Index of content:
Volume 34, Issue 2, February 2008
- LOW-TEMPERATURE MAGNETISM
34(2008); http://dx.doi.org/10.1063/1.2834257View Description Hide Description
A theoretical study of the properties of the copper spinel is carried out in the framework of the spin Hamiltonian. Only the cation sublattice , containing the ions , and the diamagnetic sites , is considered. Nearest magnetic ions are coupled by effective exchange interactions via the intermediate anions: (double exchange), , (superexchange), where . For a random distribution of ions the general physical behavior pattern of the system as a function of the concentration of the alloying element is as follows. In the region there exists an infinite ferromagnetic cluster of intercoupled ions(percolation via strong 3–4 ferromagneticbonds). At finite ferromagnetic clusters appear, oriented “up” and “down” and coupled by antiferromagneticbonds. The total magnetization vanishes, and a spin separation or a state of the cluster spin-glass type is realized. At percolation via antiferromagneticbonds appears, and practically only simple isolated ferromagnet clusters in an antiferromagnetic matrix remain, oriented “up” and “down” in equal numbers. The limit compound consists 1/4 of “holes”—diamagnetic sites with broken bonds—and 3/4 of cations with antiferromagneticbonds.Percolation via 3–4 bonds leads not only to ferromagnetism but also simultaneously to a metallic state owing to the double-exchange mechanism (the formation of a ferromagnetic half metal). In the absence of percolation via 3–4 bonds the system is an insulator. Thus as the copper spinel is alloyed with antimony, concentration phase transitions occur: a metallic ferromagnet is transformed to an insulator with spin separation , which in the limit becomes an antiferromagnetic insulator.
34(2008); http://dx.doi.org/10.1063/1.2834258View Description Hide Description
Possible mechanisms for the formation of long-period structure in the compound are considered. It is found that the main role in the theoretical treatment is played by invariants containing the first spatial derivative of the irreducible magnetic vectors to different powers. It is shown that in the formation of a spiral structure by only the antiferromagnetic vectors a spontaneous involvement of the ferromagnetic vector can occur, whereupon the orthogonality of the rotating vectors is broken. The analysis is carried out both with and without taking into account the approximate constancy of the moduli of the irreducible vectors. This makes it possible to consider the evolution of magnetic superstructure on changing temperature.