Research: Spin Dynamics

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Fig. 1: MOKE Illustration.
 

Magneto-Optical Kerr Effect
The magneto-optical Kerr effect (MOKE) is a change in the polarization state of light reflected from the surface of a magnetic material. There are three types of MOKE: polar, longitudinal, and transverse, defined by the relative orientations of the wavevector of the light and the magnetization of the material.
 

In polar MOKE, the wavevector of the incoming light is perpendicular to the surface of the material. Polar MOKE is a rotation of the polarization vector of the light proportional to the component of magnetization perpendicular to the surface. In longitudinal MOKE, the wavevector of the incoming light is not normal to the surface. Longitudinal MOKE is a rotation of the polarization vector of the light proportional to the component of magnetization parallel to the surface and parallel to the plane of the incident and reflected beams of light. In transverse MOKE, the wavevector of the incoming light is also not normal to the surface. Transverse MOKE is a change in reflectivity of the reflected light proportional to the component of magnetization parallel to the surface but perpendicular to the plane of the incident and reflected beams of light. For situations where the magnetization does not fall completely into one of the three categories just described, a combination of effects can occur. For example, the reflected light may exhibit polarization rotations due to both out-of-plane and in-plane magnetization components, and also exhibit a change of reflectivity due to the other in-plane magnetization component.

Fig 2: Diagram of Technique

Time-Resolved Experiments
Pulsed lasers are a powerful tool that allow us to study systems with characterstic times of picoseconds (10^-12 seconds). Furthermore, many of the systems we study are in thin-film form or are patterned into small structures that make it difficult to obtain measurable signals using conventional electronic techniques.

We begin with a laser which sends pulses of light every 13.6 nanoseconds (10-9 seconds), with each pulse less than 0.2 picoseconds long. Each pulse is split into two parts. One part of each pulse, known as the probe, is reflected from the sample. The polarization state of this pulse is measured to determine the magnetic state of the sample. The second part of each pulse, known as the pump, travels a variable-length path (path length is determined by the position of a corner cube, which consists of three mirrors set to reflect the beam parallel to the path by which it enters). By changing the position of our delay line, we are able to vary the time between arrival of the "probe" beam and the "pump" beam. The pump beam triggers a magnetic field pulse that perturbs the sample, and the probe beam measures the response to this perturbation. By scanning the delay, we can map out the time-domain response of the sample.

Spatial Imaging
Imaging a sample spatially requires a small optical spot size and the ability to scan the beam over the sample. . Using oil-immersion lenses we are able to focus our laser beam spot to approximately 0.5 micrometers (10-6 meters). This allows us to distinguish dynamical features on the order of our beam size, or even smaller. We also obtain fine positioning control of our sample relative to the focusing lens by use of piezoelectric controllers. Due to the short working distance of oil immersion objective lenses, these measurements have so far been done only at room temperature.

Low-Temperature Measurements
Some materials, such as iron, nickel, and cobalt, are strongly magnetic at room temperature and are suitable for study without changing the temperature. Other materials, such as some exchange-biased systems and ferromagnetic semiconductors, exhibit their most interesting magnetic properties only at temperatures below room temperature. To study these systems in their most interesting regimes, it is necessary to cool them. For reaching very low temperatures, we make use of flow cryostats with optical access.