Strain-modulated electron spin resonance ofCo2+in MgO: A comparison of extensional and flexural modes

Strain-modulated electron-spin-resonance spectra of ${\mathrm{Co}}^{2+}$-doped MgO are reported, using both a longitudinal extensional and a flexural mode of vibration. The magnitudes of the shifts of the $g$ factor and the hyperfine coupling constant $A$ are compared with ESR experiments under static uniaxial stress. For the extensional mode, good agreement is obtained for strains smaller than 5 × ${10}^{-7\mathrm{}}$. Above this threshold value dislocations break down from their pinning points. This hampers the exact determination of elastic strain from the measured gauge voltage. The $g$ shifts per unit strain obtained with the flexural mode are nearly two orders of magnitude larger than those obtained with an extensional mode. The reason for this enhancement is not clear. In addition no modulation of the hyperfine coupling constant is observed with the flexural mode.