Magnetization steps in Zn1-xMnxO:: Four largest exchange constants and single-ion anisotropy -: art. no. 125209

Magnetization steps from Mn2+ pairs in several single crystals of Zn1-xMnxO (0.0056less than or equal toxless than or equal to0.030), 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(1)/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 Cd1-xMnxS and 15% in Cd1-xMnxSe. 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 Hparallel toc and Hparallel to[10 (1) over bar0] were studied. (The [10 (1) over bar0] 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)approximate to0.080 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.