Magnetic anisotropies in ultrathin fcc Fe(001) films grown on Cu(001) substrates

Ferromagnetic resonance absorption measurements at 36.3 GHz and at room temperature have been used to determine the g factor and anisotropy parameters for a series of bilayers composed of two 3-ML-thick fcc Fe (001) films separated by a variable thickness of fcc Cu(001). The resonance field and linewidth were measured versus the out-of-plane magnetic-field angle, ${\mathrm{\theta }}_{\mathit{H}}$. The magnetic properties of these ten coupled bilayer films were found to be remarkably similar from specimen to specimen, despite the fact that each member of the bilayer was only 3 ML thick. The average g factor was found to be 〈g〉=2.08±0.02, and the average effective magnetization was found to be -5.5±0.5 kOe; i.e., the specimens were magnetized normal to the specimen plane in zero applied magnetic field. If the effective field along the specimen normal can be attributed to a second-order surface anisotropy energy of the form ${\mathit{F}}_{\mathit{s}}$=-${\mathit{K}}_{\mathit{U}1}$ ${\sin }^2$ ${\mathrm{\theta }}_{\mathit{M}}$, then 〈${\mathit{K}}_{\mathit{U}1}$〉=1.25±0.06 ergs/${\mathrm{cm}}^2$, assuming a value 4π${\mathit{M}}_{\mathit{s}}$=21.6 kOe for the saturation magnetization and using d=5.4 Å for each film thickness. (This energy includes both sides of the film; the energy corresponding to a single Fe-Cu interface is 0.63 erg/${\mathrm{cm}}^2$.) These specimens exhibited no measurable in-plane anisotropy. The linewidth was found to exhibit a sharp decrease for ${\mathrm{\theta }}_{\mathit{H}}$ near 20°. This decrease could be explained in terms of the angular dependence of inhomogeneous line broadening due to a 1% variation in the perpendicular effective field from place to place in the sample plane.