Phase-dependent and defect-driven d⁰ ferromagnetism in undoped ZrO₂ thin films

Abstract

Undoped ZrO₂ thin films are prepared on 〈100〉 Si substrates by reactive DC magnetron sputtering using a Zr target. By controlling the oxygen partial pressure during the deposition process, we can successfully control the phase structure of the as-deposited film, which can be tetragonal, monoclinic or a mixture of them. A magnetic property measurement reveals that phase-dependent d⁰ ferromagnetism exists in ZrO₂ thin films. Specifically, only tetragonal ZrO₂ thin films can be room-temperature ferromagnetic. Photoluminescence measurements, X-ray photoelectron spectroscopy analyses and post thermal annealing experiments suggest the d⁰ ferromagnetism in undoped tetragonal ZrO₂ films is mainly driven by oxygen vacancies.

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Introduction

Over the last decade, dilute magnetic semiconductors (DMSs) have developed into an important field due to their potential applications in spintronic devices, where considerable comprehensive research has been simulated.¹ In particular, the observation of spontaneous magnetization above room-temperature (RT) in a series of dilute magnetic oxides (DMOs) such as HfO₂,² ZnO,³ Al₂O₃,⁴ etc., containing no transition-metal impurities has led to tremendous research interest in clarifying the origin and mechanism of this d⁰ ferromagnetism. It is generally recognized that defects in these materials may play an important role in the room-temperature ferromagnetism (RTFM) of the non-magnetic oxides.^5–7 However, an abundance of contradicting views on the origin of the RTFM in these materials have been reported both theoretically and experimentally,^8–11 making it a most controversial research topic that requires definitely further detailed investigations.

Zirconia, as one of the most important ceramic materials with widespread applications, was predicted theoretically to be a new DMO candidate,¹² and has attracted great attention on its RTFM. A number of theoretical calculations and experimental studies have shown that doping,^13,14 phase structure^15,16 and intrinsic point defects^17,18 are all likely to influence its RTFM. As doping could create a large number of point defects¹⁹ and modify significantly the electronic structure of the host materials, it is hard to make clear, at the presence of dopants, whether intrinsic or extrinsic defects dominate the RTFM of ZrO₂. Therefore, it is of importance to clarify the origin of RTFM in undoped ZrO₂ for the development of ZrO₂ based dilute magnetic materials.

ZrO₂ has three crystalline structures, namely the monoclinic, tetragonal and cubic phases. The monoclinic phase (space group P2₁/c) is stable thermodynamically at RT. It transforms into the tetragonal phase (space group P4₂/nmc) at a temperature of ∼1400 K, and into the cubic phase (space group Fm3m) at ∼2570 K.²⁰ Thus it is a big challenge to obtain the high temperature phases at RT. The studies on doping ZrO₂ with trivalent or divalent cations suggest that the creation of enough oxygen vacancies is helpful to stabilize the high temperature phase at RT.²¹ We therefore selected reactive DC magnetron sputtering technique to prepare ZrO₂ thin films to control the phase structure for the investigation of the details of RTFM in undoped zirconia.

In this work, we utilized reactive DC magnetron sputtering to prepare undoped ZrO₂ thin films, whose phase structure could be monoclinic, tetragonal or a mixture of them by adjusting the oxygen partial pressure during deposition process. We have also performed thermal annealing treatment to modulate the phase structure and defect level on as-deposited films. Taking into account of structure analysis, defect characterization and magnetic property measurement, we found a phase-dependent room-temperature d⁰ ferromagnetism existing in undoped ZrO₂ thin films, and convinced that the ferromagnetism in tetragonal ZrO₂ films was mainly driven by oxygen vacancy.

