Investigation of field-induced metamagnetism and spin-phonon-charge coupling in Ising spin chain systems

Naik, Debismita (2026) Investigation of field-induced metamagnetism and spin-phonon-charge coupling in Ising spin chain systems. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Debismita Naik (17IP012)) 17IP012.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

This thesis investigates the magnetic properties and phase transitions of spin-chain systems, focusing on α–CoV₂O₆ and γ–CoV₂O₆. α–CoV₂O₆, exhibiting an antiferromagnetic ground state, demonstrates rich phase transitions under external magnetic fields. The study of tricritical phenomena in spin-chain systems, specifically in α–CoV₂O₆, presents a unique opportunity to explore complex magnetic phase transitions and interactions under external fields. α–CoV₂O₆, a quasi-one-dimensional material with an antiferromagnetic ground state, exhibits fascinating magnetic properties, including rich and intricate phase transitions, both at low and high temperatures. We investigate the critical exponents of the magnetic field in polycrystalline α–CoV₂O₆, finding that the values of β, γ, and δ align closely with the predictions of the Widom scaling law and follow established scaling relations, suggesting the presence of a field-induced tricritical point in the system. A detailed H-T (magnetic field-temperature) phase diagram reveals the tricritical point at which ferromagnetic, paramagnetic, and field-induced ferromagnetic phases converge, while a triple point is identified, marking the intersection of paramagnetic and two distinct antiferromagnetic states. The intricate nature of this phase diagram underscores the competition between various magnetic interactions in α–CoV₂O₆. At lower temperatures, the magnetic properties and lattice dynamics of α–CoV₂O₆ are further explored. DC magnetization and specific heat measurements confirm the presence of long-range antiferromagnetic order at the Néel temperature (T_N=15 K), along with short-range ordering below 100 K. The ground state of Co²⁺ in α–CoV₂O₆ is characterized by an effective spin of 1/2, arising from the combined effects of crystal field and spin-orbit coupling. Temperature-dependent Raman scattering measurements reveal spin-phonon coupling below 100 K, attributed to short-range order, well above T_N. Additionally, lattice dynamics show significant spin-lattice coupling, with magnetoelastic interactions occurring at T_N, influencing the magnetoelectric properties. The theoretical charge density maps confirm the presence of spin-charge coupling, showing the formation of electric dipoles between Co and O atoms in the antiferromagnetic state due to p-d hybridization. Finally, we investigate the low-temperature magnetic behavior of polycrystalline γ–CoV₂O₆ synthesized via the sol-gel method. Magnetization relaxation measurements support the presence of slow spin dynamics, indicative of spin-glass-like freezing. This phenomenon is attributed to the competition between antiferromagnetic and ferromagnetic interactions, further emphasizing the complex magnetic behavior exhibited by these materials. This work provides a comprehensive understanding of the magnetic properties, phase transitions, and the interplay between various degrees of freedom in α–CoV₂O₆ and γ–CoV₂O₆, with implications for future research into spin-chain systems and their potential applications in novel quantum materials.

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