Enhanced ferroelectricity and magnetoelectricity in 0 . 75 BaTiO 3-0 . 25 BaFe 12 O 19 by spark plasma sintering

Spark Plasma Sintering (SPS) technique was employed to synthesize 0.75BaTiO3-0.25BaFe12O19 composite. X-ray diffraction studies revealed that the composite consisted of both BaTiO3 (ferroelectric phase) and BaFe12O19 (ferrimagnetic phase), respectively. The SPS treated sample showed improved ferroelectric nature when compared to conventional sintered (CS) sample. Transformation from hard to soft magnetic nature was envisaged by magnetization measurements for SPS sample. A slim hysteresis loop was recorded with a low coercivity values (390 Oe) when compared to CS sample (3900 Oe). Mossbauer spectroscopy analysis indicated that the existence of a partial amount of γ-Fe2O3 phase in the lattice, giving rise to soft magnetic nature. The SPS sample showed slightly higher value of magnetoelectric output of 2.95 mV/cm at 3 kOe magnetic field when compared to the CS sample (1.45 mV/cm at 3 kOe). The present investigation compares the spark plasma sintered sample with the conventional sintered sample.


I. Introduction
There has been growing interest in the study of multiferroic materials, a special class of materials in which two or three kinds of order parameters i.e., ferroelectric, ferromagnetic and ferroelastic co-exist [1].Among multiferroics, the materials exhibiting ferromagnetic and ferroelectric ordering simultaneously are known as Magnetoelectric (ME) materials.In these materials one can expect the coupling between the magnetic and ferroelectric properties as well as their control by the application of magnetic and/or electric fields [2].Magnetoelectric materials have great potential for applications in novel actuators and sensors with high sensitivity.However, there are very few natural magnetoelectrics that exhibit both ferromagnetic and ferroelectric behaviour at room temperature [3].This is because the off-centre distortion responsible for ferroelectric behaviour is usually incompatible with the partially filled d-level which are prerequisite for ferromagnetic behaviour [4].
Magnetoelectric materials are usually synthesized by different sintering routes such as solid state sintering, chemical synthesis, microwave sintering, hot pressing etc., and among all the sintering routes, one of the promising sintering route is Spark Plasma Sin-tering (SPS).SPS method has received much attention, because many materials can be sintered at lower temperatures within a few minutes [13,14].In the SPS method, a powder sample is pressed in vacuum in a graphite die and heated by a pulse current.The sintering proceeds very quickly because of the spark plasma caused by the large pulse current.This method can be used in metallurgy [15], ceramics [16] and even in plastics [17].
Barium titanate, BaTiO 3 (denoted as BT), is a well studied ferroelectric perovskite and is considered to be a good candidate for high-performance lead-free piezoelectric applications [18].At high temperatures BaTiO 3 adopts paraelectric cubic phase where the large barium ions are surrounded by 12 nearest neighbour oxygen's and each titanium ion has six oxygen ions in octahedral coordination.
Barium hexaferrite, with its chemical formula BaFe 12 O 19 (denoted as BF), is the best-known representative of the hexaferrite family.BF is one of the most important hard magnetic materials, widely used in permanent magnets, magnetic recording media, and microwave applications, due to its fairly large magnetocrystalline anisotropy, high Curie temperature, relatively large magnetization, as well as excellent chemical stability and corrosion resistance [19].BF crystallizes in a hexagonal structure with 64 ions per unit cell on 11 different symmetry sites.The 24 Fe 3+ atoms are distributed over five distinct sites: three octahedral sites (12k, 2a, and 4f2), one tetrahedral site (4f1), and one bipyramidal site (2b).Its crystal structure is the so-called magnetoplumbite structure, which can be described as a stacking sequence of the basic blocks S and R [20].
In our earlier communication, we have reported the magnetoelectric coupling in BaTiO 3 -BaFe 12 O 19 composite system (containing 75 wt.%BaTiO 3 and 25 wt.%BaFe 12 O 19 , with notation 0.75BT-0.25BF)at room temperature [21].This conventionally sintered 0.75BT-0.25BFcomposite showed a magnetoelectric output of 1.45 mV/cm at 6 kOe.The aim of the present investigation is to compare the conventionally sintered sample with the SPS sample and correlate the multiferroicmagnetoelectric properties.

