Low-energy magnetic excitations in the mixed spin-( 12,52 ) chain

We present studies on the low-energy magnetic excitations in the Lieb-Mattis (LM) ferrimagnetic phase. We investigate the low-energy magnetic excitations in the mixed spin-($\frac{1}{2},\frac{5}{2}$) chain $(4\ensuremath{-}\mathrm{Br}\ensuremath{-}o\ensuremath{-}\mathrm{Me}\mathrm{Py}\ensuremath{-}\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}₄$ based on low-temperature specific heat and multifrequency electron spin resonance (ESR) measurements. A clear $\sqrt{T}$ term is observed in the specific heat, which indicates a contribution of the low-energy magnetic excitation with a ferromagnetic quadratic dispersion expected in the LM ferrimagnetic state. Through the ESR mode analysis, we examine the effects of magnetic anisotropy on the excited states in the LM plateau phase.