Experiment

ZrO₂ films were deposited on 〈100〉 silicon substrates at 500 °C by reactive DC magnetron sputtering in a vacuum chamber with a base vacuum level better than 2 × 10⁻⁴ Pa. The deposition was realized by sputtering a pure zirconium target (99.99%) in a gas stream of argon (99.999%) and oxygen (99.995%) at a flow rate of 20 SCCM. During deposition, the chamber pressure was ∼1.0 Pa, and the oxygen/argon flow rate ratio was 1/19, 2/18, 2.5/17.5, 3/17 and 4/16, respectively.

The structure of samples was characterized by X-ray diffraction (XRD, Rigaku D/max 2500 PC) using Cu Kα (λ = 0.154 nm) radiation, at the θ–2θ coupled mode with a scan step of 0.02°. The microstructure of the as-deposited films was also identified by high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2011) and the selected area electron diffraction (SAD) with a working voltage of 200 kV. The magnetic property was obtained at RT by a superconducting quantum interface device (SQUID) magnetometer (MPMS SQUID-VSM), with the magnetic field direction parallel to the surface of samples. The mass of each sample was measured by induced-coupled-plasma atomic emission spectra (ICP-AES) to normalize the magnetization. The photoluminescence (PL) spectra were measured by a Raman spectrometer (LabRam Aramis) at RT using a 325 nm He–Cd laser. The composition and chemical states of samples were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Before the measurements, the samples were sputtered by Ar⁺ for 5 minutes to avoid influence of any surface contamination. The XPS spectra were obtained using Al Kα (1486.6 eV) and calibrated using the carbon 1s core level at ∼284.6 eV.

Results and discussion

Fig. 1 shows XRD patterns of the ZrO₂ deposited at various O₂/Ar ratios. One sees that at an O₂/Ar ratio of 1/19, the film was not oxidized significantly and consisted of mainly Zr. At a ratio of 2/18, the film was completely oxidized into a high temperature phase, i.e. tetragonal phase. At ratios of 2.5/17.5 and 3/17, in addition to the tetragonal phase, the monoclinic phase was also observed. At a ratio of 4/16, the film consisted of only the monoclinic phase. We have also conducted HRTEM and SAD analysis for samples deposited at an O₂/Ar ratio of 2/18, 3/17 and 4/16, denoted as S218, S317 and S416, respectively. Fig. 2 shows the HRTEM images and corresponding SAD patterns of the three samples. One sees that the phase structure of the films was dependent on the O₂/Ar ratio. In S218, see Fig. 2(a) and (b), only the tetragonal phase was observed. In S416, as shown by Fig. 2(e) and (f), only the monoclinic phase was observed. In S317, see Fig. 2(c) and (d), both tetragonal and monoclinic were observed. This result agrees well with the XRD analysis.

Fig. 1 XRD patterns of ZrO₂ thin films deposited at different O₂/Ar ratios.

Fig. 2 SAD patterns of (a) S218, (c) S317, and (e) S416, respectively, where the upper right of each is a part of standard diffraction patterns of monoclinic phase of ZrO₂ and the lower right is that of tetragonal phase ZrO₂; the cross-sectional HRTEM images of (b) S218, (d) S317, and (f) S416.

Fig. 3 shows M–H curves of samples S218, S317 and S416 respectively. It should be mentioned that during the whole operations and measurements procedure, nonmagnetic tweezers, fixtures and containers were used to avoid any unintentional ferromagnetic contamination.²² For comparison, the M–H curves were all normalized to the mass correspondingly. One sees that the films exhibited a phase-dependent RTFM behavior, i.e. a clear hysteresis loop was observed only when the tetragonal phase was present in the films (in S218 and S317), with a coercivity of ∼60 Oe shown in the inset of Fig. 3, while the ferromagnetic was not achieved in the film consisting of only the monoclinic phase (S416). Furthermore, one can also see that the film containing more tetragonal phase has a larger saturated magnetization (M_S).

Fig. 3 M–H curves of ZrO₂ films deposited at different oxygen partial pressures; inset shows the enlarged hysteresis loop of S218.

