Magnetic and electrical characterization of TiO2 single crystalsco-implanted with iron and cobalt

C. Silva a,b, A.R.G. Costa a,c, R.C. da Silva c, L.C. Alves d, L.P. Ferreira a,e, M.D. Carvalho f,N. Franco c, M. Godinho a,b, M.M. Cruz a,b,n

a Centro de Física da Matéria Condensada, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugalb Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugalc IPFN, Instituto Superior Técnico, Universidade de Lisboa, Campus Tecnológico e Nuclear, E.N.10, 2685-066 Bobadela LRS, Portugald C2TN, Instituto Superior Técnico, Universidade de Lisboa, Campus Tecnológico e Nuclear, E.N.10, 2685-066 Bobadela LRS, Portugale Department of Physics, University of Coimbra, P-3004 516 Coimbra, Portugalf Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal, and Departamento de Química e Bioquímica,Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal

a r t i c l e i n f o

Article history:Received 21 February 2014Received in revised form3 April 2014Available online 16 April 2014

Keywords:Diluted magnetic semiconductorMagnetic nano-aggregateImplanted rutileMagnetoresistance

a b s t r a c t

Single-crystals of TiO2 rutile were co-implanted with iron and cobalt to investigate the combined role ofthese ions in the magnetic properties of the system. The implantations were carried out using an energyof 150 keV and different fluences to investigate their influence in the magnetic and electrical propertiesof the implanted samples. For the higher fluences the as implanted single crystals exhibit super-paramagnetic behaviour associated with the formation of nanosized magnetic aggregates. Annealingtreatments were performed at 673 K and 1073 K, inducing recovery of the lattice structure and theevolution of the formed phases. Iron and cobalt play different roles in the implanted region, the presenceof iron inhibiting the formation of cobalt aggregates during annealing at 1073 K.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

In the last few decades, the discovery of magnetism coexistingwith semiconductor type conductivity in cobalt doped TiO2

(anatase) [1] increased the interest in this type of materials dueto their potential application to spintronics. Extensive research hasbeen carried out in wide band gap oxide semiconductors, ZnO,TiO2, SnO2 and In2O3, doped with transition metals in the searchfor diluted magnetic semiconductors (DMS) [2].

Titanium dioxide doped with cobalt or with iron displays roomtemperature ferromagnetism (FM) both in anatase and rutileforms for different dopant concentrations [3–12]. Most of thereported work has been carried out on films and in these systemsthe magnetic behaviour at room temperature varies largely withthe fabrication method and growth conditions. It is also reportedthat aggregation occurs in the film/substrate interface [12–14] orthat the dopant concentration is higher at the film surface[7,15,16]. The debate about the origin of the room temperatureferromagnetic behaviour observed in some of these systems stillremains open. In the case of cobalt doped anatase films, low

concentration of dopant seems to lead to intrinsic magneticbehaviour, while cobalt clusters become the main contribution tothe observed ferromagnetic behaviour for higher concentrations ofcobalt [17–19] or films produced under low oxygen pressure[17,18]. It was reported that thermal treatments dissolve the cobaltaggregates [19,20]. In the case of doped rutile different authorspresent different results regarding the origin of FM, since for thesame concentration range some claim to find intrinsic magnetism[21–23] while others consider the presence of cobalt clusters theprincipal source of room temperature FM [10,24]. The formation ofcobalt aggregates was found to be enhanced by the presence ofoxygen vacancies [10].

The formation of Fe clusters dependent on the depositionconditions was reported for thin films [25,26] and iron implantedrutile [8,27], while for epitaxially grown films and polycrystallinematerial the formation of clusters of mixed TiO2 rutile and Fe3O4

was identified [28,29]. Recently, the formation of an intermediatesystem was proposed to explain the FM of Fe in rutile filmsprepared by pulsed laser deposition [30]. In films produced viathe hydrothermal method, FM was only detected after repeateddeposition and annealing steps, the observed saturation magneti-zation not being associated with the presence of a secondarymagnetic phase or iron clusters [31]. Thermal treatments invacuum lead to the formation of a ternary compound Ti–Fe–O,

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Journal of Magnetism and Magnetic Materials

http://dx.doi.org/10.1016/j.jmmm.2014.04.0220304-8853/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Fax: +351 217500977.E-mail address: mmcruz@fc.ul.pt (M.M. Cruz).

Journal of Magnetism and Magnetic Materials 364 (2014) 106–116

which is consistent with the high affinity of iron for binding withoxygen. In the case of rutile single-crystals implanted with ironions with energy of 100 keV, the formation of FeTi2O5 co-existingwith iron precipitates was observed in the as implanted samplesfor implantations with fluences of 5�1016 cm�2 and higher [32].Diffusion of iron is proposed to start at 873 K. In the case of rutileimplanted with similar fluences but higher energy, only bcc ironnanosized aggregates were observed in the as implanted state thatdisappear after an annealing treatment at 1073 K, giving place to aspinel-type phase [8,27].

In this work, Co and Fe were co-implanted in single crystals ofrutile at room temperature, in order to test the combined role ofthe two ions in aggregate formation. The high affinity of Fe forbinding with oxygen should lead to smaller concentration ofoxygen vacancies reducing the possibility of cobalt aggregationand stabilizing the ions’ position in the rutile lattice. Also, thecoexistence of both species may enhance ferromagnetic interac-tions, since iron–cobalt alloys display high values of averagemagnetic moment per ion (between 2 μB and 2.4 μB for a widerange of compositions) [33]. Furthermore an increase of themagnetization was reported in a study of TiO2 co-doped with Coand Fe produced by a solid state reaction at 1173 K [34]. The use ofsingle crystalline rutile samples allows to overcome thermody-namic equilibrium limitations and the problems created by inter-faces, low dimension and induced stress restrictions associatedwith films, and the comparison of results for different implanta-tion fluences allows studying the influence of the implanted ionsconcentration in the implanted system properties.

