MULTI-FUNCTIONAL APPLICATIONS OF Pr2NiMnO6: MAGNETIC, DIELECTRIC AND ENERGY STORAGE PROPERTIES OF RARE EARTH DOUBLE PEROVSKITE

Abstract

Multifunctional properties of double perovskite materials, with the general formula A2BB'O6, have garnered substantial attention due to their intriguing physical characteristics. These features include low-field magnetoresistance, high-temperature ferromagnetism, metal-insulator transitions, spin ordering, phase separation, and multiferroicity which is which make them promising candidates for spintronic device applications. More specifically, ferromagnetic double perovskite oxides with rare earth elements (R2NiMnO6) have emerged as a research focus, promising a plethora of physics and technological applications. The ordering of B-site cations plays a critical role in evoking unique magnetic, dielectric, and electrochemical properties in this environment. Although B-site ordered double perovskites are of paramount interest for their magneto-resistive and ferromagnetic characteristics, synthesizing a perfectly ordered double perovskite system remains challenging, owing to the similarity in ionic radii and oxidation states of different cations at the B-site, leading to mixed crystallographic occupation a phenomenon referred to as "anti-site" disorders. Griffiths phase in these compounds with a large exponent is also observed which may be attribute to the presence of anti-site disorders. These anti-site disorders can engender configurations such as Ni2+–O–Ni2+ and Mn4+–O–Mn4+, resulting in antiferromagnetic interactions in R2NiMnO6. Additionally, anti-site disorders give rise to the formation of antiphase boundaries (APBs), which are a principal contributor to antiferromagnetic coupling. Experimental evidence corroborates the presence of APBs, as indicated by a saturation magnetization (MS) consistently smaller than the expected MS (~5 μB/f.u.). These anti-site disorders exert a profound influence on the properties of double perovskite materials, leading to intriguing phenomena like exchange bias and spin glass behavior. Among the RNMO family, La2NiMnO6 (LNMO) has been extensively studied in both bulk and thin film forms, primarily due to its high Curie temperature (TC ~280 K) and notable magneto-dielectric effects. However, other members, including Pr2NiMnO6 (PNMO), remain less explored. The replacement of La with a smaller rare earth element introduces variations in the (Mn-O-Ni) bond angle, thereby modifying exchange interactions. While polycrystalline bulk PNMO has been investigated extensively for fundamental studies, the preparation of epitaxial thin films has proven to be a complex endeavor. The growth parameters of these films must be meticulously controlled to achieve the ordered double perovskite structure, underscoring the need to establish a reliable process for producing high-quality thin films of these oxides for their integration into emerging technologies. Furthermore, the structural, magnetic, and storage properties of polycrystalline bulk and epitaxial thin films of Pr2NiMnO6 are examined in this study, with a particular focus on the influence of A-site doping on these properties a crucial step in harnessing their potential for device applications. After going through the physics involved in rich magnetic properties of double perovskite compounds it is finalized that, the main objective of the present thesis is to Study the Multi-Functional Applications of Pr2NiMnO6: Magnetic, Dielectric and Energy Storage Properties of Rare Earth Double Perovskite. Chapter 1 gives the general introduction of the transition metal oxide, perovskite and double perovskites with a brief description of the origin of magnetism in these compounds. The fundamental physics involved in the magnetic, dielectric and supercapacitive properties of these compounds has been explained. The historical background of the double perovskite materials along with the current research efforts in these materials has been discussed. Among them, the rich physical properties and technological applications of R2NiMnO6 family of double perovskites have been discussed in detail. The origin of magnetism in this compound along with magnetic moment and magnetic interacions have been discussed in details. Then, a brief discussion has been done on the existence of spin glass effect, magnetic anisotropy, Griffith phase, exchange bias and antisite disorders in this compound, which are responsible for the origin of fascinating phenomena. Furthermore, importance of double perovskite in the field of energy storage, dielectric, and electronics have been discussed in detail. A literature survey has been done on the bulk Pr2NiMnO6 as well as bulk and thin films of other compounds of this family. Chapter 2 discusses the synthesis and Characterization techniques used throughout the present thesis work. During this research work, solid state method is adopted to