Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86135 Augsburg, Germany.
Phys Rev Lett. 2012 Apr 27;108(17):177203. doi: 10.1103/PhysRevLett.108.177203. Epub 2012 Apr 26.
We report a linear dependence of the phonon splitting Δω on the nondominant exchange coupling constant J(nd) in the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, NiO, and in the frustrated antiferromagnetic oxide spinels CdCr(2)O(4), MgCr(2)O(4), and ZnCr(2)O(4). It directly confirms the theoretical prediction of an exchange-induced splitting of the zone-center optical phonon for the monoxides and explains the magnitude and the change of sign of the phonon splitting on changing the sign of the nondominant exchange also in the frustrated oxide spinels. The experimentally found linear relation [symbol:see text}Δω=βJ(nd)S(2) with slope β=3.7 describes the splitting for both systems and agrees with the observations in the antiferromagnets KCoF(3) and KNiF(3) with perovskite structure and negligible next-nearest neighbor coupling. The common behavior found for very different classes of cubic antiferromagnets suggests a universal dependence of the exchange-induced phonon splitting at the antiferromagnetic transition on the nondominant exchange coupling.
我们报告了在反铁磁过渡金属氧化物 MnO、FeO、CoO、NiO 以及在受挫反铁磁氧化物尖晶石 CdCr(2)O(4)、MgCr(2)O(4) 和 ZnCr(2)O(4)中，声子分裂Δω与非主导交换耦合常数 J(nd)之间的线性关系。这直接证实了对于单氧化物，交换诱导的晶心光学声子分裂的理论预测，并解释了在受挫氧化物尖晶石中，声子分裂的大小和符号变化如何随非主导交换的符号变化而变化。实验中发现的线性关系[符号：见文本]Δω=βJ(nd)S(2)，其中斜率β=3.7，描述了这两个体系的分裂情况，与钙钛矿结构且次近邻耦合可忽略的反铁磁体 KCoF(3)和 KNiF(3)的观察结果一致。在非常不同类型的立方反铁磁体中发现的共同行为表明，反铁磁转变处的交换诱导声子分裂普遍依赖于非主导交换耦合。
