Evidence of Finite Magneto-electric Coupling in SmFeO3 – PbTiO3 Solid Solutions

: Present work investigates the detailed multiferroic properties of Pb 1- x Sm x Ti 1- x Fe x O 3 , ( x = 0.21, 0.22, 0.23, 0.24 and 0.25) synthesize through solid state reaction route. The structural, dielectric, ferroelectric and magnetic properties were measured. A sincere study is carried out to detect the magneto-electric coupling in all the samples through magneto-dielectric, magnetization after electric poling and magneto-pe response. The maximum value of coupling coefficient (γ) and magneto-dielectric response (MDR) of sample x = 0.24 has shown by magneto-dielectric properties and magnetization after electric poling whereas magneto-pe has shown that all the samples possess multiferroic nature.


Introduction:
Multifunctional materials attract great interest due to their unique functionalities which render them useful for a variety of novel applications [1][2][3]. Out of all these materials, magneto-electric multiferroics are being increasingly explored [4,5]. These special type of materials are essentially those which exhibit simultaneous magnetic and electric ordering [6]. The presence of partially filled d-orbitals is responsible for magnetic behavior, while the presence of empty dorbitals contributes to the ferroelectric behavior of multiferroics [7,8]. The multifunctionality of these materials leads to their significant potential for applications such as memory devices (FeRAMs, MRAMs) [9,10], spintronics, sensors and biomedical applications [11][12][13][14]. Mixed perovskite (doping d n ion in ferroelectric material) is one of the widely used approach to synthesize high performance multiferroic materials [15]. Lead titanate (PT) is a very important ferroelectric material which can be used as a suitable base material for synthesizing mixed perovskite multiferroics. It has high Curie temperature (Tc ~ 490°C), high ferroelectric and very high dielectric constant as compared to other ferroelectric materials. Due to its high dielectric and ferroelectric properties, PT based materials show extensive utility for sensors, transducers and memory device applications [6,[16][17][18][19]. The magnetic behavior in PT is usually generated by substituting transition metal cations (Mn 3+ , Fe 3+ , Ni 2+ ,Co 2+ ) at Ti 4+ site [20]. However, this substitution of transition metal cations is reported to enhance leakage current [20]. It has been further established that, the substitution of rare earth ions (R 3+ ) at A-site of PT based materials suppress the leakage current [21,22]. The rare earth cations are generally substituted into ferroelectric perovskites by forming their solid solutions with rare earth orthoferrites (RFeO3, where R = Sm, La, Nd) [23,24]. SmFeO3 (SF) is an important rare earth ferrite having perovskite structure. Its crystal structure is orthorhombic and space group is Pbnm [25]. Its Neel temperature (TN) and spin reorientation temperatures are reported to be ~ 670 K and 480 K, respectively [26,27]. PT-SF solid solutions offer a potential possibility of realizing efficient multiferroic materials. Few studies have been reported on PT-SF solid solutions. Barranco et al. reported dielectric studies of Pb0.88Sm0.08Ti0.48Mn0.02O3 sample [28]. In another, they reported structural and electric behavior of Pb0.88Sm0.08TiO3 ceramic sample [29]. Increased Sm doping was found to lead to decreased tetragonality in these studies. Further, in these studies, the B-site doping was either not carried out or was substituted with Mn ions. Also, the ferroelectric and magnetic studies were not reported.
Singh et al. [30] reported the multiferroic properties of (Pb0.8Sm0.2) (Ti0.8Fe0.2)O3 whereas structural and magnetic properties of (1-x)Pb(Zr0.45Ti0.55)-(x)SmFeO3, 0.10 ≤ x ≤ 0.15 have been reported by Randeep et al. [31]. From these studies, it is clear that PT-SF solid solutions have either been explored up to x = 0.20 only. The possibility of extending the composition (beyond x > 0.20) still exits. It further needs to established that at which value of x, the (PT)1-x -(SF)x system transforms to pseudo cubic structure. Similarly, in these studies the coupling between magnetic and electric has either not been explored or only demonstrated using magneto-pe studies. Magnetodielectric studies are more often used to describe the presence of M-E coupling [32]. However, Catalan et al. [33] reported that the magnetodielectric response (MDR) can also results from factors other than M-E coupling. Hence, the main aim of the present study is to synthesize and characterize the multiferroic properties of Pb1-xSmxTi1-xFexO3 (x = 0.21, 0.22, 0.23, 0.24, 0.25) solid solutions using solid state reaction route. In view of the concerns raised by Catalan et al. [34], magneto-pe and magnetization with electric poling studies have also been carried out.

Experimental:
Solid solutions of Pb1-xSmxTi1-xFexO3 (x = 0.21, 0.22, 0.23, 0.24 and 0.25) were prepared by conventional solid state reaction method. The raw materials Sm2O3, PbO, TiO2 and Fe2O3 (from Sigma Aldrich, 99.9% pure) were weighed in stoichiometric proportions and mixed well using mortar pestle for 2 hours. The powder was transferred to bottles containing acetone and zirconia balls and subject on milling for 24 hours. For phase formation, the mixed powders were calcined at 1000°C for 12 hours in high temperature furnace. Thereafter, powders were mixed with PVA binder (2 wt%) to form circular pellets of 10 mm diameter and 1mm thickness with the help of hydraulic press. Afterwards, pellets were sintered at 1150°C (after optimization studies) for 2 hours in lead environment using closed crucible arrangement to reduce weight loss due to lead volatility.
The X-ray Diffraction (XRD) data of these sintered samples were recorded using Shimadzu   The refined data for all the samples is shown in figure 3(a-e). The refined data shows a nice match with the experimental data. All the refined parameters are listed in table 1. The variation of c/a ratio, lattice parameters and cell volume as a function of composition is given in figure 4(a-c).The figure clearly shows that a, c/a ratio and cell volume decrease non -linearly with composition x. This observed decrease may be due to the mismatch in the sizes of Sm 3+ and Pb 2+ ions [29]. The tetragonality in PT based materials is attributed to internal stresses such as compressive stress and tensile stress. Due to the smaller ionic radii of Sm 3+ (1.08Å) and Fe 3+ (0.60Å) as compared to Pb 2+ (1.19Å) and Ti 4+ (0.605Å) ions, the compressive stress increases which leads to decrease in cell volume and hence results in decrease in tetragonality [36].

