Excitations of isolated clusters of magnetic ions

Abstract

The neutron scattering by isolated clusters of Mn²⁺ ions in a single‐crystal specimen of KMn_0.18Zn_0.82F₃ 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.