Fujita, K

Hirao, K

Hojo, H

Tanaka, K

Applied Physics Letters

Kyoto University Research Information Repository

Epitaxial growth of room-temperature ferrimagnetic semiconductor thin films based on the ilmenite-hematite solid solution

TitleEpitaxial growth of room-temperature ferrimagneticsemiconductor thin films based on the ilmenite-hematite solidsolutionAuthor(s)Hojo, H; Fujita, K; Tanaka, K; Hirao, KCitationAPPLIED PHYSICS LETTERS (2006), 89(8)Issue Date2006-08-21URL http://hdl.handle.net/2433/39686RightCopyright 2006 American Institute of Physics. This article maybe downloaded for personal use only. Any other use requiresprior permission of the author and the American Institute ofPhysics.Type Journal ArticleTextversionpublisher; noneKyoto UniversityEpitaxial growth of room-temperature ferrimagnetic semiconductor thinfilms based on the ilmenite-hematite solid solutionHajime Hojo, Koji Fujita,a Katsuhisa Tanaka, and Kazuyuki HiraoDepartment of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku,Kyoto 615-8510, JapanReceived 3 April 2006; accepted 29 June 2006; published online 24 August 2006Epitaxial thin films composed of 0.7FeTiO3·0.3Fe2O3 solid solution have been prepared on-Al2O3 0001 substrates by a pulsed laser deposition method, and their electrical and magneticproperties have been examined. A single phase of the ordered phase can be obtained under limiteddeposition conditions: oxygen partial pressure of 1.010−3 Pa and substrate temperature of600–700 °C. The as-deposited film is semiconducting and ferrimagnetic below room temperature,while subsequent annealing in vacuum leads to the Curie temperature above room temperature. Onthe other hand, the thin films with the disordered phase appear to be antiferromagnetic and alsoinsulating. © 2006 American Institute of Physics. DOI: 10.1063/1.2337276Magnetic semiconductors with Curie temperature TCbeyond room temperature have attracted considerable atten-tion since they are expected to be promising candidates forspin electronics applications. One approach to obtain mag-netic semiconductors is to introduce the magnetic ions intononmagnetic semiconductors, which is called diluted mag-netic semiconductor DMS. Since the discovery of room-temperature ferromagnetism in Co-doped TiO2,1 extensivestudies have been carried out concerning DMS based ontransition-metal-doped oxide semiconductors.2 Another ap-proach toward the development of magnetic oxide semicon-ductors is to explore semiconducting materials that areknown to show ferromagnetism or ferrimagnetism aboveroom temperature. Examples of such materials are solid so-lutions of ilmenite, FeTiO3, and hematite, -Fe2O3.It is known that although both FeTiO3 and -Fe2O3 areantiferromagnetic insulators,3 their solid solutions with inter-mediate compositions exhibit both semiconducting and ferri-magnetic properties.4,5 -Fe2O3 belongs to a family of co-rundum structure space group: R3¯c and is composed of adistorted hcp of anions O2− ions. Fe ions occupy two-thirdsof the available octahedral interstices, forming an alternatestack of Fe and O layers along the c axis. The crystal struc-ture of FeTiO3 is a derivative of corundum -Fe2O3; Felayers stacked along the c axis in -Fe2O3 are alternativelyreplaced by Ti layers. Due to the replacement, the spacegroup changes from R3¯c to R3¯ . Consequently, the solid so-lutions consisting of FeTiO3 and -Fe2O3 undergo the order-disorder transition between R3¯ and R3¯c symmetries. In theformer, Ti ions exist only in alternate layers along the c axisas in FeTiO3. Since each cation layer is antiferromagneti-cally coupled with adjacent layers, strong ferrimagneticproperties are observed only when the crystal structure is theordered phase. On the other hand, the disordered phasewhere cations are distributed at random, shows antiferromag-netic properties or rather weak magnetization.4The uniqueness of this system lies in the fact that theconduction type can be controlled as either p or n type bysimply changing the composition. Furthermore, recent theo-retical predictions suggested the possibility of spin-polarizedcarriers and high TC in the solid solutions.6 In spite of suchinteresting electrical and magnetic properties, however, onlya few studies