Direct Electromagnetic Generation of Compressional Waves in Metals in Static Magnetic Fields

A derivation of the electromagnetic generation of compressional waves in metals in static magnetic fields is given. It is found that the acoustic amplitude varies linearly with the amplitudes of both the incident time-varying electromagnetic field and the static magnetic field, and inversely as the acoustic frequency. An expression is obtained for the fraction of transmitted electromagnetic power converted into acoustic power. I. IKTRODUCTIOS Ll~CTROhlAG~ETIC generation of ultrasonic waves in ttletals has been ol)served under a wide variet,y of experimctntal conditions (l). In a static magnetic field both shear anti compressional waves have heen generated where the direction of polarization de- pends upon the orientation of thc field. Theoretical di+ cussion of this phenomenon with alld without a magnetir field has been given (2). For finite magnetic fields the most gcneral trcatmcnt is tht of Quinn (a). Here, using tt self-consistent semiclassical dcsrription, Quinn explains the gencrat,iorl mechanism on tlw hayis of solutions to equations for the couplctl electron-ion system. 'The ('111- phnsis is on shc:ir-wavc gcncration and propagation. hl- though the treatment is exact,, the generation luechanism may be somewhat, ohscured hy the formalism. The tlasic mechanism for sound generation is, honc~vt~r. un(lcr- shndahle via simple classical concepts. Rlodels of this type have been given and shown to he in quantitative ngreemc.nt, with experiment (3). in the gontwll trcat- ment of Quinn, these discussions were confined to the case of shear waves. It is the purpose of this paper to present a rla4Qsic:d calculation for the generation of com- pressional waves in the presence of a static magnet,ic field (4).