THEORETICAL AND EXPERIMENTAL STUDY OF MAGNETISM AND MAGNETOCALORIC EFFECT IN CERTAIN RARE-EARTH ORTHOFERRITES AND TRANSITION METAL COMPOUNDS

Abstract

The field of research in strongly correlated electron systems (SCES) has witnessed tremendous growth in recent decades. These systems display exotic electronic and magnetic properties because of the presence of strong Coulomb correlations in partially filled 3d or 4d bands of transition metal ions or 4f and 5f bands of rare earth ions. The well-known “band theory of solids” was not successful in explaining the insulating behavior of these systems. In 1949, Mott had shown that strong electron - electron Coulomb repulsion (known as strong Coulomb correlation) prevents the hopping (or delocalization) of electrons from one atomic site to the other and therefore leads to the insulating state. The simplest model describing this phenomenon is the nondegenerate Hubbard model: given by Hubbard in 1963. Many transition metal (TM) and rare earth (RE) oxides belong to this exquisite class of SCES. These TM and RE oxides form one of the active fields of research in condensed matter physics as they show variety of complex phenomena like high Tc superconductivity, colossal magnetoresistance, phenomena where the electric, magnetic, thermal and structural properties are coupled with each other such as magneto-capacitance, magnetoresistance, magnetoelasticity, magnetocaloric effect, multiferroicity and so on. One of the major characteristics of SCES is the presence of local magnetic moments due to localization of electrons. Therefore, magnetism is a dominant feature in many strongly-correlated systems. In this context, the compounds containing both transition metal (TM) and rare earth (RE) ions form one of the most interesting classes of solids with large number of novel properties arising due to the interaction between 3d and 4f states which can be explored for various applications. Rare earth orthoferrites (RE-OFs) which belongs to this category display interesting differences in their magnetic structures owing to the particular nature of interactions among the 3d states of TM and 4f states of RE ions present in the compound. Temperature induced spin reorientation transition of Fe3+ spins is one of the important phenomena observed in many RE-OFs using neutron diffraction measurements where the spin moments of Fe3+ ions reorient themselves from one crystallographic axis to the other when temperature is changed. Apart from spin reorientation, orthoferrites also possess huge potential for application in the field of magnetic refrigeration. In this thesis, we have studied the magnetic properties including the spin-reorientation and magnetocaloric effect of doped RE-OFs using various theoretical and experimental techniques. Another class of rare earth compounds we have studied in this thesis is that of Halden spin chain system Tb2BaNiO5 and Sr doped Tb2BaNiO5 which are in limelight due to their unusual multiferroic behavior which cannot be explained within conventional Dzyaloshinski-Moriya interaction based models, thereby needing further theoretical investigations. In order to understand the mechanism behind various physical phenomena observed in these materials, we have carried out first-principles density functional theory (DFT) calculations using DFT codes such as VASP and WIEN2k. The DFT calculations have been performed within various approximations such as GGA, GGA+U and GGA+SO+U as found appropriate for the problem concerned. We have also performed various experimental measurements like x-ray powder diffraction, neutron powder diffraction, magnetization measurements, specific heat and magnetic entropy measurement to investigate the structural and magnetic behavior of the compounds. Finally, to explain our experimental results on the magnetocaloric effect, we have carried out Monte Carlo (MC) simulation on model spin Hamiltonian.