have been reported on the solid solution oreven on FeTiO3 in the form of a thin film.7–10 Especially, ithas never been demonstrated that semiconducting and ferri-magnetic thin films having TC above room temperature couldbe obtained in the solid solutions. This is due to the difficultyin controlling the valence state of Fe ions as well as in sta-bilizing only the ordered phase during the film growth.9In this letter, we present the preparation of thin filmscomposed of the ordered phase of 0.7FeTiO3·0.3Fe2O3 solidsolutions using the pulsed laser deposition PLD technique.We also show that the as-deposited thin films are semicon-ducting and ferrimagnetic below room temperature, whilethe subsequent annealing treatment improves the magneticproperties; TC is raised beyond room temperature.Thin films with 0.7FeTiO3·0.3Fe2O3 composition inmolar ratio were grown on c-plane  -Al2O3 0001 sub-strates by a PLD method. The ceramic target used for PLDwas prepared by the conventional solid-state reaction amongpowders of reagent-grade Fe2O3 and TiO2. A thin film wasgrown on the substrate by focusing a KrF excimer laserwavelength: 248 nm at a repetition frequency of 5 Hz witha fluence of 2–3 J /cm2. To optimize the deposition condi-tion, the substrate temperature TS and the oxygen partialpressure PO2 were varied from 600 to 800 °C and from1.010−4 to 1.010−1 Pa, respectively. Some of the thinfilms were annealed under PO2 =2.010−4 Pa and T=700 °C for 1 h in the vacuum chamber.Rutherford backscattering spectroscopy was carried outusing 2.0 MeV He+ in order to determine the thickness andcomposition of the films. The analysis with the simulationprogram SIMNRA revealed that the film thickness was about70 nm and that the fraction of Fe and Ti was typically0.19:0.11, which means that FeTiO3:Fe2O3=0.73:0.27. Thecrystal structure of the film was analyzed by x-ray diffractionXRD measurement with Cu K radiation. Cross-sectionallattice images were obtained by means of a high-resolutiontransmission electron microscopy HRTEM. Optical trans-mission spectra were measured using a spectrophotometer.aAuthor to whom correspondence should be addressed; FAX: 81-75-383-2420; electronic mail: fujita@dipole7.kuic.kyoto-u.ac.jpAPPLIED PHYSICS LETTERS 89, 082509 20060003-6951/2006/898/082509/3/$23.00 © 2006 American Institute of Physics89, 082509-1Downloaded 31 May 2007 to 130.54.110.22. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspThe measurements of magnetization were carried out using asuperconducting quantum interference device magnetometer.The electric resistivity was measured by the van der Pauwmethod. The Seebeck coefficient was determined at roomtemperature to identify the major conduction type.Figure 1a shows XRD patterns for0.7FeTiO3·0.3Fe2O3 films grown at TS=700 °C under vari-ous PO2. One can see that the thin film grown under PO2=1.010−3 Pa is composed of the single phase of 0001-oriented FeTiO3–Fe2O3 solid solution with the orderedphase R3¯ symmetry. When PO2 is decreased to 1.010−4 Pa, the crystalline phase ascribed to a spinel-typestructure is observed as an impurity phase. On the contrary,as PO2 is increased from 1.010−2 Pa, the disordered phaseR3¯c symmetry starts to appear: one can identify the crystalsymmetry of the film, since only 0006n peaks are observedin the disordered phase with the R3¯c symmetry due to asystematic absence. The thin film grown at PO2 as high as1.010−1 Pa contains the disordered phase only. ExpandedXRD patterns for the thin films grown at 1.010−3 and1.010−1 Pa Fig. 1b reveal that the positions of diffrac-tion peaks for the disordered phase are observed at higherdiffraction angle sides, indicating that the lattice constant isreduced along the c axis. This is presumably due to the oxi-dation of Fe2+ to Fe3+ during the deposition at higher PO2,which suppresses the growth of films with the ordered phase,and also due to the change in crystal symmetry from R3¯ toR3¯c.8 We have also examined the effect of the TS on thecrystalline phase of the deposited films. The results for thefilms grown under PO2 =1.010−3 Pa are shown in Fig. 1c.The thin films grown at TS=600 and 700 °C are singlephases of the ordered phase, whereas the thin film grown atTS=800 °C contains the crystalline phase