Thermodynamics and correlations of a helical spin chain with non-magnetic impurities

The effect of non-magnetic impurities on the thermodynamics of a classical helimagnetic chain with antiferromagnetic nearest-neighbour interactions and next-nearest-neighbour interactions is studied by means of the transfer matrix method. The results for the partition function and the nearest-neighbour and the next-nearest-neighbour spin correlation functions are obtained exactly for both the isotropic Heisenberg model and the planar model as functions of temperature and concentration of non-magnetic impurities. The magnetic susceptibility is calculated up to terms linear in the concentration. It is found that a non-magnetic impurity modifies the helical spin structure in its vicinity and leads consequently to the formation of a new local equilibrium spin structure, which is called a spin complex. The consequences of this peculiar spin complex on the thermodynamics, especially on the spin correlation functions, are extensively discussed in connection with experimental results on FeMgBO ₄.