Paramagnetism: causes, paramagnetic materials, examples
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 think, magnetic properties are not reduced to ferromagnetic substances alone. 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, paramagnetics are attracted to the area where the field strength is greatest. In contrast, diamagnetics are attracted to the region of the field where the intensity is lower.
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, entropy ends the magnetic alignment that was induced.
In other words, paramagnetic materials are attracted to magnetic fields, although they are not permanently transformed into magnetized materials. Some examples of paramagnetic substances are: air, magnesium, platinum, aluminum, titanium, tungsten and lithium, among others.
Paramagnetism is due to the fact that certain materials are composed 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 missing electrons from metals and other materials that have paramagnetic properties.
In pure paramagnetism, the dipoles do not interact with each other, but are randomly oriented in the absence of an external magnetic field as a result of thermal agitation. This generates a null 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 that is added to the external field.
In any case, the alignment of the dipoles can be neutralized by the effect of temperature.
Thus, when the material is heated, thermal agitation is able to neutralize the effect that the magnetic field has on the dipoles and the magnetic moments are chaotically reoriented, reducing the intensity of the induced field.
Curie’s law was experimentally developed by the French physicist Pierre Curie in 1896. It can only be applied when high temperatures occur and the paramagnetic substance is in the presence of weak magnetic fields.
This is because it fails to describe paramagnetism when most of the magnetic moments are aligned.
The law states that the magnetization of the paramagnetic material is directly proportional to the strength of the applied magnetic field. This is what is known as Curie’s Law:
M = X ∙ H = CH / T
In the formula above, M is magnetization, H is the magnetic flux density of the applied magnetic field, T is the temperature measured in degrees Kelvin, and C is a material-specific constant called the Curie constant.
It is also evident from observation of Curie’s law that magnetization is inversely proportional to temperature. For this reason, when the material is heated, the dipoles and magnetic moments tend to lose the orientation acquired by the presence of the magnetic field.
Paramagnetic materials are all materials with magnetic permeability (the ability of a substance to attract or pass through a magnetic field) similar to the magnetic permeability of a vacuum. Such materials show an insignificant level of ferromagnetism.
In physical terms, its relative magnetic permeability (quotient between the permeability of the material or medium and the permeability of a vacuum) is said to be approximately equal to 1, which is the magnetic permeability of a vacuum.
Among paramagnetic materials, there is a particular type of material that is called superparamagnetic. Although they follow the Curie Law, these materials have a fairly high Curie constant value.
Differences between paramagnetism and diamagnetism
It was Michael Faraday who, in September 1845, realized that, in reality, all materials (not just ferromagnetic) react to the presence of magnetic fields.
In any case, the truth is that most substances are diamagnetic in nature, since pairs of paired electrons – and therefore oppositely rotating – weakly favor diamagnetism. On the contrary, only when electrons are missing does diamagnetism occur.
Paramagnetic and diamagnetic materials have a weak susceptibility to magnetic fields, but while the former is positive, the latter is negative.
Diamagnetic materials are slightly repelled by a magnetic field; on the other hand, paramagnetics are attracted, although also with little strength. 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.
As paramagnetic materials have a vacuum-like behavior in the absence of a magnetic field, their applications in industry are somewhat reduced.
One of the most interesting applications of paramagnetism is electronic paramagnetic resonance (EPR), which is widely used in physics, chemistry and archeology. It is a spectroscopy technique with which it is possible to detect species lacking electrons.
This technique is applied in fermentations, industrial polymer manufacturing, engine oil wear and beer manufacturing, among other areas. Likewise, this technique is widely used in dating archaeological remains.