2. Experimental details

TiO2 (100) rutile single crystals were implanted with cobalt andiron at room temperature (E295 K) with energy of 150 keV andthree total fluences between 3�1016 cm�2 and 1�1017 cm�2. Thesamples are separated into three groups, G1, G2 and G3, accordingto the total nominal implanted fluence and named by theircomposition (using capital letters T for TiO2, F for iron and C forcobalt). G1 consists of samples implanted with a total nominalfluence of 3�1016 cm�2, G2 consists of samples implanted with atotal nominal fluence of 8�1016 cm�2 and G3 includes samplesimplanted with a total nominal fluence of 1�1017 cm�2. Sinceprevious results indicate that the order of implantation can be animportant factor [35], two types of samples were produced ingroups G2 and G3 modifying the order of implantation of the twoions: implanted with Co before Fe, designated in the text as“TCF#”, and implanted with Fe before Co, named “TFC#” (#¼2,3being associated with G2 and G3, respectively). The same desig-nation is used in G1 samples although all samples were producedby implanting iron prior to cobalt. The corresponding implantationfluences are summarized in Table 1. To allow the use of Mössbauer

spectroscopy, 57Fe was also implanted with the same energy insamples of groups G1 and G2.

Particle Induced X-Ray Emission (PIXE) analysis, with 2 MeVprotons, was used to determine the effective amount of implantedions. Rutherford Backscattering Spectrometry (RBS) performed with2 MeV 4Heþ beam, combined with the channelling effect (RBS-C),and X-Ray Diffraction (XRD) in θ�2θ geometry using Cu-Kα radia-tion were used for structural and compositional characterization.

The temperature and isothermal field dependence of magneti-zation were measured between 3 K and 400 K in applied fields upto 5.5 T using a SQUID magnetometer. The temperature dependenceof the magnetization was measured in increasing temperature aftercooling the sample from room temperature to 3 K in zero appliedfield (zero field cooled-ZFC) and after cooling the sample down to3 K under the measurement field (field cooled-FC). The magneticmoment of all samples was considered to include two independentcontributions: a paramagnetic component assigned to the TiO2

unimplanted volume and the contribution of the implanted region.The average magnetic susceptibility of TiO2 rutile for the crystalsused was measured to be 7.9�10�10 m3/kg and independent of thetemperature, confirming the expected Van Vleck paramagneticbehaviour. All the magnetic results presented were subtracted forthe unimplanted volume contribution to isolate the behaviourassociated with the implanted region.

57Fe conversion electrons Mössbauer spectroscopy (CEMS) wasused to investigate the valence state and local surroundings of theimplanted iron ions in the near surface region (up to 150 nm). CEMSwas performed in constant acceleration mode at room temperaturewith a He–CH4 gas flow proportional counter. The spectra werefitted with Lorentzian lines using the WinNormos software. Allisomer shift values are referred to α-Fe at room temperature. DCelectrical resistivity measurements were carried out as a function oftemperature between 10 K and 300 K using four point in-linegeometry and applied dc currents in the range 1 nA–1 mA. Theisothermal magnetoresistance was measured in applied magneticfields up to 5.5 T in the same temperature range. The magnetore-sistance ratio is defined as MR¼ ðRðHÞ�Rð0ÞÞ=Rð0Þ, RðHÞ being theelectrical resistance measured under applied magnetic field H.

In all samples, both magnetic and electrical measurementswere performed along the [010] and [001] crystalline directions ofrutile, respectively.

To allow the recovery of the lattice and rearrangement of theimplanted ions, the samples underwent thermal treatments. Twoannealing stages at 673 K were carried out in vacuum, for some ofthe samples belonging to G1 and G2 (the first for 1 h and thesecond for 3 h) and one annealing stage at 1073 K for 1 h wasperformed for other samples of the three groups. The evolution ofthe systems was followed after each stage using structural,magnetic and electrical measurements.

3. Results and discussion

3.1. As implanted state

The effective implanted fluences of Fe and Co were determinedexperimentally using PIXE analysis and are presented in Table 1. Inthe G3 samples the amount of Co is smaller than the nominalfluence, while for Fe the measured fluence agrees with theexpected value irrespective of the implantation order. In the caseof samples from G1 and G2, the amount of Fe in all the samples issmaller than that of Co. This is explained by difficulties associatedwith the implantation of 57Fe. Measurements performed in theintermediate state, after the implantation of the first implantedelement, allowed to conclude that its fluence was not changed bythe implantation of the second element.

Table 1Iron and cobalt implanted fluences for the three groups of samples. The effectivefluence values were determined by PIXE analysis.

