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Room-temperature ferromagnetism in un-doped ZrO2 thin films
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 J. Phys. D: Appl. Phys. 46 445004 (http://iopscience.iop.org/0022-3727/46/44/445004) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 46 (2013) 445004 (5pp)
doi:10.1088/0022-3727/46/44/445004
Room-temperature ferromagnetism in un-doped ZrO2 thin films Shuai Ning, Peng Zhan, Qian Xie, Zhengcao Li and Zhengjun Zhang State Key laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China E-mail:
[email protected]
Received 9 June 2013, in final form 28 July 2013 Published 17 October 2013 Online at stacks.iop.org/JPhysD/46/445004 Abstract Room-temperature ferromagnetism was observed in un-doped ZrO2 thin films prepared by pulsed electron beam deposition on silicon substrates. In the obtained films, monoclinic and tetragonal phases were both stabilized, with a ratio depending on the oxygen partial pressure during deposition. The ferromagnetism was positively related to the tetragonal phase content, and had a positive linear relationship with the photoluminescence (PL) intensity of emission centred at ∼420 nm related to the singly ionized oxygen vacancies, as revealed further by the PL analysis. This study suggested that the oxygen vacancies, which stabilized the tetragonal phase at room-temperature, should be the origin of ferromagnetism in un-doped ZrO2 films. (Some figures may appear in colour only in the online journal)
reported that ferromagnetism was not achieved in Mn- and Fe-doped cubic zirconia nanoparticles at room-temperature and 5 K. Srivastava et al [18] reported that no long range ferromagnetic order was observed in Mn-doped tetragonal zirconia prepared by solid state method. Zippel et al [19], however, reported that RTF was obtained in both Mn-doped and un-doped ZrO2 thin films prepared by pulsed laser deposition and attributed this ferromagnetism to the structure defects in the films with different phase structures. Wang et al [20] also reported the observation of RTF in ZrO2 nanopowders and attributed the RTF to Zr-related vacancies. Thus it is of great interest to make clear whether the un-doped ZrO2 could be ferromagnetic and clarify the origin of the RTF in zirconia. In this study, we report on the synthesis of un-doped ZrO2 films obtained by pulsed electron beam deposition that crystalize in both monoclinic and tetragonal symmetry. Content of the tetragonal phase can be adjusted by controlling the oxygen partial pressure during deposition. The RTF observed in these films seems to be induced by oxygen vacancies produced to stabilize the tetragonal phase at roomtemperature.
1. Introduction In recent years, diluted magnetic semiconductors (DMS) and diluted magnetic oxides (DMO) have attracted great attention due to the potential applications in spintronic devices tuning simultaneously charge and spin of electrons [1]. Previous studies demonstrated that doping semiconductors and some wide band-gap oxides by transition metals or other elements could create room-temperature ferromagnetism (RTF) [2–5]. Nevertheless, the origin of the ferromagnetism especially in those oxides still remains controversial [6]. With the observation of RTF in the un-doped oxide semiconductors and insulators [7], e.g., ZnO [8], TiO2 [9] and HfO2 [10], it is believed that various defects play an important role in inducing ferromagnetism in these materials [3]. However, even for the same material such as ZnO, different defects like oxygen vacancies [11], cation vacancies [12], cation interstitials [13], etc, have been recognized to be responsible for the induced ferromagnetism. Therefore, it is of importance and necessity to clarify the origin of RTF in the DMOs. Zirconia is a material of particular interest due to its wide applications in catalytic supports, oxygen gas sensors, solid oxide fuel cells [14] and metal-oxide semiconductor devices [15]. It has been reported recently, by theoretical calculations, that Mn-stabilized cubic zirconia could be ferromagnetic above room-temperature [16]. Interestingly, experimental studies reported controversial results. For instance, Yu et al [17] 0022-3727/13/445004+05$33.00
2. Experimental Un-doped ZrO2 thin films with a thickness of ∼100 nm were deposited on 1 0 0 oriented silicon substrates by ablating a 1
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J. Phys. D: Appl. Phys. 46 (2013) 445004
S Ning et al
Figure 1. XRD patterns of ZrO2 thin films deposited at different oxygen partial pressures.
