JOURNAL OF APPLIED PHYSICS 109, 023902 共2011兲
Magnetic characterization of diluted magnetic semiconductor thin films Jian-Jun Gu,1,2,3 Li-Hu Liu,1,2 Yun-Kai Qi,2,3 Qin Xu,1,2 Hai-Feng Zhang,1,2 and Hui-Yuan Sun1,2,a兲 1
College of Physics Science and Information Engineering, Hebei Normal University, Shijiazhuang 050016, China 2 Key Laboratory of Advanced Films of Hebei Province, Shijiazhuang 050016, China 3 Department of Physics, Hebei Normal University for Nationalities, Chengde 067000, China
共Received 24 October 2010; accepted 29 November 2010; published online 19 January 2011兲 In studies of weak magnetism in diluted magnetic semiconductor thin films, there is often a significant difficulty in subtracting the background signal arising from the substrate. In the present work, an improved magnetic correction method is proposed. For Al doped ZnO and Fe doped TiO2 films, the magnetic moment and coercivity of the samples were corrected by the improved method, and the maximum fitting error due to the glass substrate was calculated. The accuracy and rationale of the improved method are discussed and compared with the traditional method. The results show that room temperature ferromagnetism is observed in Fe doped TiO2 thin films. The ferromagnetism is strongly correlated with the substitution of Ti by Fe in the TiO2 lattice, which results in a change in the crystal structure and the quality of the crystallization of the TiO2 films. The changes are responsible for altering ferromagnetism in the films. © 2011 American Institute of Physics. 关doi:10.1063/1.3532043兴 I. INTRODUCTION
Recently, diluted magnetic semiconductors 共DMSs兲 have attracted widespread attention due to their potential applications in the rapidly developing field of spintronics.1–3 This is largely due to the fact that it is possible to achieve useful semiconducting and magnetic properties within a single material system by doping semiconducting materials with small concentrations of transition metals 共TMs兲. A number of recent studies of magnetism in semiconductor oxide thin films doped with TMs have found evidence for room temperature ferromagnetism 共RTFM兲. In particular, semiconductor oxide thin films doped with nonmagnetic elements were also found to exhibit RTFM. Some notable examples include Ma et al.4 who studied Al/ZnO films deposited on a silica substrate by pulsed laser deposition 共PLD兲 and Yi et al.5 who have observed RTFM in ZnO films codoped with C and N. Likewise, Hou6 and Li7 have grown Cu doped ZnO films which showed RTFM, and Drera8 and Zhou9 have prepared N doped TiO2 and SiO2, and reported that RTFM had been observed. It has been remarked many times that in confirming the presence of ferromagnetism in DMSs thin films, the correct characterization and revision of the measured magnetization data are very important. As the magnetic moment of TMdoped oxide thin films tends to be weak, background signals arising from substrate can have a great impact on the magnetic signals assumed to originate with the thin films. It is therefore easy to mistakenly conclude that films show ferromagnetism if the background signal has not been accurately subtracted, and this will influence further analysis regarding the origin of ferromagnetism in films. Therefore, it is a prea兲
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requisite for further analysis of the origin of ferromagnetism in DMSs thin films that the background signals arising from the substrates be accurately subtracted. The measurements of the magnetic properties of DMSs have generally been performed using either a physical properties measurement system 共PPMS兲 or a superconducting quantum interference device 共SQUID兲 with measuring precisions of 10−6 or higher, but this alone does not mean a higher accuracy of the final result due to the possibility of systematic errors due to the substrate. The background signal will make measurements deviate from the true magnetic moment value of the films and the accuracy of the measurement can be negatively impacted. Therefore, how to deduct background signal accurately and eliminate systematic error arising from the background signal are key factors for obtaining accurate magnetic signals. The magnetization and coercivity are the main parameters for confirming the presence of ferromagnetism in DMSs thin films. Consequently, the correction discussed below includes correction of both the magnetic moment and the coercivity. The traditional correction method for the magnetic moment has many defects 共discussed in what follows兲, which should be improved to obtain accurate magnetic moment values. In addition, it is important that no only the magnetic moments but also the coercivity of the samples be corrected in an analysis of ferromagnetism. In theory, the M-H curve of the glass substrate goes through the coordinate origin assuming a zero coercivity. However, the actual curve shows coercivity, due to the fact that experimentally the applied field approaches but does not reach zero. 共For our glass substrate, the coercivity is about 80 Oe, as discussed in what follows.兲 If the coercivity of the substrate is ignored, it will influence further analysis of ferromagnetism in DMSs thin films. In this paper, we describe measurements made using a PPMS precision measurement system. We propose and apply
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FIG. 1. XRD patterns for pure ZnO and Al–ZnO films at different argon oxygen ratios and annealing temperatures. 共a兲 200 ° C, 共c兲, 500 ° C, 关共b兲 and 共d兲兴 magnified patterns of 共002兲 diffraction peak.
