Can a magnetic field create angular momentum?

[BucketOfFish]

I know that magnetic fields can align objects with spin. In that case, if we suspend an object and turn on a magnetic field such that a significant number of electrons become aligned with the field, could we observe a macroscopic change in angular momentum?

 

[MisterX]

Aren't you describing a permanent magnet motor with the magnet as the rotor? The short answer is yes.

 

[BucketOfFish]

I don't think I'm describing a motor. For one thing, there is no alternating current in my model, which is required even in a permanent magnet motor. I'm proposing to just dangle a block of some material in midair and then to apply an extremely strong stationary magnetic field. I know the field does interact with spins, causing the Zeeman effect, but I don't know if it's possible to see a change in angular momentum as a result.

 

[Truecrimson]

This is the Einstein-de Haas effect. The total angular momentum is conserved so the object will rotate to counteract the spin angular momentum.

 

[BucketOfFish]

Thanks, Truecrimson! This is exactly what I was thinking of!

 

[jtbell]

See also the Feynman Lectures:

http://www.feynmanlectures.caltech.edu/II_37.html#Ch37-F3

in particular Fig. 37-3 and the paragraph preceding it (you'll probably have to scroll the page up a bit).

Note that the Einstein-de Haas experiments were in 1915-16, about ten years before the invention/discovery of electron spin! They must have analyzed their results in terms of the surface bound current of the magnetized cylinder, in classical electromagnetism:

http://www.physicspages.com/2013/06/29/magnetization-bound-currents/

See Example 1 which discusses a cylinder with uniform longitudinal magnetization. If there really were a macroscopic current running azimuthally around the cylinder, it would have angular momentum due to the circular motion of the electrons.

 

[BucketOfFish]

Ah great, Feynman always catches the interesting stuff.

 

[David Lewis]

I don't think I'm describing a motor. For one thing, there is no alternating current in my model, which is required even in a permanent magnet motor.

A homopolar motor uses DC and a permanent magnet.