Institut de Chimie Moléculaire et des Matériaux d'Orsay

Laboratoire de Chimie Inorganique - LCI

 

 

 

The Quantum Design Magnetic Property Measurement System

Fundamentals of Magnetism and Magnetic Measurements. Mike McElfresh, Purdue University,
featuring Quantum Design's Magnetic Property Measurement System (MPMS®).

 

THE SQUID

 

"The SQUID in the MPMS is the source of the instrument's sensitivity, it does not detect directly the magnetic field from the sample. Instead, the sample moves through a system of superconducting detection coils which are connected to the SQUID with superconducting wires, allowing the current from the detection coils to inductively couple to the SQUID sensor. When properly configurated. the SQUID electronics produces an output voltage which is strictly proportional to the current flowing in the SQUID input coil. Hence, the thin film SQUID device, which is located approximately 11 cm below the magnet inside a superconducting shield, essentially functions as an extremely sensitive current-to-voltage convertor."
 
  1. Sample Rod
  2. Sample Transport
  3. Airlock
  4. Superconducting Magnet
  5. Detection Coils
  6. Sample
  7. SQUID Input Coil
  8. SQUID
  9. SQUID Response (output voltage vs sample position)

 

MPMS Components (fig. 1)

 

"A measurement (fig. 2) is performed in the MPMS by moving a sample (6) through the superconducting detection coils (5), which are located outside the sample chamber and at the center of the magnet (4). As the sample moves through the coils, the magnetic moment of the sample induces an electric current in the detection coils. Because the detection coils, the connecting wires, and the SQUID input coil (7) form a closed superconclucting loop, any change of magnetic flux in the detection coils produces a change in the persistent current in the detection circuit. which is proportional to the change in magnetic flux. Since the SQUID functions as a highly linear current­to-voltage convertor, the variations in the current in the detection coils produce corresponding variations in the SQUID output voltage which are proportional to the magnetic moment of the sample (9). In a fully calibrated system, measurements of the voltage variations from the SQUID detector as a sample is moved through the detection coils provide a highly accurate measurement of the sample's magnetic moment. The system can be accurately calibrated using a small piece of material having a known mass and magnetic susceptibility."

Measurement (fig. 2)

THE SUPERCONDUCTING DETECTION COIL

"The detection coil is a single piece of superconducting wire wound in a set of three coils configured as a second-order (second-derivative) gradiometer. In this configuration, the upper coil is a single turn wound clockwise, the center coil comprises two turns wound counter-clockwise, and the bottom coil is a single turn wound clockwise. When installed in the MPMS, the coils are positioned at the center of the superconducting magnet outside the sample chamber such that the magnetic field from the sample couples inductively to the coils as the sample is moved through them. The gradiometer configuration is used to reduce noise in the detection circuit caused by fluctuations in the large magnetic field of the superconducting magnet. The gradiometer coil set also minimizes background drifts in the SQUID detection system caused by relaxation in the magnetic field of the superconducting magnet. Ideally if the magnetic field is relaxing uniformly, the flux change in the two-turn center coil will be exactly canceled by the flux change in the single-turn top and bottom coils. On the other hand, the magnetic moment of a sample can still be measured by moving the sample through the detection coils because the counterwound coil set measures the local changes in magnetic flux density produced by the dipole field of the sample."

 

THE SUPERCONDUCTING MAGNET

"The MPMS system employs a superconducting magnet wound in a solenoidal configuration. An important feature of the MPMS is that the magnet is constructed as a completely closed superconducting loop, allowing it to be charged up to a specific current, then operated during a measurement in persistent mode without benefit of an external current source or power supply. To charge the magnet up to a specific current, or to change the current in the magnet when a persistent current is already flowing, the closed superconducting loop must be electrically opened by using a persistent-current switch, formed by wrapping a small heater around a short segment of the magnet's superconducting wire. When the heater is energized, the segment of wire within the heater becomes normal (no longer superconducting), thereby electrically opening the closed superconducting loop. By attaching a power supply (which essentially functions as a current source) to each side of the switch, it becomes possible to change the current in the superconducting magnet. While the current in the magnet is being provided by the power supply, the SQUID detection system in the MPMS will display a high level of noise. The detection system noise arises from fluctuations in the magnetic field of the magnet produced by current fluctuations in the current source. However, once the magnetic field is at the desired level, the switch heater can be turned off allowing the switch to return to the superconducting state. In this condition, the current from the power (…)

THE MEASUREMENT PROCEDURE

The lower portion (about 30 cm) of the sample space is lined with copper to provide a region of high thermal uniformity. Two thermometers determine the sample temperature and provide for temperature control. An extensive calibration procedure in which a standard thermometer is placed in the sample position is used to determine temperature controller constants, temperature gradients, and thermometer calibrations.

