Insulating Magnetic Chains: Ionocovalent Compounds

Abstract

The determination of the ground state of a magnetic system is of fundamental importance for the understanding of the basic physics leading to the particular ground state. In 3D-systems the ab initio calculation is usually impossible, since the interactions involved cannot be predicted quantitatively. However, phase-diagrams have been investigated in great detail theoretically as well as experimentally [1]. Such studies can be accomplished, since one deals with a system with true long range order (LRO). In this situation a thorough theoretical description is possible, and usually no particular care must be taken to consider quantum effects. Most of the effort in the studies has been devoted to the phase transitions between the different regions in the phase-diagram. The points in the phase-diagram, where phase-lines meet, are of particular interest, since special types of phase-transitions may occur: bicritrical, tricritical or tetracritical points are found. At these special points, the critical behaviour is different from a usual continuous phase transition. In particular the critical exponents have special values at these particular points [2]. In 1D-systems the situation is fundamentally different, since LRO exists at T = O only, if at all. Thus, phase-diagrams in external parameters, as magnetic field and temperature, can not be realized in one dimension. Thus at T = OK, only phase-diagrams can be considered for a variation of the parameters in the Hamiltonian: exchange energy J and/or anisotropy energy A.