an improved magnetic correction method to Al doped ZnO and Fe doped TiO2 thin films. The coercivity of the substrate was subtracted, and the maximum fitting error of the substrate was calculated. Finally, based on the corrected results, the ferromagnetism in DMSs thin films is discussed. II. EXPERIMENTAL DETAILS
Al doped ZnO and Fe doped TiO2 thin films were grown on glass substrates by reactive magnetron cosputtering in a high vacuum chamber evacuated to a base pressure of 4 ⫻ 10−5 Pa. Metallic targets of Al共99.99% purity兲, Zn共99.99% purity兲, Fe共99.99% purity兲, and Ti 共99.99% purity兲 were used in this study. Argon and oxygen were introduced into the chamber as the working and reaction gases, respectively. During deposition, the chamber pressure was fixed at 1.5 Pa. Al doped ZnO thin films were prepared on a glass substrate at a deposition temperature of 200 ° C using argon oxygen ratios of 6:1, 4:1, and 2:1. After deposition, the films formed with different Ar/ O2 ratios were annealed in vacuum at 200, 300, 400, and 500 ° C for 30 min. For all the Al doped ZnO films, the Al content was held fixed at around 8 at. %. The Ti1−xFexO2 共x = 0 – 14 at. %兲 films were deposited on glass substrates at 300 ° C at an argon oxygen ratio of 10:1, and the deposited films were annealed in air at 500 ° C for 30 min. The atomic components in the chemical composition were confirmed by x-ray energy dispersive spectroscopy. The crystalline structure was investigated by x-ray diffraction 共XRD兲 with Cu K␣ radiation. The magnetic properties were measured using by a PPMS-6700 with the magnetic field parallel to the film plane. III. RESULTS AND DISCUSSION A. The structure of Al–ZnO thin films
Figures 1共a兲 and 1共c兲 show, respectively, the XRD patterns for the Al doped ZnO films annealed at 200 ° C and at 500 ° C. The Ar/ O2 ratios used in each case are shown in the figures. 共The XRD patterns of the films annealed at 300 and 400 ° C are not given since they behave in a similar manner.兲 Figures 1共b兲 and 1共d兲 show magnified XRD spectra of the
共002兲 diffraction peaks of 共a兲 and 共c兲, respectively. The thin films are found to have a polycrystalline wurtzite structure. No secondary phases or Al oxides were detected. Compared to pure ZnO films, the diffraction peaks of the doped thin films shift slightly to the left with a decreasing argon oxygen ratio, which implies that the d-spacing of the ZnO wurzite structure increases. The sample prepared using an argon oxygen ratio of 4:1 and annealed in vacuum at 500 ° C which shows the maximum offset angle for the 共002兲 diffraction peak has a shift of about 0.4°. The angle offset is often attributed to the substitution of Zn2+ by Al3+, residual stress in the films and interstitial Al atoms. If ionic substitution is dominant in the doped films, this would lead to compression of the c-axis of the ZnO wurzite structure and the diffraction peak shifting to the right, since the ionic radius of Al3+ 共0.053 nm兲 is smaller than that of Zn2+ 共0.072 nm兲. However, this is contrary to what we observe. ZnO, with a preferred growth direction along the c-axis and an open hexagonal wurtzite crystal structure, is inverse nested by the hexagonalclose-packed structure of oxygen and zinc. Consequently, it is easier for the Al to become an interstitial atom, resulting in lattice expansion, c-axis elongation and the diffraction peak shifting to the left as observed. It also can be seen from Fig. 1 that as the argon oxygen ratio decreases, the diffraction peaks become wider, which suggests that the quality of the crystallinity becomes poorer and the grains become smaller; The intensity of the 共002兲 peak gradually decreases, the magnitude of the 共101兲 peak gradually increases and the preferred orientation of the 共002兲 peak becomes more random. These results may be attributed to the formation of a small amount of Al2O3 in the ZnO lattice as the oxygen increases during deposition. 共The amount of Al2O3 required to degrade the wurzite structure is below detection limits.兲 This leads to ZnO lattice distortion and poor crystallinity. Note that the Al–ZnO film deposited with an argon oxygen ratio of 6:1 and an annealing temperature of 500 ° C exhibits 共002兲 preferential orientation and better crystallinity.
