Magnetisation Dynamics

The understanding and control of the magnetization dynamics in ferromagnetic thin films and nanostructures is important for future applications in spintronics. When an ultrashort laser pulse (at the femtosecond time scale) interacts with a ferromagnetic material, the magnetic moment of a system can be quenched very rapidly. This is referred to as ultrafast demagnetization and there have been many experimental and theoretical works aimed at explaining the underlying mechanisms. At longer times (nanosecond time scale), precession of the magnetization is observed, and the detailed magnetization dynamics depends on the demagnetization field that results from patterning a thin film system into sub-1 ­­­μm elements.

We can characterize the magnetization dynamics of ferromagnetic thin films and mesoscopic systems using a time resolved magneto-optical Kerr effect microscope (TR-MOKE). Reflected light from a magnetic material results in a rotation of the light polarization, effectively enabling the observation of magnetization precession. Our setup is based on a two-colour collinear pump-probe geometry. The Kerr rotation of the probe beam (λ = 1030 nm and pulse width ≈50 fs) is measured after exciting the sample with another laser beam of λ = 515 nm and a pulse width ≈50 fs. Both the pump and the probe beams are focused on the sample using a microscope objective, and a magnetic field can be applied at a small angle to the sample plane. The pump pulse rapidly modifies the out-of-plane demagnetizing field and thereby induces precessional magnetization dynamics. The precessional dynamics appears as an oscillatory signal on top of the decaying time-resolved Kerr rotation.

Magnetization Dynamics in Thin Films

We have characterized the magnetization dynamics of systems with two ferromagnetic thin films coupled antiferromagnetically across a non-magnetic layer shown schematically in Fig. 1a [1] and measured the spin wave dispersion on changing the alignment between the magnetization in the two layers from nearly antiparallel to parallel with an applied magnetic field. We observed a spin wave mode in addition to the expected acoustic and optical modes, which is a result of the laser-induced decoupling of the two ferromagnetic layers. A detailed description of how the applied magnetic field influences the magnetization dynamics is given in Ref [1], providing an important guide to the implementation of antiferromagnetically coupled films in functional spintronic devices.