Cracking Magnets

Rare earth magnets are very strong despite their size. These magnets are several times stronger than an ordinary fridge magnet.

Can you hear it? The first, second and then third and fourth cracks as a magnet is brought near a magnetic (but not magnetised) material, such as a piece of cutlery? Unlike the first and second cracks during coffee roasting, which are clearly audible, it is unlikely that you would have actually heard the cracks of a magnet. To hear them you would need to amplify the effect and connect it to a loudspeaker (there’s a link to how you can do this experiment here). Nonetheless, if you were to do so, you would hear the cutlery cracking. And while these sounds are not connected to the first and second cracks in coffee roasting, they are connected, via physics, to coffee. To see why we need to think a bit more about what is causing these magnetic creaking noises.

The effect is known as the Barkhausen effect after Heinrich Barkhausen who discovered it in 1919. It turns out the the effect reveals quite a lot about how magnets work because it reveals what is going on at an atomic level in the kitchen fork. Some metals are attracted to magnets but not others. So a fridge magnet would stick onto materials containing iron but would not stick to a sheet of aluminium; we can pick up pins, paper clips and some cutlery with a strong magnet but we could not pick up a piece of kitchen foil. These iron containing metals are magnetic but not magnetised, they will be attracted to a magnet but they will not ordinarily attract other items to themselves. We may remember from school that we can make them magnetised by continuously stroking a strong magnet along the length of the pin (or fork, or paper clip) until the pin itself is able to attract other pins to it. We may even remember the explanation for this which was that for something to be magnetised, it had to have a clear magnetic orientation of North-South throughout its structure. Within the pin (or fork or paper clip) there are many small regions, called domains, which within themselves have a north-south orientation but they do not all point in the same way throughout the fork. Each little region points in a different direction to the others and so the net effect is that there is no overall North-South magnetism in the fork as a whole. As the strong magnet is used to stroke the fork, so the small regions move to align to the direction of the stroke of the magnet. The regions stop cancelling each other out and align so that the fork itself becomes a magnet with its own North-South.

inverted Aeropress and coffee stain
The link between coffee and the Barkhausen effect in magnets can be seen in this photo: a coffee spillage. It is the way that coffee evaporates and that coffee stains form that forms this physics connection between coffee and magnetism.

To return to our un-magnetised fork, you can imagine that where all these domains meet, there will be an area of confusion where the direction changes from one orientation to that of the neighbouring domain. This is called a ‘domain wall’ and it is these domain walls that are responsible for the Barkhausen effect. You can feel the effects of domains and domain walls in this experiment taken from the Institute of Physics Spark series: take two flat fridge magnets and turn them over so that the magnetic side of each faces the other. Move one of the magnets along the length of the other one. Think about how it feels to move it. Now move the same magnet perpendicular to the direction that you initially moved it in. Try it again. You will find that in one direction the movement feels smooth whereas in the other the magnets judder against each other, the movement is not smooth at all. You are feeling the effects of moving across a series of domains and domain walls, you can read more about the experiment here.

What actually happens as you bring a strong magnet towards an object such as a fork is that those domains in the fork that are aligned in the same direction as the magnet will tend to grow slightly at the expense of the ones that are not aligned with the magnet. The initial growth happens as the aligned domains get a bit bigger, a bit rounder and fatter. The domain walls bend a bit and the domains of the non-aligned regions get a bit thinner, a bit more squished. As the magnet is brought closer still, the aligned domains will actually start to grow at the expense of the non-aligned: the domain walls of the aligned domains will start to move outwards ‘eating’ into the neighbouring regions. It is at this point that you can pick up the Barkhausen effect because as the domain walls move, they can get stuck on defects in the metal rather like an elastic band would get stuck on an obstacle. The defect could be just one or two atoms that are out of place but the effect is that, just like the elastic band, the wall around the obstacle continues growing and the domain wall stretches more like an elastic band until pop – crack – the wall moves releasing a bit of energy that you pick up on the loudspeakers. This is what you hear as the Barkhausen effect. As the walls continue to grow so they will repeatedly get snagged on different defects in the metal and repeatedly ping – crack – into growth. Eventually, as the fork itself becomes magnetic* the last few non-aligned domains also start to align with the approaching strong magnet and the whole fork acts as if it is one magnet.

coffee ring, ink jet printing, organic electronics
A coffee stain. There are many experiments you can do at home with these.

The pinging domain walls have a direct link with an effect you can see in coffee, or more specifically spilled coffee. When you spill a few drops of coffee on a movable surface, you may have noticed that you can angle the surface a surprising amount before the drop starts to run down the side. You could try it now on a coaster if you have one available to you. The drop does not move because the edge is stuck, ‘pinned’, on defects on the surface of the coaster. These defects could be a crack in the material or a bit of dust or even a slight irregularity on the surface. Whatever it is, this defect acts to keep the edges of the drop in place. The first effect you would notice is that you can move the drop to a near vertical without it moving, the drop shape gets distorted but the drop itself does not move. The second effect is more subtle and is what happens if you leave the coffee drop there to dry.

Once spilled, the water in the droplet starts evaporating and eventually the droplet will dry leaving a coffee stain. The consequence of the pinning that you have just noticed is that the edges of the drop are quite stuck: the drop can’t just shrink. Instead, as the water evaporates, the drop will get flatter and because the water evaporates more quickly from the droplet edge (to see why click here), there will be a flow of water inside the drop from the centre to the edges. As the water flows outwards so it takes the coffee sediment with it which means that the dried coffee becomes a ring of sediment at the edge of the dried droplet.

Although it is on a different scale, it is the same sort of pinning that is happening in the coffee ring and in the Barkhausen effect. There are connections between physics and coffee to be found in many surprising places. Where will you find one today?

*This is an instance in which scientific English is not the same as English-English. In scientific-English, the fork is always a magnetic material it is just not fully magnetised. In English-English we tend to use the word ‘magnetic’ only for those materials that attract iron etc. to them. For ease of reading I have kept with the English-English usage here but if you are interested, you can read more in these links about magnetism and magnetic materials.

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