The impurity-induced antiferromagnetic ordering of the doped Haldane magnet $\mathrm{Pb}\left({\mathrm{Ni}}_{1-x}{\mathrm{Mg}}_x{)}_2{\mathrm{V}}_2{\mathrm{O}}_8\hspace{0.5em}\right(0\mathrm{}<~x<~0.06)$ was studied by electron-spin resonance (ESR) on ceramic samples in the frequency range 9–110 GHz. Below the Néel temperature a transformation of the ESR spectrum was found, indicating an antiferromagnetic-resonance mode of spin precession. The excitation gap of the spin-wave spectrum increases with increasing Mg concentration x in the same manner as the Néel temperature, reaching its maximum value of 80 GHz at $x>~0.04$. At small concentrations $x<\mathrm{}0.02\mathrm{}$ the signals of antiferromagnetic resonance were found to coexist with the signal of the paramagnetic resonance indicating a microscopic separation of the magnetic phases.

• Received 24 December 2001

DOI:https://doi.org/10.1103/PhysRevB.65.174422