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Comparison of ferromagnetism in n- and p-type magnetic semiconductor thin films of ZnCoO
Journal of Magnetism and Magnetic Materials 323 (2011) 1846–1850
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Comparison of ferromagnetism in n- and p-type magnetic semiconductor thin films of ZnCoO Y.H. Lee a,n, J.C. Lee a, J.F. Min a, C.W. Su b a b
Department of Physics, National Cheng Kung University, No. 1, Ta-Shuei Road, Tainan 70101, Taiwan Department of Applied Physics, National Chiayi University, Chiayi 60004, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2010 Received in revised form 6 February 2011 Available online 15 February 2011
Both n- and p-type diluted magnetic semiconductor ZnCoO are made by magnetron co-sputtering with, respectively, dopants of Al and dual dopants of Al and N. The two sputtering targets are compound ZnCoO with 5% weight of Co and pure metal Al. Sputtering gases for n- and p-type films are pure Ar and N2, respectively. These films are magnetic at room temperature and possess free electron- and holeconcentration of 5.34 1020 and 5.27 1013 cm 3. Only the n-type film exhibits anomalous Hall-effect signals. Magnetic properties of these two types of films are compared and discussed based on measurements of microstructure and magneto-transport properties. & 2011 Elsevier B.V. All rights reserved.
Keywords: Diluted magnetic semiconductor Ferromagnetism Co doped ZnO
1. Introduction Diluted magnetic semiconductor (DMS) based on wide-gap oxides such as ZnO, TiO2, and SnO2 have been reported to show room temperature ferromagnetism. Among these oxides, ferromagnetism of ZnO-DMS has been reported to be dependent on the carrier-polarity i.e. only n-type Co doped ZnO (ZnCoO) and p-type Mn doped ZnO (ZnMnO) are ferromagnetic; in contradistinction, p-type ZnCoO and n-type ZnMnO, are not [1,2]. By assuming the existence of shallow donor defects in the wide-gap oxides, Coey et al. [3] pointed out that high Curie temperature (TC) appears only when electrons associated with shallow donors are partially delocalized onto the magnetic dopants to increase the exchange coupling constants of Jsd or Jpd. However, isovalent Co2 + doping of ZnO does not itself introduce any carriers. Shallow donors in high TC ZnCoO can be invoked through native defects (oxygen vacancy (Ov), zinc interstitial (Zni)), or extrinsic donor-type impurities such as Al and Ga. According to Coey et al. [3], dopant–donor hybridization is then a crucial factor for the intrinsic carrier-mediated ferromagnetism in ZnO-DMS. For substantial hybridization between the magnetic dopant and donor impurity, the two species have to be in proximity in energy levels. Kittilstved et al. [4] further pointed out the factor that matters in the carrier-polarity dependent ferromagnetism is the energy level of the dopant-derived donor- or acceptortype ionization state i.e. one electron or hole transfers, respectively, from donor or acceptor band to Co2 + /Mn2 + ion, which results in, respectively, one electron reduced Co1+ /Mn1 + or oxidized Co3+ /Mn3 + state. It was proved by Kittilstved et al. [4] that the Co1+ /Mn3 + state is
near the conduction/valence band edge (within 0.32/0.42 eV), and the Co3 + /Mn1+ state is rather deep in energy level (1.66/2.68 eV above/ below the valence band edge). Thus, charge transfer from conduction/ valence band to Co2 + /Mn2+ and form Co1 + /Mn3+ ion is more probable, which explains why ferromagnetism exists only in n-type and not in p-type ZnCoO (or in p-type and not in n-type ZnMnO) [4]. Even so, ferromagnetic p-type ZnCoO was reported by several groups [5–7]. However, none provided any information of the holecarriers. We are interested in investigating whether ferromagnetism reported for p-type ZnCoO resulted from hole-carrier mediation. To wit, we specially made p-type ZnCoO films and endeavor to obtain explicit hole-carrier concentration to clarify this problem. Our p-type film was fabricated by using the idea of donor–acceptor (Al–N) codoping method proposed by Yamamoto et al. [8] to help to retain N acceptors and acquire as many hole-carriers as possible. Utilizing magnetron co-sputtering of two targets of ZnCoO compound and pure Al metal, we just change the sputtering gas from Ar to N2, and the film’s conductivity is changed from n- to p-type. X-ray induced photoelectron emission spectroscopy (XPS) for Co atom was taken along the film depth to verify that the ferromagnetism is intrinsic and not the result of extrinsic Co nanoclusters. Atomic substitution of N for O is confirmed by measurements of both XPS and PL (photoluminescence). Finally, we took the field-dependent Hall-effect measurements to obtain anomalous Hall-effect (AHE) information. Together with the results of microscopic characterization, information of carrier-mediated intrinsic ferromagnetism can be deduced.
2. Experiment n
Corresponding author. Tel.: +886 6 2757575; fax: + 886 6 2747995. E-mail addresses: [email protected], [email protected] (Y.H. Lee). 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.011
Both n- and p-type ZnCoO films (ZnCoO:Al and ZnCoO:(Al, N)) were made by rf-magnetron co-sputtering using different
Y.H. Lee et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1846–1850
sputtering gases (Ar for n-type and N2 for p-type) and otherwise under the same conditions. The base pressure of the vacuum system was 5 10 6 Torr. Working pressure and gas flow rate were, respectively, maintained at 10 m Torr and 20 sccm. Sputtering powers of 150 and 30 W were used for two targets of ZnCoO compound with 5% Co and pure metal Al, respectively. During deposition, the quartz substrate was heated to 250 1C and rotated at 70 rpm in order to obtain better thin film crystalline quality and homogeneity. Film thickness is about 250 nm, which was estimated using the sputtering rate prior calibration based on the thin film SEM image. Due to the carrier compensation phenomenon usually reported for p-type ZnO, we subjected thin film ZnCoO:(Al, N) to a short ( 30 min) post-deposition annealing in atmosphere at 450 1C to decrease oxygen vacancies (and thus the number of free electron-carriers) but avoid problematic formation of secondary oxide phases such as Al2O3, CoO. X-ray diffraction (XRD) spectrum and X-ray induced photoelectron emission spectroscopy (XPS) were used to inspect, respectively, the microstructure and the chemical state of each ingredient element of the film. Thin film compositions were also obtained from the analysis of XPS spectra. Optical transmittance in the wavelength range 300–1000 nm was measured by UV–vis spectrophotometer. The Hall measurement system (Ecopia HMS-3000) was used to measure electrical resistivity and carrier concentration. AHE was measured at room temperature by using the electromagnet to provide field between + 8 and 8 kOe. Field- and temperaturedependent magnetizations (M–H and M–T) were measured by superconducting quantum interference device (SQUID) with the field applied perpendicular to the film plane. PL was measured at a low temperature of 12 K using Xe lamp with wavelength of 300 nm as light source.
