Glaspell, G.

Manivannan, A.

Seehra, M. S.

English

The Research Repository @ WVU (West Virginia University)

Faculty Scholarship2003Controlled transformation of paramagnetism toroom-temperature ferromagnetism in cobalt-dopedtitanium dioxideA. ManivannanG. GlaspellM. S. SeehraFollow this and additional works at: https://researchrepository.wvu.edu/faculty_publicationsThis Article is brought to you for free and open access by The Research Repository @ WVU. It has been accepted for inclusion in Faculty Scholarshipby an authorized administrator of The Research Repository @ WVU. For more information, please contact ian.harmon@mail.wvu.edu.Digital Commons CitationManivannan, A.; Glaspell, G.; and Seehra, M. S., "Controlled transformation of paramagnetism to room-temperature ferromagnetismin cobalt-doped titanium dioxide" (2003). Faculty Scholarship. 100.https://researchrepository.wvu.edu/faculty_publications/100Controlled transformation of paramagnetism to room-temperatureferromagnetism in cobalt-doped titanium dioxideA. Manivannan,a) G. Glaspell, and M. S. SeehraDepartment of Physics, West Virginia University, Morgantown, West Virginia 26506~Received 25 June 2003; accepted 9 September 2003!Samples of Co0.1Ti0.9O22d ~anatase! prepared by the sol–gel technique are found to be paramagneticat room temperature, with the magnetic susceptibility following Curie–Weiss law in the investigatedrange of 2–370 K. However, transformation from paramagnetism to room-temperatureferromagnetism ~RTFM! is observed by hydrogenation of the sample at 573 K. The increase in thehydrogenation time from 1 to 6 h increases the remanance, and the Curie temperature .470 K isdetermined by extrapolation. X-ray photoelectron spectroscopy and transmission electronmicroscopy of the hydrogenated samples failed to detect Co nanoparticles, suggesting that theobserved RTFM in the hydrogenated samples may be intrinsic. © 2003 American Institute ofPhysics. @DOI: 10.1063/1.1622991#The recent reports1–4 of room-temperature ferromag-netism ~RTFM! in transparent semiconductingCoxTi12xO22d thin films for x>0.01, for both anatase andrutile phases, have attracted a great deal of attention in partbecause of the application of such magnetic semiconductorsin spintronics. However the cause of this RTFM remainscontroversial since more recent reports5–8 have indicated thepresence of Co nanoparticles in the thin films prepared bysputtering and pulsed laser depositions. This raises the seri-ous possibility that the observed RTFM could be due to un-detected Co nanoparticles, which because of the high Curietemperature of bulk Co (Tc.1388 K) could easily accountfor the observations. In a recent report from our laboratory,9it was observed that Co0.1Ti0.9O22d anatase films preparedby a different technique, for example, spray pyrolysis areparamagnetic at room temperature. The temperature depen-dence of the magnetic susceptibility x of this sample fol-lowed Curie–Weiss behavior, x5x01C/(T2u), withu.25 K and only for T,5 K was a hysteresis loop ob-served. In the work presented here, we report thatCo0.1Ti0.9O22d ~anatase! powder prepared by the sol–geltechnique is also paramagnetic at room temperature and fol-lows a similar Curie–Weiss law. However, by controlled hy-drogenation of this sample at 573 K, the paramagneticCo0.1Ti0.9O22d is transformed into a ferromagnet with Curietemperature Tc.470 K, without any observable change inthe crystal structure as revealed by x-ray diffraction. X-rayphotoelectron spectroscopy ~XPS! and transmission electronmicroscopy ~TEM! studies of the hydrogenated sample failedto detect the presence of any Co nanoparticles. Details fol-low.The sample of Co0.1Ti0.9O22d was prepared by mixing~reacting! appropriate amounts of titanium isopropoxide ~Al-drich! and cobalt nitrate ~Aldrich! in ethanol. The solutionwas dried in an oven and the powder obtained was heated to750 K for 1 h which yielded a dark-green sample. X-raydiffraction ~XRD! of this powder confirmed the anatase formof TiO2 , with only a trace amount of rutile phase ~Fig. 1!.Part of this powder was used for magnetic measurements andthe remainder used for 1, 3 and 6 h of hydrogenation carriedout at 573 K. The hydrogen reduction setup consisted of atubular furnace kept inside a continuously vented