Ferromagnetism in laser ablated ZnO and Mn-doped ZnO thin films: A comparative study from magnetization and Hall effect measurements

ARTICLE IN PRESS Physica B 404 (2009) 3978–3981

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Ferromagnetism in laser ablated ZnO and Mn-doped ZnO thin films: A comparative study from magnetization and Hall effect measurements Nguyen Hoa Hong a,, Ekaterina Chikoidze b, Yves Dumont b a b

Laboratoire LEMA, UMR 6157 CNRS - Universite F. Rabelais, Parc de Grandmont, 37200 Tours, France Laboratoire GeMAC, UMR 8635 CNRS - Universite de Versailles, Place A. Briand, 92195 Meudon, France

a r t i c l e in f o

a b s t r a c t

Article history: Received 16 March 2009 Received in revised form 9 July 2009 Accepted 10 July 2009

Room temperature FM was observed in pristine ZnO thin films grown by pulsed laser deposition on Al2O3 substrates. It seems to originate from other defects but not oxygen vacancies. Magnetization of thinner films is much larger than that of the thicker films, indicating that defects are mostly located at the surface and/or the interface between the film and the substrate. Data on the Fe:ZnO and Mn:ZnO films show that a transition-metal doping does not play any essential role in introducing the magnetism into ZnO. In the case of Mn doping, the magnetic moment could be very slightly enhanced. Hall effect measurements reveal that an incorporation of Mn does not change the carrier type, but decreases the carrier concentration, and increases the Hall mobility, resulting in more resistive Mn:ZnO films. Since no anomalous Hall effect was observed, it is understood that the observed FM is not due to the interaction between the free-carrier and the Mn impurity. & 2009 Elsevier B.V. All rights reserved.

Keywords: Magnetic oxides Hall effects Semiconductors Thin films

1. Introduction In 2000, Dietl et al. theoretically predicted that high temperature ferromagnetism (FM) can be obtained in ZnO, GaN, etc., if one dopes 5% of Mn into those systems, and make them p-type with a carrier concentration of about 3.5  1020 cm3. The magnetic ordering in those semiconductors was supposed to originate from the Ruderman–Kittel–Katsuya–Yoshida (RKKY) interaction via delocalized charge carriers [1]. Following this direction, experimentalists have tried to incorporate transition-metals (TM) into many semiconducting oxides such as ZnO, TiO2, SnO2, In2O3 [2]. Room temperature FM was observed in thin films grown by some special methods [3], and not in samples prepared by some other techniques [4–6]. The origin of the observed FM remains unclear, since most of these doped compounds are found to be n-type. In 2004, the first observation of FM in pristine HfO2 thin films has urged researchers in the field to re-judge the real role that a dopant can play in tailoring the magnetic properties of semiconducting and insulating oxides [7]. Recently, experimental results of various groups have confirmed that magnetism would certainly be observed in pristine oxides [8–12]. As for the TiO2, HfO2 and SnO2 films, it was shown that the FM is most probably due to oxygen vacancies. The assumption for FM due to oxygen vacancies/defects in TiO2 thin films is confirmed by the X-ray

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E-mail address: [email protected] (N.H. Hong). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.07.142

magnetic circular dichroism (XMCD) measurements [13]. Concerning ZnO system, some groups have also reported that defects could tune the FM [14,15], and having more oxygen could degrade the magnetic ordering [16]. Since ZnO is a potential semiconductor for applications in spin and opto-electronics, it is interesting to know whether the pristine ZnO can be ferromagnetic or not, and what should be the mechanism that governs it? Additionally, we aim to verify the role of a transition-metal doping. In order to elucidate these issues, we have investigated the magnetic and transport properties of pristine ZnO, Fe- and Mn- doped ZnO thin films.

2. Experiment ZnO, Fe0.01Zn0.99O and Mn0.01Zn0.99O thin films were deposited by a pulsed-laser deposition (PLD) system (KrF-248 nm, 5 Hz, 2 J/ cm2) from ceramic targets on (0 0 0 1) Al2O3 substrates. As for ZnO and Mn-doped ZnO, the substrate temperature was kept constantly at 650 1C and an oxygen partial pressure ðPO2 Þ of 106 Torr was used during deposition. As for Fe-doped ZnO, the substrate temperature was 600 1C and PO2 was 5  106 Torr. For ZnO, films with different thickness such as 10, 50, 375 nm, were made. As for Fe0.01Zn0.99O and Mn0.01Zn0.99O films, the thickness is 306 and 400 nm, respectively. Magnetic moment (M) was measured by a SQUID magnetometer in the range of temperature (T) from 5 up to 400 K and magnetic field (H) from 0 to 0.5 T. H was applied parallel to the film plane. Transport properties have been studied by temperature dependent Hall effect set-up in Van der

ARTICLE IN PRESS N.H. Hong et al. / Physica B 404 (2009) 3978–3981

Pauw configuration. Resistivity and carrier concentration were measured in the temperature range of 5–300 K.

