Material with net angular momentum (not just a magnet)

1. Jan 1, 2010

lightmass

Hi,

Why there are no materials where the net angular momentum is not zero? Permanent magnets have a net magnetic moment coming from the sum of electron spins + orbital magnetic moments. Why the net angular moment cancels out? Or in other words: Is there any material that is not rotating macroscopically but still has net angular momentum?

2. Jan 1, 2010

Staff: Mentor

A magnetized object does have a small net intrinsic angular momentum, which can be demonstrated via the Einstein - de Haas effect. Start with a magnetized object that is not rotating (zero rotational angular momentum). Reverse the direction of magnetization by "flipping" the spins of the electrons that produce the magnetization. This also reverses the direction of the object's intrinsic angular momentum. The object starts to rotate in the opposite direction so that the total angular momentum (intrinsic plus rotational) maintains the same value as before.

This is analogous to the classroom demonstration in which someone sits on a freely rotating chair while holding a spinning bicycle wheel. If the person starts out not rotating, then flips the spinning bicycle wheel 180 degrees, he begins to rotate in the opposite direction.

3. Jan 1, 2010

lightmass

jtbell, thanks for your answer. I was aware of the Einstein de Hass effect (and Barnett effect). I wanted a confirmation that a magnet indeed has some small intrinsic angular momentum (we could still have magnetic moment with 0 angular moment if nuclear moments play a role).

I would like to know if it is theoretically posible to have a material that have such strong intrinsic angular momentum that will behave macroscopically like a spinning top (or a gyroscope).

4. Jan 1, 2010

Staff: Mentor

To get an idea of the possible maximum effect of the intrinsic angular momentum, you might consider a fully-magnetized piece of iron. Take e.g. one mole of iron, assume that one electron per atom contributes to the magnetization, and calculate the net angular momentum if all those electrons are spin-aligned. Then assume e.g. that the iron is shaped as a cylinder 1 cm in diameter, and calculate how fast it would have to rotate macroscopically in order to have an equal magnitude of rotational angular momentum.

Then for fun, suppose you could align all the electrons (not just one) in each atom, what would the result be? Or if you could align all the nuclear spins also?