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Tham khảo tài liệu 'superconductivity theory and applications part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 264 Superconductivity - Theory and Applications An inhomogeneous applied field and imperfect gradiometer balance result in a crosstalk of the field to the SQUID and reduce a dynamic range of the CRSM. In SSSM a compensation coil wound on an upper part of the solenoid and supplied with an adjustable current derived from the solenoid supply current minimizes crosstalk. A careful design and construction keeps down deformation of the field affected by a proximity of magnetic or superconducting materials solder and frequency dependent eddy currents in metallic nonsuperconducting parts. The magnetic moment of the sample is m 1 V r x j d3r. 2 A vector potential of the induced or spontaneous magnetic moment m of the sample is A ụ m-. 3 The magnetic flux in the pickup coil is y A dl 4 where r is the coil circumference. The SQUID indicates difference in the flux in an upper and lower coil A upper lower and thus the SQUID output voltage is proportional to a projection of the measured magnetic moment on a gradiometer axis m t a A0 t . Since the detection system is superconducting the output voltage m t is proportional to the magnetic moment of the sample and not to a rate of change of the magnetic moment like in case of induction magnetometers ac susceptometer ACS or vibrating sample magnetometer VSM . Both the SSSM and HSSM use bulk Nb SQUID of the Zimmerman type operating at the rf frequency of about 40 MHz. The Josephson junction is a point contact type in the SSSM and thin film bridge in the HSSM. Both SQUIDs have an equivalent input flux noise density of the order of 10 4 0 i Hz 1 2 in a white noise region 1 Hz and range 500 0 i limited by a slew rate 104 0 s.3 A shielding of an external dc and time varying electromagnetic field originating from an earth magnetic field and man-made sources is necessary to utilize the extraordinary sensitivity of the SQUIDs. The shielding is ensured by a soft magnetic materials the cryostat is placed inside the shielding and .
