Giant diamagnetism in Cr, Fe selenides up to 310 K

Giant diamagnetism, with values to -1.4.10(-2) emu/g, was observed from 4.2 K to 310 K in selenides of monoclinic NiAs structure prepared with stoichi ometries FeSe . uCr(2)Se(3), u = 0.1 to 1.0. A positive component due to ma gnetic order of the spins was present from 4.2 K to about 65 K-150 K, depen ding on the sample. The magnetic order was minimized by doping with As0.3Ge 0.3. Another sample doped with Ga and Ge gave a decrease in the slope of ne gative susceptibility vs T(K) starting at 220 K; and thus an increase in th e range of diamagnetism to well over ambient temperature.