SPLIT MAGNETIZATION STEPS IN CD1-XMNXSE - INEQUIVALENT NEAREST NEIGHBORS AND THE DZYALOSHINSKI-MORIYA INTERACTION

In the wurtzite structure the twelve nearest-neighbor (NN) cations surrounding a given cation belong to two groups: six "in" NN's that are in the same c plane, and six "out" NN's that are out of the c plane. These two groups are inequivalent, by symmetry, and the NN exchange constants J1in and J1out are unequal. The difference DELTA-J1 = J1in - J1out gives rise to a splitting of the magnetization steps (MST's) associated with pairs of NN magnetic ions. Each MST splits into two "half-steps," one due to in pairs, and the other due to out pairs. The average J1, and the absolute value of the difference DELTA-J1 between the two J1's, are obtained from the fields at the half steps. Analysis of the line shape of the split MST's yields the sign of DELTA-J1 and an estimate for the Dzyaloshinski-Moriya interaction constant D. These procedures were implemented for Cd1-xMnxSe with x = 0.010 and 0.025. The first two MST's were observed at 0.5-0.6 K in experiments that used magnetic fields up to 30 T. At these temperatures only the splitting of the second MST was resolved. The splitting of the first MST was observed at 0.08 K, in experiments up to 20 T. The average NN exchange constant J1BAR = 1/2(J1in + J1out) is J1BAR/k(B) = -7.55 +/- 0.1 K, and the magnitude of the difference is \DELTA-J1\/J1BAR = 0.15 +/- 0.01. The latter value is in good agreement with a theoretical estimate by Larson. Line-shape analysis indicates that J1in is larger in magnitude than J1out, as predicted by Larson, and it places D in the range 0.10 < D/k(B) < 0.24 K. An alternative analysis of the line shape of the second MST gives the estimate D/k(B) congruent-to 0.21 K. These results for D compare well with the value D/k(B) = 0.16 K predicted by Larson and Ehrenreich.