Principles and applications of ultrafast magneto-optic sampling

In magneto-optic sampling, magnetic transients are measured via small changes in the plane of polarization of linearly polarized light upon reflection from, or transmission through, a sample (the Kerr and Faraday effects, respectively). Ultrafast studies with good signal-to-noise are possible when the magnetic dynamics can be synchronized to (triggered by) laser pulses in a conventional pump-probe implementation. Moreover, spatial resolution of better than one micrometer is easily attained through simple optical microscopy. As an example, we discuss the measurement of current pulses on a thin film transmission line. For good spatial resolution, a thin film magnetic sensor film is placed in close proximity to the line, to respond to the magnetic field generated by the current. In the simplest case, the sensor material is paramagnetic and has a spin-lattice relaxation time shorter than the fastest time constant of the current pulses. The sensor then responds linearly to the current, and the time-resolved magneto-optic signal is directly proportional to the current in the line. The magneto-optic signal is proportional to the magnetization of the sample, while depending also on factors such as the band structure. Since the magnetization in paramagnetic materials depends on the ratio of the applied magnetic field divided by the temperature, paramagnetic sensors tend not to be very sensitive at room temperature. For this reason we have further developed the method to allow the use of ferromagnetic sensor films