Magnetometry
The Magnet Lab provides five types of magnetometers for magnetic properties measurement:
- A capacitive cantilever beam
- A piezoresistive cantilever magnetometer
- A vibrating sample magnetometer (VSM)
- An AC susceptometer
- A Magneto Optical Kerr Effect probe
The cantilever and the AC pickup coils are sensitive enough to be used as de Haas - van Alphen spectrometers.
A high-sensitivity AC susceptometer is available to measure the susceptibility of samples. Due to the nature of the measurement, this instrument is mainly used for the detection of transitions in the magnetic properties rather than obtaining absolute values of susceptibility. The available temperature range so far is 35 mK to 500 mK in magnetic fields up to 18 T. An AC-probe for the temperature range between 30 mK and 300 K in fields up to 45 T has been developed. This probe also features a sample rotator so that oriented samples can be measured at a number of angles without having to remove the probe from the dewar.
The cantilever beam magnetometer is available to measure the force and torque on a magnetic moment in a magnetic field. In a typical setup it can resolve 10-4 N/uV. This is roughly equivalent to a resolution of 10-7 emu (10-10 J/T) in a field gradient of 0.1 T/cm. The torque mode is more sensitive, resolving 10-9 emu of torque at 20 T. Sample sizes are limited to 1 mm3 with a maximum weight of 10 mg. The cantilever measurement system can operate between 30 mK and 0.5 K in fields up to 18 T, and between 0.5 K and 300 K in fields up to 45 T.
Piezoresistive Cantilever Magnetometer
The piezoresistive cantilever magnetometer is usually used to measure torque and/or force on a magnetic moment of a very tiny sample. A sample is glued to a commercial piezoresistive AFM tip, the bending of which causes changes of electrical resistance. The typical sensitivity of this type of torque magnetometer is about 10-12 emu at 1 tesla for 1 mA sensing current. The piezoresistive cantilever measurement system can operate between 30 mK and 0.5 K in fields up to 18 T, and between 0.5 K and 300 K in fields up to 45 T.
De Haas - van Alphen Spectrometers
The user support staff at the MagLab has a great deal of experience in the measurement of quantum oscillations. Both the cantilever beam and the AC pickup coil apparatus have been used for dHvA measurements at the lab. A 500 G peak-to-peak modulation coil is available for use in the resistive magnets. dHvA measurements in the dilution refrigerator should be made with the cantilever since the modulation coil in that system must be run at low amplitudes to prevent excessive heating.
The Magneto-Optical Kerr Effect probe (MOK3) was designed and built by Dr. Wilhelmus J. Geerts (now at Southwest Texas State University in San Marcos) to be used in the 50 mm bore magnets at the Magnet Lab in Tallahassee. MOK3 can be used to carry out magneto-optical measurements on thin films, transparent materials, liquids and gasses in fields up to 20 tesla (or 25 tesla if the project is given time in the Keck magnet), and at temperatures from 2 K to 325 K. Typical sample diameters are between 0.5 cm and 2 cm. MOK3 has a rotation noise level of less than 0.5 mdegrees on bulk iron, and a residual system background smaller than 2 mdegrees/tesla. A better signal-to-noise ratio and lower system background are obtained for the ellipticity signal.
The VSM allows the measurement of large samples (up to 6 mm in diameter, 8 mm long) and has a resolution of 10-3 emu. The sensitivity ranges from 10-3 to 10+3 full scale. Thus only samples with a considerable moment should be measured with this technique. It can operate at temperatures between 1.2 K and 300 K and in fields up to 33 T. A large bore VSM with 10 mm maximum sample diameter can be constructed if there is sufficient user interest.
For more information about the magnetometry facilities, contact Eun Sang Choi. For information about scheduling magnet time and submitting facility requests, contact Interim DC Program Director Tim Murphy.
