Room temperature ferromagnetism in metallic Ti1−xVxO2 thin films

Transition metal doped TiO2 diluted magnetic semiconductors have attracted considerable interest due to their room temperature ferromagnetism. However, most TiO2 films are highly insulating, and thus the magnetic properties can not be controlled by tuning the carrier concentration. This will limit their application in controlling magnetization via electrical gating. Here, we deposit rutile Ti1−xVxO2 (x = 0.03 and 0.05) films with the thickness between 30 and 245 nm by the pulsed laser deposition technique, and observe an obvious room temperature ferromagnetic behavior in all films. The high resolution X-ray photoelectron spectroscopy results indicate that V substituting Ti4+ ions in the TiO2 lattice, with the +3 valence state having two unpaired d electrons, is responsible for the local spin. More importantly, the systemic investigations of transport properties for Ti1−xVxO2 films reveal that the films are n-type and have metallic conductivity with a carrier density of about 1020/cm3. Further studies suggest that the oxygen vacancies play a dual role of contributing to the metallic conductivity of the Ti1−xVxO2 films, and also providing the free electrons to mediate the long-range ferromagnetic coupling between two magnetic polarons. These findings may offer promise for gate-tunable ferromagnetism in future semiconductor spintronics.


Introduction
Diluted magnetic semiconductors (DMSs) have attracted enormous attention due to their potential applications in spintronic devices. [1][2][3] To date, the III-V based DMSs, such as Mn doped GaAs, have been well studied, however, the low values of the Curie temperature (T C ) hindered their application at room temperature. 4,5 An important step forward in the eld was the theoretical prediction by Dietl et al. of high temperature ferromagnetism in Mn doped ZnO. 6 Subsequently, the Co doped TiO 2 thin lms with the anatase structure were reported to be ferromagnetic even above 400 K with a magnetic moment of 0.32 m B per Co atom. 7 Since then, transition metal (TM) doped TiO 2 DMS has attracted particular interest as TiO 2 has many advantages, such as low cost, good dielectric properties and high chemical stability. [8][9][10][11][12] However, due to the low solubility of TMs in TiO 2 , extrinsic effects, such as magnetic clusters and impurity phases, are oen responsible for the observed ferromagnetism. [13][14][15] Furthermore, many studies have focused on the effects of the methods and growth conditions on the structural and magnetic properties of TiO 2 DMS, [16][17][18][19][20] and it is also found that most TiO 2 lms doped with different transition elements are highly insulating. [21][22][23][24][25][26] For example, Griffin et al. 21 grew a series of anatase Co:TiO 2 lms by RF magnetron sputtering, and obtained a saturation magnetic moment of 1.1 m B /Co, while all lms were highly insulating. Sharma et al. 25 showed that the Mn-doped TiO 2 lms prepared by the spray pyrolysis technique also exhibited the highly insulating nature with the resistivity of almost 10 7 U cm. It is noted that although some reports demonstrate that the incorporation of nonmagnetic element Nb and Ta in TiO 2 can lead to metallic electrical conduction, 27-32 the origin of magnetic moments is attributed to cationic vacancies. 31,32 A DMS, containing a dilute concentration of magnetic ions imbedded in the host lattices, is characterized by the free carriers mediated exchange interactions between the magnetic ions. In such systems, the magnetization can be controlled by tuning the carrier density via electrical gating. In order to meet this criterion, it is essential to obtain the conductive TiO 2 lms.
In this work, we obtain the metallically conductive Ti 1Àx V x O 2 lms with different thickness by using the pulsed laser deposition (PLD) technique with precise control of oxygen pressure at 3 Â 10 À3 mTorr. The structural, composition and magnetic results suggest that the observed room temperature ferromagnetism in Ti 1Àx V x O 2 lms is intrinsic. Further studies indicate that the oxygen vacancy not only contributes to the metallic conductivity of the Ti 1Àx V x O 2 lms, but also it provides the free electrons to mediate the long-range ferromagnetic coupling between two magnetic polarons.

