Excitations of isolated clusters of magnetic ions

The neutron scattering by isolated clusters of Mn2+ ions in a single-crystal specimen of KMn0.18Zn0.82F3 has been studied as a function of wave-vector transfer Q at 4.3 K and as a function of temperature T at Q=2πa−1(0.55,0.55,0.55). At low T we observe peaks corresponding to the ground-state transitions of isolated pairs and triplets and to two excited-state transitions of pairs. The frequencies υ of these peaks lead directly to accurate values for the exchange interaction J, e.g., 0.100±0.004 THz and 0.098±0.002 THz from the ground-state transitions of pairs (υ=2J) and triplets (υ=7J) respectively. Exact calculations of the neutron scattering by isolated pairs, triplets and quartets of Mn2+ ions interacting via nearest-neighbor antiferromagnetic exchange give a reasonably good description of the principal features of the measurements. There are, however, discrepancies that are probably attributable to the scattering by larger clusters.