STRUCTURAL AND MAGNETIC PROPERTIES OF PYROCHLORE STRUCTURED MATERIALS

Abstract

This thesis focuses on the investigations of structural and magnetic behavior of pyrochlore structured materials synthesized by standard solid-state method. The two main ideas of my work magnetism and geometric frustration are discussed in this thesis. This work explores the substitution effect at A site and B site of Ho2Ti2O7 pyrochlore on structural and magnetic properties and understands the frustration phenomenon at low temperatures. The pyrochlore structured ceramic A2B2O7 (A and B stand for rare earth elements and transition metal respectively) has fascinated an excessive deal of research interest over the last two decades. Important titanate oxides (A2Ti2O7) have a wide range of applications, such as solid catalysts, electrolytes, nuclear reactor materials, and more. Due to its unique low temperature magnetic characteristics, Ho2Ti2O7 is known for both its remarkable spin ice behaviors and its evident quantum nature. A crucial aspect of focus for theorists and experimenters is geometrical frustration in pyrochlore oxides. Strong magnetic characteristics are provided by rare-earth ions with unpaired electrons in the 4f configuration, which exhibits enormous magnetic moments. The effect of B site Ge doped pyrochlore titanate Ho2(Ti1-xGex)2O7 (x = 0.0, 0.2 and 0.4) is detailed, along with its structural and magnetic behaviors. As per structural analysis, the pyrochlore superstructural ordering improves with rising x-value in Ho2(Ti1-xGex)2O7 as seen by the increase in radii ratio (RA/RB) or variation in O48f oxygen as on increasing Ge content over Ti. Further confirmation of the pyrochlore phase with hardness of the phonon mode by Raman spectroscopy was provided by the interaction between phonons and the phonon anharmonic. Conversely, magnetic analysis of the spin-frustrated material Ho2(Ti1-xGex)2O7 pyrochlore revealed a decrease in magnetization caused by weakened ferromagnetic character and non-magnetic 16c site contributed by rise in chemical pressure. Strong crystal field generated Ising type anisotropy which results in a "two-in/two-out" arrangement in the spin [vi] configuration according to the spin ice rule produces ferromagnetic exchange interactions. Thus, pyrochlore structured compositions that are more stable exhibit less ferromagnetic character. Further, despite reducing the temperature to 2 K, no spin glass like transition was seen. Further, we have also studied the effect of A site nonmagnetic La3+ doping on magnetic and structural behavior of Ho2-xLaxTi2O7 (x = 0.0, 0.1, and 0.2) pyrochlore. Structural studies revealed that a stable single-phasic pyrochlore crystal structure had formed. Raman research has shown the red and blue shift of the phonon mode and an increase in structural ordering as a result of the inclusion of La3+ into Ho2-xLaxTi2O7, which has increased the ionic radii ratio. In all specimens, it was noted that the ferromagnetic feature continuously reduces with chemical pressure. This trend in the magnetic data is caused by variations in the dipolar and exchange interaction. A comparison of the doping at the A and B sites in the Ho2Ti2O7 demonstrates that the θCW dropped by 71% and 33% as a result of 10% doping of larger cations (La3+) at the A site and smaller cations (Ge4+) at the B site respectively. These findings support the conclusion that B site doping is more susceptible to controlling magnetic characteristics than A site doping. Ho2Ti2O7 doped with La3+ showed a significant decrease in the θCW indicating that the θCW can be tuned by doping at the A site. Finally, Probed the influence of Ho3+ substitution at A-site on structural and magnetic studies of (Gd1-yHoy)2Ti2O7 (y = 0.0, 0.2, 0.4 and 0.8) pyrochlore. Antiferromagnetically (AFM) linked Heisenberg spins are predicted to be extremely frustrating in the Gd2Ti2O7 pyrochlore lattice because there are numerous ways to minimize the exchange energy. The result of rising Ho content is that there is increase in magnetic parameters (θCW and μeff) and decrease in the ionic radii ratio (RA/RB) which leads to enhancement of ferromagnetic feature of the materials. Magnetic studies have also demonstrated the system becomes less antiferromagnetic (AFM) character as a result of the increased chemical pressure in Gd2Ti2O7. This is due to generation of ferromagnetic interaction among Gd–Ho (Ho–Ho) spins and disrupted network of Gd3+ ions. This character is considered [vii] to be relevant to the fight between FM and AFM interaction, which is correlated to local structural disorder and magnetic frustration. Although, Magnetic investigations did not reveal any evidence of a spin glassy pattern in all compositions. Based on these findings, it is logical to conclude that B site doping is more capable of controlling magnetic characteristics than A site doping. Furthermore, the nature and location of the dopant have an impact upon the value of θCW changes. As conclusion, when comparing the current work to prior findings, a higher increment in θCW was seen with an equivalent quantity of doping. This may be caused by the Gd-Ho cation's significant crystal field splitting. Structural evaluation has proven that cubic pyrochlore (impurity-free) crystals with a cell parameter that continuously shrinkage and justifies Vegard rule. The crystallite size figured from W-H graph is diminished with magnetic Ho substituting which is validate the reduction in structural ordering of prepared specimens. Raman bands show the red and blue shift of phones modes with increasing Ho content. Single ion anisotropy with dipolar exchanges was realized to harvest interesting magnetic assets. Examining this structure's whole AC and DC magnetic learning is exceedingly difficult. It would be exciting to examine the crystal structural transformations of such systems in the future at temperatures where spin ices are prone to freezing and becoming unstable.