AC Susceptibility Measurements
Magnetic susceptibility is a measure of a material's response to an external magnetic field H. Equilibrium magnetization characteristics M(H) are obtained by sweeping a DC magnetic field, whereas the magnetization's dynamic properties are captured by an AC component. For small AC excitations, the susceptibility is linearly proportional to the magnetic field. AC susceptibility measurements are used for:
- Fast magnetization characterization;
- The study of non-equilibrium processes; and
- Capturing small changes caused by magnetization dynamics or magnetic phase transitions.
Typical samples are magnetic materials and superconductors, as well as samples with time-varying magnetic properties that require the high temporal resolution afforded by the AC technique.
The weak AC signals are often buried in noise: this makes lock-in amplifiers an excellent choice to achieve the best signal-to-noise ratio. Moreover, phase-sensitive detection measures both real and imaginary parts of the susceptibility. The real part gives access to the slope of the magnetization curve M(H); the imaginary part of the susceptibility reflects the dissipative processes present in the material.
A typical measurement setup is illustrated in the figure. The static field is provided by the permanent magnet, and the lock-in amplifier drives a modulation (or primary) coil to generate an AC magnetic field and produce a time-dependent magnetic moment in the sample. The susceptometer features two pick-up coils symmetrically positioned with respect to the primary coil. One coil contains the sample, whereas the second coil is wound in the opposite direction and serves as the reference coil. With this geometry, only the signal induced by the sample's magnetization is measured and all other background contributions are subtracted.
The amplitude and phase of the measured signal are plotted as a function of the sample temperature or of the DC magnetic field. From these measurements it is possible to infer the sample's critical temperature, the magnetization curve, and the magnetic losses in the sample due to eddy currents in conducting materials and irreversible domain wall movement in ferromagnetic materials, for example.
The Benefits of Choosing Zurich Instruments
- You can perform both static (DC) and dynamic (AC) susceptibility measurements using a single MFLI Lock-in Amplifier upgraded with the MF-MD Multi-Demodulator option.
- Detecting the smallest contributions to losses faster is possible thanks to the MFLI's input noise – as low as 2.5 nV/sqrt(Hz) – and its high dynamic reserve of 120 dB.
- To study losses at various time scales, take advantage of the automated frequency sweeps realized by the LabOne® control software.