Frequency Distribution of Phonons in a Ballistic Heat Pulse Determined by Magnetic Resonance

The scattering of ballistic heat pulses by resonant spins in MgO: Fe²⁺, tuneable with a magnetic field, has been used to investigate the phonon frequency spectrum of the heat pulses at helium temperatures. A magnetic field of 60 kOe allowed phonons with frequencies up to 288 GHz to be studied. Good agreement was obtained with a Little-type calculation based on an acoustic-impedance model of interfacial thermal conduction. It is suggested that the lack of such agreement by most previous workers has often been due to inadequate preparation of surfaces.