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also obtained from measurements of XPS, optical transmittance, and PL to be presented below. Results of XPS are shown in Fig. 2. XPS is well known to be sensitive to chemical environment and provides information of the chemical state of an atom in the compound. But it is only sensitive to atoms residing near the film surface. To confirm the chemical state of Co doped in ZnO, we took XPS at every 10 nm thickness of etching. XPS of Co dopant is shown in Fig. 2(a) where the curves correspond to three arbitrary thicknesses (shown near each curve) are displayed for both films. Both experimental data (discrete points) and the fitting curves (solid lines) are exhibited. All curves are similar and show distinctive features of Co–O bonding, which are main peaks of Co 2P3/2 at 780.3 eV and 2P1/2 at 795.8 eV with their satellites, respectively, at 786 and 803 eV [9]. The main peak of 2P3/2 at 778.3 eV for Co–Co bonding,
3. Results and discussions XRD spectra for ZnCoO:Al and ZnCoO:(Al, N) film in Fig. 1 show the feature of a single strong peak, which is commonly observed for polycrystalline ZnO and assigned to (0 0 2) plane of ZnO wurzite structure. For the ZnCoO:Al film, a peak at 2y 63o corresponding to ZnO (1 0 3) plane is hardly discernible, which may have something to do with the proper selection of sputtering conditions such as substrate location and orientation. Results shown in Fig. 1 indicate a good crystallinity with strong c-axis growth direction. It also indicates hexagonal structure of ZnO remains, or atomic replacements occur with doping of Co, Al, and N. Furthermore, no obvious oxide phases of Co, Al, and N are observed. Consistent results are
Fig. 1. XRD spectra for ZnCoO:Al and ZnCoO:(Al, N) film.
Fig. 2. XPS spectra of (a) Co atom at three different thicknesses shown near the curve (b) O atom for ZnCoO:Al and ZnCoO:(Al, N) film. Inset in (b) shows the decomposition of O 1S1/2 for ZnCoO:(Al, N) film.
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observed and assigned to Co clusters by Kaspar et al. [10] in their film, does not appear at all in any spectrum of both of our films. Therefore, the doped Co atoms were certainly present as Co2 + in the cation sites and throughout the whole film, consistent with the above XRD results. This is further evidenced by the optical transmittance spectrum (OPTS, not shown) exhibiting clear three absorption peaks at the wavelengths of 568, 615 and 660 nm corresponding to d–d intra-ionic transition for Co atom in the high spin state Co2 + (d7) [11]. XPS of O 1S1/2 is displayed in Fig. 2(b). The O 1S1/2 peak for ZnCoO:Al film shown in Fig. 2(b) is at 529.3 eV. Upon doping N, i.e. for ZnCoO:(Al, N) film, the peak shifts to slightly higher energy of 539.7 eV and shows a shoulder at 531.3 eV. Since XPS peak position corresponds to the binding energy of the electronic states of atoms, the shift of the peak position indicates the variation of valence charge or chemical state of an atom. When the valence electron moves away from an atom, the atom is said to be oxidized, and the binding energy of core electrons increases due to the reduced screening of Coulomb potential. On the contrary, an atom gaining charge in the valence bond is said to be reduced, and the binding energy of core electrons decreases. In Fig. 2(b), the binding energy (529.3 eV) of O 1S1/2 in ZnCoO:Al film is smaller than that (532 eV) [12] of elemental O, indicating that O atoms are bonded (by capturing the valence electron) with either Zn, Co or Al. In ZnCoO:(Al, N) film, binding energy of O 1S1/2 increases from 529.3 to 529.7 eV signaling the N-dopant actually replaces O at the anion site. The donor–acceptor codoping method proposed to increase N solubility, and thus hole-carrier concentration, is mainly through the formation of acceptor–donor– acceptor complexes. Once 2N at near lattice sites substitute O and form N–Al–N complexes, electrons of Al seized by O are then shared with N. That is, doping N causes a decrease in the number of valence electron of O atom, thus O 1S1/2 binding energy increases. By the same token, the number of valence electron of N increases and the binding energy (398.2 eV) of N 1S1/2 decreases from that of the elemental state (399 eV) [12], as indicated in XPS of N 1S1/2 (not shown). Furthermore, those O atoms in the neighborhood of N atoms feel stronger influence from N atom, leading to a larger shift in binding energy, which results in the shoulder of the O 1S1/2 peak. Following this inference, the peak area under the shoulder should be proportional to the quantity of the doped N atoms. We decomposed the O 1S1/2 peak for ZnCoO:(Al, N) film and show the result in the inset of Fig. 2(b). The area ratio of the curve under the shoulder to the whole curve is 1/5, which is roughly equal to the ratio calculated using the atomic concentration of N and O shown below in Table 1. The XPS peak for Zn 2P3/2 and Al 2P3/2 (not shown) is, respectively, at 1021 and 73 eV for the two films, and is close to the value of Zn and Al element, respectively. It is not yet clear why cations, unlike anions, do not show explicit shifts in binding energy. The atomic concentrations listed in Table 1 for both films are obtained by XPS spectra. For concentrations less than 10%, the value cannot be taken as exact due to the sensitivity limit of such small quantities. But the relative relation between different concentrations is good. For example, concentrations of Co and
