Phonon spectroscopy and forbidden transitions in rare earths and transition metal ferromagnets

Abstract

Model of ferromagnetic clusters in amorphous rare earth and transition metal alloysOn the local magnetic moments of transitionmetal Mössbauer impurities in ferromagnetic rare earths Conventional pulse-echo techniques and spin-phonon spectroscopy have been employed to examine magnetoelastic excitations in films of Fe, Ni, Gd, Er, and crystals of Gd. With perpendicular rf and dc fields, transverse magnetoelastic excitations are observed with intensities appropriate to a coupled mode model. Recent experiments on Er, with parallel rf and dc fields applied in the film plane, demonstrate the excitation of longitudinal phonons with signal intensities which increase monotonically with an applied field, varying from 0-24 kOe at 10K. Experiments on Gd, Ni, and Fe films with the same field configuration also yield longitudinal phonons at applied fields sufficiently large to magnetize the specimens. Experiments on Gd crystals give similar results. In all cases, the phonon signal is at the rf frequency and the signal intensity varies linearly with the rf field intensity. The data support the conclusion that a first order transition is responsible for the signals; however, with this so-called "parallelpumping" configuration of the rf and dc magnetic fields, first order transitions are forbidden. An addition to the conventional Hamiltonian, which may reconcile these difficulties, is discussed.