SAMPLE TRANSPORT

The sample is mounted in a sample holder that is attached to the end of a rigid sample rod. The sample rod enters the sample space through a special type of double seal (called a hp seal) designed to allow the rod to be actuated by a drive mechanism located outside of the chamber. The component containing the lip seals is clamped onto the top of the airlock with standard 0-ring seals, forming the top of the sample space. The top of the sample transport rod is attached to a stepper-motor-controlled platform which is used to drive the sample through the detection coil in a series of discrete steps. It is possible to use discrete steps because the detection coil, SQUID input coil, and connecting wires (see Figure 2) form a complete superconducting loop. A change in the sample's position causes a change in the flux within the detection coil, thereby changing the current in the superconducting circuit. Since the loop is entirely superconducting, the current does not decay as it would in a normal conductor. During the measurement the sample is stopped at a number of positions over the specified scan length, and at each stop, several readings of the SQUID voltage are collected and averaged. The complete scans can be repeated a number of times and the signals averaged to improve the signal-to-noise ratio. The output of the SQUID as a magnetic dipole is moved through the second-order gradiomertter pickup coil. The vertical scale corresponds to an output voltage and the horizontal scale is sample position. The currents induced in the detection coil are ideally those associated with the movement of a point-source magnetic dipole through a second-order gradiometer detection coil. The spatial (position) dependence of the ideal signal is shown in Figure S. To observe this signal requires that the sample be much smaller than the detection coil and the sample must be uniformly magnetized. Uniform magnetization, however, is often not encountered with high critical-current-density ( Jc ) superconductors. This and other kinds of nonuniform magnetization can be a problem. In addition to this, the size and shape of a sample can also require special consideration. If a sample is very long, extending well beyond the coil during a scan, its motion in the gradiometer will not be observable, since there would be no net change of the flux in the detection coil. This is the reason that a long uniform tube can be used as a sample holder. In contrast to this, when the sample is short, the current in the detection coil changes with sample position. This is because different amounts of flux (the local induction B) exist in each loop of the detection coil. So, it is important to realize that there is a limit on the length of a sample for which accurate measurements can be made. Some accommodation for length is made in particular computer fitting routines used to extract the value of the moment from the SQUID output. However, the safest procedure is to calibrate the MPMS with standards having a size and shape similar to the samples to be measured.

OUTPUT SIGNAL ANALYSIS

The most accurate determinations of the moment from the SQUID output signal are made using computer fits. Three different methods for analyzing the SQUID output signal are provided. All of these methods are based on the response expected for a magnetic dipole passing through a second-order gradiometer coil. The three different methods are 1) full scan, 2) linear regression, and 3) iterative regression. In the full scan method, the area under the SQUID voltage versus position curve is integrated, since this area is proportional to the magnetic moment. This method requires that the sample be well centered and requires fairly long scan lengths to get accurate results The linear regression method makes a fit of the theoretical signal of a dipole moving through a second-order gradiometer to the actual SQUID output signal using a linear regression algorithm. A scan as short as 2 cm can be used, but this method requires that the sample remain well centered. When the sample is measured over a wide temperature range, the sample will change position due to changes in the length of the sample transport rod. In this case, the sample will no longer stay centered. Using software commands, the sample position can be changed within a program but this process requires a precise knowledge of how the sample position changes with temperature. Recent improvements in the MPMS control software now allow the user to select a tracking mode which keeps the sample properly centered over the normal MPMS operating range of 1.9 K to 400 K. The iterative regression method, on the other hand, can accommodate these position offsets using additional variables within the computer program. However, there are limitations to this method, particularly when a sample has a nonuniform magnetization or the sample signal is very small. Both regression methods have fit parameters that help to accommodate deviations from ideal behavior (e.g., sample size and shape). The MPMS reports the magnetic moment data in emu (electromagnetic units).