II. Experimental
A solid state double sintering method was employed to synthesize BaTiO 3 -BaFe 12 O 19 compositions (containing 75 wt.%BaTiO 3 and 25 wt.%BaFe 12 O 19 ) from stoichiometric amounts of BaTiO 3 and BaFe 12 O 19 powders (Sigma Aldrich with 99.9 % purity).The powders were weighed and ground together thoroughly and cold pressed into pellets.The pellets were pre-sintered at 900 °C for 2 hours, reground to powder, re-pressed into pellets and sintered at 1250 °C for 2 hours.Sintered discs were re-ground to powder form for X-ray diffraction in a Philips diffractometer at a 2q scan rate of 0.2 degree per minute and at 0.02 degree steps with Cu-K α incident radiation.The surface morphology and distribution of grains were carried out using LEO440i scanning electron microscope.
Spark plasma sintering was carried out using the model Dr. Sinter 1050 (Sumitomo Coal mining Co. Ltd., Japan).The calcined powder was poured in a cylindrical graphite die of 12 mm diameter, sintered in partial vacuum (6 Pa) for 10 min under a uniaxial pressure of 50 MPa applied throughout the sintering cycle.The sintering temperature was set at 900 °C and the heating rate was kept constant at 100 °C/min.At the end of the soaking time the power and pressure were switched off and the samples were naturally cooled inside the chamber.The SPS sintered sample was annealed at 600 °C for 1 hour for maintaining oxygen stoichiometry.
The sintered discs were polished and coated with silver paste and annealed at 600 °C for 15 minutes to obtain a good electrical contact for ferroelectric measurements at room temperature in a ferroelectric hysteresis loop tracer up to a maximum applied electric field of 25 kV/cm.The maximum polarization (P max ) was determined from the hysteresis plots.Prior to the measurement, the samples were poled electrically using a high electric field setup.Magnetization measurements were performed in DMS vibrating sample magnetometer at room temperature, up to a magnetic field of 20 kOe.The coercivity, the magnetic moment and the remanence values were recorded at room temperature.Mossbauer spectra were recorded using FAST comtec (Germany) spectrometer at room temperature in transmission geometry with a 25 mCi 57 Co source in Rhodium matrix.Prior to the experiments, the spectrometer was calibrated using a standard α-Fe foil of thickness 25 μm.The Mossbauer spectra of each powder sample were recorded for duration of about 5 days with total background counts up to 8-10 × 10 5 .The Mossbauer spectra were fitted with the PCMOS -II least-square fit program [24].Magnetoelectric measurements were carried out as discussed in our earlier reports [21].The samples were poled electrically prior to the measurements and then placed in a magnetic field and the obtained ME voltage was recorded up to a field of 5 kOe.

