The miniMOKE

The miniMOKE is a compact instrument with integrated dedicated electronics for measurements of Magneto-Optical Kerr Effects.

The miniMOKE is especially useful for routine measurements and in education. It is fast - the time to measure the graph shown was approximately one minute. Data can be stored on disk and exported to e.g. Excel for further analysis. Information on saturation, remanence and coercivity values are automatically displayed. For educational purposes and for quick measurements of a sample, the miniMOKE is an ideal and convincing solution. It has proven to be fast, accurate and easy to use. Also it is insensitive to vibrations, noise and light from the environment. No vibration isolation is necessary and no light protection is required.

The miniMOKE unit itself is 40 * 61 cm wide and 20 cm high. It weighs about 40 kg (mainly due to the magnet). In addition there is an electronic unit plus a PC. It can be placed on any table or on a small table with wheels.

Samples of maximum size 10x10 mm2 are installed in seconds. No adjustment is needed. Samples are so far at ambient temperature only. The rotation angle is controlled with a stepping motor.

The standard unit will reach maximum 0.5 Tesla , but units with lower or higher fields can also be fabricated.

The time for integration at each point can be chosen for required speed or accuracy. Letting the miniMOKE run will generate an averaged and smoothed hysteresis curve over many loops. The sensitivity of the instrument allows magnetic information to be obtained of ferromagnetic films, which are less than 5 nm thick. The displayed graph shows average values of the hysteresis loops measured. Determination of the magnetic anisotropy is possible by rotation of the sample around its surface normal.

The method of operation consists of a laser diode and a polarizer, on the incident side. A Faraday modulator, a crossed polarizer and a semiconductor detector are mounted on the exit side. The intensity of the reflected light is integrated over a variable length of time and analyzed analogically and digitally. The magnet is made of laminated sheets with essentially zero remanence and negligible hysteresis. The power supply is bipolar for changing field directions.

The angle of incidence is 45 degrees. Longitudinal MOKE is used.

A standard laser and photodiode are used. Polarizer and analyzer are of Glan-Thompson type. Modulation is made with a Faraday rod at 1000 Hz.

Shown is a hysteresis curve from a biquadratically coupled Fe(100)/Cr(100)/Cr(100) trilayer grown by MBE methods on a sapphire substrate. Specifically the sample has the film sequence: Cr cap layer 4nm/Fe 2nm/Cr 4 nm/Fe 2nm/ Cr-buffer 20nm/Nb-buffer 30nm/sapphire r-plane. All metal layers in (100) growth direction. Total thickness of the magnetic layers: 4 nm.

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