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Simulation of magnetic moments in a dilute induced-moment Heisenberg magnet

The author considers a diamagnetically diluted induced-moment Heisenberg magnet in which the active sites have a singlet electronic ground state and an excited doublet state. Moments may be induced at the active sites by mixing the doublet into the singlet through a magnetic exchange interaction or an external magnetic field Hz applied parallel to the z direction. The single-ion Hamiltonian describing this system is -H= Sigma ijJijSiSj- Delta Sigma i(Siz)2-Hz Sigma iSiz, where Delta is the single-doublet separation. Using self-consistent mean-field calculations on computer-generated lattices in one, two and three dimensions, and with connectivities z=2, 3, 4, 6 and 8, he studies the distribution of moments at T=0 as a function of the concentration of diamagnetic impurities, the ratio Jij/ Delta and H. When Hz=0, diamagnetic atoms rapidly reduce the value of the mean induced moment, leaving moments of significant size only in regions composed of active sites that are completely surrounded by other active sites. The sensitivity of the lattice towards dilution increases as z increases. The application of Hz compensates to some extent for the effect of the diamagnetic impurities, raising the mean-moment and reducing the variance of the distribution of the size of individual moments.