Study of Magnetic Excitations in Random Systems

本项目的目的是阐明具有竞争自旋各向异性和竞争交换相互作用的随机磁系统中磁激发的性质。（1）用电子自旋共振（ESR）技术研究了无规稀释XY反铁磁体Mg_<1-x>Co_xCl_2的磁激发。我们在x低于逾渗浓度（x_p）的样品中观察到几条ESR谱线。其中之一是由各向异性交换作用耦合的Co^&lt;2+&gt;对引起的。其他的模式被解释为均匀模式，其频率与磁场的关系与x高于xp的有序相中的频率与磁场的关系相同。(2)用ESR方法研究了无规稀释Ising反铁磁体Mg_ Fe_xCl_2的磁激发<1-x>。结果表明，单离子各向异性和各向异性交换作用对FeCl_2各向异性能的贡献几乎相等。(3)本文研究了典型的单自旋线性海森伯反铁磁体Ni（C_2H_8N_2）_2NO_2（ClO_4）的磁激发，其中Ni原子随机取代了Cu原子。观察到新的ESR谱线，并通过以下模型进行分析。当Cu原子被引入价键固体基态时，邻近Cu的Ni位的自旋1/2自由度恢复。我们计算了弱耦合三自旋1/2系统的激发谱，理论与实验符合得很好。

It is the aim of the present project to clarify the nature of magnetic excitations in random magnetic systems with competing spin anisotropies and with competing exchange interactions. The important findings obtained from this study are summarized below :(1) The magnetic excitations in a randomly diluted XY antiferromagnet Mg_<1-x>Co_xCl_2 are studied by Electron Spin Resonance (ESR) technique. We have observed several ESR lines in the samples with x below the percolation concentration (x_p). One of them is interpreted as arising from Co^<2+> pair coupled by anisotropic exchange interaction. The others are interpreted as a uniform mode whose frequency vs. magnetic field relation is the same as that in the ordered phase with x above x_p.(2) The magnetic excitation in a randomly diluted Ising antiferromagnet Mg_<1-x>Fe_xCl_2 are studied by ESR method. It is shown from the analysis of the result that both of the single ion anisotropy and the anisotropic exchange interaction contribute to the anisotropy energy of FeCl_2 with nearly the same amount.(3) The magnetic excitations in the typical spin one linear chain Heisenberg antiferromagnet Ni (C_2H_8N_2) _2NO_2 (ClO_4) are studied in which some of the Ni atoms are randomly substituted for Cu. Novel ESR lines are observed and are analyzed by the following model. When Cu atoms are introduced into the valence-bond -solid ground state, spin 1/2 degrees of freedom in the Ni sites neighboring the Cu recover. We have calculated the excitation spectra of the weakly coupled three spin 1/2 system and have obtained a good agreement between the theory and the experiment.