1-11 Bipolar power
3.5 Magnetization Measurement
3.5.1 SQUID Magnetometer
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Fig. 3.15 MPMS XL SQUID Magnetometer.
The major components of a SQUID magnetometer are: The RSO sample rod consists of one long, graphite sample rod; one short, graphite sample rod; and two centering plugs, superconducting magnet, superconducting detection coil, a SQUID connected to the detection coil, superconducting magnetic shield. Superconducting magnets are solenoid made of superconducting wire which must be kept at liquid helium dewar. The uniform magnetic field is produced along the axial cylindrical bore of the coil. The superconducting pick-up coil system, which is configured as a second order gradiometer is placed in the uniform magnetic field region of the solenoidal superconducting magnet. The SQUID device is usually a thin film that functions as an extremely sensitive current to voltage converter.
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RSO sample rods are specially designed to insert a drinking straw shown in Fig. 3.16, which holds a sample, into the sample chamber. The centering plugs help keep a loose sample inside the straw and protect the straw from rubbing against the inside wall of the sample chamber. RSO sample rods may be used for RSO, DC, or AC measurements. The RSO option provides an alternative way of using the MPMS to measure the magnetic moment of samples. The RSO option measures a sample by moving it rapidly and sinusoidally through the SQUID pickup coils.
The option's use of a high-quality servo motor and a digital signal processor (DSP) allow rapid measurements. The servo motor, unlike the stepper motor performing DC measurements, does not stop sample movement for each data reading. During an RSO measurement, a shaft encoder on the servo motor tracks the position of the sample. The position is recorded synchronous to the acquisition of the SQUID signal. The data is analyzed by using a nonlinear, least-squares fitting routine to fit the data to an ideal dipole response. The magnetic moment calibration for the MPMS is determined by measuring a palladium standard over a range of magnetic fields and by then adjusting the system calibration factors to obtain the correct moment for the standard. The standard is a right circular cylinder approximately 3 mm in diameter x 3 mm in height. Samples of this size or smaller effectively imitate a point dipole to an accuracy of approximately 0.1%.
Measurements are done in this equipment by moving the samples through the second order gradiometer. Hence, the magnetic moment of the sample induces an electric current in the pick-up coil system. Superconducting magnetic shield is used to shield the SQUID sensor from the fluctuations of the ambient magnetic field of the place where the magnetometer is located and from the large magnetic field produced by the superconducting magnet. It is an important feature of the instrument that one can change the magnetic field either by "oscillate mode" or "no overshoot mode". The oscillate mode is used to minimize the remanent field of the magnet, whenever an accurate value of magnetic field is needed, e.g. in case of zero field cooling. In the hysteresis measurement the no overshoot mode has been selected, in which the field is changed directly from one value to another, and the magnet is returned to its persistent mode.
The MPMS XL features the new reciprocating sample measurement system. Unlike DC measurements where the sample is moved through the coils in discrete steps the RSO measurements are performed using a servomotor, which rapidly oscillates the sample is shown in Fig. 3.17. The RSO transport moves up and down, oscillating the sample around the measurement position. If autoranging is disabled, the MPMS repeats each measurement until it locates the emu
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range that accommodates the SQUID's sensitivity. While the sample moves through the coils, MPMS MultiVu measures the SQUID's response to the magnetic moment of the sample. After a fit to the raw SQUID voltage data is performed for the specified number of cycles, MPMS MultiVu measures new SQUID voltage data and obtains another fit. This process is repeated until the measurements-to-average number is reached. Then MPMS MultiVu calculates the average sample moment from all the fits and also calculates the standard deviation if more than one measurement is averaged. The data is saved to the active data files. MPMS MultiVu uses a nonlinear, least-squares fitting routine to fit the data to an ideal dipole response in order to determine the sample position. MPMS MultiVu uses a measurement algorithm to compute the magnetic moment of the sample.
LONG SAMPLE ROD SHORT SAMPLE ROD CENTERING PLUG CENTERING PLUG
DRINKING STRAW Fig. 3.16 Attaching the Sample to the Sample Rod.
The accuracy of an RSO sample measurement is determined by how well the sample is centered within the SQUID pickup coils. If the sample is not centered, the coils read only part of the magnetic moment of the sample. Properly centering the sample is particularly important if it will be measured at the maximum slope position. During maximum slope position measurements, MPMS MultiVu cannot use autotracking or the iterative regression algorithm, which both help keep the sample correctly positioned, even when temperature changes alter the geometry of the sample rod.
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SQUID PICKUP COILS
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