Bi deficiency-tuned functionality in multiferroic Bi1-δFe0.95Mn0.05O3 films

Structural evolution and ferroelectric (FE)-to-antiferroelectric (AFE) transition behaviors were observed in Bi1-δFe0.95Mn0.05O3 (100)-textured films with a carefully controlled Bi deficiency concentration δ. Raman spectra revealed an orthorhombic structural transition induced by Mn substitution. The polarization-electric field hysteresis loops and capacitance-voltage loops of Bi1-δFe0.95Mn0.05O3 films clearly demonstrated antiferroelectric behavior with increasing δ. The responses of the domain structure of the Bi1-δFe0.95Mn0.05O3 film under positive and negative applied voltages directly suggested the coexistence of FE and AFE phases. The existence of (100) superstructure reflections and antiparallel displacements of the Bi atoms along the [100] direction observed by transmission electron microscopy unambiguously reveal the AFE phase. The chemical substitution-induced orthorhombic structural transition in BiFe0.95Mn0.05O3 film implies that as the δ concentration increases, the changes in Bi-O bonding and the stereochemical activity of Bi 6s lone pair affect both the ferroelectric distortion and the antiferrodistortive rotation and therefore drive the Bi1-δFe0.95Mn0.05O3 crystal lattice to form a PbZrO3-type orthorhombic phase with an AFE order. A continuing increase in Bi deficiency creates defect dipole complexes which produce an internal field leading to a preferred direction of the ferroelectric domain. The Bi deficiency in multiferroic BiFeO3 provides a new route by which to tune functionality.


Material and Methods
Mn (5 mol%) doped BFO thin films were grown on Pt/Ti/SiO 2 /Si substrates via a simple chemical solution deposition technique as reported before 15 . Because of the volatility of Bi in the annealing process, excess Bi is needed in the starting materials to compensate for the bismuth loss. Here, different contents, i.e., 2, 5, and 10 mol%, of excess Bi were chosen to adjust the Bi deficiency concentration in BFO films. The final thickness of the films is approximately 220 nm. Circular Au electrodes with a diameter of 0.2 mm were sputtered on the film surface using a shadow mask for electrical characterization.
X-ray diffraction (XRD, Rigaku D/max2500HB +/PC with Cu Kα radiation λ = 1.54056 Å in a θ /2θ mode) was used to check the crystal structures. Raman spectra were collected in backscattering geometry on RM2000 (Renishaw) with Ar + laser irradiation at 514 nm focused over a 5-μ m diameter area. Inductively coupled plasma mass spectrometry (ICP-MS, Thermo ICP-MS XII) was used to check the chemical compositions of the Bi 1-δ Fe 0.95 Mn 0.05 O 3 films. The Bi deficiency value δ is 0.034 for Bi content excess of 2 mol%, 0.028 for Bi content excess of 5 mol%, and 0.005 for Bi content excess of 10 mol% BFO films. Transmission electron microscopy (TEM, JEM-2100F, JEOL), high-resolution TEM (HRTEM) and nano-beam electron diffraction (NBED) images were taken at 200 kV. Ferroelectric hysteresis loops were tested using a standardized ferroelectric test system (Radiant Technology, Precision Workstation), and Agilent B1505A was utilized to take capacitance-voltage measurements. Domain structures and local piezoelectric coefficients were observed and measured using a piezoelectric force microscope (PFM, PISA XEI100) operating at an ac of 2.5 V at 17.5 kHz.

Results and Discussion
Bi 1-δ Fe 0.95 Mn 0.05 O 3 (BFMO) films with different Bi deficiency amounts all show pure perovskite phase XRD patterns, as shown in Fig. 1. An obvious (100) plane diffraction preference is observed for all BFMO films. Although the positions of the diffraction peaks do not change, the films' texture, the broadening of the peaks and the merging of the split peaks in the R3c structure make the XRD patterns insufficient to determine the crystal structure of these BFMO films. Therefore, further information on the details of the change in crystal symmetry of BFMO films with different Bi deficiencies is obtained by Raman spectroscopy.