ZrO2 target with a purity of 99.99% in the pulsed electron beam deposition (PED) system (Neocera PED180). During deposition, the discharge voltage and pulse frequency were fixed at 18.3 kV and 4 Hz, respectively, while the substrate temperature was fixed at 500 ◦ C and the oxygen partial pressure during the deposition varied in the range of 9 mTorr to 12 mTorr. The structural measurements were performed by x-ray diffraction (XRD) using a Rigaku D/max 2500 PC diffractometer at the θ –2θ scan mode with a 2θ scan step of 0.02◦ with the Cu Kα (λ = 0.1054 nm) radiation. The microstructures of the samples were investigated by transmission electron microscope (TEM) using a JEOL JEM 2011 TEM working at 200 kV. The chemical states and compositions were analysed by x-ray photoelectron spectroscopy (XPS) using an ESCALAB 250Xi. The elements content was analysed by induced-coupled-plasma atomic emission spectrum (ICP-AES) using an ELAN DRC-e ICP-AES system. The room-temperature magnetic properties of these films were measured by MPMS XL-7 superconducting quantum interference device (SQUID) magnetometer, with the magnetic field direction applied parallel to the surface of the samples. The room-temperature optical properties of the films were analysed by photoluminescence (PL) measurement using a LabRamAramis Raman Spectrometer and exciting samples by a 325 nm He–Cd laser. During the whole procedure of experiments and operations, nonmagnetic tweezers, fixtures and containers were used to avoid unintentional ferromagnetic contamination.
Figure 2. (a) A cross-sectional HRTEM image of the columnar ZrO2 film on silicon substrate deposited at an oxygen partial pressure of 12 mTorr; (b) is an enlarged HRTEM image of the interface area marked by a rectangle in (a) and (c) is an enlarged HRTEM image from an area marked by a circle in (b), showing the coexistence of the monoclinic and the tetragonal phases in the film.
higher oxygen partial pressures, i.e. 10, 11 and 12 mTorr, the content of the room-temperature monoclinic phase decreased, while the content of the high-temperature tetragonal phase increased with increasing the oxygen partial pressure during deposition. In addition, the peaks of XRD broaden slightly with the oxygen partial pressure increasing from 9 to 12 mTorr. It may result from the difference of grain size and crystalline between samples obtained at different conditions. Figure 2(a) shows a typical cross-sectional highresolution TEM (HRTEM) image of the ZrO2 film deposited on silicon at an oxygen partial pressure of 12 mTorr. Figure 2(b) shows an enlarged HRTEM image of the interface between the ZrO2 thin film and the substrate, i.e. the area marked by a rectangle shown in figure 2(a). Figure 2(c) shows an enlarged HRTEM image of the ZrO2 film, from the area marked by a circle shown in figure 2(b). From the HRTEM images one sees that the ZrO2 film is of columnar structure with a thickness of ∼100 nm, and that the interface between the film and the substrate is sharp and the thickness of the interfacial SiO2 layer is ∼2.5 nm. It is observed from figure 2(c)
3. Results and discussion Figure 1 shows XRD patterns of un-doped ZrO2 thin films deposited at different oxygen partial pressures. From the XRD patterns one observes that all ZrO2 films consisted of both the room-temperature monoclinic phase (stable below 1400 K) and the high-temperature tetragonal phase (stable within 1400– 2570 K) [21]. It is also observed that in the ZrO2 film deposited at an oxygen partial pressure of 9 mTorr, the room-temperature monoclinic phase was dominant. For ZrO2 films deposited at 2
J. Phys. D: Appl. Phys. 46 (2013) 445004
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of the tetragonal phase. For example, it was ∼0.04 emu g−1 in the ZrO2 film deposited at 9 mTorr in which the monoclinic phase was dominant, and increased to ∼0.69 emu g−1 in the ZrO2 film deposited at 12 mTorr, in which the content of the tetragonal was the highest. This positive dependence suggests that the RTF observed here might be due to the existence of the tetragonal phase in ZrO2 films, which was stabilized by the oxygen vacancies generated in the films during deposition [25]. To make sure whether this RTF was related to oxygen vacancies generated in the ZrO2 films, we carried out roomtemperature PL measurements [26] for the four samples. Figures 4(a)–(d) shows the PL spectra of the ZrO2 films deposited at an oxygen partial pressure of 9, 10, 11 and 12 mTorr, respectively. All the PL spectra can be well fitted by two Gaussian peaks centred at ∼420 nm (peak I) and ∼520 nm (peak II), respectively. With increasing the