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FIG. 2. XRD patterns for Ti1−xFexO2共x = 0 – 14%兲 films.
B. The structure of Fe– TiO2 thin films
Figure 2 shows the XRD patterns of Ti1−xFexO2 共x = 0 – 14 at. %兲 thin films annealed at 500 ° C in the air. No diffraction peaks of Ti or other impurities such as Fe, FeO, Fe2O3, or FeTiO3 were observed. The diffraction peaks of Fe doped TiO2 films are slightly shifted to the right compared with pure TiO2 films, and the crystal structure of the films changes from anatase to rutile phases with increasing of Fe concentration. This implies the substitution of Fe ions into the Ti sites as the radius of Fe3+ 共0.64 Å兲 is similar to the Ti4+ 共0.68 Å兲. In addition, the XRD results also indicate that when the Fe doping concentration is below 9%, the samples show anatase structure with a higher intensity of the 共101兲 diffraction peak; When the doping concentrations are 9% and 12%, the structure of the films exhibits mixed anatase and rutile phases, and for Fe concentrations exceeding 12%, the structure of the films completely transforms into the rutile phase. The crystallinity of the films becomes poorer and the full width at half maximum of the 共110兲 diffraction peak increases, which suggests that the structure of the TiO2 films was changed and that the film quality was disrupted by the addition of excess Fe. C. Improved correction method
The improved correction method is as follows: first, we produce a linear least-squares fit to the raw measured magnetization data for the glass substrate alone 关Fig. 3共a兲兴 and
FIG. 3. 共a兲 Magnetization loops for the glass substrate. 共b兲 The straight-line fitting error; M e = M s − M fs, for the glass substrate. 共c兲 The M-H curve of part 共a兲 shown on an enlarged scale.
obtain the linear related coefficient r = 0.99963, nonlinearity 0.037% and slope b1 = −2.06658⫻ 10−8. However, it is not sufficient merely to subtract the linearly fitted substrate data, M fs 共the slope b1 multiplied by the external magnetic field H of the sample兲, from the raw data for the substrate plus film 共M sf = M substrate + M film兲. This may be illustrated by considering the substrate data alone and calculating the error M e resulting from using the straight-line fit. Subtracting the linear fit, M fs, from the raw data for the glass substrate, M s gives the error M e = M s − M fs resulting from using the linear fit. The result is shown in see Fig. 3共b兲. For our substrate, the maximum fitting error, M me, is about 3.0⫻ 10−6 emu. Since it is difficult to ensure that the magnetic field value applied to the sample is the same as that to the glass substrate, if the magnetic moment of the pure film is calculated using the formula M sf − M s, a large error will appear and affect the accuracy of the magnetic signal of the films. Figure 3共c兲 shows the M-H curve for the glass substrate plotted on a magnified scale. The curve does not pass through the origin, but has a coercivity of about 80 Oe. Obviously, such a large coercivity cannot be ignored. Therefore, the actual coercivity of the pure film should be calculated as follows: the raw coercivity of samples 共substrate+ film兲 mi-
TABLE I. The magnetic moment 共M兲, magnetization 共M s兲, and coercivity 共HC兲 values for the Al–ZnO films under different conditions. Annealing temperature 200 ° C
300 ° C
400 ° C
500 ° C
Parameter
ZnO Ar: O2 = 6 : 1
Al–ZnO Ar: O2 = 6 : 1
Al–ZnO Ar: O2 = 4 : 1
Al–ZnO Ar: O2 = 2 : 1
M 共emu兲 共M s兲 共emu/ cm3兲 HC 共Oe兲 M 共emu兲 共M s兲 共emu/ cm3兲 HC 共Oe兲 M 共emu兲 共M s兲 共emu/ cm3兲 HC 共Oe兲 M 共emu兲 共M s兲 共emu/ cm3兲 HC 共Oe兲
0 ¯ ¯ 2.7⫻ 10−6 1.5 5 0 ¯ ¯ 0 ¯ ¯
0 ¯ ¯ 0 ¯ ¯ 3.2⫻ 10−6 1.8 5 4.0⫻ 10−6 2.2 8
0 ¯ ¯ 4.2⫻ 10−6 2.3 6 4.1⫻ 10−6 2.2 9 0 ¯ ¯
0 ¯ ¯ 2.9⫻ 10−6 1.6 7 0 ¯ ¯ 0 ¯ ¯
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FIG. 4. Magnetization loops for Al–ZnO films 共a兲 Ar: O2 = 6 : 1 vacuum annealed at 400 ° C and 500 ° C; 共b兲 Ar: O2 = 4 : 1 vacuum annealed at 300 ° C and 400 ° C. 共Improved method.兲