hood. Thesample, contained in an open glass boat, was placed insidethe tubular furnace and H2 gas was passed over the sample at573 K with the help of a gas flow control unit. Temperatureand magnetic field variations of the magnetization, M, ofthese four samples were then measured using a commercialsuperconducting quantum interference device ~SQUID! mag-netometer. These results are presented below.The temperature variation of x for the as-preparedsamples ~prior to hydrogenation! is shown in Fig. 2 wherethe solid line is fit to Curie–Weiss law: x5x01C/(T2u).This variation is similar to the one reported for the sampleprepared by spray pyrolysis.9 Note that the temperatures inFig. 2 are plotted on a log scale in order to show details ofthe x vs T behavior at lower temperatures. From the Curieconstant C52.631023 emu K/g Oe and C5Nm2/3kB ~withN the number of magnetic ions/g, kB the Boltzmann constantand m the magnetic moment!, m54.1mB for the Co21 ion isobtained. This magnitude of m is consistent with the highspin state of Co21, assuming that Co21 substitutes for theTi41 ions in the anatase unit cell for 10% Co doping, withthe balance of the charge compensated by O22d . In the insetof Fig. 2, the linear variation of M vs H at T5300 K isshown, confirming paramagnetism.After hydrogenation at 573 K, the sample acquiresRTFM. The hysteresis loops were measured for samples hy-drogenated for 1, 3 and 6 h, taking a new sample for hydro-genation each time. For the 6 h sample, Fig. 3 shows thetemperature variation of the remanance M r ~M at H50)from 5 to 350 K for the 1, 3 and 6 h samples and in the insetthe hysteresis loop measured at 350 K for the 6 h sample isshown. ~Measurements for T.350 K could not be made be-cause of experimental limitations!. These variations of M r vsa!Author to whom correspondence should be addressed; electronic mail:amanivan@wvu.eduJOURNAL OF APPLIED PHYSICS VOLUME 94, NUMBER 10 15 NOVEMBER 200369940021-8979/2003/94(10)/6994/3/$20.00 © 2003 American Institute of PhysicsDownloaded 30 Jun 2004 to 128.172.149.86. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspT are typical of a ferromagnet, with Tc.470 K determinedfrom extrapolating the M r data to zero ~Fig. 3! at highertemperatures. The primary effect of increasing the hydroge-nation time from 1 to 6 h is to increase M r , demonstrating asimilar source of the RTFM for the three samples. This isfurther confirmed by the temperature variation of the coer-civity which also tends toward zero at Tc.470 K ~Fig. 4!.Thus it appears that with the increase in hydrogenation time,more of the sample is transformed into a ferromagnet. Again,it is noteworthy that this hydrogenation does not appear toaffect the crystal structure since no significant differences areobserved in the XRD patterns of the four samples ~Fig. 1!.From the widths of the XRD lines, the average particle sizeof the anatase phase is .10~2! nm, without any majorchanges upon hydrogenation. Although in the extrapolationof Figs. 3 and 4, similar Tc is implified for the 1, 3 and 6 hsamples, the extrapolations are clearly very approximatesince M r and Hc are often nonlinear on approach to Tc .Thus an increase in Tc with an increase in hydrogenationtime is not ruled out for the data in Figs. 3 and 4.In light of the recent reports on the presence of Co nano-particles in Co/TiO2 films prepared by sputtering in differentpartial pressures of oxygen,5,8 we carried out several experi-ments in addition to XRD to check for Co nanoparticles inour samples. XPS analysis of the as-prepared and the 6 hhydrogenated samples indicated that neither Ti nor Co is in ametallic state. For the hydrogenated sample, Ti 2p peakswere observed at 457.5 and 463.2 eV and Co 2p peaks at779.5 and 795.2 eV. The corresponding values for the as-prepared samples were 458.1 and 463.8 eV for Ti 2p and780.5 and 796.2 eV for Co 2p . These magnitudes indicate21 and/or 31 states of Co and 21 and/or 41 states for Ti~the metallic Co 2p peak position should be at 778.3 eV!.High resolution TEM studies of the hydrogenated samplesclearly showed particles of about 10 nm whose energy dis-persive x-ray ~EDX! spectra yielded the presence of Co, TiFIG. 