Supplementary material for
Room-temperature exchange bias in multiferroic BiFeO3 nano- and
microcrystals with antiferromagnetic core and two-dimensional diluted
antiferromagnetic shell
Chuang Zhanga, Shou Yu Wanga,*, Wei Fang Liub,*, Xun Ling Xub, Xiu Lib, Hong Zhangc, Ju
Gaod and De Jun Lia
a
College of physics and Materials Science, Tianjin Normal University, Tianjin 300387, China
b
Tianjin Key Laboratory of Low-Dimensional Materials, Physics and Preparing Technology,
Faculty of Science, Tianjin University, Tianjin 300072, China
c
Department of Materials Science  Engineering, National University of Singapore, 9
Engineering Drive 1, 117576, Singapore
d
Department of physics, University of Hong Kong, Pokfulam Road, Hong Kong, China
*Corresponding authors.
E-mail address: [email protected] (S.Y. Wang) and [email protected] (W.F. Liu)
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1. Table S1 the amount of reagents for BFO nano- and microcrystals
Samples
Bi(NO3)3·5H2O(g)
Fe(NO3)3·9H2O(g)
KOH(g)
KNO3(g)
CTAB(g)
Temperature(℃)
Time(h)
BFO18nm
2.3283
1.9392
107.7312
0.3235
0.2915
190
12
BFO1.3μm
2.3283
1.9392
107.7312
0.3235
0
190
24
BFO33μm
2.3283
1.9392
107.7312
0.3235
0
200
24
2. The M-H loops for BFO-18nm with seven repeating cycles
Figure S1. The enlarged view of consecutively measured hysteresis loops to reveal the shift of
the loops with increasing loop number (n).
3. Electrical property
Figure S2 shows the leakage current density versus electric field (J-E) characteristics of BFO18nm and BFO-33um ceramics. It can be seen that the J-E curves display excellent symmetry
2
under positive and negative electric fields and high leakage current in the low electric field
region (<2 kV/cm). The value of leakage current density is nearly 10-1 A/cm2 at 1.5kv/cm for
BFO-33um ceramic, which is higher than BFO-18nm ceramic. Moreover, the leakage current
density of all samples prepared by hydrothermal method is approximately five orders of
magnitude higher than that by solid reaction and sol-gel method (Xi et al. 2014; Zhang et al.
2014). To further find out the origin of increased electric conductivity in BFO, we qualitatively
discussed the conduction mechanism. It is well known that the high leakage current density in
the pure BFO materials originated primarily two possible factors. First is the charge defects such
as oxygen vacancies, which come from the high volatile nature of Bi and the multiple oxidation
states of Fe ions (Fe3+ and Fe2+) (Yu et al. 2008; Wang et al. 2008). These oxygen vacancies
generate deep trap energy levels within the band gap and provide a path for thermally or
electrically stimulated charge carriers to flow under applied electric field (Makhdoom et al.
2012). The second factor is the microstructure, such as volume of grains and grain boundaries
and the density of the material (Liu et al. 2010; Agarwal et al. 2011), which dominates the leaky
behavior. Normally, the grain boundaries are known as high resistance. Then the increasing
density of grain boundaries should contribution to reduction of leakage current in BFO-18nm
ceramic (Dai et al. 2015). This trend was further supported by SEM characterization, as
discussed below (see Fig. S3). To clarify the conduction mechanism in all samples, the log J
versus log E curves were fitted by linear segment, which are plotted in inset of Fig. S2. Based on
the power law J﹠Em relationship, the value of slope in these curves gives the information about
the conduction mechanism (Wang et al. 2006). The slopes of all the samples are around 2,
indicating that the conduction is controlled by the space charge-limited conduction (SCLC)
mechanism. The SCLC is considered as a normal leakage behavior and correlates with oxygen
vacancies in BFO materials (Wang et al. 2009; Yang et al. 2008; Kawae et al. 2009).
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Figure S2. Leakage current as a function of applied electric field (J-E). The inset log J versus
log E plot for BFO-18nm and BFO-33um ceramics.
4. Scanning Electron Microscopy (SEM) for BFO samples sintered
The SEM was performed to characterize the morphology of BFO-18nm and BFO-33um after
sintering, as shown in Fig. S3a-b. It can be seen that the microstructure of all the BFO samples
changed dramatically compared to the samples before sintering. These microstructures become
rectangular grains size, which depends upon the concentration of oxygen vacancies (Pattanayak
et al. 2015). The SEM images also show that some porosity exists among the loosely connected
grains in BFO-33um ceramic. For BFO-18um ceramic, the leakage current density is lower than
BFO-33um ceramic, which is attributed to the higher density of grain boundary. Similar
phenomenon has been reported in doped BFO (Xi et al. 2014; Dhir et al. 2014).
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Figure S3. SEM images of a BFO-18nm and b BFO-33um sintered ceramics.
5. Dielectric property
Figure S4 and the inset show the frequency (f) dependence of dielectric constant (ε) and
dielectric loss (tanδ) of BFO-18nm and BFO-33um ceramics. The dielectric constants of BFO18nm ceramic are decreased sharply with the increase of frequency in low frequency region
(called the f-sensitive region) and then become almost constant at higher frequencies. Such
phenomenon can be described by Maxwell-Wagner model, which is related to the space charge
relaxation at the interface (Elissalde et al. 2001). These space charges originate from oxygen
vacancies and at low frequencies can follow the applied electric field, where as the space charges
do not have time to follow the applied field and undergo relaxation (Rodrigues et al. 2010). This
behavior is common in dielectric and ferroelectric materials (Makhdoom et al. 2012; Choudhary
et al. 2007). Very large values of dielectric constant at low frequency for BFO-18nm are
observed, which may be attributed to the interfacial polarization (Pradhan et al. 2009). The
dielectric losses of all BFO samples have similar change trend with dielectric constants. The
values of dielectric loss for BFO-33um ceramic are smallest.
Figure S4. The dielectric constant of BFO-18nm and BFO-33um ceramics at room temperature,
the inset shows the dielectric loss.
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