Acoustic paramagnetic resonance

APR is a variation of electron paramagnetic resonance (EPR) where the acoustic rather than electromagnetic waves are absorbed by the studied sample.

APR was theoretically predicted in 1952, independently by Semen Altshuler and Alfred Kastler,[1][2] and was experimentally observed by W. G. Proctor and W. H. Tanttila in 1955.

[3][4] After discovery of EPR in 1944, Evgeny Zavoisky predicted that the resonance phenomenon should not be restricted to radio or microwave absorption but could be extended to the sound waves.

[4] APR is commonly measured using the pulsed echo technique at high sound frequencies of the order 100 MHz – 100 GHz.

It generates an ultrasound wave which is detected after multiple bouncing between the flat sides, and the signal attenuation serves as the measure of the resonant absorption.