Giant negative magnetoresistance of spin polarons in magneticsemiconductors–chromium-doped Ti 2 O 3 thin filmsZhenjun Wang, Yuanjia Hong, Jinke Tang, Cosmin Radu, Yuxi Chen, Leonard Spinu, Weilie Zhou, and Le DucTung Citation: Applied Physics Letters 86, 082509 (2005); doi: 10.1063/1.1874302 View online: http://dx.doi.org/10.1063/1.1874302 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The origins of ferromagnetism in Co-doped ZnO single crystalline films: From bound magnetic polaron to freecarrier-mediated exchange interaction Appl. Phys. Lett. 95, 102501 (2009); 10.1063/1.3224911 Structures and magnetic properties of p -type Mn : TiO 2 dilute magnetic semiconductor thin films J. Appl. Phys. 106, 043913 (2009); 10.1063/1.3204493 Magnetization dependence on carrier doping in epitaxial ZnO thin films co-doped with Mn and P J. Appl. Phys. 101, 123909 (2007); 10.1063/1.2739302 Carrier induced ferromagnetism in Nb doped Co : Ti O 2 and Fe : Ti O 2 epitaxial thin film J. Appl. Phys. 99, 08M121 (2006); 10.1063/1.2177194 Magnetic structure of V : TiO 2 and Cr : TiO 2 thin films from magnetic force microscopy measurements J. Appl. Phys. 97, 10D323 (2005); 10.1063/1.1854072

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Giant negative magnetoresistance of spin polarons in magneticsemiconductors–chromium-doped Ti 2O3 thin films

Zhenjun Wang, Yuanjia Hong, and Jinke Tanga!

Department of Physics, University of New Orleans, New Orleans, Louisiana 70148

Cosmin Radu, Yuxi Chen, Leonard Spinu, and Weilie ZhouAdvanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana 70148

Le Duc TungDepartment of Physics, University of Warwick, Coventry CV4 7AL, United Kingdom

sReceived 23 April 2004; accepted 11 January 2005; published online 18 February 2005d

Epitaxial Cr-doped Ti2O3 films show giant negative magnetoresistance up to −365% at 2 K. Theresistivity of the doped samples follows the behavior expected of spinsmagneticd polarons at lowtemperature. Namely,r=r0 expsT0/Tdp, wherep=0.5 in zero field. A large applied field quenchesthe spin polarons andp is reduced to 0.25 expected for lattice polarons. The formation of spinpolarons is an indication of strong exchange coupling between the magnetic ions and holes in thesystem. ©2005 American Institute of Physics. fDOI: 10.1063/1.1874302g

Dilute magnetic semiconductors have attracted much at-tention recently because of their combined magnetic andtransport properties. In such materials, the current carriersare either strongly coupled to local moments of the magneticatoms or directly participate in the magnetism themselves.Among TiO2-related materials, experimental results haveshown evidence of room-temperature ferromagnetism in Co-doped anatase TiO2,

1,2 Fe-doped reduced-rutile TinO2n−1,3

and Co/Fe-doped SnO2−d sRefs. 4 and 5d thin films. Severalmodels, including Zener-type Ruderman–Kittel–sKasuyad–Yosida, double exchange, and spin-polarons, have been pro-posed to explain the room-temperature ferromagnetism al-though its exact origin is unclear. We have recentlyinvestigated the magnetic properties of Cr-doped Ti2O3 thinfilms and report in this letter on giant negative magnetoresis-tancesMRd due to spinsmagneticd polaron formation at lowtemperatures.

Ti2O3 has a corundumsa-Al2O3d structure. Below200 °C, Ti2O3 is a nonmagnetic semiconductor, with an en-ergy gapsEG,0.1 eVd between the top of a1g band and thebottom ofeg bands of the 3d electrons. The electrons in theconduction band are much heavier than the holes in the va-lence band, and the material exhibits a positive Hall coeffi-cient s p typed. Long ago, Mott suggested that the excitedcarriers in Ti2O3 are small polarons. Low-temperature con-ductivity shows exps−T0/Tdp sp=0.25d temperature depen-dence consistent with the variable range hoppingsVRHd na-ture of the polaron transport.6

sCrxTi1−xd2O3 s x=0, 0.04, 0.06, and 0.10d thin filmswere grown ona-Al2O3s012d substrates by pulsed-laserdeposition. Prescribed amounts of high-purity TiO2 and Crpowders were mixed, cold pressed, and sintered to make aCrxTi1−xO2 ceramic target. The films were preparedin a vacuum of 2310−6 Torr at substrate tempera-tures of 800–1000 K. The pulsed excimer laser usesKrFsl=248 nmd and produces a laser beam of intensity of1–2 J/cm2 and repetition rate of 4 Hz. The deposition rate is

between 0.3 and 0.5 Å/s, and the film thickness varies from150 nm to 300 nm. Single-phase epitaxialsCrxTi1−xd2O3

films were formed by ablating the CrxTi1−xO2 targets andreduction during the deposition.

