Investigating Magnetic Transitions and the Quantum Spin Liquid State in Complex Oxides

Abstract

Magnetic properties emerge from the interplay of spin, crystal structures, and external factors such as temperature, magnetic fields, and pressure, resulting in a diverse range of magnetic phenomena with potential applications in sensors, data storage, quantum technologies, etc. Orthoferrites (RFeO3) display magnetic properties such as spin reorientation (SRT), spin switching (SSW), ultrafast switching, magnetocaloric effect, exchange bias and magnetic compensation. Fe spins adopt magnetic configurations such as Γ1, Γ2, and Γ4 in these compounds. SRT occurs when the magnetization axis rotates from one crystallographic direction to another. ErFeO3 shows SRT between 88 and 98 K, whereas SmFeO3 exhibits SRT between 450 and 480 K. Substituting Sm into ErFeO3 allows tuning of SRT and spin-switching transitions, as revealed by magnetization versus temperature measurements on single crystals of Er1-xSmxFeO3. Additionally, the applied magnetic field and the sample history - thermal and magnetic, influence spin-switching transitions. In addition to orthorhombic structure, RFeO3 compounds can stabilize in a metastable hexagonal phase when synthesized via the sol-gel method. In a hexagonal structure, the magnetic ordering temperature (TN) rises as the c/a ratio increases, and the latter is high when the rare-earth ionic radius is small. Although the scandium ion is the smallest that can conveniently occupy the A-site, pure ScFeO3 is unstable in the hexagonal structure. This study shows that 67% Sc-substituted LuFeO3 stabilizes in the hexagonal structure, exhibiting magnetic ordering below 175 K and a high c/a ratio of 2.008. Neutron diffraction analysis indicates a mixed Γ1 + Γ2 magnetic structure, while field-dependent dielectric measurements confirm a magneto-dielectric effect of up to 1.5%. Geometric frustration in magnetic lattices hinders conventional magnetic ordering, leading to exotic states such as quantum spin liquid (QSL), which is characterized by the absence of long-range magnetic ordering, and highly entangled, fluctuating ground states that persist near absolute zero. Investigations of triangular lattice SmTa7O19 using DC and AC magnetic susceptibility, specific heat, and µSR measurements reveal no long-range order down to 30 mK, with µSR (LF) confirming a fluctuating ground state typical of a QSL. Disorder in a lattice often results in spin freezing. However, recent research suggests that disorder can enhance competing magnetic interactions and quantum fluctuations, stabilizing QSL states. Lu2CuTiO6 crystallizes in a hexagonal structure with random Cu and Ti distribution in the ab plane and forms a triangular lattice. AC susceptibility exhibits a small peak at 0.2 K, but no peak appears in the specific heat data at the same temperature. It suggests that the origin of the peak is not long-range magnetic ordering. The absence of long-range magnetic ordering or freezing down to 50 mK makes this material interesting for realizing a spin liquid state.