Magnetization steps in Zn_ {1-x} Mn_ {x} O: Four largest exchange constants and single-ion anisotropy

Magnetization steps from Mn 2 + pairs in several single crystals of Zn 1 - x Mn x O (0.0056 ≤ x ≤ 0.030), and in one powder (x=0.029), were observed. They were used to determine the four largest exchange constants (largest J's), and the single-ion axial anisotropy parameter D. The largest two exchange constants, J, /k B = -18.2′0.5 K and J' 1 /k B = - 24.3 ′ 0.5 K, were obtained from large peaks in the differential susceptibility, dM/dH, measured in pulsed magnetic fields H up to 500 kOe. These two largest J's are associated with the two inequivalent classes of nearest neighbors (NN's) in the wurtzite structure. The 29% difference between J 1 and J' 1 is substantially larger than 13% in Cd 1 - x Mn x S and 15% in Cd 1 - x Mn x Se. The pulsed-field data also indicate that, despite the direct contact between the samples and a superfluid-helium bath, substantial departures from thermal equilibrium occurred during the 7.4-ms pulse. The third- and fourth-largest J's were determined from the magnetization M at 20 mK, measured in dc magnetic fields H up to 90 kOe. Both field orientations H‖‖c and H‖‖[1010] were studied. (The [1010] direction is perpendicular to the c axis, [0001].) By definition, neighbors which are not NN's are distant neighbors (DN's). The largest DN exchange constant (third-largest overall) has the value J/k B = - 0.543′0.005 K, and is associated with the DN at r=c. Because this is not the closest DN, this result implies that the J's do not decrease monotonically with the distance r. The second-largest DN exchange constant (fourth-largest overall) has the value J/k B - 0.080 K. It is associated with one of the two classes of neighbors that have a coordination number z n = 12, but the evidence is insufficient for a definite unique choice. The dependence of M on the direction of H gives D/k B = -0.039 ′ 0.008 K, in fair agreement with -0.031 K from earlier electron paramagnetic resonance work.