Intrinsic Complex Vacancy-Induced d0 Magnetism in Ca2Nb2O7 PLD Film

Introduction

Multiferroic materials with the co-existance of ferroelectricity (FE) and ferromagnetism (FM) have received considerable attention in recent years due to the unique physical mechanism of magnetoelectric coupling and potential applications in the fields of information storage, processing and sensing (Fiebig, 2005; Chun et al., 2012). However, the natural conflict between FM and FE leads to the rarity of multiferroic material at room temperature (RT) (Hill, 2000). Ca₂Nb₂O₇ is a member of the A_nNb_nO_3n+2 family with n = 4, which has layered perovskite structure and high ferroelectric Curie temperature (above 1850°C) (Lichtenberg et al., 2001). The introduction of FM into ferroelectric Ca₂Nb₂O₇ could make it a potential multiferroic material at RT.

In order to achieve FM in ferroelectric materials, doping magnetic elements was the most frequently adopted method. So far, RTFM has been experimentally observed in the Fe-doped BaTiO₃ films (Ramana et al., 2013; Chand Verma et al., 2014), Fe-doped LiTaO₃ ceramics (Song et al., 2014) and Fe-doped K_0.45Na_0.49Li_0.06NbO₃ ceramics (Liu et al., 2015), the origin of which can be theoretically well explained by the F-center model for diluted magnetic semiconductors. Besides, nonmagnetic element doping induced d⁰ magnetism has been observed in BaTiO₃ film/ceramics, Nb-doped BaTiO₃ film and LiNbO₃ nanocrystallites, the origin of which was ascribed to the oxygen vacancy (Mangalam et al., 2009; Yang et al., 2010; Díaz-Moreno et al., 2014). However, the observed FM in Eu-doped CdNb₂O₆ powders was elucidated with the intrinsic exchange interactions between the magnetic moments associated with the unpaired 4f electrons in Eu³⁺ ions (Topkaya et al., 2017). The RTFM in K_0.5Na_0.5NbO₃ PLD film was related to the cationic K and Na vacancies (Cao et al., 2011a), while the RTFM observed in BaNb₂O₆ film was contributed mainly by the oxygen vacancy, with certain contribution by the Nb vacancy (Cao et al., 2012). In the aspect of theoretical study, both Ti and O vacancies were found to be able to induce FM in BaTiO₃ bulk material and (001) surface (Cao et al., 2009; Cao et al., 2011b) and in PbTiO₃ (Shimada et al., 2012). However, O vacancy was found to be able to induce FM in LiNbO₃ but cannot induce FM in LiTaO₃ and Sr₂AlNbO₆ (Cao et al., 2013; Li et al., 2014).

Therefore, the origin of FM in undoped ferroelectric material is still controversial in both experimental and theoretical results. Meanwhile, most of the analyses on the vacancy-induced d⁰ magnetism in ferroelectric oxide films just considered various cation vacancies or oxygen vacancy alone, the synergistic effect of cation and oxygen vacancies, i.e., the effect of complex vacancy, was seldom studied. Herein, the advantage of the identical composition between the film and ceramic target in pulsed laser deposition (PLD) was utilized to prepare stoichiometric Ca₂Nb₂O₇ single phase film and those with the complex vacancy of V_Ca+O or V_Nb+O by varying the composition of Ca₂Nb₂O₇ ceramic targets. The probability of achieving d⁰ magnetism in the nonstoichiometric Ca₂Nb₂O₇ single phase film by complex vacancy was comprehensively examined by magnetic measurement and ab initio calculations.

