MAGNETOCALORIC, MAGNETORESISTANCE AND MAGNETODIELECTRIC EFFECTS IN UNDOPED AND DOPED EuTiO3

Abstract

Rare-earth titanates (RTiO3) are mostly Mott-Hubbard type insulators at 4.2 K. While the rare earth ion is trivalent in most of the RTiO3 oxide, EuTiO3 is exceptional because Eu is in divalent state. The coexistence of magnetically active Eu2+: 4f7 and ferroelectric active Ti4+: 3d0 ions in EuTiO3 provides a route to magneto-electric coupling between localized 4f spin of Eu2+ ion and ferroelectric ordering that can be induced by off center displacement of Ti4+ ion. Although synthesis and magnetic structure of EuTiO3 has been reported in late 1970's, EuTiO3 attracted a substantial research attention only recently after the demonstration of magnetocapacitance effect in a single crystalline sample. For Ph.D. thesis, I have studied magnetocaloric effect (MCE), magnetoresistance (MR) and magnetodielectric effect (MDE) in pure and doped EuTiO3 by means of different techniques. Magnetocaloric effect refers to change in adiabatic temperature and isothermal entropy of a magnetic sample and it is much sought after magnetic refrigeration. We found that EuTiO3 exhibits the highest adiabatic temperature change among the transition metal oxides of perovskite structure. Adiabatic removal of a magnetic field of 7 T at 27 K can reduce the temperature of the sample by 10 K. This offers a great potential for liquefying hydrogen gas, which is considered to be a promising clear energy source. EuTiO3 is also found to exhibit colossal negative magnetoresistance (MR = -99.95 % under the field of 0.5 T and at the temperature of 2 K). This is the first time magnetoresistance of such a high magnitude has been found in oxides other than manganites (RMnO3 compounds doped with holes) and Tl2Mn2O7. I also show that giant positive magnetodielectric effect (~ 700 %) exists in polycrystalline EuTiO3. The effect of dilution of Eu2+ :4f7 spins on the magnetic, magnetocaloric, magnetoresistance and magnetodielectric properties was studied through Eu1-xBaxTiO3 and Eu1-xLaxTiO3 systems. While the isovalent Ba2+ substitution for Eu2+ does not dope any electron or hole in system, the substitution of aliovalent La3+ for Eu2+ seems to dope electrons into t2g orbital of Ti-3d band of EuTiO3. Interestingly, magnetoresistance was found to change sign with increasing carrier density in Eu1-xLaxTiO3 series. Correlations between magnetoresistance, magnetization and magnetodielectric properties of pure and doped EuTiO3 are explored in this thesis. Our work suggests that a strong 4f-3d interaction between Eu and Ti cations is responsible for the observed extraordinary phenomena in this family of oxides.