Magnetism & Matter
Origin seems to be a great mystery in our Universe. Even the most fundamental forces of nature seem to have concealed emergence. Magnetism, as we know it, is something which was observed naturally centuries ago. Our Physical World claims the most complex understanding of Magnetism without even having the fossils of its emergence.
If you haven’t already, please read the Structure of Atom.
Magnetism is one of the Fundamental Forces of the universe. Magnetic forces have been known to us for over a thousand years. It has evolved from simple phenomenon for amusement to something that is used to generate electricity, in electric motors, radio waves etc. It has entered in all spheres of our lives.In this page we look in to some unique properties of magnetism
Table of Contents
Diamagnetism
Diamagnetism is a property of a material to repel the magnetic force. Instead of being attracted toward a magnet, these materials give a negative response to magnets by moving away from them. Diamagnetism is present in every substance. However, it can rather be viewed as a recessive trait, i.e it can only be expressed in the absence of any other type of magnetic force such as Ferromagnetism and Paramagnetism.
Why is diamagnetism universal to matter?
Mainly because most elements and compounds have at least one pair of electrons.
How does diamagnetism exist?
One of the main phenomena involved in Magnetism is the Electronic Spin. (Electron pair)
Electrons follow the Pauli Exclusion Principle. With an integral value of 1/2, there are two possible spins for electrons, up or down.
The electron spin has yet many contradictions and still stands veiled. This is due to the fact that if electrons are treated as point particles then we cannot expect them to have a surface area to spin around which implies that electronic spin is not real. But with certain important and closely observed experiments it has been found to be real. Still due to the lack of the cause, the spin angular momentum is considered to be an intrinsic property. (Refer here)
How is Iron attracted toward a magnet?
Paired and Unpaired Electrons
Following the Pauli Exclusion Principle, no two electrons with the same spin can exist consecutively in an electron shell in order to make sure that the most stable form is obtained. However, still there are some electrons left with missing pairs.
Pauli Exclusion Principle says that electrons with opposite spins cancel out each other’s opposite magnetic fields and the net magnetic effect comes out to be zero. But, when some electrons are left unpaired, then all the unpaired electrons line up and their magnetic fields add up.
Diamagnets have all of their electrons paired due to which the net magnetic force is zero and they are not attracted by magnets. While, magnetic materials have unpaired electrons which eventually align their spins according to an external applied magnetic field and add up to their force.
Then how do diamagnets repel magnetic fields?
There’s no easy way to say this, but this can only and only be explained via maths. By defining properties of particles and building a mathematical structure, scientists made models to explain what was happening. These models, on testing, came out to be more or less true, so they went with them. Although the maths is not discussed here, we do try to give a brief (and links) to everyone interested.
Magnetic Susceptibility
Classical physics aside, quantum mechanics will tell you that materials are allowed to have what is called a negative magnetic susceptibility. The proof of this is based on Schrodinger’s equations. A negative magnetic susceptibility means if we try to magnetise something, the material produces a magnetic field in the opposite direction of the field that tried to magnetise it, which makes it repel the magnetising field.
Landau Quantization
Landau Quantization tells us that in a magnetic field, almost everything inside an atom changes. Electrons are quantized in a whole different way, which leads to a complete new picture. This picture, explained by Landau, is used to define diamagnetism.
What makes Diamagnetism Recessive?
Magnetic permeability and susceptibility affect how a material reacts to a magnet. If the two values are positive then the object shows an attraction toward the magnet, it allows its internal structure to align with that of the external magnetic field and hence the two fields add up creating a stronger bond. Whereas, the materials with negative permeability and susceptibility repel the magnet by changing their electronic alignment in order to resist the attraction. As we know Magnetism to be an interplay of the molecular magnets called electrons, Magnetism can be thought of as Gravitation. Just as the distance between the attraction or repulsion increases, the strength of the forces weakens. In the same way when two objects are attracted towards each other they keep coming closer and with a decrease in distance their attraction will keep getting stronger. While in diamagnets, when the two objects are repelling, the distance will increase and the two objects will come to their normal alignment of electrons as the strength of repulsion will weaken.
Superconductors
Superconductors have a special property of superconductivity which is attained when their temperature drops down a certain critical temperature. This leads to the electrical resistivity turn zero. With no resistance and an extremely low temperature electrons come closer and are not scattered due to impurities in the crystal lattice. Now when an external magnetic field is applied, a strong magnetic field generates opposing that of the applied one and the Meissner’s Effect kicks in. With complete opposition to the magnetic field, Superconductors manifest clear repulsion.
How are they different from normal diamagnets?
The main difference is that normal diamagnets have pre – existing paired electrons. But Superconductors obtain them at extremely low temperatures in the form of Cooper Pairs. Cooper pairs are generally the electron pairing due to a decrease in temperature because when the temperature decreases, electron scattering also decreases due to Phonon reduction. Just as the Phonon vibrations reduce, the electrons are better and more closely attracted by the positive nucleus. Then, electrons come closer too and their force of repulsion loses influence due to a much stronger nucleic attraction. The electrons in close vicinity cancel each other’s spin and become paired. The fading away of the Phonon movement is the main reason why Superconductors repel better than normal Diamagnets (because in normal diamagnets, lattice vibrations are still present due to comparatively higher temperatures. They show an unnoticeable repulsion asĀ most of the repulsive force is used to overcome other restrictions).
Magnetic Levitation
Magnetic Levitation is making an object float with no support other than magnetic field. The best property about Superconductors is that they can experience zero electrical resistance when cooled down to extremely low temperatures. After which if we induce in them an external magnetic field, it continues to exist for a significantly longer period under zero resistance (unless the temperature rises). As the electrical current produced has no friction against it, the energy dissipation is absent which allows the supeconductor to constantly keep producing the diamagnetic effect. (It should be noted that the temperature has to be extremely low).
Type I Superconductors expel all the applied magnetic field and do not allow penetration of the field even when increased. However, Type II Superconductors are yet special. They allow Flux Pinning. They have narrow Flux vortices which allow magnetic current to penetrate through them but only to a limited extent. Type I Superconductors exhibit Perfect Diamagnetism by repelling the applied field at any cost which can lead to Levitation but that would not be stable as it will not stop at one place but will keep repelling the field and the net position turns to be the result of the interaction of Gravity, the applied field and the repelling field.
We needed a more stable apparatus to obtain successful Levitation. Type II Superconductors allow it as they have both magnetic attraction and repulsion (the attraction is yet smaller). So they maintain a balance between the applied field and their Levitation by locking to their position. Also, the flux pinning can even be adjusted by changing the orientation of the superconductor. This orientation need not change by increasing the magnetic field only. It can also be done by decreasing the distance between the two objects manually, which would automatically increase the attraction or vice versa . (Look here)