Al are similar in both films because same sputtering targets and powers were used. Presumably if dopants of Co and Al were all to reside at Zn sites and N at O sites, the atomic ratio between the group of cations and anions should be close to 1:1. In Table 1, we find that the O concentration is less than the total concentration of Zn, Co, and Al for ZnCoO:Al film. But the total concentration of O and N is similar to that of Zn, Co, and Al for ZnCoO:(Al, N) film. It seems that oxygen vacancies (or Zn interstitials) exist in ZnCoO:Al film but are suppressed in ZnCoO:(Al, N) film. Probably, an extra short period of post-deposition annealing in atmosphere applied to ZnCoO:(Al, N) film is effective in reducing the O deficiency. Therefore, carrier compensation is effectively suppressed and the p-type conduction is obtained in ZnCoO:(Al, N) film as indicated below. Results of electrical properties of carrier concentration, resistivity, and mobility obtained from Hall-effect measurement are also listed in Table 1. The ZnCoO:Al film is conductively n-type semiconductor with free electron concentration of 5.34 1020 cm 3 shown in Table 1. It is an order smaller than the value of 6.17 1021 cm 3 estimated using atomic concentration of Al by assuming that Al properly replaces Zn and donates a free electron. If oxygen deficit mentioned above is assumed in ZnCoO:Al film (which also donate electrons), then about 10% or more of the doped Al atoms do not reside properly at Zn sites as donors. The p-type semiconductor with hole-carrier concentration of 5.27 1013 cm 3 was obtained for ZnCoO:(Al, N) film and shown in Table 1. Taking into account the concentration of electron/hole associated with donors/acceptors is proportional to the concentration of donor/acceptor; and in the higher concentration of N than Al in ZnCoO:(Al, N) film shown in Table 1, electrons are actually over-compensated by holes giving rise to p-type semiconductivity. The information of the existence of the acceptor band due to the doping of N is further confirmed by the results of PL (to be shown below). To the best of our knowledge, the hole-carrier concentration shown in Table 1 is the largest reported so far for p-type magnetic ZnCoO. PL spectra for n- and p-type ZnCoO film are shown in Fig. 3. The curve for n-type film in Fig. 3 shows broad peaks, respectively, at 2.35, 2.7, and 2.95 eV which are similarly observed in p-type film except with much smaller intensities. The structureless broad peak at 2.35 eV is close to the reported ( 2.4 eV) green luminescence (GL) band, which is generally reported for ZnO without intentional doping, and attributed to interstitial Zn [13] or insufficient O [14,15]. GL band is not prominent in p-type film. Peaks at 2.7 and 2.95 eV were not reported for ZnO. Whether they are related with the dopants of Co or Al needs further investigation. Here, we are more interested in the broad peak at 3.22 eV, which exists only in the p-type and not n-type film. It was attributed to donor–acceptor pair (DAP) transition by several research groups [16,17], and the acceptor was identified as unintentionally [16] or intentionally [17] doped N, which forms shallow acceptors with ionization energy of 170 meV. In addition, other N-acceptor related transitions [16,17] such as LO phonon replicas of DAP and acceptor-bound exiton at 3.02–3.15 and 3.315 eV, respectively, are within this broad peak. The crystal quality of p-type film is obviously not good enough to
Table 1 Atomic concentration and electrical properties of carrier density, conduction type, resistivity, and mobility for ZnCoO:Al and ZnCoO:(Al, N) film. Sample
ZnCoO:Al ZnCoO:(Al, N)
Composition (at%)
Electrical properties
Zn
O
Co
Al
N
Carrier density (cm 3)
Conduction type
Resistivity (O cm)
Mobility (cm2/V s)
48.21 39.06
39.87 40.24
5.60 5.78
6.24 6.55
0 8.37
5.34 1020 5.27 1013
n p
1.75 10 3 2.59 102
6.69 2.96
Y.H. Lee et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1846–1850
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Fig. 3. PL spectra measured at T¼12 K for ZnCoO:Al and ZnCoO:(Al, N) film. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
produce discrete characteristic peaks. Therefore the existence of a broad peak at 3.22 eV in p-type film signals that N in ZnCoO: (Al, N) film does replace O atoms and a shallow acceptor band is formed. Results of M–H and M–T for both films are displayed in Fig. 4. Both n-type ZnCoO:Al film and p-type ZnCoO:(Al, N) film show hysteretic M–H behavior in Fig. 4(a) and (b), respectively. Thus both films are ferromagnetic. Results of M–T are measured at 4–375 K and shown in Fig. 4(c) and (d), respectively, for ZnCoO:Al and ZnCoO:(Al, N) films. In both Fig. 4(c) and (d), there are two curves of zero-field-cool (zfc, solid symbol) and field-cool (fc, open symbol) measured at the field of 50 Oe. Both films in Fig. 4(c) and (d) exhibit M–T behavior (a fast and then a slow decrease of M with increasing T) that are similar for zfc and fc, but the curves do not coincide. This kind of M–T behavior for both n- and p-type films exhibits no feature of blocking temperature and is not superparamagnetic, according to our previous work [18,19]. Thus, we tried to describe the fc curve using 3-D spin wave model which leads to M(T)¼M0–0.117 mB(kB/2SJd2)3/2, wherein M0 is the zero temperature magnetization, and d and J are, respectively, the spacing and exchange interaction between magnetic ions. With the addition of paramagnetic phase (i.e. M–H/T), we have a good fit for fc curves shown as solid lines in Fig. 4(c) and (d) for both films. The fitting parameters are shown together in the plot. The existence of the paramagnetic phase explains the discrepancy between zfc and fc curves. It is because the paramagnetic phase responds differently at different conditions of zfc and fc. Both n- and p-type films show very similar M–H and M–T behavior. Then, the measurement of AHE is an important step to verify the intrinsic ferromagnetism. For magnetic materials, the Hall resistivity rH( ¼VHt/i, with VH the Hall voltage, i the current, and t the film thickness) is expressed as [20] rH ¼RoB +RaM, wherein Ro is the normal Hall coefficient, Ra the anomalous Hall coefficient, B the magnetic induction, and M the perpendicular magnetization. The results of field-dependent rH for n-type ZnCoO:Al film (shown in the inset of Fig. 4(a)) exhibit the prototype behavior of ordinary plus anomalous Halleffect [21]: linear rH at high field with a small vertical shift at low field. Subtracting the linear ordinary Hall-effect term, we obtain a clear signature that corresponds to the AHE, and this is shown in Fig. 4(a) (symbol). Since AHE signal was also reported [22,23] in non-ferromagnetic DMS, we must be careful and not use AHE
Fig. 4. Field-dependent magnetization (M–H) for (a) ZnCoO:Al and (b) ZnCoO: (Al, N) film. Temperature-dependent magnetization (M–T) of zfc (empty symbol) and fc (solid symbol) curves for (c) ZnCoO:Al and (d) ZnCoO:(Al, N) film.
