Hall Effect Measurements
The effect known as the Hall effect appears as the potential difference induced perpendicularly to an external magnetic field and the moving charge carriers. This effect is widely used in magnetic field sensing and in the characterization of materials.
During the characterization of a material, it is exposed to a known magnetic field B. Simultaneous measurements are taken of the Hall voltage Vxy (see figure), the voltage across the sample Vxx, along with the current IR through the material.
It is possible to infer material properties from these measurements, such as the charge carrier polarity, charge carrier density, the charge carrier mobility, and finally, the material's conductivity.
Novel physical properties of two-dimensional electron gas (2DEG) materials can also be measured with this technique by measuring the quantum Hall effect and its multiple derivatives: fractional, integer, spin, inverse spin, and many more.
The Hall effect can be used to infer the external magnetic field over many orders of magnitude when the material properties are well-known. AC measurements usually lead to faster and more accurate results, although DC voltages applied to the sample can carry out the measurements.
AC measurements have further advantages, including higher precision and sensitivity typically leading to a larger signal-to-noise ratio (SNR) over a wider measurement range.
The measurement is performed using two lock-in amplifiers, as depicted by the figure. A constant AC voltage is provided by a lock-in amplifier 1 (denoted as MFLI 1) in order to induce a current into the sample.
Often, it is sufficient to place a current-limiting resistor RL – which has a much larger resistance than all the other resistances in the circuit combined– and assume that the current is constant over the course of the measurement.
By measuring the current through the sample, too, more accurate measurements can be achieved. The Hall Voltage Vxy is measured by lock-in amplifier 1, and lock-in amplifier 2 (denoted as MFLI 2) measures the voltage Vxy across the sample.
The current can also be measured with the current input and the MF-MD Multi-Demodulator option, in the case of the Zurich Instruments MFLI Lock-in Amplifier.
The lock-in amplifiers need to be frequency-, clock- and time-stamp-synchronized in order to ensure data alignment with the magnetic field during a measurement. This is facilitated as a result of the multi-device synchronization (MDS) capability of the MFLIs.
Image Credit: Zurich Instruments
The Benefits of Choosing Zurich Instruments
- Zurich instruments allow the performing of sensitive and fast AC measurements to achieve a high SNR.
- A second instrument can be added to perform dedicated current measurements for increased accuracy.
- Zurich’s instruments offer convenient measurement acquisition and data analysis through accurate instrument synchronization, along with data alignment (with MDS) and display in the same user interface.
Principles of Lock-in Detection
Principles of Lock-in Detection
Video Credit: Zurich Instruments
This information has been sourced, reviewed and adapted from materials provided by Zurich Instruments.
For more information on this source, please visit Zurich Instruments.