synthesize the bulk material. The details about solid state method have been discussed in this chapter. Further, the details about the fabrication of the thin film using Pulsed Laser deposition technique has been given. Moreover, this is followed by the description of working principles of various analytical techniques such as X-ray diffractometer (XRD), X-ray reflectivity (XRR), Reciprocal space map (RSM), Field emission scanning electron microscope (FESEM), RAMAN and Physical Property Measurement Systems (PPMS) which include Vibrating Sample Magnetometer (VSM), Ac susceptibility, specific heat and Impedance measurements. Further, the details about the fabrication of the supercapacitor electrode has been given. In the end, the working principles of electrochemical measurement techniques (Cyclic voltammetry, Galvanostatic charge-discharge, and electrochemical impedance spectroscopy) have been discussed Chapter 3 discusses the synthesis of polycrystalline compound and study its structural magnetic and thermal properties. The present studies are based on Pr2MnNiO6 polycrystals with a considerable amount of anti-site disorder. A reduced TC~203 K, preceded by Griffith`s phase between 230 and 203 K is observed. Magnetization at 5 K in a field of 5 Tesla attains a lower value of 3.7B. The imaginary part of ac susceptibility ((T)) exhibits a frequency-dependent peak in the range 60-90 K corresponding to freezing temperature (Tf). Studies based on critical slowing down reveal that the system enters a re-entrant spin glass state with a large time constant of ~10-4 s. The spin-glass behavior is also supported by a very slow decay of thermo-remnant magnetization. Specific heat analysis reveals a broad Schottky anomaly near 10 K, along with a linear spin-glass like term which gets suppressed in a field of 5 Tesla. Chapter 4 investigates the dielectric and magneto-dielectric properties of the monoclinic bulk phase of double perovskite Pr2NiMnO6 (PNMO) as a function of the frequency and temperature. The dielectric permittivity is independent of frequency at low-temperature (less than 100 K) however, the dielectric relaxation peaks occur at high temperatures in the frequency range of 997 Hz- 997999 Hz. The peaks in dielectric relaxation spectra (tan δ) of PNMO increase gradually with temperatures from 112 K to 197 K as a function of frequency. Fine relaxation peaks are observed for dielectric relaxor behavior due to the existence of the dipole moment of the PNMO molecules. This dielectric relaxation of PNMO can be explained in terms of the electron hopping between different transition metal ions and could be well-explained by Maxwell-Wagner interfacial polarization effect. The dielectric strength, conductivity, distribution parameters, and conductivity have been analyzed by using the Cole-Cole fitting curve. The magnetic and dielectric properties of the PNMO sample have been investigated as a function of the external stimuli such as magnetic and electric field which contributes to charge transfer mobility and enhancing the ionic conductivity of ferromagnetic materials. Chapter 5 discusses an experimental realization of an instance of strain disorder and biaxial strain in the thin film of double perovskite Pr2NiMnO6, which shows a signature of two distinct ferromagnetic orders together with a local crystal symmetry distortion. In this study, we grow the epitaxial thin layers of varying thickness on different substrates, which were examined in detail with high-resolution x-ray diffraction, vibrational spectroscopic, and magnetic measurements. We observed an additional phonon excitation in Raman spectroscopic studies and higher values of saturation magnetization, which in turn confirms the stabilization of the cationic site-ordered double perovskite structure in our thin films.Chapter 6, we developed a novel approach for creating binder-free transparent supercapacitors (Trans-SC) that balance high energy density with optical transparency. Using double perovskite PNMO thin-film electrodes on a conductive indium-doped tin oxide substrate, we constructed a binder-free optically transparent symmetric supercapacitor through pulsed laser deposition. The PNMO thin-film exhibited remarkable optical transmittance of 70%. Our investigation of the electrode’s electrochemical behavior in a KOH electrolyte confirmed their transparency remained unaffected. The binder-free Trans-SC achieved a high areal capacitance of 50 mF cm–2 at 1 mA cm–2 and a maximum areal energy density of 10 mW h cm–2, with excellent cycling stability even after 20,000 charging–discharging cycles. These findings position the binder-free Trans-SC as a promising energy storage solution for transparent electronic devices, advancing the field of transparent energy storage technologies. Chapter 7 concluded the present work and contains the findings from the present research work as well as the scope of future work.