signal alone as the indicator of intrinsic ferromagnetism. Shinde et al. [22] pointed out this potential error can be avoided by combining the results of detailed microscopic characterizations with AHE signal. Our results of XRD, XPS, and optical transmittance analyzed above all come to a conclusion that most Co dopants reside at Zn site in atomic form and no indication Co
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nanoparticles exist in the n-type film. The clear M-dependent behavior of rH in Fig. 4(a) (symbol) thus characterizes the spin polarized conduction electrons, which mediate the ferromagnetic coupling between Co2 + ions. We are now sure, within the detection limit of XRD and XPS, that the n-type ZnCoO:Al film shows intrinsic ferromagnetism. The existence of intrinsic ferromagnetism in Co doped ZnO nanoparticle is also claimed by Zhang et al. [24], which used EMCD rather than AHE characterization. We have to mention here that the free electron concentration shown in Table 1 for the n-type ZnCoO:Al film is calculated using the slope of rH H at high field where normal Hall-effect dominates. Unfortunately, rH are not obtainable for p-type ZnCoO:(Al, N) film due to its large electrical resistance. The hole-concentration of p-type film (in Table 1) is obtained using commercially made Hall measurement system of model Ecopia HMS-3000. In the p-type film, there is also no evidence of Co nanoclusters (Fig. 2(a)) which is thus excluded as the origin of the ferromagnetism. The ferromagnetism is also not attributed to the free hole-carrier mediation, even though N replaces O and forms shallow acceptor band as indicated by the results of XPS and PL, because the probability of charge-transfer between the magnetic cation and the acceptor band is small if we follow the result reported by Kittilstved et al. that Co3 + is not near the valence band edge. Nevertheless, charge carriers are not needed to be conductive to mediate the ferromagnetic exchange interaction, as pointed out by both Coey et al. [3] and Kittilstved et al. [4]. We believe that donor electrons of Al become relatively immobile upon N doping. The hydrogenic radius (rH) of these electrons is large enough to cover at least two magnetic cations (estimated [3] by (4/3)pr3Hxno with rH ¼0.76 nm, x¼ 5.60% (magnetic ion concentration), and no ¼6 1022 cm 3 (oxygen density)) and forms the so called bound magnetic polaron (BMP). It is the overlap between BMPs throughout the film that results in the ferromagnetism in p-type film. Therefore, ferromagnetic order in p-type ZnCoO:(Al, N) film is not from the mediation of free hole-carriers but from that of nonconductive donor electrons.
4. Conclusions n-type ZnCoO:Al and p-type ZnCoO:(Al, N) films, respectively, with electron concentration of 5.34 1020 cm 3 and hole-concentration of 5.27 1013 cm 3, were fabricated. Both n- and p-type ZnCoO films are ferromagnetic. Signal of AHE is clearly observed only for n-type film, but is not measurable for p-type film due to its large resistance. Ferromagnetic exchange coupling between magnetic ions in the n-type film is ascertained to be
through spin polarized free electrons. Ferromagnetism in p-type film is not attributed to free hole-carriers mediation, but to the overlap of BMP which is the bound state of magnetic ions and immobile donor electrons from N doping.
Acknowledgment The authors would like to acknowledge the financial support of their research by the National Science Council under Grant nos. NSC98-2112-M-006-012 and NSC99-2112-M-006-016. References [1] K.R. Kittilstved, N.S. Norberg, D.R. Gamelin, Phys. Rev. Lett. 94 (2005) 147209. [2] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, Y. Liu, J. Appl. Phys. 101 (2007) 063918. [3] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173. [4] K.R. Kittilstved, W.K. Liu, D.R. Gamelin, Nat. Mater. 5 (2006) 291. [5] H.-T. Lin, T.-S. Chin, J.-C. Shih, S.-H. Lin, T.-M. Huang, F.-R. Chen, J.-J. Kai, Appl. Phys. Lett. 85 (2004) 621. [6] M.H.F. Sluiter, Y. Kawazoe, P. Sharma, A. Inoue, A.R. Raju, C. Rout, U.V. Waghmare, Phys. Rev. Lett. 94 (2005) 187204. [7] P. Cao, D.X. Zhao, J.Y. Zhang, D.Z. Shen, Y.M. Lu, Z.W. Fan, X.H. Wang, Physica B 392 (2007) 255. [8] T. Yamamoto, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38 (1999) L166. [9] L. Daheron, R. Dedryvere, H. Martinez, M. Menetrier, C. Denage, C. Delmas, D. Gonbeau, Chem. Mater. 20 (2008) 583. [10] T.C. Kaspar, T. Droubay, S.M. Heald, M.H. Engelhard, P. Nachimuthu, S.A. Chambers, Phys. Rev. B 77 (2008) 201303 R. [11] Y.Z. Yoo, T. Fujumura, Z. Jin, K. Hasegawa, M. Kawasaki, P. Ahmet, T. Chikyow, H. Koinuma, J. Appl. Phys. 90 (2001) 4246. [12] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King Jr (Eds.), Physical Electronics, 1995, pp. 42–88. [13] N.O. Korsunska, L.V. Borkovska, B.M. Bulakh, L.Yu. Khomenkova, V.I. Jushnirenko, I.V. Markevish, J. Lumin. 102–103 (2003) 733. [14] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [15] F.H. Leiter, H.R. Alves, N.G. Romanov, D.M. Hoffmann, B.K. Meyer, Physica B 340–342 (2003) 201. [16] K. Thonke, Th. Gruber, N. Teofilov, R. Schonfelder, A. waag, R. Sauer, Physica B 308–310 (2001) 945. [17] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [18] Y. Lee, J.C. Lee, C.W. Su, IEEE Trans. Magn. 46 (2010) 1565. [19] Y.H. Lee, T.C. Han, J.C.A. Huang, J. Appl. Phys. 93 (2003) 8462. [20] H. Ohno, J. Magn. Magn. Mater. 200 (1999) 110. [21] H. Toyosaki, T. Fukumura, Y. Yamada, K. Nakajima, T. Chikyow, T. Hasegawa, H. Koinuma, M. Kawasaki, Nat. Mater. 3 (2004) 221. [22] S.R. Shinde, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 92 (2004) 166601. [23] Q. Zu, H. Schmidt, S. Zhou, K. Potzger, M. Helm, H. HOchmuth, M. Lorentz, A. Setzer, P. Esquinazi, C. Meinecke, M. Grundmann, Appl. Phys. Lett. 92 (2008) 082508. [24] Z.H. Zhang, X. Wang, J.B. Xu, S. Muller, C. Ronning, Q. Li, Nat. Nanotechnol. 4 (2009) 523.