Morphological Studies
The morphological studies of the prepared samples were carried out using Field emission scanning electron microscopy (FE-SEM) and the micrographs are shown in figure 5(a-e).

Dielectric Properties:
The temperature dependent dielectric constant (εʹ) and dielectric loss (tanδ) for all the samples in the range 303 K ≤ T ≤ 823 K at different frequencies (1 kHz ≤ f ≤ 100 kHz) were measured. The  and has lowered the tetragonality, the effect of lone pair of Pb spontaneously decreases with increase in doping. Such a change in structure from tetragonal to cubic is also reflected from phase transition profile. The decrease in εʹ and tanδ shows that the SF substitution require less thermal energy for the ferroelectric/ paraelectric phase transition [37,38]. Therefore, the x = 0.25 sample has shown the value Tc FE below room temperature .The c/a ratio and Tc FE

Ferroelectric Properties:
The room temperature polarization versus electric field loops for all the samples Pb1-xSmxTi1-

Magneto-Electric Coupling
The coupling between magnetic and electric components of these samples is generally reported by studying the magneto-dielectric response (MDR) of these samples. Therefore, in addition to magneto-dielectric studies we have also performed magneto-polarization (MPE) and magnetization after electrical poling.

Magneto-Dielectric Properties
The MDR of these samples was determined by performing εʹ vs frequency measurements of x = 0.21 -0.24 samples at 0 T, 0.5 T, 1 T, 1.5 T fields. The data is shown in figure 10(a-d). It is clear from the data εʹ decreases with increasing frequency for given magnetic field and decreases at given frequency with increasing magnetic field in all samples. The reduction of εʹ with magnetic field at a given frequency is indicative of negative MDR. This decrease may be due to several factors including magneto-striction effect, magneto-resistance or magneto-electric coupling. Due to these factors, the magnetic phase SmFeO3 contracts on the application of magnetic field [40].
This shrinkage exerts the stress on the PbTiO3, the ferroelectric phase. Since, the nature of this strain is compressive, the samples thereby experience reduction in tetragonality and the off center positioning of Ti 4+ ion get modified to reduce the net dipole moment of unit cell. This whole mechanism results in lowering of dielectric constant of the material after applying magnetic field [41]. The %age change in dielectric constant with applied field is termed as magneto-dielectric response (MDR). The MDR values for all the samples were calculated using the following formula: absence of magnetic field, respectively. This results in lowering of the dielectric constant of the material [42]. It is clear that x = 0.24 sample exhibits highest MDR%. The thermodynamic potential in multiferroic system can be calculated using Ginzburg-Landau-Dehonshire theory [43] and expressed as: where ϕo is reference potential, α, αʹ, β, βʹ and γ are related coupling coefficients. The γP 2 M 2 term arises due to coupling between electric and magnetic components of the material [44]. The magneto-electric coupling results in being proportional to 2 as: Here M 2 is square of magnetization and γ is the magneto-electric coupling coefficient. The value of has been calculated by linear fitting of equation (3)  sample could be its Tc FE being just above room temperature.

Magnetic Moment
The impact of electric field poling on the magnetic properties of x = 0.21 -0.24 samples was also investigated. In order to check whether the electric field induces any magnetization changes in these samples or not. In this regard, the pellet was broken into two pieces for each composition.
One of these pieces were electrically poled at an applied voltage of 2kV while the other piece was kept unpoled. The first quadrant M-H loops (Virgin Curve) of these unpoled and poled samples were measured for each composition. The typical plot for x = 0.21-0.24 sample is shown in figure   12(a-d).

Magneto-polarization
In view of findings by Catalan et al. [33] that magneto-dielectric changes could also be the consequence of factors other than M-E coupling, we also carried magneto-pe measurements over Undoubtly, the obtained results reveal that all the samples have exhibited promising MPE response. Generally, the magnetic poling induced stress in the material which impose electric field on electric dipoles and hence variation in polarization comes into picture [45]. With this regard, the sample which has shown maximum polarization in the absence of magnetic poling exhibits more variation under magnetic field. Hence, for x = 0.21 sample, the MPE response has attained maximum value.

Conclusion:
Pb1-xSmxTi1-xFexO3, (x = 0.21, 0.22, 0.23, 0.24 and 0.25) have been fabricated using solid state reaction method. The tetragonal structure with P4mm symmetry has been determined by X-Ray diffraction data for x ≤ 0.24. The SEM results shows the uniform grain growth in all the samples.
A good agreement with XRD data has been established by the ferroelectric transition temperature which decrease from 412 K to 351 K for x = 0.24 (for x = 0.25, Tc FE < room temperature). The decrease in polarization and increase in magnetization has been explained through the c/a ratio and Fe 3+ content respectively. The magneto-dielectric and magnetization after electric poling studies suggested that sample x = 0.24 exhibits highest magneto-electric response.