with the spinel-type structure in addition to the ordered phase. From theresults of Figs. 1a and 1c, it is found that the crystallinephase such as Fe2TiO4 ulvöspinel-type structure also tendsto be stabilized under lower PO2 and higher TS, both deposi-tion conditions of which prompt the production of Fe2+.In Fig. 1d is compared the XRD pattern for the as-deposited thin film grown under PO2 =1.010−3 Pa and TS=700 °C with that for the thin film postannealed at 700 °Cin vacuum PO2 =2.010−4 Pa. No secondary impurityphase is detected in the annealed thin film. By the annealingtreatment, the positions of diffraction peaks shift to higherangles, indicating the reduction of the lattice constant alongthe c axis. The reduction of lattice constant is not ascribed tothe oxidation of Fe2+ to Fe3+ mentioned above, since theannealing treatment under lower PO2 favors the production ofFe2+, which would increase the lattice constant. Instead, webelieve that the improvement of the crystallinity is more sig-nificantly responsible for the decrease in lattice constant.Figure 2 shows a cross-sectional HRTEM image of0.7FeTiO3·0.3Fe2O3 solid solution thin film with the orderedphase on -Al2O3 and selected area electron diffractionSAED pattern. The HRTEM image exhibits highly epitax-ial nature of the film with no impurity phases. The SAEDpattern reveals the in-plane and out-of-plane epitaxialgrowths of the film. The epitaxial relationship is as follows:solid solution film 000111¯00//Al2O3000111¯00. The011¯2 pole figure of the thin film not shown also demon-strated this epitaxial relationship.Optical transmission spectra measured at room tempera-ture are shown in Fig. 3. The measurements were performednot only for the annealed film with the ordered phase but alsofor the film with the disordered phase. In the inset, h2 isplotted against h to evaluate the optical band gap, where and h denote the absorption coefficient and photon energy,respectively. The optical band gap, obtained from the inter-cept of the dotted extrapolated line and abscissa, is estimatedto be 3.3 eV for the ordered phase and 2.7 eV for thedisordered phase.Temperature dependence of electric resistivity  isshown in Fig. 4 for as-deposited and annealed films with theordered phase. Room-temperature  for the film with thedisordered phase is also shown for comparison. Both of theordered-phase films show thermal-activation-type tempera-FIG. 1. a Variation of XRD pattern with PO2 for the thin films grown underTS=700 °C. b XRD patterns of the thin films grown at 1.010−3 and1.010−1 Pa in the expanded scale. c Variation of XRD pattern with sub-strate temperature for the thin films grown under PO2 =1.010−3 Pa. dXRD patterns of the ordered-phase solid solution thin films before and afterannealing at T=700 °C and PO2 =2.010−4 Pa. Open circles, closed circles,and closed diamonds denote the diffraction peaks due to the ordered phase,the disordered phase, and the spinel phase, respectively.FIG. 2. Cross-sectional HRTEM image and SAED pattern of the ordered-phase film deposited on -Al2O3. The direction of the incident electron isparallel to the 11¯00 direction of -Al2O3.082509-2 Hojo et al. Appl. Phys. Lett. 89, 082509 2006Downloaded 31 May 2007 to 130.54.110.22. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspture dependence over a range of 70–350 K. The activationenergy estimated using the formula =0 expE /kT is about0.07 eV for both of the films. This value is comparable tothose in bulk and thin film samples as reported previously.5,7On the other hand,  for the disordered-phase film is threeorders of magnitude higher than that for the film with theordered phase. High  is caused by a considerably smallamount of Fe2+ ions in the disordered-phase thin film pre-pared under high PO2 because Fe3+ and Fe2+ pairs contributeto the electrical conduction in this system.5 The Seebeck co-efficient of the annealed film with the ordered phase wasfound to be almost zero; that is, within a measurable limit ofour apparatus, contributions of electrons and holes to con-duction are comparable to each other. This result is reason-able because the present composition falls on the border be-tween p and n types, and the Seebeck coefficient is expectedto be small near