Have you ever wondered why you have been told to keep your hard drives or VHS tapes away from strong magnets, lest you lose your precious stored data and memories? The culprit is the physics behind the material that these storage devices are made of, that is magnetism and its effect on so called ferromagnetic material.

We’ve all experimented with magnets, maybe you’ve picked up a chain of paper clips with one while bored, or you collect fridge magnets from the places you visit. You might have seen these paperclips remain magnetic and stick to each other after contact with a magnet or wondered how your souvenir’s stick while other metallic objects don’t to your fridge door. These phenomena are due to this magnetic ‘memory’ associated with these objects.

An impressive collection of fridge magnets [1]

How do they work then?

To establish the basics, a magnet produces a magnetic field that conventionally goes from its north pole to the south pole. This can be seen when putting iron filings around a bar magnet.

Bar magnet with iron filings demonstrating the magnetic field of the magnet [2]

We know atoms are what make up everything in the universe, these atoms have electrons that orbit them. Each electron acts like a tiny magnet and has a ‘spin’ associated with them. This spin can either be up or down. When all these spins are in the same direction in a material, this causes it to be magnetic.

Any object that sticks to a magnet is consists of a material that is considered ferromagnetic. Some ferromagnetic materials include iron, nickel, cobalt, gadolinium, and their alloys (a metal made from two or more metallic elements).

Iron nails. Watch out if you’ve got a strong magnet. [3]

These materials have a special property where the inside of the material has lots of different areas, called domains. Each individual domain has a bunch of atoms which have electrons with spin in the same direction, but each domain in the material has an overall spin that is different from each other. This means that overall, the bulk material’s domains will be randomly orientated in respect to each other. As a result, the material is usually unmagnetized, we know this from everyday life since for example an iron nail will not be magnetic on its own.

It is when an external magnetic field is applied to a ferromagnetic material, such as a magnet getting close to an iron nail, that all these domains present in the material align in the same direction parallel to the magnetic field applied. This ferromagnetic object is now magnetized. This is the basic explanation for why these ferromagnetic materials are attracted to magnets.

To open a ferromagnetic steel can of worms, the domains in the ferromagnetic material are very strong. Once they are aligned together, they end up reinforcing each other and stay aligned even after the magnetic field is removed. Apply a strong enough magnetic field, and the ferromagnetic material will stay magnetized indefinitely. Doing this allows you to obtain a fridge magnet. Depending on the magnetic field, the direction of the domains will be different. To demagnetize the material, a magnetic field in the opposite direction of the one previously applied must be used. This phenomenon is known as magnetic hysteresis. All hysteresis means is the behaviour of something depends on its history.

Visualization of the domains in a ferromagnetic material. When there’s no magnetic field applied, the domains aren’t aligned. When a magnetic field is applied, the domains align parallel to the field when the magnet is strong enough. When removing the field, the domains stay aligned. [4]

Thus, the basis for the ‘memory’ of ferromagnetic materials and the ability to write information to them comes in. You can either have a ferromagnetic object magnetized or not, and the object will remember the state it was left in. The object can also be easily changed to the other state too. This happens on an extremely microscopic scale for methods of data storage, such as hard drives, magnetic tape devices like VHS or cassette tapes, and credit cards.

Some cassette tapes in case you forgot what these look like [5]

It’s amazing that the technology that stores so much of the world’s data runs on the same physics that a mass-produced Eiffel Tower magnet on the fridge does. There’s no denying the sheer impact of magnetic storage devices on our society, whether it be the fact it was the preferred medium for music and video for years, or the fact that hard drives are still used to store huge amounts of data today. Not too bad for some humble rocks!

Image sources

Featured image: https://www.flickr.com/photos/schill/6891085910

[1] https://commons.wikimedia.org/wiki/File:Fridge_magnets_board.jpg

[2] https://commons.wikimedia.org/wiki/File:Ironfilings_cylindermagnet.svg

[3] https://pxhere.com/en/photo/1293541

[4] https://msestudent.com/what-is-magnetic-hysteresis-and-why-is-it-important/#g

[5] https://pxhere.com/en/photo/649005