Group Sample Nominal fluence (1016 cm�2) Effective fluence (1016 cm�2)

Partial Total Fe Co Total

G1 TFC1 Co – 1.5 3.0 1.3 2.4 3.757Fe – 1.5

G2 TCF2 Co – 4.0 8.0 1.8 3.1 4.9

TFC2 56Fe – 2.5 2.5 3.8 6.357Fe – 1.5

G3 TCF3 Co – 5.0 10.0 5.0 3.0 8.0TFC3 Fe – 5.0 5.0 2.4 7.4

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116 107

In Fig. 1, the RBS results for TFC2 illustrate the typical RBSspectra obtained for all the samples after implantation under twoconditions: for an incident beam aligned with the [100] TiO2 axisand for an incident beam in a random direction. The resultsobtained for an unimplanted crystal are also shown for compar-ison. In the region referred to as II, corresponding to the implantedregion of Ti, the aligned spectrum of the implanted sample has ahigh yield indicating strong dechannelling of the incident beam bythe host lattice and consequently that the implanted regionbecame highly damaged after the implantation. In the same figurethe arrows indicate the energies associated with iron, cobalt,titanium and oxygen at the surface of the sample. The presenceof Fe and Co is then confirmed by non-zero yield above 1.5 MeV,and the shape of the Ti profile in region II. From the analysis ofthese spectra the depth of the implanted region was estimated tobe approximately 140 nm for all samples.

Due to the orientation of the crystals only the (200) diffractionpeak of the rutile matrix host is observed in XRD results for allsamples. The XRD patterns for G1 and G2 samples display similartrends, illustrated in Fig. 2, for TCF2 and TFC2. The XRD results forunimplanted TiO2 are shown for comparison. After implantation ofthe first element, Co or Fe, no additional diffraction peaks aredetected but after the implantation of the second element newdiffraction peaks appear around 351 and 751, indicating the onsetof new phases. The shape of the new diffraction peaks indicatesthat they result from the overlap of two narrower peaks withdifferent relative intensities for TFC2 and TCF2, indicating thebetter crystallization of phases formed during the first implanta-tion or the formation of new phases after the implantation of thesecond element. Rotated rutile grains with (101) planes parallel tothe surface give rise to diffraction peaks at angles 2θ equal to 36.11and 76.51, and their presence cannot be excluded due to the broadshape of the diffraction peaks. Nevertheless the fact that bothmaxima are shifted to lower angles indicates that a different phasecontaining cobalt and iron is formed. Several spinel structuredoxides mixing titaniumwith iron or cobalt are compatible with thepositions of the new diffraction peaks. In the case of G3 samplesalso new diffraction peaks appear in similar positions but they arenarrower (centred in positions 35.3(1)1 and 74.7(3)1) and betterdefined in TFC3 as compared with TCF3.

Mössbauer measurements were performed for the as implantedsamples after each 57Fe implantation.

CEMS spectra of a G1 sample after Fe and FeþCo implantation,respectively, TF1 and TFC1, are shown in Fig. 3, fitted with severalLorentzian lines. The results show similar isomer shift (IS) andquadrupole splitting (QS) values before and after Co implantation,

indicating small differences in the iron environments (Table 2).Equivalent results are found if the spectra are fitted with adistribution of quadrupole doublets, the IS values obtained in bothcases being 0.98 mm/s, characteristic of Fe2þ . CEMS spectra afterFe implantation are in good agreement with the results obtainedin rutile implanted with 100 keV iron ions [9,32], where it isproposed that a phase close to ulvöspinel (Fe2TiO4) [16,36] orferro-pseudo-brookite (FeTi2O5) [37] is formed. No evidence wasfound for the presence of such phases in the XRD patterns afteriron implantation in rutile (Fig. 2b, middle pattern), indicatingthat, if they exist, grains are either very small or amorphous. Avariable neighbourhood of iron ions due to variable stoichiometryof the spinel phase or the possibility of having Fe2þ occupying

0.5 1.0 1.5 2.00.0

0.5

1.0

Fe

I

Co

Yield/103counts

E / MeV

O

Ti

randomaligned [100]:

implantedunimplanted

TFC2

II

Fig. 1. RBS spectra for sample TFC2 in the as implanted state for an incident beamin the channelling [100] direction (open squares) and random (full line). Forcomparison, the RBS results for an unimplanted crystal in channelling [100]direction are also presented (stars). The dashed line separates the non-implantedand implanted regions of Ti profile, marked as I and II, respectively.

30 40 50 60 70 802θ /degree

Counts/arb.units

TCF2

TiO2+ Co

TiO2 (200)

TiO2

30 40 50 60 70 80

2θ /degree

Counts/arb.units

TFC2

TiO2+ Fe

TiO2 (200)

TiO2

Fig. 2. XRD patterns of unimplanted rutile and after the implantation of eachelement for G2 samples (a) TCF2 and (b) TFC2.

Fig. 3. Room temperature CEMS spectra of G1 as implanted samples. TF1 and TFC1samples refer the Fe implanted sample and the same sample after Co implantation,respectively.

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116108

Ti4þ sites cannot be excluded. As can be seen in Table 2, CEMSspectrum after Co implantation differs from TF1 spectrum mainlyin the relative proportion of the various sites and only the lessoccupied iron site displays slightly increased values of IS and QS.Therefore, from Mössbauer results there is no clear evidence forthe formation of a different iron containing phase resulting fromCo addition. A slight rearrangement of the iron ions occurs whenthe sample is implanted with cobalt, probably resulting in abetter crystallization of a spinel-type structure, in agreement withX-ray data, whether involving Ti and Fe ions or Ti, Fe and Cobut keeping the close neighbourhood of iron unchanged. As thenumber of relative counts is the same in both spectra it can beconcluded that no iron loss occurred upon Co implantation inthe thickness available to CEMS (around 150 nm), in agreementwith PIXE and RBS results. Spectra acquisition at 11 mm/s wasalso performed (not shown) to look for magnetic phases asso-ciated with iron oxides, Fe or Fe–Co clusters but none wasdetected.

