Paramagnetic Resonance Detection along the Polarizing Field Direction

Strong rf power, modulated at a low frequency, was applied to a paramagnetic solid (diphenyl pycril hydrazil) in a dc field $H_z$ of up to 20 gauss. The resulting changes in $M_z$ at the modulation frequency produced a voltage in a coil oriented in the $z$ direction, to provide a measure of $M_z$. A sensitive test of the equation of motion of the spin magnetization was achieved by using 19.5-Mc/sec circularly polarized rf of up to 6 gauss in the $x-y$ plane, square wave modulated at 280 cps. The observed 280-cps signal was consistent with the Bloch equations, assuming relaxation toward the instantaneous applied magnetic field. As predicted, $M_z$ was not zero when $H_z=0\mathrm{}$. A Bloch-Seigert effect was also observed.