In particular, titanium dioxide in both anatase and rutile phases appear to be among the most promising oxide semiconductors for DMS applications. In this study, we show that ferromagnetic transition metal doped TiO₂ (110) rutile single crystals can be successfully synthesized using ion implantation by carefully controlling implantation parameters such as temperature and ion fluence. Co, Cr, Ni and Fe ions with 100 keV energy were implanted at 875 K to 1075 K and an ion fluence of ~1.25x10¹⁶ ions/cm². Vibrating sample magnetometer (VSM) measurements clearly show room temperature ferromagnetic responses with magnetic moments ranging from 0.30 to 0.70 µ _B /atom. X-ray photoelectron spectroscopy (XPS) depth profiling and Rutherford backscattering spectrometry (RBS) measurements reveal that most of the implanted dopants are uniformly distributed to a depth of ~300 nm with an average concentration of ~1 to 3 at%. K-edge x-ray absorption near edge spectra (XANES) obtained from the implants show that all of the implanted atoms are oxidized and that the formal oxidation is +2 for Co and Ni, +3 for Cr and a mixture of +2 and +3 for Fe. There is no evidence that the dopant is in the metallic state in these implanted samples. In addition, surface-sensitive total electron yield (TEY) and bulk-sensitive total florescence yield (TFY) obtained from the L-edge of Ti and implants and K-edge of O demonstrate that the structural environments in both surface and bulk regions of rutile TiO₂ are not significantly affected by the incorporation of implanted species.
11:00 AM		MI-ThM-9 MBE Growth and Room Temperature Ferromagnetism in Epitaxial Co-doped SrTiO₃ T.C. Kaspar, T.C. Droubay, S.M. Heald, C.M. Wang, V. Shutthanandan, S.T. Thevuthasan, S.A. Chambers (Pacific Northwest National Laboratory) Room temperature ferromagnetism in dilute magnetic semiconductors (DMSs) is highly desirable for practical spintronic devices. The observation of room temperature ferromagnetic behavior in doped wide bandgap semiconducting oxides such as Co-doped ZnO, Mn-doped ZnO, and Co-doped anatase and rutile TiO₂ has raised the possibility of DMS behavior in other oxide systems. Work on these materials has also revealed the critical importance of thorough materials characterization to rule out ferromagnetic contributions from metallic clusters or other secondary phases. Recently, Co-doped La_{0.5√sub 0.5}TiO₃ was found to exhibit ferromagnetic behavior at room temperature.¹ In the present study, epitaxial SrTi_1-xCo_xO₃ thin films (0 < x < 0.15) have been deposited on SrTiO₃(001) substrates by oxygen-plasma-assisted molecular beam epitaxy (OPAMBE). Smooth films free of surface particles or clusters are obtained. As-deposited films are insulating (ρ > 5 kΩ-cm); however, room temperature ferromagnetism is observed for < 5% Co doping, with a high magnetic moment and 10-25% remanence. Films doped at higher concentrations (> 5%) do not exhibit ferromagnetism. Thorough materials characterization was employed, including XPS for film composition, AFM and TEM to observe film morphology and the possible inclusion of secondary phases, and RBS and PIXE to determine the precise Co concentrations. A comprehensive study of Co K-edge XANES and EXAFS data to determine details of the Co charge state and local environment was also carried out. F-center mediated exchange will be discussed as the possible mechanism of ferromagnetic ordering in Co:STO. In addition, preliminary results of epitaxial SrTi_1-xCo_xO₃ growth on Si(001) will be presented, and the differences in deposition on oxide and semiconductor substrates will be discussed. ¹ Zhao, Y.G., et al. Appl. Phys. Lett. 83, 2201 (2003).
11:20 AM		MI-ThM-10 Characterization of Transition Metal Doped ZnO Films and Nanostructures D.H. Hill, L. Wielunski (Rutgers University); D.A. Arena (Brookhaven National Lab); R. Bartynski, P. Wu, Y. Lu (Rutgers University) A crucial element for the success of spintronics is finding a material that combines the desirable properties of ferromagnets and semiconductors. 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. ZnO, a wide bandgap (~3.3 eV) semiconductor that has received increasing attention due to its broad applications and its many desirable material properties, has recently has been identified as a promising DMS candidate for room temperature spintronics. 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. 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.
11:40 AM		MI-ThM-11 Effects of Heat Treatment on the Magnetic Properties of Nickel Cobalt Oxide Films R.R. Owings, G.J. Exarhos, T.C. Droubay, C.F. Windisch (Pacific Northwest National Laboratory) Spinel films of reactively sputtered nickel cobalt oxide exhibit an increase in saturation magnetization, up to ~5x, when rapidly quenched following heat treatment at 375° C for 10 minutes in air. The films appear to be weakly ferromagnetic with an in-plane saturation occurring in a field greater than 2000 G. Heat treatment followed by rapid quenching has also been shown to increase conductivity. Although the exact mechanism that relates the conductivity to the saturation magnetization is not well understood, the results of this work suggest that there is a relationship between the carrier movement and the spin orientation of the occupied orbitals in the octahedral or tetrahedral lattice positions. The effects of the heat treatment and cooling rate on the magnetic moment, electrical conductivity, index of refraction, IR transmission, and cation disorder are also discussed.