Fig. 4 shows the Mössbauer spectra obtained with G2 samples,TF2 referring to the sample TFC2 prior to Co implantation. Thecorresponding fitting parameters are presented in Table 3 showingIS values characteristic of Fe2þ and QS values different from thosefound for G1. Although the comparison of TF1 and TFC1 with TF2and TFC2 is difficult due to poor statistics of the latter (caused bythe lower content of 57Fe and their smaller size), it is clear that inthe case of sample TCF2 three iron sites are needed to correctlyresolve the spectra. This observation can be explained by slightlydifferent environments of the iron sites that remain in a non-magnetic environment or in superparamagnetic aggregates assuggested by the magnetization results (see below).

The magnetization of all samples was studied as a function oftemperature and magnetic field. For G1 samples the magnetizationresults (Fig. 5) indicate paramagnetic behaviour with a magneticmoment of 3:5 μB per magnetic ion. For G2 and G3 samples, thetemperature dependence of the magnetization measured at 5 mTdisplays a maximum at a temperature that depends on theimplanted fluence, following a Curie-law dependence above thattemperature, characterised by high values for the magneticmoment per implanted ion. These results indicate that the systemsare superparamagnetic and allow associating the maximum tem-perature to a blocking temperature TB. Fig. 6 illustrates themagnetization results for one sample TFC3 showing that super-paramagnetism is also supported by the scaling of the isothermalmagnetization curves with H=T above TB. Thus, the magneticbehaviour of G2 and G3 samples is dominated by the presenceof nanosized magnetic aggregates. The magnetic parameters thatcharacterize these samples are presented in Table 4. The saturationmagnetic moments per implanted ion and coercive fields are ofthe order of 0:5 μB and o9 mT for G2, and increase to 1:0 μB and14 mT for G3, respectively.

The temperature dependence of the magnetization was fittedusing a log-normal distribution of magnetic moments as describedelsewhere [38]. Assuming that the aggregates are composed ofmetallic iron and/or cobalt, with magnetic moments around 2:0 μBper ion, the results indicate a fraction between 12 and 30% of theimplanted ions incorporated in the aggregates with an averagediameter of 1 nm for G2 samples, and a fraction of the order of 75%with an average diameter of 2.4 nm for G3 samples. This resultcorroborates the dependence of aggregation with the fluence ofimplantation.

Since the blocking temperature is related to the effective anisotropyconstant Keff and nanoparticle volume V through Keff VC30kBTB,higher blocking temperatures are usually associated with largeraggregates. Accordingly, the experimental results indicate that the

Table 2Parameters extracted from fitting the Mössbauer spectra of G1 at room tempera-ture, TF1 corresponding to the result prior to cobalt implantation. QS – quadrupolesplitting; IS – isomer shift; Γ – line width; I – relative area for each component(uncertainty o2%).

Sample QS (mm s)�1 IS (mm s)�1Γ ðmm sÞ�1 I (%)

TF1 2.70(1) 0.987(4) 0.33(2) 35.22.22(2) 0.978(5) 0.33(5) 30.51.73(2) 0.991(8) 0.43(3) 28.71.12(4) 1.00(2) 0.27(6) 5.6

TFC1 2.70(1) 0.989(7) 0.31(3) 242.23(2) 0.973(6) 0.39(8) 36.61.79(2) 0.991(8) 0.34(4) 22.81.25(3) 0.97(2) 0.40(4) 16.6

Fig. 4. Room temperature CEMS spectra of the as implanted samples of G2.

Table 3Parameters extracted from fitting the Mössbauer spectra of G2 samples at roomtemperature. QS – quadrupole splitting; IS – isomer shift; Γ – line width; I – relativearea for each component (uncertainty o2%).

Sample QS (mm s)�1 IS (mm s)�1Γ ðmm sÞ�1 I (%)

TF2 2.42(3) 0.95(1) 0.45(6) 75.11.52(4) 0.96(2) 0.27(7) 24.9

TFC2 1.97(5) 0.91(2) 0.68(7) 100TCF2 2.51(4) 0.96(1) 0.34(6) 23.9

1.96(4) 0.94(1) 0.37(6) 34.91.39(7) 0.95(1) 0.48(6) 41.2

0 50 100 150 200 250 3005.0x10-10

1.0x10-9

1.5x10-9

2.0x10-9

2.5x10-9 μ0H = 5 mT ZFCFC

m/Am2 cm-2

T / K

Fig. 5. ZFC/FC magnetic moment per unit area obtained for TFC1 in a magnetic fieldof 5 mT.

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116 109

size of the aggregates increases with the implanted fluence. For TFC1,the magnetization decreases with temperature above 3 K indicatingthat aggregates, if existing in the as implanted state, correspond toTBo3 K.

The influence on the transport properties was followed bystudying the electrical resistivity behaviour. In the case of unim-planted rutile the electrical resistivity values are above ourexperimental limit ðR4100 MΩÞ, the crystals being classified asinsulators. After implantation all samples display measurablesemiconductor like conductivity. Fig. 7 shows the temperaturedependence of the electrical resistance of the as implanted TiO2

single crystals of all groups, measured in [010] direction. Resultsare well described by the Variable-Range Hopping (VRH) conduc-tion mechanism in the temperature range T4100 K, with T0(Bloch temperature), ranging between 3�105 K and 3�107 K.Similar results were obtained for the [001] direction. This beha-viour, characterised by a T �1=4 logarithmic dependence of theelectrical resistance, is observed not only in cobalt implanted rutile[27], but also for different types of implanted elements, such as Ar,Xe, Sn, W and Hg [39,40], and is explained as the result of thedisorder generated during the implantation process, which is

normally accompanied with the production of interstitials andvacancies.