Fig. 4(a) depicts a typical XPS spectrum of S218. The absence of peaks at ∼638.8, 706.8, and 777.9 eV (see inset of Fig. 4(a)) excludes the possible contamination of magnetic elements like Mn, Fe, and Co in the samples. The chemical states of Zr and O in the films were investigated by XPS analysis. Fig. 4(c)–(e) show the Zr 3d core level spectra of the three samples. Fig. 4(f)–(h) show the corresponding O 1s core level spectra. For all the samples, the Zr 3d core level spectra can be well fitted by two pairs of peaks, i.e. A (∼178.3 eV) and B (∼180.5 eV) ascribed to 3d_5/2 and 3d_3/2 of Zr⁰, and C (∼182.3 eV) and D (∼184.5 eV) ascribed to 3d_5/2 and 3d_3/2 of Zr⁴⁺ respectively. The O 1s core level spectra can be well fitted by two Gaussian peaks, i.e. one at a binding energy of ∼530.0 eV (peak a) ascribed to O²⁻ in the lattice sites, and the other at a higher binding energy of ∼531.0 eV (peak b) due to O²⁻ in the oxygen-deficient regions in the ZrO₂.⁸ Fig. 4(b) plots the area proportion of peaks C and D (black triangle) related to Zr⁴⁺ and the area proportion of peak b (blue circle) related to oxygen vacancy, respectively. It is seen that all the three samples are oxygen-deficient and that the number of oxygen vacancy reaches a maximum in S218 with a tetragonal structure, and a minimum in S416 with a monoclinic structure.

Fig. 4 (a) XPS survey scans of ZrO₂ films after Ar⁺ sputtering; inset depicts an enlarged range from 600 eV to 800 eV. (b) Plots the integrated area of peak C and D related to Zr⁴⁺ and the proportion of peak b reflecting the amount of oxygen vacancies in different samples from the high resolution XPS of Zr 3d core level of (c) S218, (d) S317, and (e) S416, respectively; and the high resolution XPS of O 1s core level of (f) S218, (g) S317, and (h) S416, respectively.

Fig. 5(a) shows the PL spectra of the three samples measured at RT. One sees that all PL spectra can be well fitted by two Gaussian peaks centered at ∼410 nm (peak I) and ∼520 nm (peak II), respectively. According to the literature, peak I is assigned to oxygen vacancy, and peak II is ascribed to lower valence cation (Zr³⁺ or other doping cations).^23–25 The intensity of peak I is the highest in S218 with a tetragonal structure, and decreases monotonically in S317 and S416, in which the content of tetragonal phase get decreased. Fig. 5(b) plots the normalized M_S of all the samples versus the integrated intensity of peak I, where one observes a good positive linear relationship. It indicates that oxygen vacancies play an important role in the origin of the RTFM in the films.

Fig. 5 (a) Room-temperature PL spectra of all samples. (b) The saturated magnetization versus the integrated area of peak I from PL.

The XPS and PL results indicate that the RTFM in undoped ZrO₂ depends on not only phase structure but also oxygen vacancy within the films. To clarify their respective effects, two series of annealing experiments were carried out: (1) anneal the S218 (a tetragonal phase with a large number of oxygen vacancies) at 350 °C for 1 h in air, to reduce oxygen vacancies in the annealed sample (denoted as A218); (2) anneal the S416 (a monoclinic phase with negligible oxygen vacancy) at 350 °C for 1 h in Ar to introduce more oxygen vacancy in the annealed sample (denoted as A416). XRD analysis of S218 and A218 indicates that no structure change in the film is induced by thermal annealing shown in Fig. 6(a). The Zr 3d core level XPS spectra of S218 and A218 are shown in Fig. 6(c), from which one observes a significant increase in the proportion of peaks related to Zr⁴⁺ after air annealing. Fig. 6(d) compares the O 1s core level XPS spectra of two samples, where one sees a significant decrease in the number of oxygen vacancies in A218 comparing with that of S218. Fig. 6(b) shows the comparison of the normalized M–H curves of A218 and S218. One sees that, due to the striking reduction of oxygen vacancies in A218, the RTFM almost disappears. It suggests that the number of oxygen vacancies in tetragonal ZrO₂ is very crucial in enhancing the RTFM. As for A416 annealed in argon, XRD shown in Fig. 7(a) indicates no observable change in phase structure. Fig. 7(c)–(d) compare the XPS spectra of Zr 3d and O 1s core level in S416 and A416 respectively, from which a decrease in the proportion of peaks related to Zr⁴⁺ and an increase in the number of oxygen vacancies was observed. However, as shown by Fig. 7(b), RTFM was still not achieved in A416 with a monoclinic structure, although its oxygen vacancy level is close to that in S218 with a tetragonal structure. Thus, it suggests that phase structure of ZrO₂ film plays a dominant role in the RTFM driven by oxygen vacancy in undoped ZrO₂ thin films.

