We have studied the system with alternating strong and weak exchange coupling of constants J 1 and J 2 and found how the magnetic properties depend on the ratio J 2 / J 1 . The results obtained for smaller J 2 / J 1 by calculation on the basis of the model of pair and lone spins much differ from those obtained by the simple molecular field approximation.

Magnetism of Spin System with Alternating Strong and Weak Exchange Coupling. I. Theoretical Study

On the basis of the dynamical correlated effective-field approximation (DCEFA) the authors study the magnetic properties of an antiferromagnetic system with crystal structure of the hexagonal BaTiO3 type which has two kinds of inequivalent magnetic-ion sites. They calculate the Neel temperature and the sublattice magnetisation for various sets of exchange parameters. The results of calculation are compared with those obtained by the simple molecular-field approximation and the pair and lone spin model. One of the advantages of the DCEFA is that it can treat moment reduction of both kinds of inequivalent spins. The spin wave dispersion is also calculated.

Theoretical study of the magnetic properties of an antiferromagnetic system with two kinds of inequivalent magnetic-ion sites

MAGNETISM OF ANTIFERROMAGNETIC RbMnCl 3

The ensemble consists of N clusters each containing two spin 1/2. The energy spectrum is derived from an exchange interaction J between the two spins of one cluster, a Weis-molecular-field Hm = γm (m averaged magnetization of the random ensemble), and a Zeeman-term in an external magnetic field Hext. In special cases the four-level-system can be approximated by a two-level-system. Averages are performed over the two parameters J and γ. The averaged magnetization m of the ensemble as function of Hext and T is obtained from a nonlinear self-consistency-problem. The susceptibility χ(T, Hext) is also calculated. The application to experimental results is discussed.



Das Ensemble besteht aus N Clustern, die jeweils aus zwei Spin-1/2-Einheiten aufgebaut sind. Das Energiespektrum leitet sich aus einer Austauschwechselwirkung J zwischen den zwei Spins eines Clusters, aus einem Weisschen Molekularfeld Hm = γm (m gemittelte Magnetisierung des randomisierten Ensembles) und einem Zeeman-Term im auseren Magnetfeld Hext ab. Das 4-Niveau-System kann in speziellen Fallen durch ein 2-Niveau-System approximiert werden. Uber die beiden Parameter J und γ werden Mittelungen durchgefuhrt: Die gemittelte Magnetisierung m des Systems als Funktion von Hext und T wird aus einem nichtlinearen Selbstkonsistenz-Problem gewonnen. Ergebnisse fur die Suszeptibilitat χ(T, Hext) werden ebenfalls angegeben. Die Anwendung auf experimentelle Resultate wird diskutiert.

Magnetic Properties of a Random Ensemble of Two-Spin Clusters

A coherent potential approximation (C.P.A.) theory has been developed in order to examine the bond dilution problem in an antiferromagnet which contains alternating strong and weak magnetic exchange interactions. Specifically, we have calculated the concentration dependence of the transition temperature at which the system displays long‐range order. This is done by examining the Green function for the system in the paramagnetic phase. The existence of the strong exchange interaction creates pairs of highly correlated spins in the paramagnetic state. The transition temperature is identified by introducing the weak coupling between these spin pairs and locating the temperature at which the excitation of a pair from the singlet ground state to the lowest excited states becomes soft. When only interpair bonds are broken, a linear concentration vs transition temperature behavior is predicted. This prediction is compared with recent experimental results for the diluted paired antiferromagnet, RbMn1−xMgxCl3.

T. Egami

Papers

C.P.A. theory for a dilute paired antiferromagnet