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Comparison of ferromagnetism in n- and p-type magnetic semiconductor thin films of ZnCoO Y.H. Lee a,n, J.C. Lee a, J.F. Min a, C.W. Su b a b
Department of Physics, National Cheng Kung University, No. 1, Ta-Shuei Road, Tainan 70101, Taiwan Department of Applied Physics, National Chiayi University, Chiayi 60004, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 October 2010 Received in revised form 6 February 2011 Available online 15 February 2011
Both n- and p-type diluted magnetic semiconductor ZnCoO are made by magnetron co-sputtering with, respectively, dopants of Al and dual dopants of Al and N. The two sputtering targets are compound ZnCoO with 5% weight of Co and pure metal Al. Sputtering gases for n- and p-type films are pure Ar and N2, respectively. These films are magnetic at room temperature and possess free electron- and holeconcentration of 5.34 1020 and 5.27 1013 cm 3. Only the n-type film exhibits anomalous Hall-effect signals. Magnetic properties of these two types of films are compared and discussed based on measurements of microstructure and magneto-transport properties. & 2011 Elsevier B.V. All rights reserved.
Keywords: Diluted magnetic semiconductor Ferromagnetism Co doped ZnO
1. Introduction Diluted magnetic semiconductor (DMS) based on wide-gap oxides such as ZnO, TiO2, and SnO2 have been reported to show room temperature ferromagnetism. Among these oxides, ferromagnetism of ZnO-DMS has been reported to be dependent on the carrier-polarity i.e. only n-type Co doped ZnO (ZnCoO) and p-type Mn doped ZnO (ZnMnO) are ferromagnetic; in contradistinction, p-type ZnCoO and n-type ZnMnO, are not [1,2]. By assuming the existence of shallow donor defects in the wide-gap oxides, Coey et al. [3] pointed out that high Curie temperature (TC) appears only when electrons associated with shallow donors are partially delocalized onto the magnetic dopants to increase the exchange coupling constants of Jsd or Jpd. However, isovalent Co2 + doping of ZnO does not itself introduce any carriers. Shallow donors in high TC ZnCoO can be invoked through native defects (oxygen vacancy (Ov), zinc interstitial (Zni)), or extrinsic donor-type impurities such as Al and Ga. According to Coey et al. [3], dopant–donor hybridization is then a crucial factor for the intrinsic carrier-mediated ferromagnetism in ZnO-DMS. For substantial hybridization between the magnetic dopant and donor impurity, the two species have to be in proximity in energy levels. Kittilstved et al. [4] further pointed out the factor that matters in the carrier-polarity dependent ferromagnetism is the energy level of the dopant-derived donor- or acceptortype ionization state i.e. one electron or hole transfers, respectively, from donor or acceptor band to Co2 + /Mn2 + ion, which results in, respectively, one electron reduced Co1+ /Mn1 + or oxidized Co3+ /Mn3 + state. It was proved by Kittilstved et al. [4] that the Co1+ /Mn3 + state is
near the conduction/valence band edge (within 0.32/0.42 eV), and the Co3 + /Mn1+ state is rather deep in energy level (1.66/2.68 eV above/ below the valence band edge). Thus, charge transfer from conduction/ valence band to Co2 + /Mn2+ and form Co1 + /Mn3+ ion is more probable, which explains why ferromagnetism exists only in n-type and not in p-type ZnCoO (or in p-type and not in n-type ZnMnO) [4]. Even so, ferromagnetic p-type ZnCoO was reported by several groups [5–7]. However, none provided any information of the holecarriers. We are interested in investigating whether ferromagnetism reported for p-type ZnCoO resulted from hole-carrier mediation. To wit, we specially made p-type ZnCoO films and endeavor to obtain explicit hole-carrier concentration to clarify this problem. Our p-type film was fabricated by using the idea of donor–acceptor (Al–N) codoping method proposed by Yamamoto et al. [8] to help to retain N acceptors and acquire as many hole-carriers as possible. Utilizing magnetron co-sputtering of two targets of ZnCoO compound and pure Al metal, we just change the sputtering gas from Ar to N2, and the film’s conductivity is changed from n- to p-type. X-ray induced photoelectron emission spectroscopy (XPS) for Co atom was taken along the film depth to verify that the ferromagnetism is intrinsic and not the result of extrinsic Co nanoclusters. Atomic substitution of N for O is confirmed by measurements of both XPS and PL (photoluminescence). Finally, we took the field-dependent Hall-effect measurements to obtain anomalous Hall-effect (AHE) information. Together with the results of microscopic characterization, information of carrier-mediated intrinsic ferromagnetism can be deduced.
2. Experiment n
Corresponding author. Tel.: +886 6 2757575; fax: + 886 6 2747995. E-mail addresses: [email protected], [email protected] (Y.H. Lee). 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.011
Both n- and p-type ZnCoO films (ZnCoO:Al and ZnCoO:(Al, N)) were made by rf-magnetron co-sputtering using different
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sputtering gases (Ar for n-type and N2 for p-type) and otherwise under the same conditions. The base pressure of the vacuum system was 5 10 6 Torr. Working pressure and gas flow rate were, respectively, maintained at 10 m Torr and 20 sccm. Sputtering powers of 150 and 30 W were used for two targets of ZnCoO compound with 5% Co and pure metal Al, respectively. During deposition, the quartz substrate was heated to 250 1C and rotated at 70 rpm in order to obtain better thin film crystalline quality and homogeneity. Film thickness is about 250 nm, which was estimated using the sputtering rate prior calibration based on the thin film SEM image. Due to the carrier compensation phenomenon usually reported for p-type ZnO, we subjected thin film ZnCoO:(Al, N) to a short ( 30 min) post-deposition annealing in atmosphere at 450 1C to decrease oxygen vacancies (and thus the number of free electron-carriers) but avoid problematic formation of secondary oxide phases such as Al2O3, CoO. X-ray diffraction (XRD) spectrum and X-ray induced photoelectron emission spectroscopy (XPS) were used to inspect, respectively, the microstructure and the chemical state of each ingredient element of the film. Thin film compositions were also obtained from the analysis of XPS spectra. Optical transmittance in the wavelength range 300–1000 nm was measured by UV–vis spectrophotometer. The Hall measurement system (Ecopia HMS-3000) was used to measure electrical resistivity and carrier concentration. AHE was measured at room temperature by using the electromagnet to provide field between + 8 and 8 kOe. Field- and temperaturedependent magnetizations (M–H and M–T) were measured by superconducting quantum interference device (SQUID) with the field applied perpendicular to the film plane. PL was measured at a low temperature of 12 K using Xe lamp with wavelength of 300 nm as light source.
