Magnetoelectricity in LaYFe₂O₆ and its Derivatives for Energy Harvesting

Abstract

Amid the rapid development of technology, magnetoelectric (ME) multiferroic materials provide a fertile playground to explore fascinating electric and magnetic properties. In this backdrop, the development and establishment of magnetoelectricity in a new material or insight may prove beneficial in the evolution of this field. As a variant of perovskite structure, double perovskite oxides have been coveted much research attention in recent years because of various intriguing properties. In this thesis, double perovskite LaYFe2O6 and its derivatives are studied for magnetoelectric applications. Here, an ordered double perovskite LaYFe2O6 is prepared by the sol-gel auto combustion method and its structural, magnetic, and magnetoelectric properties have been studied depending upon three different sintering temperatures (800, 1000, and 1200 °C). The phase purity of all the samples is checked by the X-ray diffraction (XRD) technique followed by the Rietveld refinement method. The diffraction studies establish orthorhombic symmetry having the space groups of P21nm (~ 90%) and Pbnm (~ 10%). The effect of sintering temperature is discernible in the surface morphology, which is inspected by a Field Emission Scanning Electron Microscope (FESEM). The grain size increases from ~ 50 nm to ~ 150 nm by the influence of sintering temperature. Magnetization study reveals antiferromagnetic (AFM) ordering of the spins at the room temperature (RT) having the transition temperature ~ 700 K. Double perovskite formation and AFM ordering is also asserted by the neutron diffraction (ND) measurement. With the increasing temperature, spin canting appears and become maximum near the magnetic transition, which is suggested by the isothermal magnetization study. The enhanced crystallinity with the increasing sintering temperature (displayed in XRD profile and FESEM depiction) is also substantiated by the magnetization measurement. The highest converse magnetoelectric coefficient ~ 2.26(6) mOe ∙ cm V-1 and direct magnetoelectric coefficient ~ 0.45(3) mV cm-1 Oe-1 is recorded at RT. Nonetheless, this material exhibits linear magnetoelectric effect even for temperature as high as 400 K. To enhance the ME coefficient further, magnetic ‘Sm’ is substituted in place of non-magnetic ‘La’ (La1 xSmxYFe2O6; 0 ≤ x ≤ 1). To inspect the role of Sm in this compound, the structural, electric, magnetic, and ME properties have been investigated. Here, XRD study confirms the successful formation of double perovskite structure (P21nm phase only). The surface morphology of x = 0.75 sample asserts good crystallinity along with the sharp edges of the grains, indication of improved structural order. It is observed that, AFM ordering (~ 700 K) exist in pristine sample become enriched by the Sm substitution, which is also substantiated by the depletion of short-range ordering in the form of non-Griffith’s phase. The spin-reorientation transition observed in low temperature region (in lower Sm content sample) moved towards the RT region, which is very effective for application purposes. The x = 0.75 sample displayed 1st-order direct ME coupling coefficient ~ 0.59(4) mV cm-1 Oe-1 (~ 31 % higher than x = 0) at RT. The induced magnetoelectricity in this composition is found to be mediated by spin-lattice coupling. Thereafter, polymer composite of LaYFe2O6 and poly(vinylidene fluoride) hexafluoropropylene [LaYFe2O6/P(VDF-HFP)] is synthesized in the form of thick film by the solution casting method. The prepared films are then characterized via XRD, FESEM, Fourier Transform Infrared Spectroscopy (FTIR) techniques. Depending upon the wt% of LaYFe2O6 nanoparticles (NPs), the excellent phase to phase connectivity and enhanced beta phase fraction in the composite film is ascertained. The strong interaction between polymer matrix and magnetic nanoparticles is also substantiated by the enhanced ferroelectric response (electric-field dependent polarization measurement). Interestingly, 10 wt% NPs based nanocomposite manifests the maximum ME voltage coefficient of ~ 2.92(5) mV cm-1 Oe-1 at RT (~ 1 order higher than the pristine LaYFe2O6). Now, to meet the need for Internet of Things (IoT), miniaturization, and power efficiency, a flexible, lightweight, cost-effective, and portable ME nanogenerator (MENG) is made by this composite film. This MENG can be deployed as an energy harvester to harvest wasted magnetic energy into electric energy with an efficiency of 1.5 % and can charge the capacitor followed by the glowing of a light-emitting diode (LED).