1. X-ray diffraction patterns of the as-prepared ~0 h! sample ofCoxTi12xO22d and the sample hydrogenated for 1, 3 and 6 h at 573 K. Theexpected line positions for the anatase and rutile phases are shown.FIG. 2. Temperature dependence of the magnetic susceptibility of the as-prepared sample of CoxTi12xO22d . The solid line is fit to the equationshown. In the inset is the M vs H variation at 300 K.FIG. 3. Temperature variation of remanance M r of the sample hydrogenatedfor 1, 3 and 6 h at 573 K. The dotted lines are extrapolations, indicatingTc.470 K where M r goes toward zero. In the inset, the hysteresis loop forthe 6 h sample at 350 K is shown.FIG. 4. Same as in Fig. 3 except that the plot is for coercivity Hc .6995J. Appl. Phys., Vol. 94, No. 10, 15 November 2003 Manivannan, Glaspell, and SeehraDownloaded 30 Jun 2004 to 128.172.149.86. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspand oxygen, again ruling out the presence of metallic Co.Also, no exchange bias could be detected at 5 K in the 6 hsample when cooled from 300 to 5 K in H520 kOe, indicat-ing the absence of a ferromagnetic/antiferromagnetic inter-face ~e.g., Co/CoO! in the system.10 Finally, we measured xvs T for the 6 h sample under zero-field-cooled ~ZFC! andfield-cooled ~FC! conditions ~Fig. 5!. The absence of a peakin x for the ZFC cases, which is a signature for the blockingtemperature, TB , for Co as reported recently for thin films ofCo/TiO2 prepared by sputtering,6 provides additional assur-ance of the absence of Co nanoparticles in our samples. Thereported TB values of Co nanoparticles of different sizes areTB.20 K (3 nm), TB.50 K (6 nm), TB.100 K (8 nm) andTB.260 K (11 nm).11,12Theoretically, it has been suggested that oxygen holesnear Co in CoxTi12xO22d are essential to provide the ex-change coupling between Co ions, and lead to intrinsicRTFM.13 This indeed was the motivation for our hydrogena-tion experiments, since H2 is likely to extract oxygen fromthe sample and produce oxygen holes. From our experimentsdescribed above, we were unable to detect the presence ofCo nanoparticles, thus suggesting the intrinsic nature ofRTFM in our hydrogenated samples. Additional experimentssuch as accurate measurements of electrical conductivitywith time of hydrogenation would be useful to confirm theseobservations.The authors express their thanks to C. Stinespring forproviding facilities for the hydrogenation experiments and J.Poston of National Energy Technology Laboratory, Morgan-town, for the XPS experiments. TEM measurements weredone by Advanced Materials Engineering Inc. Sunnyvale,CA. This research was supported in part by the U.S. AirForce Office of Scientific Research.1 Y. Matsumoto et al., Science 291, 854 ~2001!.2 S. A. Chambers et al., Appl. Phys. Lett. 79, 3467 ~2001!.3 Y. Matsumoto et al., Jpn. J. Appl. Phys., Part 2 40, L1205 ~2001!.4 W. K. Park, R. J. Ortega-Hertogs, J. Moodera, A. Punnoose, and M. S.Seehra, J. Appl. Phys. 91, 8093 ~2002!.5 P. A. Stampe, R. J. Kennedy, Y. Xin, and J. S. Parker, J. Appl. Phys. 93,7864 ~2003!.6 A. Punnoose, M. S. Seehra, W. K. Park, and J. S. Moodera, J. Appl. Phys.93, 7867 ~2003!.7 D. H. Kim et al., J. Appl. Phys. 93, 6125 ~2003!.8 B. Z. Rameev, F. Yildiz, L. R. Tagirov, B. Aktas, W. K. Park, and J. S.Moodera, J. Magn. Magn. Mater. 258–259, 361 ~2003!.9 A. Manivannan, M. S. Seehra, S. B. Majumder, and R. S. Katiyar, Appl.Phys. Lett. 83, 111 ~2003!.10 A. E. Berkowitz and K. Takano, J. Magn. Magn. Mater. 200, 552 ~1999!,and references therein.11 J. P. Chen, C. M. Sorensen, K. J. Klabunde, and G. C. Hadjipanayis, Phys.Rev. B 51, 11527 ~1995!.12 C. B. Murray, S. Sun, H. Doyle, and T. Betley, MRS Bull. 26, 985 ~2001!.13 M. S. Park, S. K. Kwon, and B. I. Min, Phys. Rev. B 65, 161201 ~2002!.FIG. 5. Temperature variation of the magnetic susceptibility of the 6 hsample under zero-field-cooled and field-cooled conditions.6996 J. Appl. Phys., Vol. 94, No. 10, 15 November 2003 Manivannan, Glaspell, and SeehraDownloaded 30 Jun 2004 to 128.172.149.86. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

Controlled transformation of paramagnetism to room-temperature ferromagnetism in cobalt-doped titanium dioxide

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Controlled transformation of paramagnetism to room-temperature ferromagnetism in cobalt-doped titanium dioxide

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