Intrinsic Complex Vacancy-Induced d0 Magnetism in Ca2Nb2O7 PLD Film

Introducing magnetism into the ferroelectric Ca2Nb2O7 with high Curie temperature can make it a potential multiferroic material at room temperature. Stoichiometric Ca2Nb2O7, nonstoichiometric Ca1.9Nb2O7-δ and Ca2Nb1.9O7-δ single phase films were deposited on STO (110) substrate by pulsed laser deposition under appropriate conditions. The films were characterized by XRD, FE-SEM, Element mapping and XPS. Both stoichiometric Ca2Nb2O7 and Ca1.9Nb2O7-δ films were diamagnetic in the magnetic measurement and ab initio calculations, while the Ca2Nb1.9O7-δ film with the complex vacancy of VNb+O exhibited ferromagnetic behavior at room temperature, with the saturated magnetization of 3.6 emu/cm3. Calculations on the Ca2Nb2O7 (010) surface indicate that the VNb+O can induce spin polarization on the residual O atoms around the Nb vacancies, and the system was most stable when the Nb and O vacancies were the 4th nearest-neighbored, with FM coupling energetically more stable than the AFM coupling. Our work verified experimentally and theoretically the feasibility of introducing ferromagnetism into Ca2Nb2O7 film by the intrinsic complex vacancy of VNb+O.


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). Ca 2 Nb 2 O 7 is a member of the A n Nb n O 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 Ca 2 Nb 2 O 7 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 BaTiO 3 films (Ramana et al., 2013;Chand Verma et al., 2014), Fe-doped LiTaO 3 ceramics (Song et al., 2014) and Fe-doped K 0.45 Na 0.49 Li 0.06 NbO 3 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 d 0 magnetism has been observed in BaTiO 3 film/ceramics, Nb-doped BaTiO 3 film and LiNbO 3 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 CdNb 2 O 6 powders was elucidated with the intrinsic exchange interactions between the magnetic moments associated with the unpaired 4f electrons in Eu 3+ ions (Topkaya et al., 2017). The RTFM in K 0.5 Na 0.5 NbO 3 PLD film was related to the cationic K and Na vacancies (Cao et al., 2011a), while the RTFM observed in BaNb 2 O 6 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 BaTiO 3 bulk material and (001) surface (Cao et al., 2009;Cao et al., 2011b) and in PbTiO 3 (Shimada et al., 2012). However, O vacancy was found to be able to induce FM in LiNbO 3 but cannot induce FM in LiTaO 3 and Sr 2 AlNbO 6 (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 d 0 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 Ca 2 Nb 2 O 7 single phase film and those with the complex vacancy of V Ca+O or V Nb+O by varying the composition of Ca 2 Nb 2 O 7 ceramic targets. The probability of achieving d 0 magnetism in the nonstoichiometric Ca 2 Nb 2 O 7 single phase film by complex vacancy was comprehensively examined by magnetic measurement and ab initio calculations.

MATERIALS AND METHODS
CaCO 3 and Nb 2 O 5 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 Ca 2 Nb 2 O 7 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). SrTiO 3 (110) substrate in the size of 5 mm × 5 mm was chosen due to its smaller lattice mismatch with the Ca 2 Nb 2 O 7 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). Figure 1 shows the XRD patterns of the STO substrate and the asdeposited films. Aside from the (110) and (220) diffraction peaks from the substrate (see Figure 1A), the only appearance of (040) diffraction peaks from orthorhombic Ca 2 Nb 2 O 7 (PDF#70-2006) in Figure 1B means that single phase of stoichiometric Ca 2 Nb 2 O 7 was obtained under the current deposition conditions without impurity phase. The Ca 2 Nb 2 O 7 (020) diffraction peaks in Figures 1C,D indicated that the single phase of Ca 2 Nb 2 O 7 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 Ca 2 Nb 2 O 7 lattice, therefore, the nonstoichiometric Ca 2 Nb 2 O 7 films were denoted as Ca 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 7-δ , respectively. The film thickness of the three samples was close and approximated to be 100 nm. The SEM images for the Ca 2 Nb 2 O 7 , Ca 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 7-δ surfaces are displayed in the Supplementary Figure S1.

Structural and Surface Characterizations
Smooth and even surface could be observed for the Ca 2 Nb 2 O 7 film, while bumps and hollows were present on the surface of nonstoichiometric films, with the highest degree of roughness obtained by the Ca 2 Nb 1.9 O 7-δ surface. The corresponding element mapping images show that the elements Ca and Nb from the Ca 2 Nb 2 O 7 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 2 shows the core level XPS for the Ca 2 Nb 2 O 7 , Ca 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 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 Ca 2 Nb 2 O 7 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 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 7-δ films, respectively. With respect to the stoichiometric Ca 2 Nb 2 O 7 film, the Ca 1.9 Nb 2 O 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 Ca 2 Nb 1.9 O 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 Ca 2 Nb 1.9 O 7-δ film FIGURE 2 | C 1s (A), Ca 2p (B), Nb 3d (C) and O 1s (D) core level XPS for the Ca 2 Nb 2 O 7 , Ca 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 7-δ films.
FIGURE 3 | Original MH curves for the STO substrate, Ca 2 Nb 2 O 7 , Ca 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 7-δ films on STO measured at room temperature (A), and the MH curve for the Ca 2 Nb 1.9 O 7-δ film after subtraction of the diamagnetic signal from the substrate (B).  Figure 3 displays the MH curves for the STO substrate, Ca 2 Nb 2 O 7 , Ca 1.9 Nb 2 O 7-δ and Ca 2 Nb 1.9 O 7-δ films on STO measured at room temperature. Compared with the diamagnetic behavior of the STO substrate, the stronger diamagnetic signals in the Ca 2 Nb 2 O 7 and Ca 1.9 Nb 2 O 7-δ films on STO suggest the presence of diamagnetism in the obtained films. However, the Ca 2 Nb 1.9 O 7-δ film on STO showed much weaker diamagnetic signal. As shown in Figure 3B, MH hysteresis loop was observed for the Ca 2 Nb 1.9 O 7-δ film after subtraction of the diamagnetic signal from the substrate, meaning that the Ca 2 Nb 1.9 O 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 Ca 2 Nb 1.9 O 7-δ film.

Theoretical Calculation
In order to explore the origin of FM in the Ca 2 Nb 1.9 O 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 Ca 2 Nb 2 O 7 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 Ca 2 Nb 2 O 7 (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 Ca 2 Nb 2 O 7 (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 Ca 2 Nb 2 O 7 (010) surface is nonmagnetic.
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 Ca 2 Nb 2 O 7 (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 Ca 2 Nb 2 O 7 (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, FIGURE 6 | Total and partial DOSs of Ca 2 Nb 2 O 7 (010) surface with the complex vacancy of VNb+O in the distributions of Nb 1 +O 1 (A), Nb 1 +O 2 (B), Nb 1 +O 3 (C), and Nb 1 +O 4 (D). The insets show the corresponding three-dimensional iso-surfaces of magnetization density.
Frontiers in Materials | www.frontiersin.org August 2021 | Volume 8 | Article 736011 5 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.

CONCLUSION
Nonstoichiometric Ca 2 Nb 2 O 7 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 Ca 2 Nb 2 O 7 (020) single phase film, while V Nb+O can induce spin polarization on the residual O atoms around the Nb vacancies, and make the Ca 2 Nb 2 O 7 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 Ca 2 Nb 2 O 7 film, and this method might be applicable to other A 2 Nb 2 O 7 -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.