Experimental method
The Ti 1Àx V x O 2 lms (x ¼ 0.03 and 0.05) with the thickness of 30-245 nm were grown on SrTiO 3 (100) substrate by the PLD technique at a temperature of 800 C and an oxygen partial pressure of 3 Â 10 À3 mTorr. The laser pulses were supplied by a KrF excimer source (l ¼ 248 nm) with an energy density of 2.5 J per cm 2 per shot and a frequency of 10 Hz. The nominal Ti 1Àx V x O 2 targets were prepared by a solid-state reaction method using TiO 2 (99.99%) and V 6 O 13 (99.97%) powders, and they were ablated for 5 minutes to eliminate surface contamination before deposition. Aer deposition, the lms were annealed in situ for 30 minutes, and then cooled down to room temperature slowly at the same oxygen pressure. The crystal structures of the lms were analyzed by q-2q X-ray diffraction (XRD) with using Cu Ka radiation (l ¼ 0.15406 nm). The chemical composition was determined by X-ray photoelectron spectroscopy (XPS) with a monochromatic Al Ka radiation as the X-ray source. The magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer. The transport properties of the lms were determined in the four-point probe conguration using a Quantum Design physical properties measurement system (PPMS) as a function of temperature. Here, the XRD pattern for pure TiO 2 lm deposited as the same condition is also placed at the bottom of gure for comparison. The spectra are plotted on a logarithmic scale to discern any minor secondary phase with small intense reections. The results show that the Ti 1Àx V x O 2 lms are epitaxial with single-phase rutile phase character, with only (200) and (400) reections detectable. It is noted that the undoped TiO 2 lm is epitaxial and of anatase phase with the (00l) orientation. It has been proposed that two Ti-O bonds break in the anatase structure, allowing the rearrangement of the Ti-O octahedra, which leads to a smaller volume and the rutile phase. 33 The breaking of these bonds is accelerated by the lattice disruptions, which can be introduced by the presence of dopant ions, the oxygen vacancies, and the method of synthesis. 34 In our study, the Ti 1Àx V x O 2 lms were deposited at a high vacuum (3 Â 10 À3 mTorr), resulting in a large amount of oxygen vacancies in the lms. This will presumably reduce the strain energy that must be overcome before the rearrangement of the Ti-O octahedral can occur, 33 and thus promotes the phases transformation.

