Transport properties in chromium-doped Ti 2 O 3 thin films

In this paper, we report the transport properties of Cr-doped Ti2O3 thin films. The thin films were grown on α-Al2O3 (012) substrates by pulsed-laser deposition. X-ray diffraction and transmission electron microscopy results show that the films are single corundum phase. All of (CrxTi1−x)2O3 show semiconducting behavior. Without doping, pure Ti2O3 thin films show positive magnetoresistance (MR) of 23% at 2K. The MR behavior changed dramatically after doping with Cr. Sample (Cr0.1Ti0.9)2O3 shows MR=−360% at 2K. All of the Cr-doped films are ferromagnetic up to room temperature.

In this paper, we report the transport properties of Cr-doped Ti 2 O 3 thin films.The thin films were grown on ␣-Al 2 O 3 ͑012͒ substrates by pulsed-laser deposition.X-ray diffraction and transmission electron microscopy results show that the films are single corundum phase.All of ͑Cr x Ti 1−x ͒ 2 O 3 show semiconducting behavior.Without doping, pure Ti 2 O 3 thin films show positive magnetoresistance ͑MR͒ of 23% at 2 K.The MR behavior changed dramatically after doping with Cr.Sample ͑Cr 0.1 Ti 0.9 ͒ 2 O 3 shows MR= −360% at 2 K.All of the Cr-doped films are ferromagnetic up to room temperature.© 2005 American Institute of Physics.͓DOI: 10.1063/1.1852855͔

I. INTRODUCTION
2][3][4] Following the study, we have investigated Crdoped Ti 2 O 3 thin films.The microstructure, transport and magnetic properties of Cr-doped Ti 2 O 3 thin films are reported in this paper.
0][11][12][13][14] The electrons in the conduction band are much heavier than the holes in the valence band, and the material exhibits positive Hall coefficient ͑p type͒.
Previous study reported that between Cr 2 O 3 and CrTiO 3 the sesquioxides form a homogeneous corundum-type solid solution.In ͑Cr x Ti 1−x ͒ 2 O 3 ͑0.5Ͻx Ͻ 1͒ the Cr 2 O 3 -type antiferromagnetic ͑AF͒ structure is found with the Ti 3+ ion acting as a simple diluent.Electron-spin resonance ͑ESR͒ gives a signal width= 1600 G and g = 2.00 for both x = 0.9 and 1.0. 15

II. EXPERIMENTS
͑Cr x Ti 1−x ͒ 2 O 3 ͑x = 0, 0.04, 0.06, and 0.10͒ thin films were grown on ␣-Al 2 O 3 ͑012͒ substrates by pulsed-laser deposition ͑PLD͒.Prescribed amounts of high-purity TiO 2 and Cr powders were mixed, cold pressed, and sintered to make Cr x Ti 1−x O 2 ceramic targets.The films were prepared in vacuum of 2 ϫ 10 −6 torr at substrate temperatures of 800-1000 K.The pulsed excimer laser uses KrF ͑ = 248 nm͒ and produces a laser beam with an intensity of 1-2 J/cm 2 and a repetition rate of 4 Hz.The deposition rate is between 0.3 and 0.5 Å / s, and the film thickness varies from 150 to 300 nm.The crystalline structure was investigated by x-ray diffraction ͑XRD͒ with Cu K␣ radiation and transmission electron microscopy ͑TEM͒.The magnetic properties were studied with a superconducting quantum in-terference device ͑SQUID͒ magnetometer.The transport properties were measured with a physical property measurement system ͑PPMS͒ from Quantum Design.