In our samples the comparison between the different groupsshows that the electrical resistance decreases with the increase ofthe total implanted fluence but seems to be independent of theorder of implantation. The parameters characterizing the electricaltransport along with the electrical resistivity values at 300 K arepresented in Table 5.

The low values of room temperature electrical resistivity (of theorder 10�3Ω cm) cannot be accountable only by the defects producedby the implantation (creation of Ti3þ interstices and vacancies) [39],indicating that the implanted ions should contribute to the density oflocalized electronic states available for the electric conduction.

Using a reasonable value for the localization length ðλC10�9 mÞ,the density of participating localized states at the Fermi level NðϵF Þcan be estimated from the Bloch temperature T0 [41]. The obtainedvalues are also presented in Table 5 along with the average hoppinglength ðR¼ 3

8 λðT0=TÞ1=4Þ and the average value of the hopping energyðw¼ ðkB=4ÞT1=4

0 T3=4Þ. The density of localized states increases forhigher implanted fluence and the hopping energy decreases, asexpected due to the increase in the number of defects.

To study possible polarization of the carriers, magnetoresistancemeasurements under fields up to 5.5 T were performed on all samples.Only G3 samples display measurable magnetoresistance and only fortemperatures below 30 K (Fig. 8). Both TCF3 and TFC3 samples exhibitnegative magnetoresistance that increases with the applied magneticfield and with the decrease of the temperature.

Since the blocking temperature for these samples is of theorder of 30 K, this behaviour is compatible with spin disorderscattering resulting from the interaction between the spins of 3delectrons belonging to the implanted transition metals and the sp

0 50 100 150 200 250 300 3500

1x10-8

2x10-8

3x10-8

4x10-8

2 4 6 8 10 12

1

2TFC3

ZFCFC

m/Am2 cm-2

T / K

μ0H = 5 mT

0.00 0.01 0.02 0.03 0.04 0.05 0.060

2x10-7

4x10-7

6x10-7

100 K150 K200 K250 K300 K330 K380 K

m/Am2 cm-2

0HT -1 / TK-1

Fig. 6. (a) ZFC/FC magnetic moment per unit area for TFC3 in a magnetic field of5 mT and (b) isothermal magnetization curves for the same sample. (Results forTCF3 are similar.)

Table 4Summary of the magnetic results for the as implanted samples.

Sample TB ðKÞ H//[001] T¼5 K H//[010] T¼5 Kms ðμB) μ0Hc ðmTÞ ms ðμBÞ μ0Hc ðmTÞ

TFC2 7 0.4 8.0 0.25 8.0TCF2 5 0.65 6.5 0.4 6.5TFC3 20 1.19 14.0 0.86 13.0TCF3 33 0.89 14.0 1.23 14.0

0.25 0.30 0.35 0.40 0.45 0.50 0.55

102

103

104

105

106

107256 123 67 39 24 16 11

T / K

T -1/4 / K-1/4

R/Ω

TFC1TCF2TFC2TCF3TFC3

Fig. 7. Electrical resistance of iron and cobalt co-implanted TiO2 single crystalsversus T �1=4. Experimental data can be described by the VRH mechanism forT4100 K.

Table 5Parameters characterizing the electrical behaviour of all samples in the as implantedstate, electrical resistivity at 300 K (ρ) and parameters obtained assuming VRHconduction above 100 K: Bloch temperature (T0), average hopping energy (w),average hopping length ðRÞ and density of states at Fermi level ðNF ¼NðϵF ÞÞ.

Sample ρ ðΩ cmÞ T0 (K) w ðmeVÞ R (nm) NF ðeV�1 cm�3ÞT¼300 K T4100 K T¼300 K T¼300 K

TFC1 3.0�10�2 2.5�107 109 6.4 2.7�1018

TCF2 9.7�10�3 5.8�106 76.3 4.4 1.1�1019

TFC2 9.7�10�3 3.9�106 68.9 4.0 1.7�1019

TCF3 2.7�10�3 5.3�105 41.9 2.4 1.3�1020

TFC3 2.9�10�3 3.0�105 36.3 2.1 7.0�1020

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116110

electrons of rutile, occupying states localized by disorder thatcontribute to the hopping transport.

Both TFC3 and TCF3 are superparamagnetic, but the smallernanosized aggregates were detected for TFC3 indicating a highernanoaggregate density in this case that corresponds to a strongersp–d exchange interaction and explains the higher value ofmagnetoresistance in TFC3 (MRC62% at 5 T).

3.2. Annealed state

To study lattice recovery and the diffusion of the implantedions, the samples were submitted to annealing treatments at 673 K(G1 and G2) and 1073 K (G1–G3).

3.2.1. Annealing at 673 KAfter annealing for 4 h at 673 K, a small recovery of the rutile

lattice is detected by RBS (Fig. 9), with no significant modificationsin the implanted region (II). From this result we can conclude thatat 673 K cobalt and iron do not diffuse out of the implanted region,the modifications being attributed to the reorganization of thelattice and implanted ions.

