Submillimeter wave ESR study of magnetic excitations in the Ising ferromagnetic chain CoNb2O6

Magnetic excitations in the Ising ferromagnetic chain CoNb2O6 have been stu died by submillimeter wave electron spin resonance in strong transverse mag netic fields. A distinct non-linear field dependence of the resonance branc hes and also zero field splittings have been observed. A large residual int ensity of the spin-cluster resonance was found which was caused by the stro ng mixing among the different n-fold spin-clusters. In addition to a series of n-fold spin-cluster excitations, we also discovered a new set of spin-c luster excitation which can be attributed to non-integer n such as n simila r to 1.5 or n similar to 2.5 for the first time. This effect is explained b y the change of the quantization axis of the system in strong transverse ma gnetic fields. These new aspects of spin-cluster excitation show the signif icant effect of transverse magnetic fields on the magnetic excitation in an Ising ferromagnetic chain.