The ferromagnetism is the property that gives some substance intense and permanent magnetic response. In nature, there are five elements with this property: iron, cobalt, nickel, gadolinium and dysprosium, the last rare earths.
In the presence of an external magnetic field, such as that produced by a natural magnet or an electromagnet, a substance responds in a characteristic way, according to its internal configuration. The magnitude that quantifies this response is magnetic permeability.
Magnets forming a bridge. Source: Pixabay
Magnetic permeability is a dimensionless quantity given by the ratio between the strength of the magnetic field generated inside the material and that of the magnetic field applied externally.
When this response is much greater than 1, the material is classified as ferromagnetic. On the other hand, if the permeability is not much greater than 1, the magnetic response is considered weaker, they are paramagnetic materials.
In iron, magnetic permeability is on the order of 10 4 . This means that the field inside the iron is about 10,000 times larger than the field applied externally. Which gives you an idea of how powerful this mineral’s magnetic response is.
How does the magnetic response originate in substances?
It is known that magnetism is an effect associated with the movement of electrical charges. This is exactly the electrical current. So where do the magnetic properties of the bar magnet come from, with which a note got stuck in the fridge?
The magnet material and also any other substance contain within it protons and electrons, which have their own motion and generate electrical currents in various ways.
A very simplified model assumes the electron in a circular orbit around the nucleus formed by protons and neutrons, thus forming a small current loop. Each loop is associated with a vector magnitude called “orbital magnetic moment”, whose intensity is given by the product of the current and the area determined by the loop: the Bohr magneton.
Obviously, in this small circuit, the current depends on the charge on the electron. As all substances contain electrons in their interior, they all have, in principle, the possibility of expressing magnetic properties. However, not all do.
This is because their magnetic moments are not aligned, but randomly arranged inside, so their magnetic effects at the macroscopic level are canceled out.
The story doesn’t end here. The product of the magnetic moment of the electron’s movement around the nucleus is not the only possible source of magnetism on this scale.
The electron has a kind of rotational movement around its axis. It is an effect that results in an intrinsic angular momentum. This property is called electron rotation .
Of course, it also has an associated magnetic momentum and is much more intense than orbital momentum. In fact, the biggest contribution to the atom’s net magnetic moment is through spin, however, both magnetic moments: translation plus intrinsic angular momentum contribute to the atom’s total magnetic moment.
These magnetic moments are those that tend to align in the presence of an external magnetic field. And they also do it with the fields created by neighboring moments in the material.
Now electrons usually pair up in atoms with many electrons. Pairs are formed between oppositely rotating electrons, resulting in cancellation of the magnetic rotational moment.
The only way that spin contributes to the total magnetic moment is if some are missing, that is, the atom has an odd number of electrons.
One might ask what happens to the magnetic moment of protons in the nucleus. They also have a rotational moment, but it is not considered to contribute significantly to an atom’s magnetism. This is because the moment of rotation is inversely dependent on mass and the mass of the proton is much greater than that of the electron.
magnetic domains
In iron, cobalt and nickel, the triad of elements with high magnetic response, the net rotation moment produced by the electrons is not zero. In these metals, the electrons in the 3d orbital, the outermost ones, contribute to the net magnetic moment . That’s why these materials are considered to be ferromagnetic.
However, this individual magnetic moment of each atom is not enough to explain the behavior of ferromagnetic materials.
Within strongly magnetic materials, there are regions called magnetic domains , whose length can vary between 10 -4 and 10 -1 cm and which contain billions of atoms. In these regions, the moments of net rotation of neighboring atoms manage to get tightly involved.
When a magnetic domain that contains material approaches a magnet, the domains align, intensifying the magnetic effect.
This is because domains, like bar magnets, have magnetic poles, also called North and South, so that like poles repel and attract opposite ones.
As the domains align with the external field, the material emits inflections that can be heard through adequate amplification.
This effect can be observed when a magnet attracts the sweet iron nails and these behave like magnets attracting other nails.
Magnetic domains are not static boundaries established within the material. Its size can be changed by cooling or heating the material, as well as subjecting it to the action of external magnetic fields.
However, domain growth is not unlimited. When it is no longer possible to align them, the saturation point of the material is said to have been reached. This effect is reflected in the hysteresis curves that appear below.
Material heating causes loss of alignment of magnetic moments. The temperature at which magnetization is completely lost varies with the type of material, so a bar magnet is usually lost at around 770°C.
Once the magnet is removed, the magnetization of the nails is lost due to thermal agitation present at all times. But there are other compounds that have permanent magnetization because they have spontaneously aligned domains.
Magnetic domains can be observed when a flat area of non-magnetized ferromagnetic material, such as soft iron, is cut and polished very well. Once this is done, sprinkle with powder or fine iron filings.
Under the microscope, it is observed that the chips are grouped in the mineral-forming regions with a very well defined orientation, following the magnetic domains of the material.
The difference in behavior between various magnetic materials is due to the way the domains inside behave.
magnetic hysteresis
Magnetic hysteresis is a characteristic that only materials with high magnetic permeability have. It is not presented by paramagnetic or diamagnetic materials.
Represents the effect of an applied external magnetic field, which is denoted as H, on the magnetic induction B of a ferromagnetic metal during a magnetization and demagnetization cycle. The graph shown is called the hysteresis curve.
Initially at point O, there is no applied field H or magnetic response B , but as the intensity of H increases , induction B progressively increases until reaching the magnitude of saturation B s at point A, which is expected.
Now the intensity of H gradually decreases until reaching 0, with this point C, but the magnetic response of the material does not disappear, maintaining a remaining magnetization indicated by the value B r . This means that the process is not reversible.
Thereafter, the intensity of H increases, but with the polarity reversed (negative sign), so that the remaining magnetization is nullified at point D. The required value of H is indicated as H c and is called the coercive field .
The magnitude of H increases until it reaches the saturation value at E again and immediately the intensity of H decreases until it reaches 0, but there is a magnetization remaining with the opposite polarity as described above, at point F.
Now the polarity of H is reversed again and its magnitude is increased until the magnetic response of the material at point G is canceled out. Following the GA path, its saturation is obtained again. But the interesting thing is that you didn’t get there by the original path indicated by the red arrows.
Magnetically hard and soft materials: applications
Soft iron is easier to magnetize than steel, and tapping the material further facilitates mastery alignment.
When a material is easy to magnetize and demagnetize, it is said to be magnetically soft and, of course, if the opposite occurs, it is magnetically hard . In the second, the magnetic domains are small, while in the first they are large, so they can be seen through a microscope, as detailed above.
The area enclosed by the hysteresis curve is a measure of the energy needed to magnetize – demagnetize the material. The figure shows two hysteresis curves for two different materials. The one on the left is magnetically soft, while the one on the right is hard.
A soft ferromagnetic material has a small and narrow H c coercive field and high hysteresis curve. It is a suitable material to place in the core of an electrical transformer. Examples of this are mild iron and ferro-silicon and ferro-nickel alloys, which are useful for communication equipment.
On the other hand, it is difficult to demagnetize magnetically hard materials once magnetized, as is the case with alnico alloys (aluminum-nickel-cobalt) and rare earth alloys from which permanent magnets are made.
