• density matrix renormalization group;
  • low temperature;
  • spin chains;
  • spin waves


We study the low-temperature properties of a mixed equation image alternating quantum spin chain with antiferromagnetic–ferromagnetic bond alternation and single-ion anisotropy using spin-wave theory, density-matrix renormalization group calculations and exact diagonalization of finite clusters. An instance in this system is the recently synthesized bimetallic chain equation image [LCuIICoII(NCS)2], which shows a novel magnetic behavior, namely, the equation imageversusequation image curve decreases rapidly at low temperatures after displaying a pseudo-plateau around a certain intermediate temperature. There are two different mechanisms which could explain this unconventional feature: the zero-field splitting of the ground state and/or the ferromagnetic nature of the interdimer interactions. We clarify the role of these two kinds of mechanisms in the observed properties of the system by deviating from the otherwise-expected ferrimagnetic ground state and considering a slight deviation from the decoupled-dimer limit, namely the description of the system by means of effective spin-1 ions at each unit cell with residual antiferromagnetic interactions, which is consistent with the antiferromagnetic–ferromagnetic bond alternation.