ESR Study of the Triangular Lattice Heisenberg Antiferromagnets VX2 (X=Cl, Br and I)

ESR signals from the 2D triangular lattice antiferromagnet VX 2 have been studied in detail in paramagnetic and ordered phases. At high T , the shift and the width are nearly isotropic and an intense exchange narrowed Lorentzian peak is observed. When T is lowered toward the transition point T s , rapid increase in width and down shift of the resonance field as seen in the spin-glass occur. For T < T s , however, an extremely strong absorption line reappears when the field is applied normal to the c -axis. This signal becomes sharp and shifts rapidly toward the lower field with T →0 K. Many unusual behaviors which can not be observed in the non-frustrated systems are observable though we can not yet interpret them successfully.