Structure Evolution and Multiferroic Properties in Cobalt Doped Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-δ Intergrowth Aurivillius Compounds

Here, we report the structure evolution, magnetic and ferroelectric properties in Co-doped 4- and 3-layered intergrowth Aurivillius compounds Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-δ. The compounds suffer a structure evolution from the parent 4-layered phase (Bi4NdTi3FeO15) to 3-layered phase (Bi3NdTi2CoO12-δ) with increasing cobalt doping level from 0 to 1. Meanwhile the remanent magnetization and polarization show opposite variation tendencies against the doping level, and the sample with x = 0.3 has the largest remanent magnetization and the smallest polarization. It is believed that the Co concentration dependent magnetic properties are related to the population of the Fe3+ -O-Co3+ bonds, while the suppressed ferroelectric polarization is due to the enhanced leakage current caused by the increasing Co concentration. Furthermore, the samples (x = 0.1–0.7) with ferromagnetism show magnetoelectric coupling effects at room temperature. The results indicate that it is an effective method to create new multiferroic materials through modifying natural superlattices.


I. Statistical analysis of second phase magnetic inclusions: volume fractions and magnetic contributions
In previous researches, M. Palizdar et al., 1 L. Keeney et al. 2 and M. Schmidt et al. 3 provided an original methodology for the detection, localization and quantification of second phase inclusions in Aurivillius phase via energy selective backscatter (ESB) image and energy dispersive X-ray analysis (EDX). The backscatter electron is sensitive to the mean atomic number, 4 therefore it is one of the ideal techniques for searching for possible secondary phases. In an ESB image, high atomic number regions appear bright, while low atomic number regions appear dark. 1 To determine the magnetic contribution from the ferromagnetic secondary inclusions, the scanning electron microscopy (SEM) images (including secondary-electron (SE) images and ESB images) and EDX were performed for all Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-δ (BNTFC-x, x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) samples on Zeiss Gemini SEM 500 equipped with an ESB detector and an Oxford X-Max 80 detector.2 Taking x = 0.3 sample as an example, Fig. S1 shows the ESB image and corresponding Bi, Co and Fe intensity maps (acquired from the EDX mapping). The circled region (with black contrast in Fig. S1 (a)) are Co and Fe rich (Bi poor) areas, a) Author to whom correspondence should be addressed. Electronic mail: lixg@ustc.edu.cn and yyin11@unl.edu demonstrating the existence and location of the Fe and Co-rich secondary phase. 1    (Fe3-yCoyO4), and therefore in ESB images, Bi12TiO20 phase should be brighter and Fe3-yCoyO4 inclusions should be darker.
Take x=0.5 in Fig. S5 for example, the black regions (such as the black area marked by a red circle) represent the Fe/Co rich spinel magnetic inclusion phase. While the gray regions, which have the largest area, denote the main phase. In addition, as discussed in the main text, with increasing the Co doping level x, an impurity phase Bi12TiO20 with higher Bi composition appears, which is the brightest in the ESB images (e.g. the white area marked by a blue circle). These are also supported by the local EDS    Generally, the Fe and Co-rich magnetic inclusions have a chemical formula Fe3-yCoyO4 (0 ≤ y ≤ 3), and its remanent magnetization at room temperature decreases as Co content increases. [5][6][7][8][9][10] In order to give a reasonable estimation of the magnetic contribution from the inclusions, the chemical compositions of the inclusions for all samples were calculated from the EDX mapping. The mole ratios of Fe/Co (Δ) in the inclusions can be obtained by analyzing the inclusion intensities of the EDX mapping.  MIr of 0.73 emu/g (Fe0.5Co2.5O4). 10 For x = 0.9 and 1.0, the Fe/Co mole ratios of the inclusions are determined to be 0. Thus the inclusions should be Co3O4, which is paramagnetic at 300K, 10,11 and the MIr could be set to 0 emu/g at 300K. Besides, the Bi/Ti mole ratio of the white region calculated from a point EDS (Fig. S10) is about 11.7/1, confirming that the white regions represent Bi12TiO20 phase. In addition, it is also worth mentioning that the ESB images were acquired at a set resolution of 1024768 pixels. If the smallest detectable size of inclusions across all scanned areas is defined as 22 pixels, which can be distinguished easily, the minimal detectable diameters for scanning areas of 40000 μm 2 (largest scan area) and 4 μm 2 (smallest scan area) are 450 nm and 4.5 nm, respectively. The ferromagnetic contributions from Fe3O4 5,12 and CoFe2O4 13 inclusions with grain sizes smaller than 5 nm, having a blocking temperature below 300 K, could be neglected at 300 K. In our case, no Fe/Co rich area with a size smaller than 200 nm was observed for all samples.
This may be attributed to the high synthesis temperature and long sintering time (presintered at 850 °C for 20 h and calcined at 900 °C for 20 h), which would make the grain size of inclusions grow large. 14 Assuming that all inclusions are spherical, and following the statistical method proposed by M. Schmidt et al. 3 , the upper limit of volume fraction fk,u for the inclusions with the sizes between dk and dk-1 (dk-1>dk) can be calculated by 3 1 , with a confidence of γ = 99.5%, and Vj is an individual scan of a series scans (k = 1, …,

K).
In order to apply the statistical method to the ESB or EDX images of our samples, formula (1) can be modified to 2-dimension model as follow: In an isotropic ceramic system, it is reasonable to assume that the inclusions evenly  Table S1.
Given the volume fraction of the ferromagnetic inclusions, the maximal contribution from the inclusions (Mi) with diameter between [dk, dk-1] can be calculated by: where MIr,k is the remanent magnetization of the inclusions, fm,k is the mass fraction of the inclusions, fV,main and ρmain are the volume fraction and density of the main phase respectively, fV,sillenite and ρsillenite are the volume fraction and density of sillenite   The actual stoichiometries (mole ratio, normalized by Ti element) of the samples determined by energy dispersive X-ray analysis (EDX) mapping are listed in Table S2.

II. Actual stoichiometry from EDX analysis for Bi 4 NdTi 3 Fe 1-x Co
The mole ratios of Bi, Nd and Ti change little, while those of Fe, Co vary with the nominal doping level. The volume fractions for 4-and 3-layered phases and Bi12TiO20 listed in Table S2 are modified by calculating the franction of magnetic inclusions, thus these values have a small difference with those obtained by XRD refinements.