nus that of substrate. The M-H curves given below are corrected and the coercivity of the substrate subtracted. D. Magnetic characterization of Al–ZnO thin films
The M-H curves of the Al–ZnO films were corrected by the above method. Table I shows the revised magnetic moments, magnetizations and coercivity values of the samples. The saturated magnetic moment is taken to be 0 for samples whose magnetic moment is lower than the M me 共defined above兲. Figures 4共a兲 and 4共b兲 show the M-H curves of the films in which saturation moments are higher than M me. In order to facilitate comparison, the M-H curves of the samples whose saturation magnetic moments were lower than M e are shown in Fig. 5. If the M-H curves are taken at face value, all the films show clear hysteresis, and one might draw the conclusion that these films possess ferromagnetism, which is not clear since M e was not considered. Considering the M e and coercivity, we found that the magnetic moments
FIG. 5. Magnetization loops for ZnO films formed with Ar: O2 = 6 : 1 and Al–ZnO film with Ar: O2 = 2 : 1 and vacuum annealed at 300 ° C corrected by the improved and traditional method, respectively.
of the films were on the order of 10−6 and close to M me, and that their coercivities were only a few oersted, although there are a few samples whose magnetic moments were slightly higher than M e. After applying the corrections, therefore, the conclusion that the films exhibit ferromagnetism is unconvincing. The traditional method for correcting magnetic data is to fit the raw magnetization data of the sample 共substrate + film兲 and obtain the slope b2. Then, the raw magnetic moment of the sample M sf minus the fitted magnetic moment of sample the M fsf 共slope b2 multiplied by the applied magnetic field H兲 is taken to be the magnetic moment of pure films, i.e., M f = M sf − M fsf. Comparing the M-H curves shown in Fig. 5 corrected by the traditional method and the improved method, one can see that the saturation magnetizations of two samples are about 4.1 and 3.7 emu/ cm3 according to the traditional method, whereas the saturation magnetizations are only 1.6 and 1.5 emu/ cm3 using the improved correction method. Obviously, the magnetic moment obtained by traditional method is larger than that obtained using the improved
FIG. 6. Magnetization loops for Ti1−xFexO2共x = 0 – 14%兲 films. 共The inset shows the M-H curves showing open hysteresis on an enlarged scale.兲
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TABLE II. The magnetic moment 共M兲, magnetization 共M s兲, and coercivity 共HC兲 values of the Ti1−xFexO2共x = 0 – 14%兲 films corrected by the improved method. Doping concentration M 共emu兲 M s 共emu/ cm3兲 HC 共Oe兲
0%
4%
7%
9%
12%
14%
6.1⫻ 10−6 3.4 10
9.2⫻ 10−6 5.1 30
2.1⫻ 10−5 11.2 100
1.6⫻ 10−5 8.9 115
1.2⫻ 10−5 6.7 135
7.7⫻ 10−6 4.3 65
method, and, in particular, is higher than M me. It is therefore easy to mistakenly conclude that films show ferromagnetism. All samples measured in this work were also corrected by the traditional method. The same result was observed in all cases; that is, the magnetic moment value calculated using the traditional method was systematically larger than the value obtained with our new method. The principal reason is that the linear correlation coefficient becomes smaller and the fitting deviation becomes larger, when the raw magnetization data of the sample 共substrate+ film兲 are fitted. E. Magnetic characterization of Fe– TiO2 thin films
