Tuning of Spin Reorientation and Compensation Transitions in Ferrites

Abstract

Ferrites have attracted the attention of scientists and engineers in the last few years because of a combination of ferrimagnetic and insulating character. This makes them suitable for high frequency devices in telecommunication and radar systems. The subsets of orthoferrites and spinel ferrites form the focus in this thesis. Magnetic interactions between 4f and 3d electrons of rare earth and transition metal based perovskite oxides give rise to several exciting properties, such as magnetoelectric effect, multiferroicity, spin-reorientation transition, magnetic compensation, magnetization reversal, and spin switching. Perovskite structure of orthoferrites allows a great deal of flexibility for doping in the A and B sites. By choosing a suitable dopant with optimum concentration, many properties of the parent compound can be tuned. We were motivated to raise the spin reorientation transition temperature of RFeO3 to as near as the room temperature by doping either sites. In this study, several interesting phenomena of doped orthoferrites will be discussed. Alongside, multifunctional spinel ferrites offering novel electrical and magnetic properties are explored. Their structural and magnetic properties were tuned by varying the cation distribution which has an influence on many magnetic properties and magnetic compensation phenomena. The work presented in this thesis is broadly divided into three parts. In the first part, B-site doping (50% Cr) and its effect on the magnetic properties of orthoferrites have been detailed, especially changes in the spin reorientation transition temperature of TbFeO3 (TbFe0.5Cr0.5O3). A clear evidence of Griffiths phase was observed which was presumably due to short range spin fluctuations. This was later confirmed from the results of neutron diffraction and thermal conductivity measurement. Further, a signature of spin-phonon coupling was observed in the Raman spectroscopy data. In the second part, variations in the magnetic properties of SmFeO3 single crystal have been illustrated by doping the A-site with Yttrium. SmFeO3 has the highest spin reorientation transition temperature (420 K–460 K) and it is known to reorient from Γ4 to Γ2 magnetic spin configuration. When doped with a non-magnetic element as Yttrium, not only was this transition temperature brought to room temperature but also a new spin configuration was induced (Γ3) that was not observed in any of the parent RFeO3 compounds. The third and final part of the thesis discusses the results of a spinel ferrite LiFe5O8 whose cation distribution has been mapped in the octahedral and tetrahedral sites. A study of the effect of Cr doping in LiFe5O8 and the distribution of the dopant ions in tetrahedral and octahedral sites has resulted in interesting magnetic transitions which are highlighted here.