Exchange coupling of molecular-beam-epitaxy-grown Fe/Al/Fe trilayers by dynamic techniques

Abstract

Network analyzer ferromagnetic resonance (NA-FMR) spectroscopy and Brillouin light-scattering (BLS) techniques are used to probe, in frequency domain, the interlayer exchange coupling (bilinear and biquadratic) of ultrathin Fe/Al/Fe trilayer films epitaxially grown by molecular-beam epitaxy (MBE). These two dynamic techniques provide the possibility to determine the magnetic excitation frequency f (acoustic and optic modes) as well as the frequency linewidth Δf without perturbing the magnetization orientation. A large advantage of the NA-FMR technique compared to conventional FMR is that it can detect the complex magnetic signal at a fixed magnetic field so that the magnetic state of the exchange-coupled trilayer is not perturbed during the frequency sweep. As the spin-wave frequencies are very sensitive to the strength of the static magnetic field and the orientation of the magnetizations, their field dependence reflects the reorientation transitions taking place in a coupled trilayer system. The exchange-coupling constants are derived from the magnetic-field variation of mode positions both from NA-FMR and BLS. We report very strong (>2.5 erg/cm2) antiferromagnetic exchange coupling in the Fe/Al/Fe system. The biquadratic coupling of 1.0 erg/cm2 that we observed at 6 Å Al spacer thickness is much stronger than earlier results for the same system.