The x-ray diffractionsXRDd and transmission electronmicroscopysTEMd results show that thesCrxTi1−xd2O3 filmsare single phase. Figure 1sinsetd shows the XRD patterns ofsCr0.10Ti0.90d2O3 and Ti2O3 films grown ona-Al2O3. Onlythe reflections from thes012d family of corundum phase areobserved. The Cr content dependence of theR-axis latticespacingfds012dg is shown in Fig. 1. The value ofds012ddecreases linearly with the Cr concentration, indicating thatthe Cr ions gradually substitute for the Ti ions in the filmswithout changing the corundum structure. The Cr concentra-tions of the films were measured with energy dispersivex-ray spectroscopy in TEM mode, and they were consistentwith those of the targets.

Figure 2 shows the magnetizationM curves forsCr0.06Ti0.94d2O3 at 2 K and 300 K. The magnetization ofsCr0.06Ti0.94d2O3 at 2 K increases with the applied field and

adAuthor to whom correspondence should be addressed; electronic mail:jtang@uno.edu

FIG. 1. The peak position of thes012d reflection of sCrxTi1−xd2O3 filmsshifts to higher angles with increasing Cr concentrationx. Theds012d spac-ing decreases withx. Inset: XRD patterns forsCr0.10Ti0.90d2O3 and Ti2O3

films. Only the peaks of thes012d family of corundum phase are observed.

APPLIED PHYSICS LETTERS86, 082509s2005d

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does not reach saturation at 50 kOe. It apparently consists oftwo components. One is a low-field ferromagnetic compo-nent, which shares the same origin as the ferromagnetic sig-nals at high temperatures. The other is a paramagnetic com-ponent that corresponds to the upturn at low temperature inthe M –T curve supper-left-hand side inset of Fig. 2d. Themoment at 2 K and 50 kOe is 0.5, 0.91 and 0.87mB/Cr forx=0.04, 0.06, and 0.10, respectively. It initially increaseswith x, then decreases slightly upon a further increase in Crdoping. The room-temperature moments are much smaller,e.g., 0.2mB/Cr for x=0.06. The origin of the room-temperature ferromagnetism is not clear and may be frommagnetic impurities. The lower-right-hand side inset of Fig.2 shows the Curie–Weiss behavior of the susceptibility atlow temperature after the ferromagnetic component issubtracted. A negative paramagnetic Curie temperaturesu=−0.6 Kd suggests an antiferromagnetic ground state forthe system.

In Fig. 3, we compare the temperature dependence of

resistivity for the undoped Ti2O3 andsCr0.06Ti0.94d2O3 films.The films exhibit increasing resistivity with decreasing tem-perature. The carriers arep type for both doped and undopedfilms. A carrier density of 1.231021/cm3 was estimated forsCr0.06Ti0.94d2O3 from the Hall measurements. All films showa rapid increase in resistivity at very low temperaturessbe-low about 30–40 Kd, which is due to the VRH of polaronsand spin polarons as will be discussed next.

The MRs=sRH-R0d /RH3100%d was measured for Ti2O3

and sCrxTi1−xd2O3 films. The upper-right-hand side inset ofFig. 3 shows the MR curves measured at 2 K. The MR forTi2O3 is positive 23% at 140 kOe. This large positive MR isin agreement with previous data and is explained with a two-band model where both electrons and holes participate in thetransport.7 For dopedsCrxTi1−xd2O3, the MR is negative andlarge. Samplex=0.06 has a giant value of −80% and −107%at 50 kOe and 140 kOe, respectively. The MR forx=0.10reaches −365% at 140 kOe. Forx=0.04, positive MR domi-nates in high fields. The MR correlates well with the magne-tization shown in Fig. 2. Its giant negative values cannot beexplained in the framework of scattering by magnetic impu-rities, scattering by magnetic granules, or magnetic tunnelingbetween two granules. Rather, its origin lies in the quenchingof spin smagneticd polarons by the applied field, in whichextremely large negative MR is expected and observed.8

We now discuss the origin of the giant negative MR inconjunction with the temperature dependence of the resistiv-ity at low temperature. As mentioned at the beginning of thisletter, Ti2O3 exhibits VRH conductance at low temperatureand its resistivity follows an expf1/T0.25g behavior. Our un-doped Ti2O3 film exhibits a slightly smaller exponentp of0.12 as determined from a least-squares fit. The deviationfrom the expected value of 0.25 is not surprising consideringthe diverse values ofps0.18–0.7d seen for VRH in variousmaterials.9,10 We have plotted the resistivity in logarithmicscale of Ti2O3 along with that ofsCr0.06Ti0.94d2O3 as a func-tion of 1/T0.25 in Fig. 4. The small departure from a linearbehavior is noticeable. For dopedsCr0.06Ti0.94d2O3, the resis-tivity in zero field does not follow exps1/Td0.25 law as seenin the upturn at low temperatures. Instead, its resistivity wasfound to follow r=r0 expsT0/Td0.4 at low temperature.

