Magnetism in neptunium borides

Magnetization measurements were performed on NpB2, NpB4, and NpB12 over the temperature range 2-18020K. NpB2 is ferromagnetic below T/sub C/ = 99.5 +--20K; NpB4 antiferromagnetic below T/sub N/ = 52.5 +--20K. These compounds exhibit approximate Curie-Weiss behavior above their ordering temperatures. The corresponding Pu compounds as well as PuB6 were found not to exhibit magnetic ordering above 20K. The susceptibility of NpB12 is relatively independent of temperature. Attempts to prepare NpB6 in the arc furnace were unsuccessful. The ferromagnetism of NpB2 is apparently itinerant in nature, the moment being unsaturable at 52 kOe and 20K and having magnitude approximately 0.3 μ/sub B/ per Np atom. The very occurrence of magnetism in the compound is somewhat surprising considering the relatively small separation between nearest-neighbor Np atoms in the compound, d/sub Np-Np/ = 3.16 A. (auth)