this composition.5Figure 5 shows magnetization M as a function of anexternal magnetic field H at room temperature for the as-deposited and annealed films with the ordered phase and thefilm with the disordered phase. Large M and a well-definedhysterisis loop are observed only for the annealed film withthe ordered phase. Dependence of M on temperature T isdisplayed in the inset of Fig. 5. The measurements were per-formed under a field-cooled condition, while H of 8500 Oewas applied parallel to the film surface. For the as-depositedand annealed films with the ordered phase, M increases al-most linearly with a decrease in temperature below a criticaltemperature. The TC is below room temperature for the as-deposited film, while the annealing raises the TC above roomtemperature. A similar M-T curve was observed for the bulksamples of FeTiO3–Fe2O3 solid solution and ascribed to theR-type ferrimagnetsim.4 However, we observed that zero-field-cooled M deviates from field-cooled M at a certain tem-perature when a low H such as 300 Oe is applied, which isindicative of finite-range ferrimagnetic ordering. This phe-nomenon may arise since the distribution of Fe and Ti ions inthe film with the ordered phase is slightly different from theideal arrangement as Ishikawa11 described previously, bywhich long-range ferrimagnetic ordering can be developed.For the film with the disordered phase, on the other hand, Mis very small and exhibits almost no temperature depen-dence, suggesting that the film is antiferromagnetic in thistemperature range.In conclusion, we have grown 0.7FeTiO3·0.3Fe2O3 solidsolution thin films on -Al2O3 0001 substrates using thePLD method to examine their electrical and magnetic prop-erties. The growth of solid solution thin film consisting of theordered phase R3¯ symmetry is reproducibly achieved bytuning the oxygen partial pressure and substrate temperatureduring the deposition. We also found that the ordered-phasefilms annealed at 700 °C in vacuum are ferrimagnetic withTC above room temperature.One of the authors K.F. acknowledges financial supportfrom Yazaki Foundation.1Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M.Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, and H. Koinuma, Science291, 854 2001.2T. Fukumura, H. Toyosaki, and Y. Yamada, Semicond. Sci. Technol. 20,S103 2005.3To be exact, -Fe2O3 shows canted spin magnetism. L. Néel, Ann. Phys.Paris 4, 249 1949.4Y. Ishikawa and S. Akimoto, J. Phys. Soc. Jpn. 12, 1083 1957.5Y. Ishikawa, J. Phys. Soc. Jpn. 13, 37 1958.6W. H. Butler, A. Bandyopadhyay, and R. Srinivasan, J. Appl. Phys. 93,7882 2003.7F. Thou, S. Kotru, and R. K. Pandy, Thin Solid Films 408, 33 2002.8T. Fujii, M. Kayano, Y. Takada, M. Nakanishi, and J. Takada, J. Magn.Magn. Mater. 272-276, 2010 2004.9T. Fujii, M. Kayano, Y. Takada, M. Nakanishi, and J. Takada, Solid StateIonics 172, 289 2004.10Z. Dai, P. Zhu, S. Yamamoto, A. Miyashita, K. Narumi, and H. Naramoto,Thin Solid Films 339, 114 1999.11Y. Ishikawa, J. Phys. Soc. Jpn. 17, 1835 1962.FIG. 3. Optical transmission spectrum for the annealed film with the orderedphase solid curve and the film with the disordered phase dashed curve.The inset shows the plot of h2 vs h for estimation of direct allowedband gap closed circles: ordered phase; open circles: disordered phase.FIG. 4. Temperature dependence of  for films with the ordered phase opencircles: as-deposited; closed triangles: annealed. Room-temperature  forthe film with the disordered phase is also shown a closed circle.FIG. 5. Room-temperature M-H curves for the thin films with the orderedphase open circles: as-deposited; closed triangles: annealed and that withdisordered phase closed circles. The inset shows temperature dependenceof M measured at H=8500 Oe under a field-cooled condition.082509-3 Hojo et al. Appl. Phys. Lett. 89, 082509 2006Downloaded 31 May 2007 to 130.54.110.22. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/39686/1/ApplPhysLett_89_082509.pdf

Epitaxial growth of room-temperature ferrimagnetic semiconductor thin films based on the ilmenite-hematite solid solution

Abstract

Similar works

Full text

Available Versions

Kyoto University Research Information Repository

Kyoto University Research Information Repository