Quantum effects and Haldane gap in magnetic chains with alternating anisotropy axes

The isotropic quantum Heisenberg model with alternating uniaxial anisotropy axes is analyzed numerically by the density-matrix renormalization-group (DMRG) method. In the classical version, the model is applied to describe the magnetic properties of the S=2 zigzag chain containing Mn(III) acetate meso-tetraphenylporphyrin complexes coupled by the phenylphosphinate ligands which transmit antiferromagnetic interactions. Although the tensors representing the uniaxial magnetic anisotropy D and g factors are non-diagonal in the global coordination system, the DMRG approach has been successfully applied to this complex model in the entire temperature region studied. The predictions of our quantum approach are compared to those previously obtained from the classical one and the importance of quantum effects for analysis of the single-crystal susceptibility and magnetization is demonstrated. At low temperatures the magnetization in the field applied along the c direction increases much more slowly than the classical counterpart. The magnetization behavior is very sensitive to temperature. Moreover, the presence of a magnetization jump in the limit T→0 at the field H=3.8 Tesla can be an indication of the Haldane gap of the order of 10.2 K. The considerable differences are demonstrated for the temperature dependent single-crystal susceptibilities, but surprisingly they disappear after averaging over the three crystallographic directions which has not been reported before.