Ultrafast Magnetization Dynamics

Magneto-Optical Kerr Effect (MOKE)

Setup to observe the longitudinal magneto-optical Kerr effect

To probe the dynamics of magnetic materials on a picosecond time scale, our lab has developed a free space pump probe technique based on using half-cycle pulses to induce precession in a magnetic sample such as Permalloy thin films. This precession is then monitored using a probe pulse split off from the same femtosecond oscillator. Due to MOKE [ref. 1], magnetization in the sample results in a rotation of the polarization of the probe pulse which is then monitored as a function of the time delay between the pump and probe pulse.

MOKE Data from a Permalloy substrate reveals precession of the individual spins induced by a input half-cycle pulse

As compared to existing techniques [ref. 2] which combine the sample and magnetic pulse generation on the same wafer, this scheme will provide much enhanced flexibility by decoupling the sample from the free-space pulse generation. However the tradeoff is in a much reduced magnetic pulse (on the order of 1 Gauss instead of hundreds of Gauss). In the future, the magnetic field amplitude could be increased by over an order of magnitude by moving from a femtosecond oscillator to an amplified system.

Coming Soon

Current techniques for probing ultrafast magnetic behavior in general all probe the magnetization of a substrate. These techniques, such as MOKE, all depend on the sample having specific characteristics. For example, for MOKE this is the Kerr angle that determines the optical rotation for a given magnetization. We are currently developing a technique using Rubidium atoms that will measure the magnetic field on femtosecond time scales. This is independent of the specific sample and will provide a general technique for measuring dynamics where it is either extremely difficult or impossible to use MOKE.

References

1. M. R. Freeman, R. R. Ruf, R. J. and Gambino, "Picosecond pulsed magnetic fields for studies of ultrafast magnetic phenomena," IEEE Transactions on Magnetics, 27, 6, pp. 4840-4842.
2. W. K. Hiebert, A. Stankiewicz, and M. R. Freeman, "Direct Observation of Magnetic Relaxation in a Small Permalloy Disk by Time-Resolved Scanning Kerr Microscopy," Phys. Rev. Lett. 79, 1134-1137 (1997).