3. Results and discussions Magnetization versus magnetic field at 300 K for a 375 nm-thick as-grown and post-annealed ZnO, Fe-doped ZnO, and Mn-doped ZnO films is shown in Fig. 1(a). One can see that the pristine ZnO film is room temperature ferromagnetic. However, the saturated magnetization (Ms) of ZnO films is rather modest (about 1 order smaller than that of TiO2 and HfO2 films with the same thickness [8]). The observed FM in ZnO films is surprising, since neither Zn2+ nor O2 is magnetic. Different from the cases of TiO2, HfO2 and SnO2 films, where one could find a strong relation between oxygen vacancies and magnitude of magnetic moment, the oxygen post-annealing does not give any obvious effect on the magnetism of the undoped ZnO films. From Fig. 1(a), one also can see that after annealing in an O2 atmosphere for 10 h, the magnetic moment of the undoped ZnO films does not change. This suggests that the magnetism in ZnO films is governed

Magnetization (emu/cm3)

10 ZnO O2 annealed ZnO MnZnO FeZnO

5

0

-5

-10 -0.4

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200

375 nm 10 nm 50 nm

100

0

-100

-200

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Field (T) Fig. 1. Magnetization versus magnetic field taken at 300 K for (a) as-grown ZnO, O2 annealed ZnO, Fe-doped ZnO, and Mn-doped ZnO films, and (b) ZnO films with various thicknesses. Field was applied parallel to the film plane.

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by another mechanism, and nothing is directly related to oxygen vacancies. This assumption is in agreement with a theoretical model for FM due to oxygen vacancies in oxide films that concluded that the case of pristine ZnO is completely different from that of TiO2, HfO2, or In2O3 [17]. Most probably for laser ablated films, by depositing on a substrate under some specific conditions, new defects or/and impurities are induced, so that it results in the observed FM. In order to verify the real role of magnetic impurities, we have investigated two examples of doping: Fe- and Mn-doped ZnO films. From Fig. 1(a), one can see that the Fe0.01Zn0.99O film has the same magnetization as that of the pristine ZnO film. We must say that the Fe doping indeed does not play any important role in tailoring the magnetic properties of the ZnO host. Concerning Mn doping, it is known that the case should be more complicated. Some theoretical work suggested that doping Mn alone could not result in any FM in n-type ZnO, and in order to obtain FM, one must co-doped Mn with Cu in order to have additional carriers [18]. However, experimentally, it was shown that doping with 10% of Mn alone in ZnO films could result in FM due to oxygen vacancies that were formed during the film growth [19]. In this work, we investigated the case of doping with only 1% of Mn. The M(H) curve for a Mn0.01Zn0.99O film taken at 300 K was shown in Fig. 1(a). The film is obviously ferromagnetic at room temperature with the Ms of about 7 emu/cm3, which is about 3 times larger than that of the pristine ZnO films. Different from the Fe doping case, doping with 1% of Mn can certainly enhance the magnetic moment of the ZnO host (i.e. Mn doping brings some additional contribution). The assumption for a different mechanism that Mn doping can bring into ZnO is supported by the different behavior of its anisotropy. When the magnetic field was applied perpendicular to the film plane, the Mn-doped ZnO film has a smaller magnetic moment, but certainly it is still well ferromagnetic (not diamagnetic as in the ZnO and Fe-doped ZnO cases) [3]. In Mn-doped ZnO films, there must be two different sources for magnetism: a major one comes from the Mn situated on the Zn sites of the ZnO host, and another one related to the ZnO matrix with undetermined defects. From Fig. 1(b), one also should notice that very thin films of ZnO (i.e. 10–50 nm-thick) are room temperature ferromagnetic, and they have the magnetization of about 2 orders of magnitude larger than that of the 375 nm-thick one. It implies that if the magnetism in ZnO films comes from defects, then those defects must be mostly located near the interface between the film and the substrate, or at the film’s surface. Therefore, the observed FM is kind of ‘‘skin effect’’ but not any bulk’s property. This is very similar to what observed for TiO2 and HfO2 films [8,9]. To verify the relation between magnetization and spin carrier polarization, anomalous Hall effect (AHE) measurements should be performed. Viewing the two cases of Fe and Mn dopings, one may expect to see some difference in the carriers in those compounds. Hall effect and resistivity measurements, therefore, were carried out to correlate transport properties and the observed FM. All the undoped ZnO, Fe and Mn-doped ZnO films are n-type. At room temperature, while in the case of Fe-doped ZnO film, resistivity, electron concentration and mobility (r ¼ 1.88  101 O cm, n ¼ 2.43  1018 cm3 and m ¼ 13.7 cm2 V1 s1) are found not much different from those of the pure ZnO film (r ¼ 7.58  101 O cm, n ¼ 8.25  1018 cm3 and m ¼ 15.1 cm2 V1 s1), as for Zn0.99Mn0.01O, they are very different with r ¼ 4  101 O cm, n ¼ 9.2  1017 cm3 and m ¼ 29.3 cm2 V1 s1, indicating that Mn doping makes the carriers decreased while it increases the resistivity and Hall mobility. Since Fe does not seem to be electrically active in ZnO, and behaves like Fe2+, it is surprising that the Mn is electrically active regarding to ZnO, and

ARTICLE IN PRESS N.H. Hong et al. / Physica B 404 (2009) 3978–3981

43.5 300 41.4

180

120

100

50

Ln (n)

ZnMnO

40.8 40.6 42.5

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ZnO

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1/T (K-1) Fig. 3. n-Type carrier density for the ZnO and Mn-doped ZnO (inset) films as a function of inverse temperature.