oxygen partial pressure during deposition, as seen from figures 1 and 4, the content of the tetragonal phase in the films and the intensity of peak I increased, while the content of the monoclinic phase and the intensity of peak II decreased. It has been reported in the literature that two types of intrinsic defects could be responsible for the PL of un-doped ZrO2 within the 400–600 nm range [27]. One is the singly ionized + ) in the tetragonal phase with an emission oxygen vacancy (VO around peak I; the other involves trivalent cations (Zr3+ or other doping cations) whose emission was around peak II [28–30]. From the above observations it is clear that the RTF observed here could not have originated from the defects responsible for the PL emission around peak II, and could not be related to the monoclinic phase. It is suggested that this RTF might be induced by the singly ionized oxygen vacancies in the tetragonal ZrO2 phase, which are responsible for the PL emission around peak I. To make clear the relationship of the RTF and the singly ionized oxygen vacancies in the ZrO2 tetragonal phase, we plotted the saturation magnetization of the as-deposited ZrO2 films versus the PL intensity of peak I (centred at ∼420 nm) in figure 5. It is seen from the figure that there exists a good linear relationship between the Ms value and the PL intensity of the emission centred at ∼420 nm. This suggests that the singly ionized oxygen vacancies in the ZrO2 tetragonal phase (stabilized by oxygen vacancies generated during the film deposition) should be the origin of the RTF observed here in these un-doped ZrO2 thin films.
Figure 3. (a) M–H loops of ZrO2 thin films on silicon substrates deposited at various oxygen partial pressures; inset shows the enlarged hysteresis loop of the ZrO2 film deposited at 11 mTorr, with a coercivity of ∼100 Oe; (b) shows a typical XPS spectrum of the ZrO2 films; inset is an enlarged part in the vicinity of the Fe 2p core-level emission.
that the room-temperature monoclinic phase and the hightemperature tetragonal phase coexisted in the columnar ZrO2 film, which is in good agreement with the XRD analysis shown by figure 1. The existence of the tetragonal phase at roomtemperature, as reported in literature, was probably stabilized by the oxygen vacancies generated in the films during the deposition process [22, 23]. Figure 3(a) shows the magnetic hysteresis loops of the ZrO2 thin films as-deposited at various oxygen partial pressures, which was normalized to their mass. It is seen that all ZrO2 films exhibited RTF. The inset of figure 3(a) shows an enlargement of the hysteresis loop of the ZrO2 film deposited at an oxygen partial pressure of 11 mTorr, from which one observes clearly a coercivity of ∼100 Oe. Figure 3(b) shows typical XPS spectrum of the un-doped ZrO2 films and inset is an enlargement of the vicinity of the Fe 2p core-level emission, from which one sees that contamination could not be the origin of the RTF observed in these un-doped ZrO2 thin films [24]. The saturated magnetization (Ms ) of the films, as shown by figure 3(a), was found to be positively dependent on the oxygen partial pressure during deposition or on the content
4. Conclusions In short, RTF was observed in un-doped ZrO2 films deposited on silicon substrates by PED at different oxygen partial pressures, where coexistence of the room-temperature monoclinic phase and the high-temperature tetragonal phase was observed. This RTF was positively related to the content of the tetragonal phase, and the PL emission intensity from the oxygen vacancies that stabilized the tetragonal phase at room-temperature. These suggest that oxygen vacancies in the tetragonal phase should be the origin of RTF observed here in un-doped ZrO2 thin films. 3
J. Phys. D: Appl. Phys. 46 (2013) 445004
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Figure 4. Room-temperature PL spectra of the ZrO2 films deposited at various oxygen partial pressures: (a) 9 mTorr, (b) 10 mTorr, (c) 11 mTorr and (d) 12 mTorr, respectively.
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Figure 5. Linear relationship between the saturation magnetization of ZrO2 films deposited at various oxygen partial pressures and the integrated intensity of the PL emission centred at ∼420 nm attributed to the singly ionized oxygen vacancies in the tetragonal phase.
Acknowledgments The authors are grateful for the financial support by the National Natural Science Foundation of China (Grant Nos 51072094 and 51372135) and the Tsinghua University Initiative Scientific Research Program. 4
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