From the comparison of the XRD patterns obtained before andafter the thermal treatment (Fig. 10) the decrease of the diffractionpeaks intensity at 351 and 751 and the definition of a diffractionpeak at 371 are clear. Since the difractograms' intensities arenormalized by (200) TiO2 diffraction, the decrease of the peaks at351 and 751 is consistent with the recovery of the rutile lattice in theimplanted region and the appearance of the diffraction peak at 371indicates that modifications occurred in the spinel-type phases.

CEMS spectra and the Mössbauer fitting parameters of sampleG1 after the referred annealing stages are shown in Fig. 11 and

Table 6, respectively. The obtained QS values are lower than thoseobtained before annealing for the three iron sites fitted, suggestingsome reorganization of the implanted ions, in agreement withRBS. Two of the iron sites are still characterised by IS values ofFe2þ (around 0.9 mm/s and 1.0 mm/s) but, after 4 h annealing, thethird one corresponds to a new iron environment with a lower,well defined, IS (0.48 mm/s). Such decrease of the IS value maycorrespond to an iron oxidation state between Fe2þ and Fe3þ , in adifferent magnetic spinel-type phase involving iron and cobalt.

Magnetization results indicate that TFC1 has a superparamagneticcomponent after the 1 h annealing stage (Fig. 12), displaying differentblocking temperature when the magnetic field is applied alongdifferent crystallographic directions, namely 30 K when the field isapplied along the [010] direction of rutile and 7 K when it is appliedparallel to the [001] direction. Anisotropic behaviour is also observedin the measurements of magnetic moment as a function of appliedmagnetic field, resulting in coercive fields of 40 mT when the field isparallel to [010] and 15mT with the field applied along the [001]direction. The saturation magnetic moment increases to approxi-mately 1:4 μB per implanted ion. After the 3 h annealing stage nosignificant modifications occurred, the blocking temperatures for eachdirection of the crystal, and the coercive field and saturation magneticmoment remaining nearly unaltered.

TFC2 and TCF2 remain superparamagnetic after the 1 h anneal-ing treatment with an increase of the blocking temperature toTB ¼ 15 K, of the coercive field to approximately 20 mT, and of thesaturation magnetic moment to a value of the order of 1:0 μB perimplanted ion (Fig. 13). After the 3 h annealing stage, furtherincrease of the blocking temperature ðTBC30 KÞ and also of thecoercive field and saturation magnetic moment, to 35 mT and1:5 μB per implanted ion is observed. This increase of TB andsaturation magnetic moment indicates that a higher fraction of theimplanted ions is included in magnetic nanoaggregates. In thesetwo samples no magnetic anisotropy was observed when compar-ing the results obtained with magnetic field applied along the two

0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0 TCF2

Yield/103counts

E / MeV

random:As impl.

aligned [100]:As impl.1 h @ 673K4 h @ 673KI II

Fig. 9. Evolution of RBS results for TCF2 depending on the annealing stage at 673 K.Since all the random spectra overlap, only the as implanted random spectra isplotted. Similar results were obtained for TFC1 and TFC2.

Table 6Hyperfine parameters obtained from room temperature Mössbauer results of G1samples after annealing stages at 673 K: QS – quadrupole splitting; IS – isomershift; Γ – line width; I – relative area for each component (uncertainty o2%).

G1 QS (mm s�1) IS (mm s�1) Γ ðmm s�1Þ I (%)

1 h 2.37(2) 1.03(2) 0.29(2) 17.81.92(2) 0.90(3) 0.61(3) 64.90.73(2) 0.59(3) 0.44(3) 17.2

(1þ3) h 2.39(2) 1.04(2) 0.25(2) 9.22.01(2) 0.90(3) 0.75(3) 71.30.61(2) 0.48(2) 0.37(2) 19.5

-6 -4 -2 0 2 4 6-30

-20

-10

0

10TCF3

MR/%

0H / T

10 K5 K2 K

-6 -4 -2 0 2 4 6

-60

-40

-20

0

MR/%

0H / T

10 K5 K2 K

TFC3

Fig. 8. Magnetoresistance results for (a) TCF3 and (b) TFC3.

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116 111

different in plane directions, [010] and [001], unlike what wasobserved for TFC1. The fit of the ZFC results indicates that TFC2samples have aggregates with an average diameter of 4 nm,similar to the 3 nm average diameter obtained for TCF2.

As for the electrical behaviour, G1 and G2 samples exhibit adecrease of the electrical resistance after the 1 h annealing stage at673 K, the electrical behaviour being still described by VRH in thetemperature range T4150 K, with T0C106 K (Fig. 14a). The new

values for the hopping distance, density of hopping sites andaverage hopping energy were calculated and are presented inTable 7, indicating that the decrease of the resistivity after 1 hannealing can be associated with a lower energy barrier forhopping, and the increase after 4 h with the annealing out ofdefects that were contributing to the conduction, resulting in acorresponding increase of the average hopping length.

After the first annealing stage (Fig. 15a and b), samples of G2exhibit magnetoresistance below 50 K. Like the samples of G3 inthe as implanted state, negative magnetoresistance appears below

0 100 200 3000.0

2.5x10-8

5.0x10-8

7.5x10-8

1.0x10-7

m/Am2 cm-2

T / K

H // [010] H // [001]ZFC ZFCFC FC

μ0H = 5 mT

TFC1

Fig. 12. Magnetic moment as a function of temperature for TFC1 with magneticfield applied along the [010] (squares) and [001] (triangles) directions of the rutilecrystal after 1 h annealing at 673 K.

