Study of Magnetic Excitations in Random Systems

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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.