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also obtained from measurements of XPS, optical transmittance, and PL to be presented below. Results of XPS are shown in Fig. 2. XPS is well known to be sensitive to chemical environment and provides information of the chemical state of an atom in the compound. But it is only sensitive to atoms residing near the film surface. To confirm the chemical state of Co doped in ZnO, we took XPS at every 10 nm thickness of etching. XPS of Co dopant is shown in Fig. 2(a) where the curves correspond to three arbitrary thicknesses (shown near each curve) are displayed for both films. Both experimental data (discrete points) and the fitting curves (solid lines) are exhibited. All curves are similar and show distinctive features of Co–O bonding, which are main peaks of Co 2P3/2 at 780.3 eV and 2P1/2 at 795.8 eV with their satellites, respectively, at 786 and 803 eV [9]. The main peak of 2P3/2 at 778.3 eV for Co–Co bonding,
3. Results and discussions XRD spectra for ZnCoO:Al and ZnCoO:(Al, N) film in Fig. 1 show the feature of a single strong peak, which is commonly observed for polycrystalline ZnO and assigned to (0 0 2) plane of ZnO wurzite structure. For the ZnCoO:Al film, a peak at 2y 63o corresponding to ZnO (1 0 3) plane is hardly discernible, which may have something to do with the proper selection of sputtering conditions such as substrate location and orientation. Results shown in Fig. 1 indicate a good crystallinity with strong c-axis growth direction. It also indicates hexagonal structure of ZnO remains, or atomic replacements occur with doping of Co, Al, and N. Furthermore, no obvious oxide phases of Co, Al, and N are observed. Consistent results are
Fig. 1. XRD spectra for ZnCoO:Al and ZnCoO:(Al, N) film.
Fig. 2. XPS spectra of (a) Co atom at three different thicknesses shown near the curve (b) O atom for ZnCoO:Al and ZnCoO:(Al, N) film. Inset in (b) shows the decomposition of O 1S1/2 for ZnCoO:(Al, N) film.
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observed and assigned to Co clusters by Kaspar et al. [10] in their film, does not appear at all in any spectrum of both of our films. Therefore, the doped Co atoms were certainly present as Co2 + in the cation sites and throughout the whole film, consistent with the above XRD results. This is further evidenced by the optical transmittance spectrum (OPTS, not shown) exhibiting clear three absorption peaks at the wavelengths of 568, 615 and 660 nm corresponding to d–d intra-ionic transition for Co atom in the high spin state Co2 + (d7) [11]. XPS of O 1S1/2 is displayed in Fig. 2(b). The O 1S1/2 peak for ZnCoO:Al film shown in Fig. 2(b) is at 529.3 eV. Upon doping N, i.e. for ZnCoO:(Al, N) film, the peak shifts to slightly higher energy of 539.7 eV and shows a shoulder at 531.3 eV. Since XPS peak position corresponds to the binding energy of the electronic states of atoms, the shift of the peak position indicates the variation of valence charge or chemical state of an atom. When the valence electron moves away from an atom, the atom is said to be oxidized, and the binding energy of core electrons increases due to the reduced screening of Coulomb potential. On the contrary, an atom gaining charge in the valence bond is said to be reduced, and the binding energy of core electrons decreases. In Fig. 2(b), the binding energy (529.3 eV) of O 1S1/2 in ZnCoO:Al film is smaller than that (532 eV) [12] of elemental O, indicating that O atoms are bonded (by capturing the valence electron) with either Zn, Co or Al. In ZnCoO:(Al, N) film, binding energy of O 1S1/2 increases from 529.3 to 529.7 eV signaling the N-dopant actually replaces O at the anion site. The donor–acceptor codoping method proposed to increase N solubility, and thus hole-carrier concentration, is mainly through the formation of acceptor–donor– acceptor complexes. Once 2N at near lattice sites substitute O and form N–Al–N complexes, electrons of Al seized by O are then shared with N. That is, doping N causes a decrease in the number of valence electron of O atom, thus O 1S1/2 binding energy increases. By the same token, the number of valence electron of N increases and the binding energy (398.2 eV) of N 1S1/2 decreases from that of the elemental state (399 eV) [12], as indicated in XPS of N 1S1/2 (not shown). Furthermore, those O atoms in the neighborhood of N atoms feel stronger influence from N atom, leading to a larger shift in binding energy, which results in the shoulder of the O 1S1/2 peak. Following this inference, the peak area under the shoulder should be proportional to the quantity of the doped N atoms. We decomposed the O 1S1/2 peak for ZnCoO:(Al, N) film and show the result in the inset of Fig. 2(b). The area ratio of the curve under the shoulder to the whole curve is 1/5, which is roughly equal to the ratio calculated using the atomic concentration of N and O shown below in Table 1. The XPS peak for Zn 2P3/2 and Al 2P3/2 (not shown) is, respectively, at 1021 and 73 eV for the two films, and is close to the value of Zn and Al element, respectively. It is not yet clear why cations, unlike anions, do not show explicit shifts in binding energy. The atomic concentrations listed in Table 1 for both films are obtained by XPS spectra. For concentrations less than 10%, the value cannot be taken as exact due to the sensitivity limit of such small quantities. But the relative relation between different concentrations is good. For example, concentrations of Co and
