Magnetometry is the basic characterization of a magnetic sample. It is the measurement of the total magnetic moment of a sample, in a given direction, as a function of various external parameters, the most usual ones being the magnetic field and the temperature.
From such measurements, we extract the following parameters:
- the spontaneous magnetization, for example, as a function of temperature (see illustration below for a ferromagnetic sample, where this variation is not simple). This is the most important parameter of a magnetic sample.
- the type and magnitude of the magnetic anisotropy of the sample. As the sample gives rise to an internal demagnetizing field, it is in fact the effective anisotropy of the sample, the combination of the intrinsic anisotropy and the one due to the sample shape, that is measured. The precise measurement of the anisotropy is delicate, as the orientation of the applied field has to be precisely controlled.
- in addition, from the shape of the hysteresis loop (magnetization vs. field, when the latter is cycled from positive to negative and back) and from what we know about the sample, we can get an idea of the magnetization reversal processes (domain nucleation, domain walls propagation). This has to be confirmed by magnetic imaging.
The magnetometry tools we use are:
- The magneto-optical magnetometry setup. This does not give access to the sample magnetization, but is well adapted to measure hysteresis loops very rapidly (one minute timescale). The advantages of magneto-optical measurements are its insensibility to magnetic contaminations, and the possibility to measure the properties locally. The field is limited (1T perpendicular, less in the planar geometry).
- The MPMS XL SQUID magnetometer within the Physical Properties Measurement Platform at LPS. The magnetization can be obtained. This is a more time consuming measurement, especially when the field is scanned (several hours). The main advantage of the SQUID is the access to a very wide temperature (0.3 to 400 K) and field (7 T) ranges.
- In Paris-Saclay University, we can use the VSM at Ecole Normale Supérieure, and the AGFM at Centre de Nanosciences et de Nanotechnologies. The measurement time is intermediate (a few tens of minutes). Like SQUID, these are global measurements, i.e., of the full sample plus its contaminations. The absolute calibration of the magnetization is more difficult compared to SQUID, especially for VSM. The AGFM works at RT, up to 2T. The VSM has the possibility to vary the temperature.