Materials and Methods

CaCO₃ and Nb₂O₅ were used as raw material and weighted according to the atomic ratio of Ca/Nb as 0.95/1, 1/1, and 1/0.95, respectively. Stoichiometric and nonstoichiometric Ca₂Nb₂O₇ ceramic targets with intrinsic cation vacancies were then obtained by the conventional solid state reaction. Ablation of the targets were achieved using a KrF excimer laser source (λ = 248 nm, pulse duration = 20 ns, energy of pulse = 200 mJ, frequency of pulse = 3 Hz). SrTiO₃ (110) substrate in the size of 5 mm × 5 mm was chosen due to its smaller lattice mismatch with the Ca₂Nb₂O₇ lattice. The deposition duration was 0.5 h, the distance between the target and substrate was 60 mm, the substrate temperature (T_S) was 650°C, and the oxygen pressure (P_O) was 1 mTorr for all depositions.

The crystal structure was examined by High Resolution X-ray Diffractometer (D8 discover, Bruker AXS GmbH, German) using Cu Kα radiation. The SEM and element mapping on the surface were performed by Scanning Electron Microscope (SU8010, Hitachi, Japan). X-ray photoelectron spectra were obtained by X-ray Photoelectron Spectrometer with monochromated Al Kα radiation (Escalab 250, Thermo Electron Corporation, United States). Magnetic properties were measured by Magnetic Property Measurement System (MPMS-5XL, Quantum design, United States) with magnetic field parallel to the surface of film. The film thickness was checked by Stylus Surface Profiler (Dektak 150, Veeco Metrology, France).

Results and Discussion

Structural and Surface Characterizations

Figure 1 shows the XRD patterns of the STO substrate and the as-deposited films. Aside from the (110) and (220) diffraction peaks from the substrate (see Figure 1A), the only appearance of (040) diffraction peaks from orthorhombic Ca₂Nb₂O₇ (PDF#70-2006) in Figure 1B means that single phase of stoichiometric Ca₂Nb₂O₇ was obtained under the current deposition conditions without impurity phase. The Ca₂Nb₂O₇ (020) diffraction peaks in Figures 1C,D indicated that the single phase of Ca₂Nb₂O₇ was maintained using the targets with 5% Ca or Nb deficiency. To maintain the charge neutrality, the Ca or Nb vacancy would lead to the concomitant appearance of oxygen vacancy in the Ca₂Nb₂O₇ lattice, therefore, the nonstoichiometric Ca₂Nb₂O₇ films were denoted as Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ, respectively. The film thickness of the three samples was close and approximated to be 100 nm. The SEM images for the Ca₂Nb₂O₇, Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ surfaces are displayed in the Supplementary Figure S1. Smooth and even surface could be observed for the Ca₂Nb₂O₇ film, while bumps and hollows were present on the surface of nonstoichiometric films, with the highest degree of roughness obtained by the Ca₂Nb_1.9O_7-δ surface. The corresponding element mapping images show that the elements Ca and Nb from the Ca₂Nb₂O₇ films were uniformly distributed on the surface of STO substrate for all samples, while the element O exhibited higher degree of density due to the simultaneous contribution from the film and substrate.

FIGURE 1

FIGURE 1. XRD patterns of the STO substrate (A), the as-deposited films using stoichiometric Ca₂Nb₂O₇ target (B), nonstoichiometric Ca₂Nb₂O₇ target with 5% Ca deficiency (C) and Nb deficiency (D).

Figure 2 shows the core level XPS for the Ca₂Nb₂O₇, Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ films. All spectra were charge corrected according to the C-C peak at the binding energy (BE) of 284.8 eV. In Figure 2A, the C-C and C-O-C peaks come from the adventitious carbon contamination, while the small amount of O-C=O peaks originate from the carbonate formed on the surface. From the Ca 2p and Nb 3d XPS in Figures 2B,C, Ca and Nb ions were in the valence states of +2 and +5 respectively. Since there is no other valence states present for Ca and Nb, the existence of Ca or Nb vacancy in the Ca₂Nb₂O₇ lattice can only induce oxygen vacancies to maintain the charge neutrality, hence the complex vacancy of V_Ca+O and V_Nb+O should be present in Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ films, respectively. With respect to the stoichiometric Ca₂Nb₂O₇ film, the Ca_1.9Nb₂O_7-δ film exhibited 0.17 eV lower BE for the Ca 2p_3/2 peak but 0.51 eV higher BE for the Nb 3d_5/2 peak, while the Ca₂Nb_1.9O_7-δ film exhibited 0.63 eV lower BE for the Ca 2p_3/2 peak but 0.05 eV lower BE for the Nb 3d_5/2 peaks. As for the O 1s XPS, lattice oxygen (O_L) and adsorbed oxygen (O_ads) were present in all samples, but the relative content of O_ads for the Ca₂Nb_1.9O_7-δ film was 58.8% which was much higher the 40.53 and 40.01% for the Ca₂Nb₂O₇ and Ca_1.9Nb₂O_7-δ films, respectively.

