Quantum spin state in the rare-earth compound YbAl ₃C ₃

Magnetic properties of YbAl ₃C ₃ with the hexagonal ScAl ₃C ₃-type structure have been investigated by the magnetization (M) and specific-heat (C) measurements under magnetic fields (H). YbAl ₃C ₃ is reported to show a spin-gap state, which is considered to be ascribed to a magnetic dimer formed in the orthorhombic phase below the structural transition temperature (T _s= 77 K). Present study has revealed history-dependent magnetic properties below T _s and field-induced anomalous magnetic states at low temperatures. The former is considered to be attributed to the cross correlation between the structural deformation and the magnetic field similar to those observed in multiferroic materials, although YbAl ₃C ₃ below T _s is not in the ferromagnetic state but certainly in the dimer state. The latter is partially similar to that observed in the field-induced ordered phase (FIOP) of d-electron dimer systems. The M versus H curve at low temperatures exhibits a kink at a certain magnetic field and then it increases in proportion to the field like that of FIOP. However, neither kink nor peak suggesting the emergence of FIOP is observed in the temperature dependence of both M(T) and C(T). Instead, C/T shows an anomalous increase proportional to -lnT with decreasing temperature in a finite field range, suggesting an anomalous disordered state such as a non-Fermi-liquid state in a strongly correlated f-electron system. These anomalous magnetic states may be partially relevant to characteristics of f-electron dimer system.