III. RESULTS AND DISCUSSION
The results of XRD and TEM show that single phase epitaxial ͑Cr x Ti Transport measurements with PPMS show that the films exhibit a semiconducting behavior-increasing resistivity with decreasing temperature.The carriers are p type for both doped and undoped films.The Hall effect of the films was measured with a four-probe method at room temperature.A carrier density of 1.2ϫ 10 21 cm 3 was estimated for ͑Cr 0.06 Ti 0.94 ͒ 2 O 3 from the Hall-effect measurements.All films do not exhibit anomalous Hall effect at room temperature.It is underway to measure the Hall effect at higher and lower temperatures, trying to observe the anomalous Hall effect at various temperatures.
The magnetoresistance ͑MR= ͑R H − R 0 ͒ / R H ϫ 100% ͒ was measured with magnetic field perpendicular to the film plane for Ti 2 O 3 and ͑Cr x Ti 1−x ͒ 2 O 3 at various temperatures and the results are shown in Fig. 2. The MR at 2 K for Ti 2 O 3 reaches +23% at 140 kOe, as Fig. 2͑a͒ shows.This large positive MR is in agreement with previous data and is explained with a two-band model where both electrons and holes participate in the transport. 16igure 2͑b͒ shows MR for x = 0.04 samples, at lower field, the MR is negative, while positive MR dominates in high fields at 2 K. Positive MR was observed at 6 and 10 K for x = 0.04 films.Inset of Fig. 2͑b͒ shows the thermooscillation of magnetoresistance.With 14-T constant magnetic fields, the temperature sweep caused two peaks in the magnetoresistance at about 7 and 84 K.Such peaks in the magnetoresistance were observed in Hg 1−x Mn x Te. 17 The strong temperature dependence of the energies at which the Landau levels occur suggests that at a constant magnetic field ͑that provides the Landau quantization͒, the temperature sweep will cause the crossing of various Landau levels with the Fermi energy, if the carrier concentration in a sample is properly chosen.These crossings will then lead to successive peaks in the magnetoresistance of the sample as a function of temperature. 18he MR is negative and takes a giant value of −107% and −365% at 2 K in 140 kOe for x = 0.06 and 0.10, respectively, as shown in Figs.2͑c͒ and 2͑d͒.The magnitude of the negative MR of ͑Cr 0.06 Ti 0.94 ͒ 2 O 3 and ͑Cr 0.10 Ti 0.90 ͒ 2 O 3 in high magnetic field decreases with increasing temperature.The inset of Fig. 2͑c͒ shows the low-field region of the MR curve for x = 0.06.At 10 K, the MR for x = 0.06 is positive in H Ͻ 5 kOe, reaching the maximum around 2 kOe, and turns to negative as the magnetic field goes above 5 kOe.Similar MR behavior in low fields was observed in ͑Ga 0.947 Mn 0.053 ͒As thin films and the small positive magnetoresistance disappeared when magnetic field was applied parallel to the sample plane. 19While in our sample x = 0.06, the small positive magnetoresistance in lower field was found in field orientations both perpendicular and parallel to the film plane.
In order to understand the magnetoresistance, we have studied the magnetic properties of the films.The results from SQUID measurements show that all of the Cr-doped films are ferromagnetic up to 400 K. Figure 3 shows magnetic hysteresis loops for ͑Cr 0.06 Ti 0.94 ͒ 2 O 3 at 2 and 300 K.The saturation moment M s at 2 K is 0.5, 0.91, and 0.87 B / Cr for x = 0.04, 0.06, and 0.10, respectively.M s initially increases with x then decreases slightly upon further increase of Cr doping.The room-temperature saturation moments are smaller, e.g., M s = 0.2 B / Cr for x = 0.06.Its coercivity is 50 Oe at 300 K as compared to 350 Oe at 2 K.The magnetization of ͑Cr 0.06 Ti 0.94 ͒ 2 O 3 film at 2 K does not reach saturation at 50 kOe.The M-H curve consists of two components.One is a ferromagnetic part which corresponds to the lower field hysteresis loop and the other is the paramagnetic component.
To discuss the giant negative magnetoresistance for the x = 0.06 and 0.1 film, one possible explanation is based on polaron theory.The bound magnetic polaron ͑BMP͒ is the characteristic collective state of diluted magnetic semiconductors.1][22][23][24] A bound polaron consists of the localized carrier and surrounding cloud of Cr spins polarized via p-d exchange interaction.Without magnetic field, the magnetic polarons have to move through the sea of Cr more or less randomly oriented spins.Their motion involves flipping many Cr spins, making the polarons massive and immobile.With increasing magnetic field, all Cr spins are gradually aligned to the direction of the applied magnetic field, increasing the mobility of the carriers.The negative magnetoresistance is attributed to smearing of the polaronic cloud with increasing magnetic field, 25 corresponding to the slowly increasing part of magnetization in the high-field region shown in Fig. 3.

IV. CONCLUSIONS
The experimental results show that Cr ions systematically substituted for the Ti ions in ͑Cr x Ti 1−x ͒ 2 O 3 films.The ͑Cr 0.10 Ti 0.9 ͒ 2 O 3 films show a negative giant magnetoresistance as large as −365% at 2 K, while the undoped film shows a positive giant magnetoresistance of +23%.The Crdoped samples exhibit ferromagnetism up to 400 K.They are p-type semiconductors with a carrier density of about 1.2ϫ 10 21 cm 3 for the x = 0.06 sample.
1−x ͒ 2 O 3 films were formed by ablating the Cr x Ti 1−x O 2 targets and reduction during the deposition.Fig-a͒ Electronic mail: jtang@uno.edu1. ͑a͒ XRD patterns for ͑Cr x Ti 1−x ͒ 2 O 3 ͑x = 0, 0.04, 0.06, and 0.10͒ films.Only the reflections from the ͑012͒ family of corundum phase are observed.͑b͒ The peak position of the ͑012͒ reflection of ͑Cr x Ti 1−x ͒ 2 O 3 films shifts to higher angles with increasing Cr concentration x.Inset shows the d ͑012͒ values that decrease linearly with the Cr concentration.ure 1͑a͒ shows the XRD patterns of ͑Cr x Ti 1−x ͒ 2 O 3 ͑x =0, 0.04, 0.06, and 0.1͒ films grown on ␣-Al 2 O 3 .Only the reflections from the ͑012͒ family of corundum phase are observed.Figure 1͑b͒ shows XRD peaks for the ͑012͒ reflection of the ͑Cr x Ti 1−x ͒ 2 O 3 films with different Cr concentration.For ͑Cr 0.04 Ti 0.96 ͒ 2 O 3 , the ͑012͒ peak shifts to a higher angle relative to the ͑012͒ peak of Ti 2 O 3 , which indicates the decrease in the lattice parameters of ͑Cr x Ti 1−x ͒ 2 O 3 .This shift is further enhanced with increasing Cr content, as shown in the patterns for ͑Cr 0.06 Ti 0.94 ͒ 2 O 3 and ͑Cr 0.10 Ti 0.90 ͒ 2 O 3 .The Cr content dependency of the R-axis lattice constants ͓d ͑012͒ value͔ is shown in the inset of Fig. 1͑b͒.It is observed that the value of d ͑012͒ decreases linearly as the Cr concentration increases, which indicates that the Cr ions gradually substitute for the Ti ions in the films without changing the corundum structure.