FIGURE 2

FIGURE 2. C 1s (A), Ca 2p (B), Nb 3d (C) and O 1s (D) core level XPS for the Ca₂Nb₂O₇, Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ films.

Magnetic Measurement

Figure 3 displays the MH curves for the STO substrate, Ca₂Nb₂O₇, Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ films on STO measured at room temperature. Compared with the diamagnetic behavior of the STO substrate, the stronger diamagnetic signals in the Ca₂Nb₂O₇ and Ca_1.9Nb₂O_7-δ films on STO suggest the presence of diamagnetism in the obtained films. However, the Ca₂Nb_1.9O_7-δ film on STO showed much weaker diamagnetic signal. As shown in Figure 3B, MH hysteresis loop was observed for the Ca₂Nb_1.9O_7-δ film after subtraction of the diamagnetic signal from the substrate, meaning that the Ca₂Nb_1.9O_7-δ film exhibited weak ferromagnetic behavior. Given the film thickness of 100 nm, the saturated magnetic moment of 9×10⁻⁶ emu corresponds to the magnetization of 3.6 emu/g for the Ca₂Nb_1.9O_7-δ film.

FIGURE 3

FIGURE 3. Original MH curves for the STO substrate, Ca₂Nb₂O₇, Ca_1.9Nb₂O_7-δ and Ca₂Nb_1.9O_7-δ films on STO measured at room temperature (A), and the MH curve for the Ca₂Nb_1.9O_7-δ film after subtraction of the diamagnetic signal from the substrate (B).

Theoretical Calculation

In order to explore the origin of FM in the Ca₂Nb_1.9O_7-δ film, the density functional theory (DFT) calculations were performed using the plane-wave pseudopotential method in the Vienna Ab initio Simulation Package (VASP) (Kresse and Hafner, 1993a; Kresse and Joubert, 1999). The Projector Augmented Wave (PAW) (Kresse and Hafner, 1993b; Blöchl, 1994) potentials were employed, and General Gradient Approximate (GGA) was used to describe the exchange correlation energy. According to the XRD result, an orthorhombic Ca₂Nb₂O₇ 1 × 2 × 1 supercell containing 176 atoms was first relaxed to get the most stable structure (see Supplementary Figure S2A) until the total energy in the optimized structure was converged to 1.0 × 10⁻⁴ eV/atom and the Hellman-Feynman force was smaller than 0.01 eV/Å. Then four different (010) planes were cleaved along the b axis, and a 10Å vacuum layer which was thick enough to isolate the atom layers was added above the supercell for further optimization. Among the four (010) surfaces, the configuration in the Supplementary Figure S2B showed the lowest energy, on which further calculations were performed.

The stable configuration of Ca₂Nb₂O₇ (010) surface with labeled Ca, Nb, and O atoms to be removed for vacancy study is displayed in Figure 4A, among which O1 is connected only to one Nb atom, O2 is coordinated between two neighbored Nb atoms, O3 connects one Nb atom in the outermost layer with another Nb atom in the second layer, while O4 denote the one between two Nb atoms in the second layer. Stoichiometric Ca₂Nb₂O₇ (010) surface was firstly studied and the total DOS was showed in Figure 4B. No spin polarization could be observed around the Fermi level, meaning that the stoichiometric Ca₂Nb₂O₇ (010) surface is nonmagnetic.

