Magnetic coupling across a ferromagnetic-metal-alkali-metal interface

We report on an experimental study of the interfacial magnetic coupling in metallic bilayers consisting of an alkali metal and a thin ferromagnetic film. The alkali metals used were either potassium or lithium, with the ferromagnetic metal consisting of either iron or Permalloy. This study began as part of a larger project consisting of a study of Larmor waves in potassium. Our observations will also be of interest to those studying magnetic multilayers, in which the nature of the magnetic coupling between ferromagnetic and nonmagnetic metals plays an important role. In this paper, we show how the degree of magnetic coupling in these bilayer samples can be examined by studying the interaction of the ferromagnetic resonance of the ferromagnetic film with the conduction-electron spin resonance (CESR) of the alkali-metal layer. These studies are carried out at cryogenic temperatures (down to 1.1 K) using microwave reflection spectroscopy at 12 GHz. We have also examined the temperature dependence of the magnetic resonances. Between 1.1 and 25 K, for both lithium and potassium, an unexpected narrowing of the CESR lines occurs with increasing temperature.