Using ferromagnetic resonance to measure the magnetic moments of ultrathin films

A technique that uses ferromagnetic resonance (FMR) to measure the saturation magnetization of ultrathin ferromagnetic films is described. It has been used to measure the layer averaged magnetic moments of ultrathin Fe films located in Ag/Fe/Ag, Au/Fe/Ag. Cu/Fe/Ag, Pd/Fe/Ag, and Ni/Fe/Ag structures relative to that of an Au/5.7 ML Fe/Ag reference film. The ratios obtained using method have a total measurement error of a little over 1%. The measurements were carried out to investigate theoretical predictions that Fe atoms located at or near surfaces and interfaces should possess enhanced magnetic moments compared those of Fe atoms in the bulk. All of the bcc Fe(001) films used in our work were approximately 5 monatomic layers (ML) thick and were all grown on bulk fee Ag(001) substrates. The covering layers of Cu, Pd, and Ag were all 7 ML thick while the covering layers of Ni were 2-3 ML thick and the covering layers of Au were 20 ML thick. All FMR measurements were carried out at a temperature of 77 K. An Ag/5.5 ML Fe/Ag specimen and an Ag/10.9 ML Fe/Ag specimen (the thick Fe specimen) were determined to have a moment ratio of 1.06 +/- 0.01. This compared well to a layer averaged, ground state, moment ratio of 1.05 calculated for the two films using results obtained from published first principles calculations. Ratios of 1.08 +/- 0.01 and 1.10 +/- 0.01 were obtained for Au/5.7Fe/Ag and Cu/5.8Fe/Ag structures, respectively, when compared to the thick Fe specimen. In a similar manner, ratios of 1.11 +/- 0.01 and 1.12 +/- 0.01, respectively, were determined for Pd/5.6 Fe/Ag and Pd/5.7Fe/Ag sandwiches. Finally, the layer averaged moment of a 2Ni/5.7Fe/Ag structure was in a ratio of 1.15 +/- 0.01 with the thick Fe specimen while two 3Ni/5.7Fe/Ag sandwiches were in ratios of 1.24 +/- 0.01 and 1.20 +/- 0.01, respectively. All measured values were in excellent agreement with published first-principles calculations and with the layer averaged moments obtained from polarized neutron reflection (PNR) studies carried out on many of our specimens.