Curie Point Measurements by TGA and DSC - METTLER TOLEDO

Curie Point Measurements by TGA and DSC

Many materials exhibit temperature-dependent magnetic properties. These properties can be measured by thermogravimetry (TGA) and differential scanning calorimetry (DSC). The materials are ferromagnetic at low temperatures, but lose their macroscopic magnetic behavior above the Curie point or Curie temperature.

arrangement of electron spins in a ferromagnetic material

 

Introduction

Magnetism is an important property of a material and has many practical uses in everyday life. The applications range from refrigerator magnets, compasses, hard disks to sorting machines to separate valuable metals and waste material.

The general term called magnetism is in fact the macroscopic effect of a microscopic property known as ferromagnetism. Ferromagnetism is a physical phenomenon in which the individual electron spins align parallel to one another in the same direction within a microscopic region (domain). This phenomenon is shown schematically in Figure 1.

Besides ferromagnetism, there are also several other related magnetic properties, for example ferrimagnetism, paramagnetism and diamagnetism.

In ferrimagnetism, some spins are arranged antiparallel and others parallel. The opposing magnetic moments do not however cancel each other out because the magnetic moment is stronger in one direction. This gives rise to a weak magnetic field.

In paramagnetic materials, the spins are randomly oriented so that no macroscopic magnetism results. Diamagnetic materials react to an external magnetic field by creating an opposing magnetic field that weakens it.

The topic of this article is however ferromagnetism and in particular the ferromagnetic-paramagnetic transition temperature or Curie point and its determination.

If a material is microscopically ferromagnetic, it need not necessarily have macroscopic magnetic properties. It is possible that the material consists of differently oriented ferromagnetic domains whose magnetic moments neutralize one another. This means that macroscopically the material is in a disordered state.

These separate magnetic domains are called Weiss domains and their size is in the range of a few micrometers to less than one millimeter. The orientation of domains can be influenced in a production process, for example in the production of permanent magnets...



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Conclusions

TGA and DSC experiments allow the Curie point to be simply and reproducibly determined. The measured Curie point depends only to a small extent on the strength of the magnet used. The sample mass and heating and cooling rates have a larger influence, namely indirectly via the heat capacity and hence time lag of the sample temperature. The advantage of the TGA method is that the intensity of the TGA signal does not depend on the heating rate. This means that exact measurements can be made at very low heating rates where the time lag approaches zero.

If materials with known Curie points and previously evaluated measurement parameters are available, the Curie phase transition can be used to adjust thermogravimetric instruments that do not produce a DSC signal. The measurement conditions must be precisely investigated to find possible reference materials. Table 2 presents an overview of the different measurement conditions and evaluations.

If the TGA curves for the different sample masses and magnet strengths are normalized, the curves are exactly the same. It is therefore best to use a small sample mass and a weaker magnet because the Curie temperature is hardly influenced by the strength of the magnet and a larger sample mass results in a longer time lag.

Curie Point Measurements by TGA and DSC | Thermal Analysis Application No. UC362 | Application published in METTLER TOLEDO Thermal Analysis UserCom 36