Abstract
The thermal magnetoresistance of iron-doped Al2O3 shows sharp minima when the frequency of the Delta MJ'=2 transition of Fe2+ crosses the frequencies of the two Delta MJ'=1 transitions. From the positions of these minima, a value for the trigonal splitting of the ground state of D=3.75+or-0.05 cm-1 is obtained. Measurements made at 1.15K, when only the population of the lower levels is significant, show that D is positive; that is, the singlet is lowest in zero field.