MAGNETIZATION STEPS OF SPIN QUARTETS

Magnetization steps (MST's) arising from quartets consisting of four i dentical spins, each of magnitude S, are discussed. It is assumed that the only interactions are (1) antiferromagnetic exchange bonds betwee n quartet members, with only one exchange constant J, and (2) the Zeem an interaction with the magnetic held H. There are six quartet types. Each type gives rise to steps in the magnetization curve at low temper atures, KBT<\J\. For five quartet types analytical expressions for the magnetic fields H-n at the MST's are given, for any S. These results are based on expressions for the quartets' energy levels. For the sixt h quartet type (called ''string''), numerical results for H-n are give n assuming S = 5/2, which is appropriate for Mn2+ and Fe3+ ions. The p robabilities that a magnetic ion is in any of the six quarter types ar e given for an fee lattice. Magnetization curves for Zn1-xMnxS (x = 2. 5 and 3.8%) and Zn1-xMnxSe (x = 3.1 and 5.3%) were measured at 0.6 K i n de magnetic fields up to 190 kOe. The data show an MST which is a su perposition of the first MST from ''string quartets'' and the first MS T from ''funnel quartets'' (two of the six quartet types). The predict ed fields for these two MST's differ by only 5%, so that the two MST's overlap in the experiment. The magnetic field at the center of the ob served MST is close to the predicted value in all cases. The size of t he magnetization rise at the observed MST is consistent with the calcu lated probabilities of finding a spin in strings and funnels.