Dynamics, damping and defects in ferromagnetic thin films.
R. D. McMichael
National Institute of Standards and Technology,
Gaithersburg, MD USA
One interesting consequence of the link between the magnetic moment
and the spin angular momentum of electrons is that the magnetization
precesses around the effective field in a way that is analogous to the
precession of a top in the gravitational field. Typical precession
frequencies are in the GHz range. Damping processes allow the
magnetization to come to equilibrium in a few nanoseconds, a time that
is becoming slow relative to increasing data rates in disk drives,
magnetic memory chips and sensors.
Ferromagnetic resonance experiments use the width of the peak in
susceptibility at the magnetization precession frequency to measure
magnetization damping. These measurements are generally clouded by
the presence of material inhomogeneities that broaden the
ferromagnetic resonance lines. In this talk, I will give an
introduction to ferromagnetic resonance and three models of the added
line width due to inhomogeneities: one that ignores magnetic
interactions and assumes that the resonance is local, one that handles
weak inhomogeneities while accounting for interactions, and a new
model that links the two older models. This new model uses the
eigenvalues and eigenmodes of the spin wave Hamiltonian to calculate
the susceptibility spectrum, and shows that the localization
transition depends on both the film thickness and the perturbation
strength. Experimental examples of the effects of different kinds of
defects will be given.