Keywords: Multiferroics, Magnetization, Ferroelectrics and Magnetoelectric coupling
 

*corresponding author      email: rr@vet.ac.in    Telephone: +91 452 2465289 (Extn: 208)

 

1. Introduction

Materials possessing multiferroic properties have potential applications in information storage, sensors, spintronics, etc. Even though there are many multiferroic materials now available, BiFeO3 stands distinct in the sense that there is strong coupling between ferroelectricity and ferromagnetism.  BiFeO3 is of current interest for researchers, as magnetic and ferroelectric orderings are possible above room temperature. But practical applications of this material have many limitations. So, doping or co-doping in this BiFeO 3 is being tried nowadays to make it successful for multiferroic applications. Amit Kumar and Yadav [\citet{Kumar_2013}] have studied the enhancement of magnetoelectric properties La, Lu co-doped BiFeO3 as BiFeO3 is a known magnetoelectric material. They have also added small quantity  of La at Bi site to reduce the Bi3+ volatilization and oxygen vacancies. With this, they could report an enhancement in the magnetoelectric capacitance is nearly three times the bare BiFeO3. Due to the ferroelectric Curie temperature of Tc ~ 830 0and G-type antiferromagnetic Neel Temperature (TN) of 370 0C, on doping with La and Lu, it is found that TN shifts towards lower temperature, thereby room temperature coupling is possible. This also makes it as a useful material for practical applications.
Recently in 2017, Abdelkafi et al [\cite{Abdelkafi_2017}] have report dielectric relaxation study in NiTi co-doped BiFeO3 at Fe site. The dielectric anomaly at 247 C near the Neel temperature (TN) supports the strong magnetoelectric coupling.  Among the various methods, the method of doping is found to be successful to modify the crystal structure and tune the physical properties.
Most of the reported studies on divalent ion doped BiFeO3 have focussed on the investigation of modifications in the crystal structure and the corresponding changes in magnetization and electric polarization [\cite{Tirupathi_2013}]. Rarely reported the results of dielectric constant changes near the magnetic transition. These works lead to the present investigation on the multiferroic properties of Ho, Sm co-doped BiFeO3 at Bi site.

2. Experimental Details

2.1. Synthesis

Polycrystalline BiFeO3, Bi0.9Sm0.1FeO3 and Bi0.85Sm0.1Ho0.05FeO3 are prepared by high temperature solid
state reaction.  High purity powders such as Bi2O3, Fe2O3, Sm2O3 and Ho2O3 (99.99% pure from Sigma Aldrich & Alfa Aesar) are taken in the stoichiometric ratio and ground vehemently with mortar & pestle made of agate for 30 minutes for homogeneity. Then these mixed powders were calcined in two step process at 600° C for 2 hours and then at 835° C for 2 hours with intermediate grinding and consequently quenched towards room temperature to avoid oxygen deficiency [\cite{Zhang_2005}]. These residues of mixed powders are pelletized using polyvinyl alcohol as a binder. Three distinct cylindrical pellets are prepared by a hydraulic press at 30 MPa. These pellets were then sintered finally at 835° C for 2 hours at the rate of 10 °C / minute with an accuracy of 0.02 °C.  Finally, the sintered pellets were coated with silver paint. 

2.2. Characterization

Phase identification of the samples is examined from the powder X-ray diffraction (XRD) using PANalytical X’Pert Pro Plus Goniometer PW3050/60 X-ray diffractometer with CuKα radiation in a wide scanning range of 10° to 90° with a step size of 0.0170.  Rietveld refinement on XRD pattern is performed using GSAS II (General Structure Analysis System) software to analyse the crystallographic structure present in the sample. The surface morphology of the sample is investigated using FEG quanta 250 scanning electron microscope (SEM).
Temperature dependent dielectric analysis is done for frequencies of 1 Hz to 107 Hz at the temperature of 30° C to 500° C using Alpha A impedance analyser (Novocontrol, Germany).  Ferroelectric hysteresis loops of the samples are measured using Precision Premier II Multiferroic test system (Radiant Technology, USA). The leakage current of the samples is measured using Keithley 6517 electrometer. Magnetoelectric measurements are observed using Polytronic electromagnet HEM 100along with Lakeshore 425 Gaussmeter up to a field of 9 kOe and magnetization measurement by using Lakeshore VSM7410 vibrating sample magnetometer. 

3.  Results and Discussion

3.1. Structural  and microstructural analysis