Abstract

The modulated structures involving the ordered arrangements of electrons, spins, and orbitals can lead to complex and fascinating physical properties in materials. This thesis focuses on the spin-ordered material YBaCuFeO5 (YBCFO) and the charge-ordered material Ir2In8Se (IIS).

In YBCFO, neutron scattering and resonant elastic x-ray scattering experiments established a low-temperature double-helical magnetic structure composed of the spin arrangements of Fe³⁺ and Cu²⁺. Neutron scattering experiments with an applied magnetic field revealed complex magnetic phase transitions, indicating the formation of magnetic domain structures below the phase transition temperature and competition among the domains. Moreover, a memory effect induced by the magnetic field was observed.

For IIS, temperature-dependent resistivity measurements revealed a change in slope around 200 K, and heat capacity measurements indicated the presence of long-range ordered structures below the same temperature. X-ray scattering experiments identified two distinct modulated structures at low temperatures with qCM = (0.5, 0.5, 0) and qICM ~ (0.24, 0.24, 0). Temperature-dependent X-ray scattering experiments showed that the modulated structure ICM undergoes a second-order phase transition, while the CM structure exhibits an inverse order-disorder transition around 150 K, suggesting the interaction of two different order parameters within the system. It is speculated that the ICM modulated structure is driven by charge density waves, while the CM structure arises from long-range order formed by phonon condensation.