Investigation of Magneto electric Effect in CoFe2O4-BiFeO3
Core-Shell Nano particles

Abstract

This thesis consists the details about the development of magnetoelectric voltage measurement set up and studies of various properties such as magnetic,magnetoelectric,
magneto resistance and magneto impedance of CoFe2O4-BiFeO3 core-shell nanoparticles.A magnetoelectric(ME)voltage measurement set up has been developed following the
dynamic method.This method is widely used as one can measure linear as well as quadratic magnetoelectric coefficients.In this setup,ME measurement can be performed by varying dc magneticfield(±1T)and temperature(120K400K).The calibration of this set up is executed successfully.For the case study,ME measurement is performed on BiFeO3 and its cobalt substituted(1%and2%)systems.The temperature dependent first order ME coefficients of BiFe1􀀀xCoxO3 (x=0,0.01,0.02)suggests the involvement of magnetic disorder can enhance the ME couplings by many folds.In the intrinsic region a nearly same value of the firs to rdercoefficient for all the three samples are found.The behavior of higher order coefficients shows that parent BiFeO3 is of mono-glassy character whereas indications of multi glass nature is seen in the cobalt substituted BiFeO3.This study suggests that,a very light substitution of Coat the Fesite may leave the first order coefficients unperturbed but may initiate multi glassiness in the sample,which is highly
desirable for enhancing the magneto electricity.The subsequent part of the thesis deals with the investigation of magneto electricity in CoFe2O4 -BiFeO3 core-shell nanoparticles. The core-shell nanoparticles are prepared via sol- gelauto- combustion technique,by varying
the relative molar concentration of core and shell materials(40% CoFe2O4-60% BiFeO3,
50% CoFe2O4-50% BiFeO3 and 60%CoFe2O4-40%BiFeO3).The core-shell nature is confirmed from transmission electron micros copy measurement.Whereas,the Rietveld refinement of X ray diffraction pattern confirms the presence of BiFeO3 (R3c) and CoFe2O4 (Fd-3m). Isothermal magnetic field dependent magnetization measurement indicates the ferromagnetic nature of the core-shell nano particles,but at room temperature and above they exhibit super paramagnetic behavior.Signature of two types of spin dynamics:(a) particle size dependent spin blocking and(b)spin-disorder is found from derivative of
ΔM(=field cooled magnetization-zero field cooled magnetization)vs temperature plot. These two spin dynamic processes are found to contribute independently in the generation of magneto electric voltage.A very clear,first order and second order magneto electric voltages
are recorded.Resemblance of the first order magneto electric coefficient vs temperature plot to that of building up of order parameters in the mean field theory,suggests that spin
disorder can act like one of the essential ingredient in building the magneto electric coupling.Best result is obtained for 50-50 composition sample,may be due to better coupling of magneto strictive CoFe2O4, and piezo electric BiFeO3, because of optimum thickness of shell and core.Magneto resistance(MR)measurement is carried out near room temperature for CoFe2O4-BiFeO3 core-shell nano particles.The analysis of MR data reveals that the MR
is guided by an isotropic magneto resistance(AMR)and giant magneto resistance(GMR) effect.A clear GMR and AMR effect has been observed in parallel and perpendicular mode
of magneto resistance measurement.GMR effect is found predominant in sample having maximum shell thickness whereas;the AMR effect dominates the least shell thickness and bare CoFe2O4. Analysis of the MR data in conjunction with the resistivity,suggests that the electrical conduction in these samples is due(a)tunneling and(b)hopping of electron across the domains.Impedance and modulus spectra analysis shows that the core-shell particles as a whole is participating in the relaxation process,thus indicating a strong coupling between the core and shell.Temperature dependent impedance spectroscopy ofCoFe2O4-BiFeO3
core-shell nano particles is investigated.The relaxation mechanism near room temperature is found to be significantly affected by the magnetic disorder.The effect of disorder becomes insignificant when the temperature is raised above 380K and the De bye like relaxation is established.The phenomenon of transition from disordered to De bye relaxation is of activated nature having activation energy0.45eV,whichislowerby0.1eV,compared to the activation energy of De bye-like relaxation sit self.Magneto impedanced at a shows as trong dependence of relaxation spectra on the external magnetic field,there by manifesting the signature of magneto electric(ME)effect.The relative concentration of core-shell is found to play a prominent role in determining the nature of ME coupling,there by offering a
possibility of tailor tuning the ME ffect.