Study of Tailored La2NiMnO6 and its Derivatives for Magnetoelectric Applications

Abstract

With the ever increasing demand for up-gradation in technology, there has been a sudden gush for the materials exhibiting magnetoelectric coupling. In this regard, La2NiMnO6 and its derivatives are synthesized and studied for magnetoelectric applications in this thesis.
Crystal symmetry phase diagram of La2NiMnO6 prepared via sol-gel route and sintered under ambient conditions at various high temperatures and times are studied via powder X-ray diffraction (laboratory source and synchrotron source), powder neutron diffraction and X-ray Photoelectron Spectroscopy (XPS). The XPS results suggest emergence of 2+ oxidation state of Mn with increase in sintering temperature. The diffraction studies establish rhombohedral (R-3c) symmetry for the samples sintered below 800°C, whereas above this temperature, the monoclinic (P21/n) phase gradually increases. Also, a thermodynamic phase diagram of symmetry evolution as a function of sintering temperature and time is drawn. However, crystallographic phase transition from mixed (P21/n + R-3c) to R-3 is seen at high temperature (at ~650 K). Since this transition does not follow the group-subgroup relationship, it is suspected to be of displacive type and chances of it being of first order phase transition is not ruled out. Interestingly this structural transition is also accompanied by slight magnetic ordering. An explicit dependence of relative crystallographic phases on the measured magnetoelectricity is observed. The maximum value of magnetoelectricity is observed when the two phases are in nearly equal proportion i.e., the sample sintered at 1000°C for 6 hours. The results obtained, suggests interfacial magnetoelectricity in these samples.
The prepared ceramic sample 4H is found to be highly porous and it is difficult to study the true magnetoelectric coupling associated with it because of the leakage current. So poly(vinylidene fluoride) (PVDF) impregnated 4H samples are synthesized and a comparative study with its parent sample (4H) and 24H sample (which is relatively more compact) were done. DC magnetization study reveals the single magnetic transition in 4H sample whereas double magnetic transition in 24H sample. In impregnated sample, beta phase crystallization of PVDF is observed in FTIR spectra. The magnetoelectric voltage is measured as a function of temperature, and it is found that the room temperature region is dominated by the magnetoconductivity / magnetoloss which results in low ME voltage. At low temperatures (T < 200 K) the coefficient of magnetoelectric voltage improves, the loss component becomes negligible so that the true magnetoelectricity is observed. Comparative study on all the three samples (4H, 24H and 4H+PVDF) shows significant enhancement in the magnetoelectricity in the 4H+PVDF sample. The highlight of this work is the unique way to arrest the porosity by PVDF impregnation into the pores of La2NiMnO6, so that leakage is minimized.
Thereafter, various forms of multiferroic polymer composites of La2NiMnO6 and PVDF were synthesized in the form of discs and thick films, via a two-step method and solution casting method respectively. And all the prepared composite samples are studied via powder X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Field Emission Scanning Electron Microscope (FESEM). The XRD result suggests the biphasic (P21/n + R-3c) nature of La2NiMnO6 nanoparticles for disc samples and single phase (R-3c) for film sample. The FTIR analysis indicates strong interactions of La2NiMnO6 nanoparticles with polymer molecules in terms of electro-active beta-phase formation. Maximum fraction of beta-phase PVDF (~37 wt. %) with zero porosity was found in polymer composite film through FTIR and FESEM analysis. Interestingly, significant ME voltage is observed in all the polymer composites up to 400K (reported as the upper end of short-range magnetic ordering of the LNM sample). The maximum magnetoelectric voltage (10mV/cm-Oe) at room temperature is found in film composite samples. The results indicate that this synthesis technique integrates the formation of electro-active beta-phase PVDF with reduced porosity and hence large magnetoelectric voltage were obtained as an outcome.
Thin film of La2NiMnO6 is deposited successfully on Pt/TiOx/SiO2/Si substrates using pulsed laser deposition technique. The X-ray diffraction, Raman spectra, and X-ray photoelectron spectroscopy hints toward the formation of ordered double perovskites. Interestingly, significant magnetoelectric voltage up to 400 K is measured. Metallic behavior of the thin film is observed in the resistivity vs. temperature measurement.