FIG. 2. Magnetization vs field curves ofsCr0.06Ti0.94d2O3 film measuredwith a superconducting quantum interference device atT=2 K and 300 K.The upper-left-hand side inset shows the temperature dependence of themagnetization measured atH=300 Oe. The Curie–Weiss behavior at lowtemperature, after subtraction of the ferromagnetic component, is shown inthe lower-right-hand side inset.

FIG. 3. The temperature dependence of the resistivity for the films, showingsemiconducting behavior. Insets show MR curves for Ti2O3 andsCrxTi1−xd2O3 at 2 K.

FIG. 4. The resistivity in logarithmic scale of as a function of 1/T0.25. Thedata were taken in zero field for Ti2O3 and in H=0 and 50 kOe forsCr0.06Ti0.94d2O3. Inset shows lnr vs 1/T0.4 plots for the same three datasets.

082509-2 Wang et al. Appl. Phys. Lett. 86, 082509 ~2005!

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Figure 4 sinsetd shows the resistivity ofsCr0.06Ti0.94d2O3 inlogarithmic scale as a function of 1/T0.4. A nearly perfectstraight line is obtained. A least-squares fit gives an exponentof p=0.391±0.007. This value is very close to 0.4 and isdistinctively different from 0.5 expected for VRH with Cou-lomb blockade or tunneling transport in granular systems.The 1/T0.4 temperature dependence has been predicted byFoygelet al.11 for bound spinsmagneticd polarons. Tradition-ally, spin polaron transport has been assumed to follow asimple activation process. Recent study demonstrates that, inthe systems with significant disorder of magnetic energies,the spin polaron hopping also adopts a variable range natureand obeysr=r0 expsT0/Tdp law, wherep=2/sd+2d andd isthe dimensionality of the system. The thickness of our filmss150–300 nmd falls into the 3d range, thereforep=0.4 isexpected. The agreement between the spin polaron theoryand our experimental data is excellent.

Further evidence for spin polaron transport comes fromthe resistivity data obtained in high magnetic field. The tem-perature dependence of the resistivity atH=50 kOe revertsback to the expsT0/Td0.25 behaviorssee the linear behaviorFig. 4d, suggesting that the spin polarons are quenched byhigh field and the transport returns to that of standard non-magnetic lattice polarons. An exponentp=0.238±0.005 wasobtained from the fit. The same data show negative curvatureon a log R versus 1/T0.4 plot sFig. 4, insetd. In a field of140 kOe,p was found to be 0.284±0.001. All data analysisand curve fitting were done over the temperature range 2 K,T,15 K. The crossover fromp=0.4 for magnetic po-larons top=0.25 for standard Mott lattice polarons in highmagnetic field has been predicted in Ref. 11. It should bementioned that the observed field dependence of exponentpis unique for magnetic polarons. The opposite trend in thefield dependence ofp is expected for lattice polaron VRH,wherep generally increases from 0.25 in zero field to 0.33 inhigh fields in the absence of Coulomb interaction.12

The giant negative MR observed insCrxTi1−xd2O3 ischaracteristic of bound spin polarons. Bound spin polaron orbound magnetic polaron is the characteristic collective stateof diluted magnetic semiconductors.13 Exchange interactionof localized carrierssholesd with magnetic Cr ions leads tothe formation of bound spin polarons, which consists of thelocalized hole and surrounding cloud of Cr spins polarizedvia the exchange interaction. Without a magnetic field, thespin polarons have to move through the sea of Cr with some-what randomly oriented spins. Their motion involves flippingmany Cr spins, making the polarons massive and immobile.

With the field applied, all Cr spins are increasingly aligned tothe direction of the field, which increases the mobility of thepolarons.

In conclusion, our experiments show spin polaron for-mation in Cr-doped Ti2O3 films at low temperatures. TheCr-doped films exhibit giant negative MR as large as −365%at ,2 K, which is in contrast to the positive MR in theundoped film. The giant negative MR is indicative of thespin polaron formation. Analysis of the temperature depen-dence of the resistivity both in zero field and in high fieldsconfirms the spin polaron interpretation of the data. The for-mation of spin polarons reveals the strong exchange couplingbetween the magnetic ions and the holes. It shows thatCr-doped Ti2O3 is a magnetic semiconductor where the car-riers participate in the magnetism. We wish to note that spinpolarons may provide a mechanism for the room-temperatureferromagnetism in doped semiconductors reported recently.Giant negative MR has been reported in the well-establishedferromagnetic semiconductorsGa,MndAs and explained onthe basis of bound magnetic polarons.14

This work was supported by Sharp Laboratories ofAmerica and by the Louisiana Board of Regents SupportFund sGrant No. LEQSFs2004-07d-RD-B-12d.

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