Results and discussion
The Ti 1Àx V x O 2 lms with different thickness exhibit the obvious room temperature ferromagnetism. The in-plane room temperature magnetic hysteresis (M À H) loops of the lms are shown in Fig. 2 There is no evidence of the blocking temperature in the whole temperature range of 10-300 K, suggesting that the tiny ferromagnetic nano-clusters are not present in the lms. 37,38 Moreover, the ZFC/FC curves are distinctly separated from each other without any phase transition from 10 to 300 K, indicating that the T C of the Ti 1Àx V x O 2 lms is higher than 300 K.
The four-point probe geometry was used to obtain the transport properties of the Ti 1Àx V x O 2 lms. The results indicate that all of the lms show n-type conductivity and the carrier concentration is about 10 20 /cm 3 . The temperature (T) dependence of resistivity (r) is measured down to 10 K, which is shown in Fig. 3. All the resistivity versus temperature curves show positive slope, indicating an metallic conductivity, and the resistivity slightly increases as the lms thickness increases. Hong et al. 39 deposited the V-doped TiO 2 lms on LaAlO 3 substrates by the PLD method, and they found that the lms were semiconductors and the resistivity at room temperature was as high as 10 7 U cm, which is very different from our lms. This may be due to the inuence of preparation or processing conditions and the resulting defects on the transport properties of V-TiO 2 lms. Osorio-Guillén et al. 40 studied theoretically the electronic behaviors in V doped anatase TiO 2 , and showed that V Ti introduced deep levels in the gap due to the low 3d energy of the V atoms, resulting in the nonconductive for V-doped TiO 2 . Now, the possible origins of room temperature ferromagnetism and metallic behavior in the Ti 1Àx V x O 2 lms will be explored. If some portion of V is in the +3 or +4 valence states, or Ti is in the +3 valence state, and then the V 3+ , V 4+ or Ti 3+ will act as a localized spin, which is prerequisite to induce the magnetic ordering. 41 The n-type donors of V 5+ and oxygen vacancies may contribute to the metallic conductivity of the lms. In order to examine these possibilities, the XPS measurement was performed to determine the chemical states of Ti, O and V in the Ti 1Àx V x O 2 lms. Fig. 4(a) shows the XPS survey spectra of Ti 0.95 V 0.05 O 2 lm with the thickness of 180 nm. No additional peaks corresponding to secondary phases are detected, which is in accordance with the XRD and ZFC/FC measurements. Fig. 4(b) shows the Ti 2p spectrum for the same sample with Ti 2p 3/2 and Ti 2p 1/2 located at 458.5 and 464.3 eV, respectively, suggesting that Ti is in the +4 state. 25,42 The peak separation between the 2p 3/2 and 2p 1/2 lines is 5.8 eV, which is also consistent with the Ti 4+ oxidation state. 43,44 The binding  energies of V 2p 3/2 and V 2p 1/2 shown in Fig. 4(c) are 515.2 and 523.4 eV, respectively, indicating that V is in the +3 state. 45,46 Fig. 4(d) displays the spectra of O1s, which are divided into two peaks, referred to as O1 and O2. The peaks near 530.2 and 531.9 eV correspond to the binding energy of lattice oxygen in TiO 2 and oxygen defects, respectively. 47 It can be seen that an amount of oxygen vacancies exists in the Ti 0.95 V 0.05 O 2 lm, which can be ascribed to the lms deposited at a very low deposition oxygen pressure (3 Â 10 À3 mTorr). Additionally, the substitution of Ti 4+ by V 3+ ions will also increase the concentration of oxygen vacancies due to the necessity for the charge balance. Based on the XPS results, it is reasonable to claim that in the Ti 1Àx V x O 2 lms the V 3+ ions provide the local magnetic moment and the metallic conductivity can be attributed to the ionized donors of oxygen vacancies. This result is consistent with the theoretical calculations by Osorio-Guillén et al. that V dopants could convert nonmagnetic TiO 2 into a ferromagnet as V Ti can introduce a partially occupied, spin-polarized level, which could promote ferromagnetism. 40 There are different mechanisms for ferromagnetic coupling in the literature for TiO 2 -based DMS, such as carrier-mediated exchange 48 and bound magnetic polaron (BMP) model. 49 Tian et al. 36 speculated that the ferromagnetic coupling between V ions mediated by oxygen vacancies at interfaces may account for the observed room temperature ferromagnetism in V-doped TiO 2 nanoparticles. Hong et al. 50 reported the large value of magnetic moment of 4.2 m B /V for the V:TiO 2 lms and suggested that the room temperature ferromagnetism did not come from V clusters but from V-doped TiO 2 matrix. Du et al. 51 used a rst principles to study the magnetic properties of anatase Ti 1Àx V x O 2 , and showed that the oxygen vacancy induced magnetic polaron could produce long-range ferromagnetic interaction between largely separated V impurities. In the present work, V is chosen as a dopant because it is impossible to form any ferromagnetic secondary phase of V metal and V oxide, ruling out the extrinsic origin of the ferromagnetism. Indeed, the XRD, XPS and ZFC/FC results suggest that the observed room temperature ferromagnetism in Ti 1Àx V x O 2 lms is intrinsic. Moreover, there are amount of oxygen vacancies in the Ti 1Àx V x O 2 lms, and the lms exhibit the metallic behavior ( Fig. 3) with the high carrier concentration of 10 20 /cm 3 . In this regard, we propose that the doped V 3+ ions ferromagnetically couple with the electrons trapped by the oxygen vacancies, and form the BMPs, the carriers mediated the long-range ferromagnetic coupling between the magnetic polarons is a more possible mechanism in Ti 1Àx V x O 2 lms. This is in agreement with our previous theoretical results. 52

Conclusions
In summary, we have prepared rutile Ti 1Àx V x O 2 (x ¼ 0.03 and 0.05) lms with different thickness by using the pulsed laser deposition technique, and observed ferromagnetism at room temperature. The structural, composition and magnetic results suggested that the room temperature ferromagnetism in Ti 1Àx V x O 2 lms was intrinsic. More importantly, the Ti 1Àx V x O 2 This journal is © The Royal Society of Chemistry 2018 lms showed n-type and metallic conductivity. Further studies indicate that the oxygen vacancy not only contributes to the metallic conductivity of the Ti 1Àx V x O 2 lms, but also it provides the free electrons to mediate the long-range ferromagnetic coupling between two magnetic polarons.

Conflicts of interest
There are no conicts to declare.