national high magnetic field laboratory
PULSED FIELD FACILITY
specialization and methods
Most materials exhibit a change of resistance with applied magnetic fields (magnetoresistance) that is one of the most common transport measurements made at NHMFL. Magnetoresistance measurements contain information about a material's electronic structure, carrier mass, electron-phonon coupling, etc. Ordinarily, resistance is measured by placing two probes of an ohm-meter across the ends of the sample to be measured. 2 wire measurements do not work very well for samples located in our experimental set-up because the samples are located at the bottom of a 1.5 meter long probe. For best signal-to-noise performance, 4 wires need to be attached to a sample to measure its magnetoresistance.
The leads connecting the sample to the outside world have a resistance of ~25W. This makes it impossible to measure accurately the resistance of just the sample using the 2 lead measurement method if the sample resistance is less than 25W. The 4 wire resistance measurement method solves this problem.
For the 4 wire method, we pass a current through the sample (2 contacts), then measure the potential difference across the sample (2 more contacts). We then know that R = V/I between the voltage leads, which gives us just the resistance of the sample without the leads and contacts.
One of the biggest obstacles to overcome in the quest for accurate measurements is excess noise and interference that obstructs the signal from your sample. Note, however, that the connections between the magnet and data acquisition devices are arranged to help reduce as much noise as possible. A central component of this arrangement is the Lock-In Amplifier.
Lock-In Amplifiers detect the signal and any noise associated with it (including thermal, shot and 1/f noise). Because of this, it is necessary to take noise into consideration when choosing a frequency for the measurement. There is substantial 1/f noise at zero frequency, making DC undesirable. Measuring at a higher frequency usually results in a less noisy environment. However, if you go too high in frequency, cable capacitance and inductance may obscure the signal. Therefore, it is best to test your system at chosen frequencies to determine the appropriate one for your particular sample and measurement. Typically signal frequency fall between 50 kHz and 500 kHz in pulsed field measurements.
During your experiment, the digitizer records the voltage changes at a fixed rate of time (converting it from analog to digital). These multichannel digitizers are capable of recording the signals from multiple experimental channels.
In order to record the magnetic field at the sample position when the magnet is pulsed, a solenoid, “dB/dt coil”, is placed near the sample on the end of the probe. When the magnet is pulsed, there is a specific coil callibration constant for each probe that will help calculate the data. When the magnet is pulsed, the magnetic flux induces voltage in the dB/dt coil which is proportional to rate of change in magnetic flux. The waveform of dB/dt signal is recorded by the digitizer and then integrated by DAQ software to obtain a "Filed Time Profile." The resulting profile is a waveform showing the magnetic field at each moment in time. The profile can now be used to help interpret any data received from your sample in accordance with your experimental specifications.