Electricity: measuring and testing – Magnetic – Magnetometers
Patent
1989-06-08
1991-03-19
Snow, Walter E.
Electricity: measuring and testing
Magnetic
Magnetometers
324260, G01R 3302, G01R 3300
Patent
active
050014269
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a magnetometer with an inhomogeneous magnetic field which timewise changes in its intensity periodically and acts on the sample to be measured, which sample is attached to the free end of a tongue that can be caused to oscillate and whose oscillations can be recorded by at least one piezo element connected with an evaluation circuit that features a measuring amplifier for acquisition of the amplitudes of the piezo signals, and features a low frequency generator which through a power amplifier is connected to a coil setup which generates the periodically changing magnetic field, with the sample being surrounded by a compensating current loop.
A magnetometer of that type, without compensating current loop, is described in "Review of Scientific Instruments" 51 (5), May 1980, pages 612, 613 and enables qualitative measurements of the magnetic moments of very small samples, by registering with the aid of a piezo element the oscillation amplitude of a tongue constructed from gold wire and a glass fiber. The output signal of the piezo element is fed to a phase-sensitive amplifier whose output signal represents the measuring signal.
The oscillation amplitude of the prior system, and thus the output signal, depend not only on the magnitude of the magnetic moment to be determined but also on the elasticity of the materials used the inertia of the oscillator and the magnitude as well as frequency of the field gradient Therefore, only qualitative measurements for microscopically small samples, without registering the temperature contingency of the magnetic moment, are possible with the prior magnetometer.
From the 1984 activities report of the Fraunhofer Institute for applied solid state physics, page 136, it is known that the piezo signals can be compensated for by the magnetic moment of a current loop. How this is to be accomplished in practice, however, does not derive from the activities report.
Basing on this prior art, the problem underlying the invention is to provide a magnetometer of the initially mentioned type which allows the performance of quantitative measurements of magnetic moments at various temperatures, also on samples having dimensions in the range of several millimeters.
This problem is inventionally solved in that the low frequency generator is activated by an automatic frequency control circuit by which the frequency of the magnetic field can be adjusted to the respective resonance frequency of the tongue, and in that the magnitude of the compensating direct current can be registered by the compensation current loop for the quantitative determination of the magnetic moment, at which compensating current the magnetic moment of the sample, and thus the excitation of the tongue, is compensated for.
Since a measurement takes place after the tongue oscillations have been zeroed, a quantitative determination of the magnetic moment of a sample is possible independently of the amplitude of the output signal of the measuring amplifier and thus independently of the system magnitudes which have an influence on the amplitude. By tuning the exciting frequency to the resonance frequency it is possible to measure magnetic moments across large temperature ranges at a very high sensitivity.
In a favorable embodiment of the invention, the sample is contained in a hollow cylinder from quartz whose longitudinal axis extends in the direction of the magnetic field between the two magnetic poles and on which a wire loop with a winding from a thin gold wire is applied as compensating current loop. The tongue to be oscillated with the hollow cylinder from quartz is housed in a tail part of a cryostat which protrudes into the gap between the two poles of the electromagnet generating the magnetic field.
The measuring amplifier is preferably a phase-sensitive amplifier with a first output for a first control circuit that contains the compensating current loop. A second output of the phase-sensitive amplifier for signals phase-shifted by 90.degree. controls the input of a circuit for an automatic freq
REFERENCES:
patent: 4005358 (1977-01-01), Foner
Tatigkeitsbericht IAF 1984, pp. 135-137.
Zeitschriftangenwandte Physik 26. Bd. Heft 1, 1969, pp. 84-87, W. Hellent and U. Lotter.
Roos et al, "High Sensitivity Vibrating Reed Magnetometer", Rev. Sci. Instrum. 51(5), 5/1980, pp. 612-613.
Balev et al, "Automated Magnetometer with Superconducting Solenoid", 8/64, Instruments and Experimental Techniques, May-Jun. 1985, No. 3, part 2, pp. 685-686.
"Thermal Conductivity at Very Low Temperatures", published by Jacob N. Haasbroek, Jun. 1971, pp. 44-45.
Frey Thomas
Jantz Wolfgang
Stibal Rudolf
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Snow Walter E.
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