What is going on inside my magnets?

[RobBer5]

Hi All,

I'm currently trying to make a diamagnetic levitation system for manipulating microdroplets for my Ph.D. As I was carefully grinding down my magnets to an appropriate size I noticed that the strength of the field didn't change that much. One magnet before the grinding produced a maximum magnitude of ~0.46T and after grinding the field on the same side produced a maximum magnitude of ~0.45T! I measured the field with a Teslameter and ground down the magnets using a file and ethanol for cooling.

Can anyone explain this to me (I'm only a self-taught physicist)? I'm thinking it's something to do with the magnetic domains...

Thanks

 

[Simon Bridge]

You are self taught and doing a PhD?!
Way to go!

Have you told your supervisor about what is happening to your magnets?

Anyway - you are thinking, I imagine, that if you join two small magnets together you should get one magnet of greater strength - so grinding them down should weaken them?

Conceptually: sketch the field lines for two equal strength bar magnets lined up like so:

[N1S][N2S]

... all the lines from the N of 1 go into the S of 2, and all the lines from the N of 2 go into the S of 1. So what happens to the magnet strength close to the poles of the combined magnet compared with the separated magnets?

You know how permanent magnets work right?

 

[RobBer5]

Thank you for replying, and yes I know how permanent magnets work. Although it doesn't really answer my question, since I was talking about a single magnet before and after grinding.

I really should have explained about how diamagnetic levitation works (at least how it should work in my case) to save time:

The magnetization of the permanent magnets will be directed in-plane and opposite in direction to each other:

[S1N] [N2S]

The idea is to produce a large field gradient between the magnets. When a diamagnetic droplet/particle comes into a contact with the field it produces a small induced magnetic moment directed opposite to the magnetic field. Using strong enough magnets, the interaction of this moment with the magnetic field gradient provides the force which levitates the droplet/particle. Due to the orientation of the magnets the minimum should lie along a line between the magnets.

 

[Simon Bridge]

Excuse me: I thought the grinding was just to resize the magnets ... if you made them smaller radially I'd expect a decrease in strength near the poles but not smaller vertically.

Grinding is the action of cutting magnets into lots of smaller bits ... and one big bit.
The simplification for illustration purposes is just slicing the magnet in half to get two equal strength magnets.

Isn't the field gradient supposed to be created by the fact you are shoving like-poles together? i.e. not by shaping the magnets? What did you expect the grinding to do to the field? (i.e. what is it you want explained?)

 

[Rob D]

Do you have some reliable data on field strength variance with mass of the magnet? Shape of the final ground magnet would probably have little effect other than affecting field propagation. How or how much I denna ken.

The above are the WAGs of yet another self-taught physicist.

Best of Luck,
Rob