0 100 200 300

0

1x10-8

2x10-8

3x10-8

4x10-8

5x10-8

6x10-8

m/Am2 cm-2

T / K

As implantedZFCFC

Annealed 1h @ 673KZFCFC

μ0H = 5 mT

TFC2

-6 -4 -2 0 2 4 6

-2x10-6

-1x10-6

-5x10-7

0

5x10-7

1x10-6

2x10-6

m/Am2 cm-2

0H / T

H // [010] H // [001]As implanted As implantedAnnealed Annealed

T = 5 K

Fig. 13. Comparison of the results in the as implanted and after 1 h annealing statesat 673 K for TFC2: (a) magnetic moment as a function of temperature for [010]rutile direction (similar results for [001]) and (b) magnetic moment as a function ofapplied magnetic field along [010] and [001] rutile directions.

TCF2TFC2

4 h @ 673 K

1 h @ 673 K

TFC1

As implanted

1 h @ 673 K

As implanted

4 h @ 673 K

As implanted

1 h @ 673 K

4 h @ 673 K

Counts/a.u.

30 40 50 60 70 30 40 50 60 70

2θ / degree30 40 50 60 70 80

Fig. 10. Evolution of XRD patterns for samples from G1 and G2 with annealing treatment at 673 K.

Fig. 11. Room temperature CEMS spectra of G1 samples after 1 h and 4 h annealingat 673 K.

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116112

the blocking temperature and increases with the applied magneticfield. Sample TCF2 displays lower magnetoresistance ratios whencompared with TFC2, having at 5 K a value of E20% for TCF2 andE40% for TFC2 samples.

After the second annealing stage the temperature dependence ofthe electrical resistance for TFC1 changes drastically (inset in Fig. 14b)and follows the same trend as reduced unimplanted TiO2 [10]. Thisresult indicates that the electrical resistance of the implanted layer isno longer dominated by disorder and that the conduction is carriedout through the TiO2 extended band states.

In the case of G2 samples, after 4 h annealing at 673 K, theelectrical resistivity at room temperature increases along with theBloch temperature indicating that the lattice recovery implies a

decrease of the density of localized electronic states participatingin the transport.

The different room temperature electrical resistivities andelectrical behaviours for temperatures below 150 K can be attrib-uted to the different total retained fluences (Fig. 14b). Themeasurable magnetoresistance decreases to values below 5% inthe temperature range To50 K. The magnetoresistance depen-dence with field for TCF2 continues to be explained by the spindisorder scattering added with the effect of weak localization, butfor TFC2 this effect dominates displaying a maximum value ofE3.5% at 20 K for 0.5 T, saturated above that value. The differentsizes of the aggregates detected from the magnetization resultscombined with the experimental implanted fluence imply that alarger number of aggregates exist in the case of TCF2 and explainthe stronger spin disorder scattering in this case and the lowervalues for electrical resistivity of TCF2.

3.2.2. Annealing at 1073 KAfter a thermal treatment at 1073 K in vacuum for 1 h, carried out

for G1, G2 and G3 samples, RBS analysis shows significant structuralrecovery (Fig. 16) with a clear decrease in yield in region II. Therecovery of the lattice depends on the fluence, since it is clear thatthe greatest recovery occurred for the sample with the lowestfluence, TFC1.

The XRD patterns obtained after annealing (Fig. 17) showsignificant differences when compared to the as implanted state,the peaks at 351 and 751 remaining present after the thermaltreatment and becoming sharper. This indicates the improvementof the crystalline quality of the associated phase.

Fig. 18 and Table 8 display the CEMS spectrum and thecorresponding fitting parameters of a G1 sample annealed at1073 K. Despite some background noise or a poorly resolved

-6 -4 -2 0 2 4 6

-20

-10

0

0H / T

MR/%

100 K50 K10 K5 K

TCF2Annealed 1 h @ 673 K

-6 -4 -2 0 2 4 6

-40

-30

-20

-10

0

0H / T

MR/%

50 K10 K5 K

TFC2Annealed 1 h @ 673 K

Fig. 15. Magnetoresistance results for samples TCF2 and TFC2 after annealing at673 K for 1 h.

Table 7Values of fitting parameters obtained by adjusting the experimental curvesconsidering the VRH mechanism for the temperature range considered.

Sample ρ ðΩ cmÞ T0 (K) w ðmeVÞ R (nm) NF ðeV�1 cm�3ÞT¼300 K T4100 K T¼300 K T¼300 K

1 h at 673 KTFC1 2.7�10�2 9.2�106 86.0 5 7.3�1018

TFC2 6.0�10�3 2.1�106 59.4 3.4 3.1�1019

TCF2 7.9�10�3 1.5�106 54.0 3.2 4.5�1019

(1þ3) h at 673 KTFC2 2.7�10�2 6.8�106 79.2 4.6 1.1�1019

TCF2 1.3�10�2 2.7�106 63.0 3.7 2.5�1019

0.25 0.30 0.35 0.40 0.45 0.50 0.55102

103

104

105

106

107256 123 39 24 16 11

T / KR/Ω

T -1/4 / K-1/4

TFC1TFC2TCF2

Annealed 1 h @ 673 K

0.25 0.30 0.35 0.40 0.45 0.50 0.55101

102

103

104

105

106

107256 123 39 24 16 11

T / K

R/Ω

T -1/4 / K-1/4

TFC2TCF2TFC1

Annealed (1+3) h @ 673 K

0 100 200 300103104105106

R/Ω

T / K

Fig. 14. Electrical resistance of iron and cobalt co-implanted TiO2 single crystals ofG1 and G2: (a) after the first annealing and (b) after the second annealing stageat 673 K.