Eight vibration modes are clearly identified in the Raman spectra of all the BFMO films shown in Fig. 2(a). Here, the quite strong E(TO) vibrational modes at 622 and 480 cm −1 resemble those observed in orthorhombic manganite perovskites that display Jahn-Teller distortions, as noted by a recent study on BiFeO 3 -LaMnO 3 solid solution thin films 16 . Mn ions are implicated as the main cause of octahedral distortion that induces the transition to an orthorhombic BFO structure. Thus, it is reasonable to deduce that the structural distortion arisen in BFO films due to the substitution of 5 mol% Mn shows the appearance of orthorhombic phase. Meanwhile, the characteristic peaks of rhombohedral phase still retained. Raman spectra were also used to analyze the Bi deficiency because the Raman peaks between 120 and 250 cm −1 are attributed to Bi-O vibrations. No Raman peak shift was observed in BFMO films with different Bi deficiency concentrations; however, the intensity ratios of the peaks changed. As shown in the inset of Fig. 2, the intensity ratio between the A 1 (TO) mode at 141 cm −1 and the E(TO) at 622 cm −1 , which are the characteristic peaks of the orthorhombic phase, increases with increasing Bi deficiency; the intensity ratio between the two A(TO) modes at 141 cm −1 and 169 cm −1 follows the same trend. The change in Raman mode intensity ratios that represent the vibration of Bi-O bonds with the Bi deficiency amount directly confirms that Bi deficiency influences Bi-O bonding in the BFO lattice.
The electrical properties of BFMO films were examined via polarization-electrical field (P-E) hysteresis loops and capacitance-voltage (C-V) loops, as shown in Figs 3 and 4, respectively. As shown in Fig. 3(a), the BFMO film with the almost stoichiometric composition (i.e., δ = 0.005) exhibits a normal P-E hysteresis loop with a remanent polarization of 37 μ C/cm 2 , displaying ferroelectric characteristics. The gap between the initial and the final polarizations may be due to partial domain depolarization after a presetting voltage pulse. As shown in Fig. 3(b,c), with increasing Bi deficiency concentration, i.e., δ = 0.028 and 0.034, the square-shaped P-E hysteresis loops become distorted and develop into doubled hysteresis loops that are the typical characteristic of an antiferroelectric material.
Point defects or defect dipoles have long been considered to be responsible for the observed double P-E loops in ferroelectrics 17 . A general microscopic mechanism was proposed by Ren to explain the strange double hysteresis in doped or irradiated ferroelectrics after aging 18 . Here, the Bi deficiency was intentionally introduced, and oxygen vacancies are inevitable in perovskite films fabricated using a chemical method. According to the defect reaction → + δ + δ ′′′ + δ + δ O , Bi deficiency ( ′′′ ) V Bi will promote the formation of an oxygen vacancy ( ) Thus, the oxygen vacancy and Bi deficiency are likely to form defect dipole complexes that will lead to domain pinning.
As seen from the C-V loop of a Bi-sufficient BFMO film presented in Fig. 4(a), a shoulder-peak-like plateau was observed in the process of either increasing or decreasing voltage. Similar C-V loops were reported for BFO films in a previous study and were attributed to the ′′ − 19 . Such defect-dipole complexes produce an internal field leading to a preferred direction of the ferroelectric domain. Comparison to the BFMO film with δ = 0.005 shows that as δ increases to 0.028 and 0.034, clear double butterfly-shaped C-V loops showing two capacitance maxima for both increasing and decreasing voltages appear in Bi-deficient BFMO films [see Fig. 4(b,c)], again showing the typical characteristics of antiferroelectricity.
Consideration of the C-V loop together with the P-E behavior of BFMO film for a stoichiometric composition shows that defect dipole complexes alone cannot account for the all the observations. Because antiferroelectric phase transition has been proven to exist in Mn-doped BFO material, it is quite reasonable that the phase transition may be another origin of the antiferroelectric-like behavior. Belik et al. discovered an antiferroelectric orthorhombic PbZrO 3 -type phase in BiFe 1-x Mn x O 3 (0.15 ≤ x ≤ 0.4) solid solutions 11 . The deformation of the (Fe,Mn)O 6 octahedra is primarily driven by the covalent Bi-O bonding and the related stereochemical activity of the lone pair. Therefore, when we increase the amount of Bi deficiency, the Bi-O bonding and the stereochemical activity of Bi 6s lone pairs in the crystal cell of the BFMO film are changed so as to drive the deformation of the (Fe,Mn)O 6 octahedra into a structure resembling an orthorhombic PbZrO 3 -type phase.