The M-H curves of the Ti1−xFexO2 films were corrected by the improved method, and are shown in Fig. 6. The insert shows the data near zero field on an expanded scale. Table II shows the magnetic moments, magnetizations and coercivity values of the films. The magnetic moment of pure TiO2 is very close to the M me, and its coercivity is about 10 Oe 共within the range of the correction error兲, so the presence of ferromagnetism in pure TiO2 is doubtful. However, Fe– TiO2 thin films, whose magnetic moments are obviously higher than M me, have definite coercivity and exhibit RTFM. The XRD patterns of Fe– TiO2 thin films show no diffraction peaks due to Fe or other impurities, and the diffraction peaks of the doped films are slightly shifted to the right compared to pure TiO2 films. That the films show ferromagnetism may be attributed to the substitution of Fe ions into the Ti sites.10,11 In terms of the bound magnetic polaron 共BMP兲 model for the intrinsic magnetism of the DMSs films,12,13 the magnetic Fe ions substitute at Ti sites and defects exist around the magnetic ions.14 With increasing Fe doping concentration, there is more Fe2+ 共or Fe3+兲 substitution for Ti4+, and a stronger magnetic interaction between the magnetic Fe ions and the defects.15,16 This leads to longrange ferromagnetic order, and ferromagnetism in the sample.12 It can be seen from the XRD patterns that the crystallinity of the films becomes poorer with increasing Fe doping concentration, and the structure shifts phase from anatase to rutile. This change in structure may be the main reason leading to the weakening of the ferromagnetism at higher Fe doped concentration. We are currently developing the theoretical basis underpinning the influence of the structural phase transition on the ferromagnetism in doped TiO2 films. Based on the above analysis, failure to properly consider the fitting errors and the influence of the coercivity of the substrate may lead to incorrect conclusions regarding ferromagnetism in thin film samples. Results obtained by traditional methods, even though taking into account the calculated value of M me, may mistakenly be interpreted as
indicating ferromagnetism especially for those samples with magnetic moment values near the M me. In addition, considering only the magnetization, without consideration of the coercivity is not satisfactory. It is, as demonstrated here, therefore necessary for confirming ferromagnetism in DMS films to employ the improved correction method, calculate the fitting error and revise the coercivity. From the above, the magnetic moments of Al doped ZnO films are smaller and closer to the M me compared to the Fe doped TiO2 films, which may be ascribed to the fact that Fe is a magnetic metal, but Al is nonmagnetic. The above analysis suggests that in reports of ferromagnetism in thin film semiconductors doped with nonmagnetic elements,4–9 there may therefore be some question as to the observed ferromagnetism if the M me was not calculated and the coercivity of the substrate was not subtracted, or if the traditional method was used. IV. CONCLUSIONS
An improved magnetic correction method has been proposed in this paper. The measured magnetization data for Al–ZnO and Fe– TiO2 thin films were corrected by the improved method, and the maximum fitting error duo to the glass substrate was calculated. The accuracy and rationale of the improved method have been discussed and compared with the traditional method. The results show the presence of RTFM in Fe– TiO2 thin films, and suggest that changes in the crystal structure and crystallization of TiO2 films strongly affect the ferromagnetism of the films. The experimental results show that the improved method provides a new approach for obtaining accurate magnetic signals and reliable data for further analysis of the origin of ferromagnetism in DMSs. ACKNOWLEDGMENTS
This work is supported by the Natural Science Foundation of Hebei Province 共Grant No. A2009000254兲, and the Ph.D. fund from Hebei Normal University 共Grant Nos. L2006B10 and L2009Y03兲. The authors wish to thank Dr. Norm Davison for helpful discussion. 1
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