20 ZnO

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not Zn2+-isovalent. To study transport properties more profoundly (concerning carrier concentrations and scattering of electrons), we performed temperature dependence measurements. Temperature dependence of resistivity is different for pristine ZnO and Zn0.99Mn0.01O films (Fig. 2). At low temperatures, resistivity of ZnO does not change significantly, showing the behavior of semiconductor, which is close to degeneration, while for Zn-doped MnO in the same temperature region, the change of resistivity is more pronounced, i.e. Zn0.99Mn0.01O is still far from degenerated state. From the temperature dependence of carrier concentration (Fig. 3), activation energy for the centers that is responsible for conductivity has been determined as of 9 meV (very shallow) for ZnO film, and 24 meV for Zn0.99Mn0.01O. As was reported previously, activation energy as of 15 meV was found for thermo-dynamical MOCVD Zn0.098Mn0.02O samples [20]. Hall mobility curves are shown in Fig. 4(a, b). It is obvious that for both ZnO and Zn0.99Mn0.01O, the mobility decreases with temperature in all studied intervals (8–300 K). For both samples, ionized impurity scattering is the dominant scattering mechanism. Similar behavior was observed for Mn (x) ¼ 0.12 and 0.17 thin films, which can be explained by a high concentration of impurity centers introduced by manganese in ZnO matrix [20]. In Zn-doped MnO nanowires, where similar Hall mobility temperature dependence in high temperature region was obtained, it was attributed to the degeneration of that semiconductor [21]. Usually for undoped ZnO at high temperature region, Hall mobility decreases as temperature increases, showing a dominant scattering by polar optical phonons [20]. It is surprising that we do not observe this kind of scattering in the presented ZnO thin films. Moreover, the maximum of the m(T) curve for pristine ZnO appears at a higher temperature than that for Zn0.99Mn0.01O, indicating that the undoped ZnO has scattering centers with a higher concentration. This fact deserves further investigation to determine both scattering centers and how the Mn can be electrically active in these films. Anomalous Hall effect was not observed either for the Mn-doped ZnO film (Fig. 5), or for the pristine ZnO. The Hall voltage is linearly proportional to magnetic field, implying that the observed FM is not due to the interaction between the free-carrier and the spin moment [22].

Hall mobility (V-1S-1cm2)

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150 T (K)

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30 ZnMnO

Hall mobility (V-1S-1cm2)

ρ (ohm.cm)

101

100 ZnO

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ZnMnO

20

15

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5 0.00

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1/T (K) Fig. 2. Resistivity for the ZnO and Mn-doped ZnO films as a function of the inverse temperature.

0

50

100

150 T (K)

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250

300

Fig. 4. Temperature dependence of Hall mobility for (a) the ZnO and (b) Zn0.99Mn0.01O films.

ARTICLE IN PRESS N.H. Hong et al. / Physica B 404 (2009) 3978–3981

almost degenerated. Mn doping seems to be electrically active. Further investigations on the nature of these almost degenerated donors in pristine ZnO and Mn-doped ZnO are needed. Since no anomalous Hall effect has been observed, it is understood that the observed FM does not relate to any interaction between the freecarrier and the spin moment.

0.0000

Mn-doped ZnO

-0.0001

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T = 10 K

VH (V)

-0.0002

References -0.0003 [1] [2] [3] [4]

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[6]

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0.2

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0.4

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[7] [8] [9]

Fig. 5. Hall voltage versus magnetic field taken at 10 K for the Zn0.99Mn0.01O film.

[10]

4. Conclusion

[11] [12] [13]

In conclusion, room temperature FM was observed by SQUID magnetometer in pristine ZnO and Mn:ZnO and Fe:ZnO thin films. It does not seem to stem from oxygen vacancies but other defects that are mostly located at the surface and/or the interface between the film and the substrate. Transition-metal doping does not seem to play any important role in introducing the FM into the ZnO host. However, when doping Mn, the magnetic moment could be slightly enhanced. Hall effect measurements show that the Mn doping does not change the carrier type, but decreases the carrier concentration. It also increases the resistivity and the Hall mobility. Thin films exhibit a very large scattering of electrons (from the temperature dependence of mobility), while they are

[14] [15] [16] [17]

[18] [19] [20] [21] [22]

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