FIGURE 4

FIGURE 4. Stable configuration of Ca₂Nb₂O₇ (010) surface with labeled Ca, Nb and O atoms to be removed for vacancy study (A), and the total DOS of stoichiometric Ca₂Nb₂O₇ (010) surface (B).

The complex vacancy of V_Ca+O in four different distributions, i.e., Ca1+O1, Ca1+O2, Ca1+O3, and Ca1+O4, was then studied. The relative stability (ΔE) of the Ca₂Nb₂O₇ (010) surface with these four types of complex vacancy was 3.51, 4.18, 0, and 2.21 eV respectively, meaning that the system with V_Ca1+O3 was most stable. However, no spin polarization could be observed in the DOSs around the Fermi level in Figure 5, indicating that the complex vacancy of V_Ca+O cannot induce magnetism in the Ca₂Nb₂O₇ (010) surface. On the other side, the complex vacancy of V_Nb+O in four different distributions all induced spin polarization around the Fermi level and impurity bands in the forbidden gap (see Figure 6), which were contributed mainly by the O 2p electrons. The three-dimensional iso-surfaces of magnetization density in the inset of Figure 6 show that the complex vacancies of V_Nb+O mainly induced spin polarization on the residual O atoms around the Nb vacancies. The total net magnetic moment (M_tot) of the system with V_Nb1+O1, V_Nb1+O2, V_Nb1+O3, and V_Nb1+O4 was 1.0, 0.99, 0.96, and 1.0 μ_B, respectively. Among which, the system with V_Nb1+O4 was the most stable one with the lowest energy, and the ΔE of the system with V_Nb1+O1, V_Nb1+O2 and V_Nb1+O3 was 1.04, 27.46, and 27.90 eV, respectively. More importantly, FM coupling was energetically more favorable than AFM coupling in the cases of V_Nb1+O1 and V_Nb1+O4, with the relative energy between FM and AFM as 7 and 45 meV, respectively.

FIGURE 5

FIGURE 5. DOSs of Ca₂Nb₂O₇ (010) surface with the complex vacancy of VCa+O in the distributions of Ca₁+O₁(A), Ca₁+O₂(B), Ca₁+O₃(C), and Ca₁+O₄(D).

FIGURE 6

FIGURE 6. Total and partial DOSs of Ca₂Nb₂O₇ (010) surface with the complex vacancy of VNb+O in the distributions of Nb₁+O₁(A), Nb₁+O₂(B), Nb₁+O₃(C), and Nb₁+O₄(D). The insets show the corresponding three-dimensional iso-surfaces of magnetization density.

Conclusion

Nonstoichiometric Ca₂Nb₂O₇ single phase films with the complex vacancy of V_Ca+O or V_Nb+O were deposited on STO (110) substrate under appropriate deposition conditions using nonstoichiometric ceramic targets. The V_Ca+O cannot induce magnetism in the diamagnetic stoichiometric Ca₂Nb₂O₇ (020) single phase film, while V_Nb+O can induce spin polarization on the residual O atoms around the Nb vacancies, and make the Ca₂Nb₂O₇ film exhibit FM behavior at RT. Our work demonstrated experimentally and theoretically that the introduction of intrinsic complex vacancy during deposition should be a feasible way to induce ferromagnetism in the ferroelectric Ca₂Nb₂O₇ film, and this method might be applicable to other A₂Nb₂O₇-type niobate ferroelectric films as well.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

EC contributed to conception and design of the study. LW, YZ and ZN contributed to the acquisition, analysis and interpretation of data. All authors contributed to manuscript revision, read, and approved the submitted version.

Funding

This work was supported by National Natural Science Foundation of China (11404236, 11604236, and 11974258), Natural Science Foundation of Shanxi Province (201901D111117 and 201901D111126).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling Editor declared a past co-authorship with one of the authors NZ.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

Part of this study has been performed using facilities at IBS Center for Correlated Electron Systems, Seoul National University.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2021.736011/full#supplementary-material

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