Johnson noise in SQUID magnetometers

Savicki, Joseph P.

doi:10.1063/1.1138914

Review of Scientific Instruments

1986

DOI: 10.1063/1.1138914

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Johnson noise in SQUID magnetometers

Joseph P. Savicki

Abstract: Conducting tubes threading the pick-up coils in SQUID magnetometers may generate magnetic flux noise larger than the intrinsic SQUID noise. A method for calculating the level of this noise is presented which can aid in the design of low noise systems. Application to a system in this lab predicts a noise level of 0.001 fluxoids rms/(Hz)1/2, in good agreement with the observed level of 0.001 fluxoids rms in a 1-Hz bandwidth.

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1993

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Image and sample geometry effects in SQUID magnetometers

Zięba

1993

Review of Scientific Instruments

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Articles you may be interested inErratum: ''Effect of sample and magnetometer dimensions on the flux detected by a SQUID magnetometer' ' [Rev. Sci. Instrum. 5 4, 1380] Rev. Sci. Instrum. 55, 818 (1984); 10.1063/1.1137973 Effect of sample and magnetometer dimensions on the flux detected by a SQUID magnetometer Rev. Sci. Instrum. 54, 1380 (1983); 10.1063/1.1137249 Superconducting magnet image effects observed with a vibrating sample magnetometer Rev. Sci. Instrum. 54, 137 (1983);This review concerns the interpretation of the signal provided by SQUID magnetometers designed for measurements of magnetization and magnetic susceptibility. "Image effect" refers to the influence of superconducting elements in the vicinity of the sample and detection coil. Several exact and approximate methods are presented for calculation of how the device sensitivity G(r) varies with position r of a unit dipole sample in the presence of a superconducting shielding tube. In particular, the relative decrease of G(r) is approximately given by the third power of the ratio of the detection coil and shield diameters, and the effect of the shield's finite length is found to be negligible in practical situations. A survey of complex detection coils (Helmholtz, saddle-type, gradiometer, and second derivative coil) includes calculation of the optimum spacing of the Helmholtz pair in the presence of a superconducting cylinder. The image effects due to the properties of the SQUID circuit and the field-dependent effects in high-field SQUID magnetometers are also discussed. Change in the instrument calibration due to sample size, shape, and location is considered for arbitrary samples as well as for specific cases of small, medium, and very long samples. A spherical harmonic expansion of G(r) makes it possible to derive simple formulas describing sample geometry effects for medium-sized samples with regular geometries (thin rod, cylinder, and rectangular parallelepiped). The results are compared to published experimental data.

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Image and sample geometry effects in SQUID magnetometers

Zięba

1993

Review of Scientific Instruments

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Johnson noise in SQUID magnetometers

Cited by 1 publication

References 3 publications

Image and sample geometry effects in SQUID magnetometers

Image and sample geometry effects in SQUID magnetometers

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