PARAMAGNETISM: CAUSES, PARAMAGNETIC MATERIALS, EXAMPLES - PHYSICAL - 2024

The paramagnetism is a form of magnetism which certain materials are weakly attracted by an external magnetic field and form internal magnetic fields induced in the direction of the applied magnetic field.

Contrary to what many people often think, magnetic properties are not only limited to ferromagnetic substances. All substances have magnetic properties, even in a weaker way. These substances are called paramagnetic and diamagnetic.

In this way, two types of substances can be distinguished: paramagnetic and diamagnetic. In the presence of a magnetic field, the paramagnetic ones are attracted towards the zone where the intensity of the field is greater. Instead, diamagnetics are attracted to the region of the field where the intensity is lowest.

When in the presence of magnetic fields, paramagnetic materials experience the same type of attraction and repulsion that magnets experience. However, when the magnetic field disappears, the entropy ends the magnetic alignment that had been induced.

In other words, paramagnetic materials are attracted by magnetic fields, although they do not become permanently magnetized materials. Some examples of paramagnetic substances are: air, magnesium, platinum, aluminum, titanium, tungsten and lithium, among others.

Causes

Paramagnetism is due to the fact that certain materials are made up of atoms and molecules that have permanent magnetic moments (or dipoles), even when they are not in the presence of a magnetic field.

Magnetic moments are caused by the spins of unpaired electrons in metals and other materials that have paramagnetic properties.

In pure paramagnetism the dipoles do not interact with each other, but are oriented randomly in the absence of an external magnetic field as a consequence of thermal agitation. This generates a zero magnetic moment.

However, when a magnetic field is applied, the dipoles tend to align with the applied field, resulting in a net magnetic moment in the direction of that field and adding to that of the external field.

In either case, the alignment of the dipoles can be counteracted by the effect of temperature.

In this way, when the material is heated, the thermal agitation is able to counteract the effect that the magnetic field has on the dipoles and the magnetic moments reorient themselves in a chaotic way, reducing the intensity of the induced field.

Curie's law

Curie's law was developed experimentally by the French physicist Pierre Curie in 1896. It can only be applied when high temperatures are present and the paramagnetic substance is in the presence of weak magnetic fields.

This is so because it fails to describe paramagnetism when a large part of the magnetic moments are aligned.

The law states that the magnetization of the paramagnetic material is directly proportional to the intensity of the applied magnetic field. It is what is known as Curie's Law:

M = X ∙ H = CH / T

In the above formula M is the magnetization, H is the magnetic flux density of the applied magnetic field, T is the temperature measured in degrees Kelvin and C is a constant that is specific to each material and is called the Curie constant.

Observing Curie's law also shows that magnetization is inversely proportional to temperature. For this reason, when the material is heated, the dipoles and the magnetic moments tend to lose the orientation acquired by the presence of the magnetic field.

Paramagnetic materials

Paramagnetic materials are all those materials with magnetic permeability (ability of a substance to attract or cause a magnetic field to pass through) similar to the magnetic permeability of a vacuum. Such materials show a negligible level of ferromagnetism.

In physical terms, it is stated that its relative magnetic permeability (quotient between the permeability of the material or medium and the permeability of a vacuum) is approximately equal to 1, which is the magnetic permeability of a vacuum.

Among paramagnetic materials there is a particular type of materials that is called superparamagnetic. Although they follow Curie's Law, these materials have a fairly high value of the Curie constant.

Differences between paramagnetism and diamagnetism

It was Michael Faraday who, in September 1845, realized that in reality all materials (not just ferromagnetic ones) react to the presence of magnetic fields.

In any case, the truth is that most substances have a diamagnetical character, since the pairs of paired electrons - and, therefore, with opposite spin - weakly favor diamagnetism. On the contrary, only when there are unpaired electrons does diamagnetism occur.

Both paramagnetic and diamagnetic materials have a weak susceptibility to magnetic fields, but while in the former it is positive in the latter it is negative.

Diamagnetic materials are slightly repelled by a magnetic field; on the other hand, paramagnetics are attracted, although also with little force. In both cases, when the magnetic field is removed, the effects of magnetization disappear.

As already mentioned, the vast majority of the elements that make up the periodic table are diamagnetic. Thus, examples of diamagnetic substances are water, hydrogen, helium and gold.

Applications

Since paramagnetic materials have a similar behavior to vacuum in the absence of a magnetic field, their applications in industry are somewhat limited.

One of the most interesting applications of paramagnetism is Electronic Paramagnetic Resonance (RPE), which is widely used in physics, chemistry and archeology. It is a spectroscopy technique with which it is possible to detect species with unpaired electrons.

This technique is applied in fermentations, in the industrial manufacture of polymers, for the wear of motor oils and in the manufacture of beers, among other areas. Similarly, this technique is widely used in the dating of archaeological remains.

References

1. Paramagnetism (nd). In Wikipedia. Retrieved on April 24, 2018, from es.wikipedia.org.
2. Diamagnetism (nd). In Wikipedia. Retrieved on April 24, 2018, from es.wikipedia.org.
3. Paramagnetism (nd). In Wikipedia. Retrieved on April 24, 2018, from en.wikipedia.org.
4. Diamagnetism (nd). In Wikipedia. Retrieved on April 24, 2018, from en.wikipedia.org.
5. Chang, MC "Diamagnetism and paramagnetism" (PDF). NTNU lecture notes. Retrieved on April 25, 2018.
6. Orchard, AF (2003) Magnetochemistry. Oxford University Press.