Fig. 6 (a) XRD pattern; (b) M–H curves; (c) high resolution XPS of Zr 3d core level; (d) high resolution XPS of O 1s core level of S218 and A218.

Fig. 7 (a) XRD pattern; (b) M–H curves; (c) high resolution XPS of Zr 3d core level; (d) high resolution XPS of O 1s core level of S416 and A416.

There are lots of theoretical studies on the defect-driven RTFM mechanisms corresponding to the magnetic ordering in DMOs. Generally, two models can explain the phenomenon to some extent, i.e. the percolation model of bound magnetic polaron (BMP)²⁶ and charge transfer ferromagnetism (CTF).²⁷ In the BMP model, the percolation threshold for magnetic ordering is determined by the vacancy level. Specifically, more oxygen vacancies help to produce more BMPs and yield a larger overall volume occupied by BMPs. When the density of oxygen vacancy exceeds a certain threshold, it will result in an overlap of BMPs thus enhancing ferromagnetic behavior.²⁸ However, it could not explain why RTFM was not achieved in A416 with a monoclinic structure and a number of oxygen vacancies but achieved in S218 with a tetragonal structure. In the CTF model, the basic idea is that a narrow, structured local density of states N_S(E) is associated with defects, but the Fermi level will not normally locate at a peak in N_S(E). A local charge reservoir such as oxygen vacancy occupied with electrons or cations of different charge states, provides a possibility for electron transfer to raise the Fermi level to a peak in N_S(E), leading to Stoner splitting of N_S(E) and triggering the ferromagnetism.²⁹ Put simply, the CTF not only depends on the vacancy levels; it is also associated with the band structure. As the electronic band structure of tetragonal ZrO₂ is significantly different from that of the monoclinic one,^21,30,31 the phase-dependent RTFM induced by oxygen vacancy in undoped ZrO₂ films observed here may be explained by the CTF model.

Conclusions

Undoped ZrO₂ thin films were deposited by reactive DC magnetron sputtering of a zirconium target. By adjusting the oxygen partial pressure during deposition process, the phase structure of as-deposited film could range from monoclinic to tetragonal. When deposited at a low oxygen partial pressure, the film was more inclined to be a tetragonal phase and exhibit room-temperature ferromagnetism. The difference in magnetic behaviors between monoclinic and tetragonal ZrO₂ films indicates that the RTFM depends very much on the phase structure of film. Furthermore, the disappearance of RTFM after reducing the number of oxygen vacancy in tetragonal ZrO₂ thin films suggests that the RTFM in undoped tetragonal ZrO₂ is mainly driven by defect, specifically oxygen vacancy. Our study provides a clue to understand the d⁰ ferromagnetism in undoped oxide films and opens up a new way on the preparation of ZrO₂-based DMOs materials.

Acknowledgements

We are grateful to the financial support by the National Natural Science Foundation of China (Grant no. 51372135) and the Ministry of Education of the People's Republic of China (Grant no. 113007A) and the Tsinghua University Initiative Scientific Research Program.

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