Al are similar in both films because same sputtering targets and powers were used. Presumably if dopants of Co and Al were all to reside at Zn sites and N at O sites, the atomic ratio between the group of cations and anions should be close to 1:1. In Table 1, we find that the O concentration is less than the total concentration of Zn, Co, and Al for ZnCoO:Al film. But the total concentration of O and N is similar to that of Zn, Co, and Al for ZnCoO:(Al, N) film. It seems that oxygen vacancies (or Zn interstitials) exist in ZnCoO:Al film but are suppressed in ZnCoO:(Al, N) film. Probably, an extra short period of post-deposition annealing in atmosphere applied to ZnCoO:(Al, N) film is effective in reducing the O deficiency. Therefore, carrier compensation is effectively suppressed and the p-type conduction is obtained in ZnCoO:(Al, N) film as indicated below. Results of electrical properties of carrier concentration, resistivity, and mobility obtained from Hall-effect measurement are also listed in Table 1. The ZnCoO:Al film is conductively n-type semiconductor with free electron concentration of 5.34 1020 cm 3 shown in Table 1. It is an order smaller than the value of 6.17 1021 cm 3 estimated using atomic concentration of Al by assuming that Al properly replaces Zn and donates a free electron. If oxygen deficit mentioned above is assumed in ZnCoO:Al film (which also donate electrons), then about 10% or more of the doped Al atoms do not reside properly at Zn sites as donors. The p-type semiconductor with hole-carrier concentration of 5.27 1013 cm 3 was obtained for ZnCoO:(Al, N) film and shown in Table 1. Taking into account the concentration of electron/hole associated with donors/acceptors is proportional to the concentration of donor/acceptor; and in the higher concentration of N than Al in ZnCoO:(Al, N) film shown in Table 1, electrons are actually over-compensated by holes giving rise to p-type semiconductivity. The information of the existence of the acceptor band due to the doping of N is further confirmed by the results of PL (to be shown below). To the best of our knowledge, the hole-carrier concentration shown in Table 1 is the largest reported so far for p-type magnetic ZnCoO. PL spectra for n- and p-type ZnCoO film are shown in Fig. 3. The curve for n-type film in Fig. 3 shows broad peaks, respectively, at 2.35, 2.7, and 2.95 eV which are similarly observed in p-type film except with much smaller intensities. The structureless broad peak at 2.35 eV is close to the reported ( 2.4 eV) green luminescence (GL) band, which is generally reported for ZnO without intentional doping, and attributed to interstitial Zn [13] or insufficient O [14,15]. GL band is not prominent in p-type film. Peaks at 2.7 and 2.95 eV were not reported for ZnO. Whether they are related with the dopants of Co or Al needs further investigation. Here, we are more interested in the broad peak at 3.22 eV, which exists only in the p-type and not n-type film. It was attributed to donor–acceptor pair (DAP) transition by several research groups [16,17], and the acceptor was identified as unintentionally [16] or intentionally [17] doped N, which forms shallow acceptors with ionization energy of 170 meV. In addition, other N-acceptor related transitions [16,17] such as LO phonon replicas of DAP and acceptor-bound exiton at 3.02–3.15 and 3.315 eV, respectively, are within this broad peak. The crystal quality of p-type film is obviously not good enough to
Table 1 Atomic concentration and electrical properties of carrier density, conduction type, resistivity, and mobility for ZnCoO:Al and ZnCoO:(Al, N) film. Sample
ZnCoO:Al ZnCoO:(Al, N)
Composition (at%)
Electrical properties
Zn
O
Co
Al
N
Carrier density (cm 3)
Conduction type
Resistivity (O cm)
Mobility (cm2/V s)
48.21 39.06
39.87 40.24
5.60 5.78
6.24 6.55
0 8.37
5.34 1020 5.27 1013
n p
1.75 10 3 2.59 102
6.69 2.96
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Fig. 3. PL spectra measured at T¼12 K for ZnCoO:Al and ZnCoO:(Al, N) film. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
produce discrete characteristic peaks. Therefore the existence of a broad peak at 3.22 eV in p-type film signals that N in ZnCoO: (Al, N) film does replace O atoms and a shallow acceptor band is formed. Results of M–H and M–T for both films are displayed in Fig. 4. Both n-type ZnCoO:Al film and p-type ZnCoO:(Al, N) film show hysteretic M–H behavior in Fig. 4(a) and (b), respectively. Thus both films are ferromagnetic. Results of M–T are measured at 4–375 K and shown in Fig. 4(c) and (d), respectively, for ZnCoO:Al and ZnCoO:(Al, N) films. In both Fig. 4(c) and (d), there are two curves of zero-field-cool (zfc, solid symbol) and field-cool (fc, open symbol) measured at the field of 50 Oe. Both films in Fig. 4(c) and (d) exhibit M–T behavior (a fast and then a slow decrease of M with increasing T) that are similar for zfc and fc, but the curves do not coincide. This kind of M–T behavior for both n- and p-type films exhibits no feature of blocking temperature and is not superparamagnetic, according to our previous work [18,19]. Thus, we tried to describe the fc curve using 3-D spin wave model which leads to M(T)¼M0–0.117 mB(kB/2SJd2)3/2, wherein M0 is the zero temperature magnetization, and d and J are, respectively, the spacing and exchange interaction between magnetic ions. With the addition of paramagnetic phase (i.e. M–H/T), we have a good fit for fc curves shown as solid lines in Fig. 4(c) and (d) for both films. The fitting parameters are shown together in the plot. The existence of the paramagnetic phase explains the discrepancy between zfc and fc curves. It is because the paramagnetic phase responds differently at different conditions of zfc and fc. Both n- and p-type films show very similar M–H and M–T behavior. Then, the measurement of AHE is an important step to verify the intrinsic ferromagnetism. For magnetic materials, the Hall resistivity rH( ¼VHt/i, with VH the Hall voltage, i the current, and t the film thickness) is expressed as [20] rH ¼RoB +RaM, wherein Ro is the normal Hall coefficient, Ra the anomalous Hall coefficient, B the magnetic induction, and M the perpendicular magnetization. The results of field-dependent rH for n-type ZnCoO:Al film (shown in the inset of Fig. 4(a)) exhibit the prototype behavior of ordinary plus anomalous Halleffect [21]: linear rH at high field with a small vertical shift at low field. Subtracting the linear ordinary Hall-effect term, we obtain a clear signature that corresponds to the AHE, and this is shown in Fig. 4(a) (symbol). Since AHE signal was also reported [22,23] in non-ferromagnetic DMS, we must be careful and not use AHE
Fig. 4. Field-dependent magnetization (M–H) for (a) ZnCoO:Al and (b) ZnCoO: (Al, N) film. Temperature-dependent magnetization (M–T) of zfc (empty symbol) and fc (solid symbol) curves for (c) ZnCoO:Al and (d) ZnCoO:(Al, N) film.