The dynamic responses of BFMO films to the applied electric field were investigated via the observation of domain switching behaviors. The topography and the corresponding out-of-plane (OP) PFM amplitude images of BFMO film with δ = 0.005 are presented in Fig. 5(a,b) and show that the grains of Bi-sufficient film are quite small, with the size of approximately 20 nm, and each grain contains a single domain with either an OP polarization direction (i.e., perpendicular to the film surface and pointing either downward or upward). The domain structures obtained after the film were polarized by the PFM probe in an area of 1 μ m × 1 μ m under negative and positive 10 V, are shown in Fig. 5(c,d), respectively, and show that all domains in the polarized area were completely switched to align to the direction of the applied electric field. However, when δ increases to 0.028 and 0.034, regions showing a near-zero piezoelectric response (circled by the dashed line) were observed in the domain structures of the BFMO films. When an external electric field is applied, the FE domains are switched to follow the external electric field orientation, while the non-responsive areas are hardly changed by the applied voltage. Moreover, we found some connection between the lack of response of the non-polarized regions and their corresponding microstructures, as shown in Fig. 5(a,b) with δ = 0.028 and 0.034, as indicated by the dashed lines. The AFE domains generally show clusters of grains with a higher topography on the film surface; this is probably caused by the lattice mismatch between the matrix FE rhombohedral phase and the transformed AFE    To further understand the origin of the antiferroelectric-like behavior, HRTEM and electron diffraction were used to characterize the atomic arrangement of the Bi-deficient BFMO film. Figure 6(a) shows a cross-sectional TEM image of Bi 1-δ Fe 0.95 Mn 0.05 O 3 film with δ = 0.028. Nano-beam electron diffraction (NBED) was used to obtain the electron diffraction of the region that corresponds to the AFE domain showing a higher topography on the film surface. Figure 6(b) presents an image of NBED along the < 011 > zone axis of the framed region in Fig. 6(a); the (100) superstructure reflection marked by arrows are typically directly associated with the antiphase tilting of oxygen octahedra. These reflections are attributed to the antiphase rotations of the oxygen octahedral 9,20,21 . The corresponding HRTEM image of the framed region is shown in Fig. 6(c), where the alternating shifts of Bi ions left and right along the [100] direction are indicated by blue and red circles. Antiparallel displacements of the Bi atoms along the [100] direction clearly demonstrate the presence of an antipolar order in Bi-deficient BFMO film and confirm that the film is in the AFE phase.
We also carried out measurements of local piezoelectric coefficients (d 33 ) of the BFMO films, with the results shown in Fig. 7. A sharp decrease in the absolute value of d 33 (from 31 pm/V for δ = 0.005 to 18 pm/V δ = 0.028 BFMO film) was observed upon the appearance of the AFE phase in the BFMO film. As Bi deficiency concentration increased to δ = 0.035, a decrease in the absolute value of d 33 (~15 pm/V) A as well as an obvious shift of the hysteresis loop toward a positive bias and negative piezoelectric coefficient were observed, indicating the dominating preferred polarization direction in the film. Such observation in the d 33 hysteresis loop of BFMO film with high Bi deficiency δ = 0.035 implies that the defect dipole complexes probably produce an internal field leading to a preferred direction of the ferroelectric domain. The observed domain structures for δ = 0.035 were consistent with this hypothesis, while the polarization of some domains is pre-oriented up ( Fig. 7(b)) and even after the application of the maximum downward voltage (− 10 V in our experiment), not all the domains are reversed downwards, as shown in Fig. 7(c); it is much easier for all the domains to orient upwards as shown in Fig. 7(d).
Therefore, the introduction of Bi deficiency tuned the structural transition such that Bi 1-δ Fe 0.95 Mn 0.05 O 3 films undergo a phase change from FE to AFE due to the interaction between (Fe,Mn)O 6 octahedra and Bi-O bonding and the stereochemical activity of the Bi 6s lone pair; under high Bi deficiency concentrations, domain structures of the FE phase show a preferred polarization orientation due to the influence of Bi vacancy and oxygen vacancy defect dipole complexes.