ferromagnetism, diamagnetism, and paramagnetism

[eLiu]

Ok...my physics book doesn't really have much along the lines of explanations for these things...so I have a few questions I'd like to clear up:

Diamagnetism is a direct result of Lenz's Law, right? Like, in the presence of a magnetic field, the electrons in the (otherwise neutral) diamagnetic material will begin to spin in the opposite direction to resist the incoming field, which results in an induced, anti-parallel B-field. Is that correct?

Now, if that's the case...if I place a diamagnetic material in a constant B-field, after some amount of time, the 'net change' effect would settle down and eventually stop...wouldn't it? And then in that case, wouldn't the induced dipoles under go a torqe-ing force in the same way that paramagnetic materials do? Or does this not happen b/c diamagnetic materials have induced dipoles? In that case, would it work more like...once the 'change' stops, if the induced-dipoles move back to their normal, non-magnetic state, lenz's law will cause them to return to their previous state (antiparallel to the comparatively strong magnet)? Sorry...I'm confused, lol

As for ferrogmagnets & paramagnets...why is it that ferromagnets are so much stronger than paramagnets? I read on google about the whole 'domain' theory and this "long range order" of electrons that creates domains, but I don't follow why that makes ferromagnets so much stronger. I mean, in a weak B-field, I can see why the already strong domains would experience significantly more torque than the individual dipoles of a paramagnet, which would result in a larger total # of dipoles reinforcing the external B-field...right?

However, if we have an extremely strong external magnet acting on both materials, wouldn't the net B-fields of both systems be about equal? I mean, in an extremely strong (infinitely strong?) magnetic field, one might expect all of the dipoles of both to allign in the same direction. In that case, would the ferromagnet and paramagnet still behave differently? I mean, if all the domains are pointing in the same direction (ferro), wouldn't that mean all the individual dipoles are pointing in the same direction...just like what would be happening in the paramagnet...??

I hope I'm not missing something incredibly obvious here...lol

TIA,
-Eric

Edit: ferro/paramagnets both experience the diamagnetic effects right? Is it just that the net-field generated by the parallel-aligned dipoles dominates the diamagnetic effect?

Edit2: Does the strength of ferromagnets have something to do with the fact that they don't lose strength under high temperature (well, until the Curie temp)? B/c I take it that paramagnets will grow weaker as the temperature rises, due to higher random movement of the atoms...

 

[TGHI]

My attention span is too small to read all that....but paramagnet sounds like what they used in Ghostbusters to trap them nasty ghoulies.

 

[cquark]

Paramagnetism is the effect where electron spins/dipoles align in parallel with an external magnetic field. Diamagnetism is a similar effect due to their orbital angular momentum. Both of these effects can be explained by classical physics. In both (pure) paramagnetism and diamagnetism, the electron dipoles interact with the external field and not each other.

However, ferromagnetism is a quantum mechanical effect involving the free electrons in a metal. In classical physics, the dipoles of two close electrons would align in opposite directions, but quantum effects cause aligned spins to be a lower energy state (and thus more stable). IIRC, the difference is called the exchange energy and is due to the Pauli Exclusion Principle. However, over sufficiently large distances, classical effects supersede quantum effects, so the spins are aligned only over small areas called domains. An external field can cause all of the domains to align and remain aligned after the field is removed. As you can see, in ferromagnetism, the electron dipoles interact with each other to reinforce the magnetic field, which is why it's stronger.

 

[eLiu]

Do you mean like..in ferromagnets, the dipoles have a much greater level of "order" than they do in paramagnets? Like, in ferromagnets, the dipoles act more in "unison" and create chunks (the domains) that are much stronger than a paramagnet's single dipoles?

(I take it that the exact workings of a ferromagnet are beyond what's taught in an introductory E&M course...lol)

But uh, in an extremely strong B-field...would the reinforcing effects of a paramagnet and a ferromagnet be approximately equal? I mean, if both are subjected to an extremely strong field, wouldn't that result in all the dipoles (or domains for ferro) pointing in the same direction?

Also, why would you say diamagnets & paramagnets are similar? I mean, to my understanding, the permanent dipoles in paramagnets undergo a torque (u X B, where u means teh greek letter mu) which causes them to allign with the external B-field. However, the induced dipoles of the diamagnet will spin opposite to the external B-field (Lenz's Law) to resist the changing field (producing a reducing effect). Is there something I'm missing?

And what about my question about diamagnets? Like...once you stop moving the diamagnet in the external B-field, why don't the induced dipoles experience the same torque that paramagnets experience and re-orient themselves? Is it b/c of Lenz's Law again...? (Like, if the dipoles attempt to rotate, would that "resisting" force would cause them to return to their anti-parallel orientation...?)

 

[cquark]

Originally posted by: eLiu
Do you mean like..in ferromagnets, the dipoles have a much greater level of "order" than they do in paramagnets? Like, in ferromagnets, the dipoles act more in "unison" and create chunks (the domains) that are much stronger than a paramagnet's single dipoles?

Basically, yes.

(I take it that the exact workings of a ferromagnet are beyond what's taught in an introductory E&M course...lol)

You'd want a solid state physics class or a graduate E&M class to go into ferromagnetism at a fundamental level.

But uh, in an extremely strong B-field...would the reinforcing effects of a paramagnet and a ferromagnet be approximately equal? I mean, if both are subjected to an extremely strong field, wouldn't that result in all the dipoles (or domains for ferro) pointing in the same direction?

If the magnetic field was sufficiently strong enough to align all of the dipoles in the paramagnetic substance, the strength of the paramagnetic field would be equal to that of the same amount of a ferromagnetic substance in the same field.

Also, why would you say diamagnets & paramagnets are similar? I mean, to my understanding, the permanent dipoles in paramagnets undergo a torque (u X B, where u means teh greek letter mu) which causes them to allign with the external B-field. However, the induced dipoles of the diamagnet will spin opposite to the external B-field (Lenz's Law) to resist the changing field (producing a reducing effect). Is there something I'm missing?

You're not missing anything. I was just saying on the particle level, both types of magnetism derive from the particle's angular momentum, one of the spin angular moment, the other of the orbital. Also, both types can also be explained by classical physics, whereas ferromagnetism cannot.

I'll try to get back to your other question later.

 

[eLiu]

Oh okay, that clears up things a bit...it's starting to come together for me now, lol

Good to know I'm not missing something terribly obvious, lol. I'm learning this stuff so I take a placement test in the fall and hopefully pass out of the introductory E&M course at my college. I already took AP Physics C in high school, so I've already seen most of the material in the intro E&M class. But yeah...there's some stuff (like this) that's new to me/caught my curiosity.

Thanks for the help man
-Eric