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116 113

magnetic contribution, a doublet and a broad singlet are clearlydefined in the spectrum. Again, the IS values point to Fe2þ withthe QS ones indicating much less asymmetric iron environments.

As for magnetic behaviour, the sample TFC1 displays paramag-netic behaviour like in the as implanted state, however with muchlower effective moment – approximately 0:8 μB per implanted ion.For G2 and G3 samples, anisotropic weak ferromagnetic behaviouris detected, consistent with the formation of an uncompensatedantiferromagnetic phase (Figs. 19 and 20). These results allowconcluding that annealing at 1073 K causes the destruction of theferromagnetic behaviour observed before the thermal treatment,and the formation of an antiferromagnetic compound.

The electrical resistivity of the annealed samples depends onthe implanted fluence but generally comes close to the one of theunimplanted annealed TiO2 crystal, exhibiting the typical beha-viour of a doped semiconductor (Fig. 21). Despite this change, theelectric resistivity, at room temperature, kept the same order of

magnitude, approximately 2.3�10�3 Ω cm for both G2 and G3.No magnetoresistive behaviour was observed corroborating thatthis effect is associated with the presence of the FM aggregates.

4. Conclusions

Co-implantation of iron and cobalt of 150 keV into singlecrystalline TiO2 modifies the near surface region of the rutile toa depth of the order of 140 nm and its magnetic and electricalproperties. In no case does XRD analysis detect the formation ofaggregates after the implantation of the first element, however,new phases are detected after the implantation of the secondelement. This behaviour is observed irrespective of the

40 50 60 70 80

2θ / degree

Counts/arb.units

TFC3

TFC2

TFC1

Fig. 17. XRD patterns for samples TFC1, TFC2 and TFC3 after annealing at 1073K for 1 h.

Fig. 18. Room temperature CEMS spectrum of sample G1 after 1 h annealingat 1073 K.

Table 8Parameters extracted from fitting the Mössbauer spectrum of TFC1 sample afterannealing at 1073 K. QS – quadrupole splitting; IS – isomer shift; Γ – line width;I – relative area for each component (uncertainty o2%).

QS (mm s�1) IS (mm s�1) Γ ðmm s�1Þ I (%)

1 h 0.49(2) 1.07(1) 0.25(1) 71at 1073 K 0(1) 0.74(4) 0.53(2) 29

0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0Yield/103counts

E / MeV

random:As impl.Anneal.

aligned [100]:As impl.Anneal.

TFC1

I II

0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

II

Yield/103counts

E / MeV

random:As impl.Anneal.

aligned [100]:As impl.Anneal.

TFC2

I

0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

II

Yield/103counts

E / MeV

random:As impl.Anneal.

aligned [100]:As impl.Anneal.

TFC3

I

Fig. 16. RBS spectra in the as implanted and after annealing states for samples(a) TFC1, (b) TFC2 and (c) TFC3.

C. Silva et al. / Journal of Magnetism and Magnetic Materials 364 (2014) 106–116114

implantation order. Mössbauer and XRD results not only supportthe formation of ulvöspinel FeTi2O4 or ferro-pseudobrookiteFeTi2O5 spinel aggregates but are also compatible with severaltitanium-cobalt or cobalt–iron oxides. The fact that the com-pounds can only be identified after the implantation of both ionspoints to the existence of spinel like phases with both Co and Fe.

The magnetic results indicate the existence of magnetic aggre-gates in the as implanted state that grow upon annealing at 673 K.Mössbauer results do not detect iron aggregates or iron in cobaltaggregates, but do not exclude the existence of iron in oxidenanosized aggregates. A small recovery of the lattice structure is

observed in parallel with the increase of the magnetic aggregatesdiameter.

The electric characterization shows that electrical resistivitydepends on implanted fluence and globally decreases with increas-ing fluence, the temperature dependence indicating that electricaltransport occurs essentially by hopping through localized states. G3samples in the as implanted state and G2 samples after 1 hannealing at 673 K exhibit negative magnetoresistance understoodin terms of the spin disorder induced scattering. In both cases, G2and G3, the spin disordered scattering is associated with thepresence of aggregates, indicating that carriers follow low resis-tance paths through the aggregates. For G2, the magnetoresistancedecreases after 4 h annealing at 673 K.

Annealing in vacuum at 1073 K for 1 h results in strongdiffusion of the implanted ions and significant recovery of thelattice in the implanted region for all samples. A spinel type phaseassociated with the diffraction peaks at 351 and 751 remainspresent but the global magnetic behaviour changes. Samples ofG1 are paramagnetic with lower magnetic moment, while G2 andG3 exhibit anisotropic and antiferromagnetic behaviours.

The annealing out at 1073 K of the FM aggregates formed in theas implanted state with the appearance of an antiferromagneticspinel-type phase is also observed in iron implanted rutile. On thecontrary, for cobalt implanted rutile the same thermal treatmentfavours the growth of the ferromagnetic aggregates. Consistentlythe results indicate that iron plays the dominant role in thebehaviour of Co and Fe co-implanted samples inducing theformation of an AFM spinel phase.

Acknowledgements

This work was carried out with the support of the PortugueseFCT through projects PTDC/FIS/66262/2006, PEst-OE/FIS/UI0261/2011 and PEst-OE/QUI/UI0536/2011.

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