On microwave phonon echo patterns using nickel-film transducers

It is shown that when Ni-film transducers are used to generate transverse microwave phonons in an acoustically birefringent medium, extra echoes appear in the pulse-echo pattern which arise from ultrasonic mode conversion on reflection from the Ni-film-sample interface. This mode conversion is caused by changes in the effective acoustic impedance of the Ni-film close to ferromagnetic resonance. When the medium is not acoustically birefringent, minute errors inevitably introduced in the sample preparation can cause strong phase cancellation effects in the pulse-echo pattern.