III. Results and discussion
X-ray diffraction patterns of BaTiO 3 , 0.75BT-0.25BFconventionally sintered sample, SPS treated sample and BaFe 12 O 19 are shown in Fig. 1.All the peaks could be indexed as the BT (ferroelectric) and the BF (ferrimagnetic) phases respectively.Interestingly, no difference is noted in the peak positions for the conventionally sintered (CS) and spar plasma sintered (SPS) samples, except some change in the peak intensities.It indicates that the phase formation is achieved by SPS at lower temperature and in short-er sintering intervals.The structure and composition remains the same for both the phases, as there are no peak shifts even after the SPS treatment and annealing.Similar kind of results was reported in Multiferroic compounds in PZT + NiZn ferrite system and also for BF systems respectively [22,23].
Figure 2 shows the SEM images of the CS and SPS treated sample of 0.75BT-0.25BF.It is observed that the grains are distributed homogenously all over the sample for SPS treated sample.When compared to the CS sample, the SPS sample grains are formed in more compacted manner.Moreover, the hexagonal patterned grains which are significant for BF phase is evidenced in the CS treated sample, whereas, in the SPS treated sample, all BT and BF grains are nurtured giving rise to flakes/platelets kind.The reason may be due to the rapid heating rate (100 °C/min) and cooling rates (50 °C/min) during the SPS sintering.Another interesting observation arising from the Fig. 2 is that the SPS disks were nearly free of pores, with only a few pores This result indicates that a short holding time period is an essential factor to obtain 0.75BT-0.25BFcomposite with uniform and fine grains by the SPS process.Similar phenomena have been reported earlier for (NiZn) Fe 2 O 4 [25][26][27].
Figure 3 shows the magnetic hysteresis loops for the CS and SPS sintered samples at room temperature.The saturation magnetization (M s ) and coercivity (H c ) of the SPS and CS samples are enumerated in Table 1.The SPS sample showed a high saturation magnetization value (38 emu/g), when compared to CS sample (15 emu/g).Since the two samples have the same composition, the possible cause could be a difference in microstructure [28].On the other hand, the SPS sample showed a very low coercivity value of 390 Oe when compared to 3900 Oe for CS treated sample.It shows that SPS treatment has a favourable effect on obtaining the soft magnetic material.It is well known that coercivity decreases with increasing density [29].Such a decrease in samples coercivities can be attributed to the secondary crystallization which induced higher anisotropy constant and intragranular pores inhibiting the motion of domain walls [30].
Figure 4 shows the ferroelectric hysteresis loops for the CS and SPS sintered composite.The maximum polarization attained for the CS sample is 6.9 μC/cm 2 at 22 kV/cm and for the SPS sample it is 16.89 μC/cm 2 at 17.5 kV/cm.The CS sample showed a slim hysteresis loop whereas the SPS treated sample showed a broad hysteresis loop.Table 1 gives the maximum polarization, remanent polarization and coercivity values for the CS and SPS samples.All the ferroelectric properties viz maximum polarization, remanance and coercivity are higher for the SPS samples as compared to the CS sample.The possible reasons for low values for the CS sample may be due to the piezoelectric phase which is susceptible to electrical shunting and the loss of piezoelectrically generated charges due to low resistivity for hexaferrite and also due to possible presence of Fe 2+ in the ferrite phase which could further enhance the leakage current through the sample.For the SPS sample, the density and the resistivity is much higher than for the CS sample giving a better hysteresis and ferroelectric properties for SPS sample.
Figure 5 shows the Mossbauer spectra for 0.75BT-0.25BFsample (CS), 0.75BT-0.25BF(SPS) and BF (SPS) samples.In order to understand the effect of SPS process, the Mossbauer spectra of three samples are shown.The barium hexaferrite has five crystallographically in-equivalent Fe-sites and the Mossbauer spectra were analysed with five different sextets corresponding to the five Fe-sites.The Mossbauer parameters obtained from the fitting match well with the reported values in the literature [31].However, the Mossbauer spectra of the SPS treated samples were fitted with two or more sextets in addition to five sextets corresponding to barium-hexaferrite.The additional two sextets were necessary to obtain a satisfactory fitting and the hyperfine field values of these sextets are 490 and 500 kOe, respectively.These sextets are attributed to the presence of γ-Fe 2 O 3 phase as the field values match well with the field values for this compound.The Mossbauer study indicates that the high temperature metastable γ-Fe 2 O 3 phase might have formed during SPS.The Mossbauer data for the samples is presented in Table 2.
The presence of Fe 3+ and Fe 2+ ions has rendered piezomagnetic materials dipolar in nature.Since in piezomagnetic materials, the rotational displacement of  Fe 3+ ↔Fe 2+ dipoles results in orientational polarization that may be visualized as exchange of electrons between the ions, the dipoles align themselves with the alternating field.When magnetic field is applied to the sample, the magnetoelectric output tends to increase with applied magnetic field and on decreasing the magnetic field, the output slightly increase and then decreases with a half butterfly loops indicating the strains produced during the magnetic field are not linear and a kind of relaxation (phenomenon) is taking place, giving rise to non-linearity in the system.Further studies are needed to understand the relaxation nature of the SPS treated sample.The magnetoelectric effect is the product property of magnetostrictive and piezoelectric constituents of the composites.The ME coefficient depends on the mechanical coupling, resistivity, mole fraction of the constituent phases and importantly on sintering technique.The magnetic field dependent variations in the ME voltage is presented in Fig. 6.Table 1 gives the magnetoelectric output values of the CS and SPS treated samples at different fields.The ME output increases linearly with applied magnetic field up to a field of 5 kOe and shows a value of 2.95 mV/cm at 3 kOe when the field is reversed.The rise in output is attributed to the enhancements in elastic interactions between the piezoelectric and magnetostrictive phases.The magnetization and associated strain produce a constant electric field in the piezoelectric phase beyond the saturation limit.The possible reason for the attaining higher ME values for the SPS sample could be due to the enhancement of hyperfine fields at 12k and 2b sites as strengthening in the Fe 3+ -O-Fe 3+ superexchange interaction giving higher value of magneto-electric output.After ME output reaches to a maximum, the decrease in ME can be caused by two effects such as magnetic dilution with changing of the Fe 3+ (high spin) valence state to Fe 2+ (low spin) state on 2a site by substitution of the Ba 2+ site ions and existence of spin canting which is promoting the reduction of super-exchange fields [32].The Fe 3+ -O-Fe 3+ super-exchange interaction is disrupted and weakened by Fe 2+ ions and canted spins, which could be produced by resulting in lower coercivity and magnetization for SPS treated samples [32,33].

IV. Conclusions
The SPS treated composite have showed enhanced ferroelectric properties when compared to the conventional sintered composites.The hard magnetic phase of the composite has transformed into soft phase by SPS methodology.In addition to these, the magnetoelectric output has shown high values at low fields when compared to conventional sintering.From these results, one can conclude that the SPS is favourable technique for improvement of the multiferroic and magnetoelectric properties in composites.