Nonmonotonic thickness dependence of spin wave energy in ultrathin Fe films: Experiment and theory

In this work we present an experimental attempt to determine directly and systematically the properties of highenergy SWs in ferromagnetic Fe thin films of varying thickness. We discover that for the investigated in-plane momentum transfers q between 0.5 and 0.7 A 1 the energy of SWs, nq, is a nonmonotonous function of the thickness n expressed in ML, with a distinct maximum for a 2-MLthick film. An accompanying first-principles theoretical study of the SW excitations uses the clear experimental trend to choose between two complementary scenarios of polarized electron scattering. The theory suggests that the SWs excited in the experiment are localized at the surface of the film. Both the hybridization with substrate and the atomic relaxation of the Fe film are important for the nonmonotonous behavior of the SW energy. This paper is structured as follows: In Sec. II we introduce the experiments on preparation, characterization, and SPEELS measurements of the Fe thin films on W110 .I n Sec. III we present the experimental and theoretical results focusing the discussion on the nonmonotonous thickness dependence of the spin wave energies. The conclusions will be finally given in Sec. IV.