Nonmonotonic inelastic tunneling spectra due to emission of surface magnons in itinerant electron ferromagnetic junctions

Abstract

We propose an itinerant electron theory of nonmonotonic inelastic tunneling spectra of ferromagnetic junctions observed in several recent experiments. Assuming that the Coulomb interaction between electrons across the barrier is poorely screened due to the exponential suppression of the band electron density inside the barrier, we consider a process where a hot electron from one ferromagnet during the tunneling excites a magnon at the surface of the other ferromagnet in the course of the exchange scattering with a Fermi sea electron from the opposite side. For this process one can derive an inelastic transfer Hamiltonian in which the matrix elements of the electron- magnon coupling depend on the magnon wave-vector (and hence on its energy) via the long-range Coulomb potential. This dependence can account for the sharp peaks observed in the low-temperature tunneling spectra of a number of ferromagnetic junctions at the excitation energies much smaller than the Curie temperature of the ferromagnets.