Modified spin-wave description of the nuclear spin relaxation in ferrimagnetic Heisenberg chains

We make a modified spin-wave description of the nuclear spin relaxation in Heisenberg alternating-spin chains with antiferromagnetic exchange coupling. In contrast with the conventional one-dimensional antiferromagnetic spin-wave theory, which is plagued with the divergence of the sublattice magnetization even in the ground state, the present spin-wave description is highly successful over a wide temperature range. The temperature dependence of the relaxation rate $T_1^{-1}$ significantly varies with the crystalline structure, exhibiting both ferromagnetic and antiferromagnetic aspects. $T_1^{-1}$ further shows a unique dependence on the applied field, which turns out an indirect observation of the quadratic dispersion relations.