signal alone as the indicator of intrinsic ferromagnetism. Shinde et al. [22] pointed out this potential error can be avoided by combining the results of detailed microscopic characterizations with AHE signal. Our results of XRD, XPS, and optical transmittance analyzed above all come to a conclusion that most Co dopants reside at Zn site in atomic form and no indication Co
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nanoparticles exist in the n-type film. The clear M-dependent behavior of rH in Fig. 4(a) (symbol) thus characterizes the spin polarized conduction electrons, which mediate the ferromagnetic coupling between Co2 + ions. We are now sure, within the detection limit of XRD and XPS, that the n-type ZnCoO:Al film shows intrinsic ferromagnetism. The existence of intrinsic ferromagnetism in Co doped ZnO nanoparticle is also claimed by Zhang et al. [24], which used EMCD rather than AHE characterization. We have to mention here that the free electron concentration shown in Table 1 for the n-type ZnCoO:Al film is calculated using the slope of rH H at high field where normal Hall-effect dominates. Unfortunately, rH are not obtainable for p-type ZnCoO:(Al, N) film due to its large electrical resistance. The hole-concentration of p-type film (in Table 1) is obtained using commercially made Hall measurement system of model Ecopia HMS-3000. In the p-type film, there is also no evidence of Co nanoclusters (Fig. 2(a)) which is thus excluded as the origin of the ferromagnetism. The ferromagnetism is also not attributed to the free hole-carrier mediation, even though N replaces O and forms shallow acceptor band as indicated by the results of XPS and PL, because the probability of charge-transfer between the magnetic cation and the acceptor band is small if we follow the result reported by Kittilstved et al. that Co3 + is not near the valence band edge. Nevertheless, charge carriers are not needed to be conductive to mediate the ferromagnetic exchange interaction, as pointed out by both Coey et al. [3] and Kittilstved et al. [4]. We believe that donor electrons of Al become relatively immobile upon N doping. The hydrogenic radius (rH) of these electrons is large enough to cover at least two magnetic cations (estimated [3] by (4/3)pr3Hxno with rH ¼0.76 nm, x¼ 5.60% (magnetic ion concentration), and no ¼6 1022 cm 3 (oxygen density)) and forms the so called bound magnetic polaron (BMP). It is the overlap between BMPs throughout the film that results in the ferromagnetism in p-type film. Therefore, ferromagnetic order in p-type ZnCoO:(Al, N) film is not from the mediation of free hole-carriers but from that of nonconductive donor electrons.
4. Conclusions n-type ZnCoO:Al and p-type ZnCoO:(Al, N) films, respectively, with electron concentration of 5.34 1020 cm 3 and hole-concentration of 5.27 1013 cm 3, were fabricated. Both n- and p-type ZnCoO films are ferromagnetic. Signal of AHE is clearly observed only for n-type film, but is not measurable for p-type film due to its large resistance. Ferromagnetic exchange coupling between magnetic ions in the n-type film is ascertained to be
through spin polarized free electrons. Ferromagnetism in p-type film is not attributed to free hole-carriers mediation, but to the overlap of BMP which is the bound state of magnetic ions and immobile donor electrons from N doping.
Acknowledgment The authors would like to acknowledge the financial support of their research by the National Science Council under Grant nos. NSC98-2112-M-006-012 and NSC99-2112-M-006-016. References [1] K.R. Kittilstved, N.S. Norberg, D.R. Gamelin, Phys. Rev. Lett. 94 (2005) 147209. [2] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, Y. Liu, J. Appl. Phys. 101 (2007) 063918. [3] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173. [4] K.R. Kittilstved, W.K. Liu, D.R. Gamelin, Nat. Mater. 5 (2006) 291. [5] H.-T. Lin, T.-S. Chin, J.-C. Shih, S.-H. Lin, T.-M. Huang, F.-R. Chen, J.-J. Kai, Appl. Phys. Lett. 85 (2004) 621. [6] M.H.F. Sluiter, Y. Kawazoe, P. Sharma, A. Inoue, A.R. Raju, C. Rout, U.V. Waghmare, Phys. Rev. Lett. 94 (2005) 187204. [7] P. Cao, D.X. Zhao, J.Y. Zhang, D.Z. Shen, Y.M. Lu, Z.W. Fan, X.H. Wang, Physica B 392 (2007) 255. [8] T. Yamamoto, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 38 (1999) L166. [9] L. Daheron, R. Dedryvere, H. Martinez, M. Menetrier, C. Denage, C. Delmas, D. Gonbeau, Chem. Mater. 20 (2008) 583. [10] T.C. Kaspar, T. Droubay, S.M. Heald, M.H. Engelhard, P. Nachimuthu, S.A. Chambers, Phys. Rev. B 77 (2008) 201303 R. [11] Y.Z. Yoo, T. Fujumura, Z. Jin, K. Hasegawa, M. Kawasaki, P. Ahmet, T. Chikyow, H. Koinuma, J. Appl. Phys. 90 (2001) 4246. [12] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray photoelectron spectroscopy, in: J. Chastain, R.C. King Jr (Eds.), Physical Electronics, 1995, pp. 42–88. [13] N.O. Korsunska, L.V. Borkovska, B.M. Bulakh, L.Yu. Khomenkova, V.I. Jushnirenko, I.V. Markevish, J. Lumin. 102–103 (2003) 733. [14] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [15] F.H. Leiter, H.R. Alves, N.G. Romanov, D.M. Hoffmann, B.K. Meyer, Physica B 340–342 (2003) 201. [16] K. Thonke, Th. Gruber, N. Teofilov, R. Schonfelder, A. waag, R. Sauer, Physica B 308–310 (2001) 945. [17] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [18] Y. Lee, J.C. Lee, C.W. Su, IEEE Trans. Magn. 46 (2010) 1565. [19] Y.H. Lee, T.C. Han, J.C.A. Huang, J. Appl. Phys. 93 (2003) 8462. [20] H. Ohno, J. Magn. Magn. Mater. 200 (1999) 110. [21] H. Toyosaki, T. Fukumura, Y. Yamada, K. Nakajima, T. Chikyow, T. Hasegawa, H. Koinuma, M. Kawasaki, Nat. Mater. 3 (2004) 221. [22] S.R. Shinde, S.B. Ogale, J.S. Higgins, H. Zheng, A.J. Millis, V.N. Kulkarni, R. Ramesh, R.L. Greene, T. Venkatesan, Phys. Rev. Lett. 92 (2004) 166601. [23] Q. Zu, H. Schmidt, S. Zhou, K. Potzger, M. Helm, H. HOchmuth, M. Lorentz, A. Setzer, P. Esquinazi, C. Meinecke, M. Grundmann, Appl. Phys. Lett. 92 (2008) 082508. [24] Z.H. Zhang, X. Wang, J.B. Xu, S. Muller, C. Ronning, Q. Li, Nat. Nanotechnol. 4 (2009) 523.