Magneto-Optics Research

Magneto-optics is the study of effects arising from the interaction of light with magnetized media. Initially linearly polarized light, after interaction with such materials, can exhibit both ellipticity and a rotation of the polarization state. These effects are generally catagorized into two phenomena, the Faraday Effect which occurs when electro-magnetic radiation is transmitted through a magnetized media, and the Kerr Effect which deals with reflections from the magnetized media. The Magneto-Optical Kerr Effect (MOKE) is further catagorized by the direction of the magnetization vector with respect to the reflection surface and the plane of incidence.

The following illustration shows the three different geometries for MOKE experiments.

If the magnetization vector is perpendicular to the reflection surface and parallel to the plane of incidence, the effect is called the polar Kerr effect. As a matter of simplification, near normal incidence is usually employed when doing experiments in the polar geometry. In the longitudinal effect the magnetization vector is parallel to both the reflection surface and the plane of incidence. When the magnetization is perpendicular to the plane of incidence and parallel to the surface it is said to be in the transverse configuration.

The experiment shown above allows for convenient measurement of the Kerr rotation and the Kerr ellipticity. The light source used is a diode laser that emits at a wavelength of 635 nm. The light then passes through a polarizer set to 45°. The photoelastic modulator (PEM) head is aligned such that the modulation axis is along the y-axis. This insures that linearly polarized light will be incident upon the PEM at an angle of 45° with the modulation axis. Light that passes through the PEM will receive a periodically changing retardation, given by

where is the retardation amplitude, and is the modulation frequency, 50 kHz for this PEM model. The PEM controller must be set to the proper wavelength, 635 nm, and retardation, l/4 wave, in order to produce the alternating procession between right-handed and and left-handed circularly polarized light.

The modulated light is then reflected by the sample, which is situated in the center of the electromagnet poles. In the polar configuration, a magnet with a hole bored through one of the poles is used. This gives the desired condition of a near normal angle of incidence. For longitudinal measurements, light enters from the side, between the magnet poles. With the current setup, an angle of incidence of approximately 20° is possible.
The reflected beam then passes through an analyzer. Although any angle could be used, setting the analyzer angle to 0° greatly simplifies the mathematical relationship between the intensity ratios measured and the magneto-optical parameters. A detector is placed at the end of the light path. In the current experiment, the detector is a silicon photo-diode, although a photomultiplier may also be used.

The signal from the detector is sent to two measuring devices. One line connects to a DC voltmeter to record the DC component of the modulated, reflected light. The other goes to a lock-in amplifier to measure the AC components. The lock-in also requires a reference signal at the same frequency as the light modulation. The PEM supplies a frequency reference signal which is connected to the lock-in's reference input. Use of a bipolar power supply to drive the magnet simplifies the process of sweeping the magnetic field smoothly from positive to negative extremes. A gaussmeter probe is placed behind the sample to monitor field levels during the experiment.

The command center for the entire experiment is a personal computer running National Instruments' LabView 5.0. Using GPIB, and custom made programs, LabView is able to control all of the essential electronics associated with the experiment. By using a DAC board in conjunction with the power supply's current programming input, it is possible to precisely control magnetic field sweeps. All relevant data is read from the lock-in, DC voltmeter, and gaussmeter, and stored into a spreadsheet format for later processing.

Magneto-optical effects have seen widespread use in the field of data storage. Traditional magnetic storage media records data bits as a series of small magnetic domains with reversed magnetizations in the plane of the media. The problem of increasing the bit density involves decreasing the size of the domains used and further reducing the head/surface seperation. Storage schemes based on MO rely on domains magnitized perpendicular to the plane, and the laser's ability to be focused to a point from a remote location eliminates the problem of head/surface separation.

MO disk recording systems have been available since 1988, and in 1992 Sony introduced its Minidisk™ system which had the ability to store up to 74 minutes of digitized music on an inexpensive disk.