The magnetization is a vector quantity describing the magnetic state of a material and is defined as the amount of dipolar magnetic moments per unit volume. A magnetic material - iron or nickel for example - can be considered as being made up of many small magnets called dipoles.
Normally these dipoles, which in turn have north and south magnetic poles, are distributed with a certain degree of disorder within the volume of the material. The disorder is less in materials with strong magnetic properties such as iron and greater in others with less obvious magnetism.
Figure 1. Magnetic dipoles are arranged randomly inside a material. Source: F. Zapata.
However, by placing the material in the middle of an external magnetic field, such as that produced within a solenoid, the dipoles are oriented according to the field and the material is able to behave like a magnet (Figure 2).
Figure 2. Placing a material such as a piece of iron for example, inside a solenoid through which a current I passes, the magnetic field of this aligns the dipoles in the material. Source: F. Zapata.
Let M be the magnetization vector, which is defined as:
Now, the intensity of the magnetization in the material, product of being immersed in the external field H, is proportional to this, therefore:
M ∝ H
The constant of proportionality depends on the material, it is called magnetic susceptibility and is denoted as χ:
M = χ. H
The units of M in the International System are ampere / meter, like those of H, therefore χ is dimensionless.
Orbital and spin magnetic moment
Magnetism arises from moving electrical charges, therefore to determine the magnetism of the atom, we must take into account the movements of the charged particles that constitute it.
Figure 3. The motion of the electron around the nucleus contributes to the magnetism with the orbital magnetic moment. Source: F. Zapata.
Starting with the electron, which is considered to be orbiting the atomic nucleus, it is like a tiny loop (closed circuit or closed current loop). This movement contributes to the magnetism of the atom thanks to the orbital magnetic moment vector m, whose magnitude is:
Where I is the current intensity and A is the area enclosed by the loop. Therefore, the units of m in the International System (SI) are amps x square meter.
Vector m is perpendicular to the plane of the loop, as shown in Figure 3, and is directed as indicated by the rule of the right thumb.
The thumb is oriented in the direction of the current and the four remaining fingers are wrapped around the loop, pointing upwards. This small circuit is equivalent to a bar magnet, as shown in Figure 3.
Spin magnetic moment
Apart from the orbital magnetic moment, the electron behaves as if it were rotating on itself. It doesn't happen exactly this way, but the resulting effect is the same, so this is another contribution that needs to be taken into account for the net magnetic moment of an atom.
In fact, the spin magnetic moment is more intense than the orbital moment and is mainly responsible for the net magnetism of a substance.
Figure 4. The spin magnetic moment is the one that contributes the most to the net magnetization of a material. Source: F. Zapata.
The spin moments align in the presence of an external magnetic field and create a cascade effect, successively aligning with neighboring moments.
Not all materials exhibit magnetic properties. These are because the electrons with opposite spin form pairs and cancel their respective spin magnetic moments.
Only if any are unpaired is there a contribution to the total magnetic moment. Therefore, only atoms with an odd number of electrons have a chance of being magnetic.
The protons in the atomic nucleus also make a small contribution to the total magnetic moment of the atom, because they also have spin and therefore an associated magnetic moment.
But this is inversely dependent on mass, and that of the proton is much larger than that of the electron.
Examples
Inside a coil, through which an electric current passes, a uniform magnetic field is created.
And as described in figure 2, when placing a material there, the magnetic moments of this align with the field of the coil. The net effect is to produce a stronger magnetic field.
Transformers, devices that increase or decrease alternating voltages, are good examples. They consist of two coils, the primary and the secondary, wound on a soft iron core.
Figure 5. In the core of the transformer a net magnetization occurs. Source: Wikimedia Commons.
A changing current is passed through the primary coil that alternately modifies the magnetic field lines within the core, which in turn induces a current in the secondary coil.
The frequency of the oscillation is the same, but the magnitude is different. In this way, higher or lower voltages can be obtained.
Instead of winding the coils to a solid iron core, it is preferable to put a filling of metal sheets covered with varnish.
The reason is due to the presence of eddy currents inside the core, which have the effect of overheating it excessively, but the currents induced in the sheets are lower, and therefore the heating of the device is minimized.
Wireless chargers
A cell phone or an electric toothbrush can be charged by magnetic induction, which is known as wireless charging or inductive charging.
It works in the following way: there is a base or charging station, which has a solenoid or main coil, through which a changing current is passed. Another (secondary) coil is attached to the brush handle.
The current in the primary coil in turn induces a current in the coil of the handle when the brush is placed in the charging station, and this takes care of charging the battery that is also in the handle.
The magnitude of the induced current is increased when a core of ferromagnetic material, which may be iron, is placed in the main coil.
For the primary coil to detect the proximity of the secondary coil, the system emits an intermittent signal. Once a response is received, the mechanism described is activated and the current begins to be induced without the need for cables.
Ferrofluids
Another interesting application of the magnetic properties of matter are ferrofluids. These consist of tiny magnetic particles of a ferrite compound, suspended in a liquid medium, which can be organic or even water.
The particles are coated with a substance that prevents their agglomeration, and thus remain distributed in the liquid.
The idea is that the flowability of the liquid is combined with the magnetism of the ferrite particles, which by themselves are not strongly magnetic, but acquire a magnetization in the presence of an external field, as described above.
The acquired magnetization disappears as soon as the external field is withdrawn.
Ferrofluids were originally developed by NASA to mobilize fuel within a spacecraft without gravity, giving impulse with the help of a magnetic field.
Currently, ferrofluids have many applications, some still in the experimental phase, such as:
- Reduce friction on the mufflers of the speakers and headphones (avoid reverberation).
- Allow the separation of materials with different density.
- Act as seals on the shafts of the hard drives and repel dirt.
- As a cancer treatment (in the experimental phase). Ferrofluid is injected into cancer cells and a magnetic field is applied that produces small electrical currents. The heat generated by these attacks the malignant cells and destroys them.
References
- Brazilian Journal of Physics. Ferrofluids: Properties and applications. Recovered from: sbfisica.org.br
- Figueroa, D. (2005). Series: Physics for Science and Engineering. Volume 6. Electromagnetism. Edited by Douglas Figueroa (USB). 215-221.
- Giancoli, D. 2006. Physics: Principles with Applications. 6th Ed Prentice Hall. 560-562.
- Kirkpatrick, L. 2007. Physics: A Look at the World. 6th abridged edition. Cengage Learning. 233.
- Shipman, J. 2009. Introduction to Physical Science. Cengage Learning. 206-208.