Multiferroicity and metal-insulator transitions in strongly correlated electron systems

Abstract

In this thesis, electric and magnetic properties of exotic condensed matter materials are explored via studies of La₂Ti₂₋ₓVₓO₇, CaMn₇O₁₂ and PrRu₄P₁₂. In order to form a proper multiferroic, single phase La₂Ti₂₋ₓVₓO₇ (x = 0, 0.0625, 0.125) was synthesised successfully by a vacuum annealing plus water quenching method. The magnetic properties of the doped samples were studied and compared with that of undoped La₂Ti₂O₇. The results reveal a primary paramagnetic-like behaviour and a weak magnetic order rather than the ferromagnetic order for the doped samples, which is explained by a vanadium monomer phase formed at the high synthesis temperature. To obtain a single phase with ferromagnetic order, other novel synthesis method with a lower synthesis temperature has to be used. The multiferroic CaMn ₇O₁₂ was synthesised successfully by a flux method and isotope substitution effect on magnetic properties was studied. The 18O concentration was estimated to be 61 at.% by Raman Spectroscopy and 64 at.% from the mass difference. Smaller magnetization was observed for 50 < T < 120 K wherein two characteristic magnetic transitions occur and ferroelectric polarization develops. This can be considered as direct evidence that oxygen affects the magnetic properties and the ferroelectric polarization. However different divergence temperatures were also observed. Two additional experiments are suggested that would further clarify these changes in magnetization with oxygen mass. To investigate the order parameter of Pr below TMl = 62.3 K in PrRu₄ P₁₂ temperature dependent resonant soft x-ray diffraction in combination with x-ray absorption spectroscopy were performed at the Pr M 4,5 edges. A resonance enhancement of the (100) superlattice reflection signalling the order parameter of the Pr 4f shells was observed below T MI, with a steady increase with decreasing temperature. The experimental spectra and subsequent analysis rule out the existence of magnetic and/or orbital order as well as any Pr lattice displacement. The order parameter below TMI is likely due to charge disproportionation. Our results also indicate that a synergistic coupling of Pr 4f – Ru states plays an important role in the metal-insulator transition associated with charge density wave state formation.