How do magnets work?

How do magnets work?

We all are very familiar with magnets! Everyone gets so curious when they play with magnets and feel the attractive/repulsive forces.Typically when we place other objects like piece of wood or stones, they don’t really interact with each other (in the sense they don’t attract or repel). But if we place magnets near each other they are attracted to each other (or repelled). So what really happens in magnets? What makes magnets, magnets!?

We are familiar with the illustrations we see, of how the magnetic lines of forces originate from north pole and end at south pole. Opposite poles attract and like poles repel each other due to magnetic lines of force.

Magnetic Field Lines.

But what creates this force in the first place? Lets go down to atomic level and see why. Every atom has certain number of electrons whizzing around the nucleus in specific orbits. Every electron has fundamental properties like mass, charge, spin etc. Now to explain the origin of magnetism one needs to use quantum mechanics (which we are not going to do right now). In simple terms we imagine electrons as charged particles which create magnetic field due to their motion in orbitals. Except these don’t actually contribute to the magnetic field. The outermost shells of some atoms which remain partially filled or half filled contribute to the majority of magnetic field of the atom. The electronic configuration of atoms follows Hund’s rule in which spin up electrons occupy energy levels first and then spin down. So in some atoms the outermost shells remain half filled and the intrinsic magnetism of individual electrons actually creates majority of magnetic field of the atom.

Electronic Configuration of Iron which shows partially filled outermost shell.

Okay so now that we have established that outermost electrons of an atom actually creates magnetic field we can say that each atom works like a tiny magnet (very tiny magnet). When very very large number of such magnetic atoms come together to form a solid we call it a magnet! So one natural question pops up in our mind that why are there few elements which can be magnets ?

Which brings us to the topic of domains. Now when magnetic atoms come together to form solids they have got options. They can align their magnetic fields in one particular direction which creates a magnet, or they can align in alternating fashion which essentially cancels out the magnetic field. They align themselves in a fashion requiring least energy to do so. A permanent magnet is a Ferromagnet i.e the magnetic field of every atom is aligned in one direction. Whereas Anti-Ferromagnets are the one’s in which every atom is aligned in an alternating way such that the magnetic field cancels out and makes the solid very non-magnetic.

(a) Ferromagnetic
(b) Anti- Ferromagnetic

But these are the two extreme cases, it may happen that some elements are somewhat magnetic or can be magnetised.Paramagnets are materials in which the direction of magnetic field of individual atoms is random. Domains are chunks or very small part of solid consisting of electrons aligned in the same direction as other in that chunk. It is possible in solids that one chunk of it has its atoms magnetic field pointed in one direction and other chunks in slightly different or opposite direction. However if we apply a very strong magnetic field we can force all the atoms in these domains to align with the external magnetic field hence creating a magnet!

Domains

That’s why only few elements like iron, nickel, cobalt, etc can be very good magnets. Other elements of the periodic table have completely or mostly filled outermost shells so those elements are not magnetic. Ferromagnetic materials are the only ones attracted to a magnet strongly enough to be considered as magnetic but all other elements do respond weakly to magnetic fields.

Atom which have full or partially full outermost shells are not magnetic but atoms which are at the middle of major blocks having partially filled shells are magnetic.
Image Credits: Minute Physics.

The elements in red are magnetic. But Chromium as an atom is very magnetic but as a solid its one of the most anti-ferromagnetic element as the atoms align to opposite directions in alternating fashion cancelling out the total magnetic field. Rare earth elements (Lanthanides) have partially filled outermost shells and as these are huge atoms they are very magnetic. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets. But these rare earth elements which make best magnets and have endless applications are somewhat expensive. The United States Department of Energy has identified a need to find substitutes for rare-earth metals in permanent-magnet technology, and has begun funding such research.

So lets just summarise how magnets work:

  • Each atom of magnet has to have half-filled or partially filled outermost shell of electrons.
  • These atoms have to be aligned with each other in the same direction of magnetic field.
  • And all the domains have to be aligned.

So yeah that’s a very short description of how magnets work! If you go deep enough and ask why do electrons have magnetic field in the first place, the answer is no one actually knows! Its just the way the universe works and we know this because of observations made that every particle having charge has magnetic field. The case of electromagnets is totally different and it works due to special relativity which is a topic for another post.

 

 

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