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

Magnetic excitations in the Ising ferromagnetic chain CoNb₂O₆ have been studied by submillimeter wave electron spin resonance in strong transverse magnetic fields. A distinct non-linear field dependence of the resonance branches and also zero field splittings have been observed. A large residual intensity of the spin-cluster resonance was found which was caused by the strong 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-cluster excitation which can be attributed to non-integer n such as n ∼ 1.5 or n ∼ 2.5 for the first time. This effect is explained by the change of the quantization axis of the system in strong transverse magnetic fields. These new aspects of spin-cluster excitation show the significant effect of transverse magnetic fields on the magnetic excitation in an Ising ferromagnetic chain.