Ferromagnetism versus Paramagnetism

[Chemist@]

I know that the different magnetic properties arise from the spin of the electron, I also did some determinations whether a molecule is paramagnetic or diamagnetic (using MO theory), but I don't know how to make a difference between ferromagnetism and paramagnetism. Could someone explain it? And one more: In one text I saw that iron is paramagnetic and in another that it is ferromagnetic, what's true? I suppose that the answer for the first question will reveal the answer of the second one. Thanks in advance!

 

[Vanadium 50]

Is there something here that is particularly confusing? http://en.wikipedia.org/wiki/Paramagnetism

 

[Chemist@]

Okay, read that. In ferromagnetics the dipoles are interacting strongly in a suficient area to make domains and those domains produce a ferromagnetic. Why and how are the dipoles interacting?
By the electronic structure Fe should be paramagnetic, but "ferro"magnetism refers to iron. Still confused here.

 

[Chemist@]

Could someone help, I'll have a test soon?
Are ferromagnetics maybe a subgroup of paramagnetics?

 

[DrDu]

In a normal paramagnetic, the dipoles are moving independently from each other, they tend to orient in a magnetic field so as to increase the field strength. In ferromagnetics the dipoles also orient so as to increase the field. However the dipoles are not independent from each other. If dipoles are oriented in some direction, the other dipoles will preferentially orient parallel to these.
Hence if a ferromagnetic is brought even into a very weak magnetic field, almost all dipoles in the ferromagnet will orient in the direction of the field. Hence a ferromagnet will enormously increase the field strenght.
When temperature is rised, the thermal motion of the dipole will counteract the tendency to orient parallel to each other. As some temperature, the Curie point, the ferromagnet will become an ordinary paramagnet.

 

[Chemist@]

Thanks for the answer. I guess that ferromagnets can be called a subgroup of paramagnets, as I don't find a way how to differ them, by looking at the electronic structure of a material.
What causes the ferromagnets dipoles to make a domain structure and paramagnets not?

 

[SSJ2]

really, it comes down to the spin of the system.

Ferromagnetic implies that all the spin of the electrons point in the same direction.

anti-ferromagnetism is when half of the electrons are pointing in the same direction and the other half are pointing in the opposite direction. essentially canceling them out giving a spin of 0

paramagnetism - spin of the electrons has no order and can point in whatever direction they want. the solid solution of uranium carbide has been reported to have this property

 

[DrDu]

the solid solution of uranium carbide has been reported to have this property

I would have problems to invent a more exotic example.

 

[DrDu]

What causes the ferromagnets dipoles to make a domain structure and paramagnets not?

Compare ferromagnetism with crystalline solid and paramagnetism with a gas or liquid.
In a crystal you have crystalline domains while a gas or liquid is homogeneous.

 

[Chemist@]

Thanks both.
"ferromagnetism with crystalline solid and paramagnetism with a gas or liquid"
Does this mean that ferromagnetism is only exhibited by solids and paramagnetism is only exhibited by gas or liquid? The opposite is impossible?

 

[DrDu]

No, but the transition from a paramagnetic state to a ferromagnetic state is a phase transition comparable to the liquid-solid transition. In the paramagnetic states, the spins do not (or only weakly) interact, like the atoms do in a gas or liquid, while in the ferromagnetic state the spins interact so as to orient spontaneously in some direction, analogously to the atoms ordering when forming a crystal.

 

[M Quack]

Most ordinary solids that contain unpaired electrons ("magnetic ions") are paramagnetic at high temperature ("high" meaning closse to their melting point). Some of them undergo a paramagnetic-ferromagnetic phase transition when cooled. Some undergo other phase transitions such as antiferromagnetic, charge density wave, or superconducting.

I do not know of any ferromagnetic liquids or gaes.

Ferrofluids are suspensions of ferromagnetic solid particles.

 

[Chemist@]

I don't see a way how to distinguish para and ferromagnetics. For example, if I was asked to determine the magnetic properties of the substances: C2, Fe, O2, Al... I could only say (by looking at the electronic structure) that they are para or diamagnetic.Could the ferromagnetics really be called as a type of paramagnetics?

I suppose that the magnetic dipoles(with N and S) in ferromagnets interact with each other similar as the electric dipoles (with + and -). After all it is the same force. By heating the electrons have more kinetic energy so they travel more chaotic, so the magnetic dipoles don't interact so well, making the material a paramagnetic. Is this true?

 

[Dickfore]

Could someone help, I'll have a test soon?
Are ferromagnetics maybe a subgroup of paramagnetics?

If your professor did not explain this, then it won't be on the test.

 

[DrDu]

I do not know of any ferromagnetic liquids or gaes.

Some years ago supercooled ferromagnetic melts were created.

 

[Chemist@]

It was today and it was much easier than I thought , ferromagnetism wasn't almost mentioned. Still, I want to understand this topic, so I would need answers for the two last questions I posted:

I don't see a way how to distinguish para and ferromagnetics. For example, if I was asked to determine the magnetic properties of the substances: C2, Fe, O2, Al... I could only say (by looking at the electronic structure) that they are para or diamagnetic.Could the ferromagnetics really be called as a type of paramagnetics?

I suppose that the magnetic dipoles(with N and S) in ferromagnets interact with each other similar as the electric dipoles (with + and -). After all it is the same force. By heating the electrons have more kinetic energy so they travel more chaotic, so the magnetic dipoles don't interact so well, making the material a paramagnetic. Is this true?

 

[DrDu]

It was today and it was much easier than I thought , ferromagnetism wasn't almost mentioned. Still, I want to understand this topic, so I would need answers for the two last questions I posted:

While it is possible in simple cases to decide whether a substance is para- or diamagnetic, ferromagnetism (or other collective magnetic phenomena) is much harder to predict.
The mechanism you are mentioning (i.e. interaction of the dipoles via their magnetic fields) is hardly ever of importance. Rather, the exchange forces between the dipoles are mostly due to the Pauli effect, which penalizes two electrons of the same spin coming close together.

 

[M Quack]

Some years ago supercooled ferromagnetic melts were created.

Interesting. Do you have a reference?

As for distinguishing paramagnets and ferromagnets:

Microscopically, ferromagnets are characterized by a spontaneous magnetization=magnetic moment without applied external field. Paramagnets never have spontaneous magnetization.

Macroscopically that is not always observable, due to the formation of domains. In many cases you can still identify ferromagnets by measuring a hysteresis curve. If there is any hysteresis=remnant magnetization=magnetization when the field is brought back to zero, then it is ferromagnetic.

http://en.wikipedia.org/wiki/Magnetic_domain
http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/MagneticDomains.htm

Now "soft" ferromagnets do not necessarily show hysteresis, but microscopically they will still be made up of domains that have a spontaneous magnetization.

Domain sizes are usually between ~30 nanometers and many micrometers, i.e. they contain a large number of atoms. It is possible to see domains using Kerr microscopy, magnetic force microscopy, x-ray circular dichroism, polarized neutron scattering and several other techniques.

There are of course borderline subjects like the magnetism of largish molecules like Mn-12-acetate or of nanoclusters. But let's not get into that before we have sorted out the basics.

http://en.wikipedia.org/wiki/Ferromagnetism

 

[Chemist@]

Dr Du, when I need to predict the magnetic properties of a substance only by looking at the electronic structure, I should say that it is diamagnetic or "maybe" paramagnetic (for covalent molecules I could say that they are paramagnetic if the electronic structure shows so, and if I know that it is either a gas or a liquid, because the molecules are moving chaotically). You said about "supercooled ferromagnetic melts", I don't think that in the questions I get, some extreme conditions are taken into consideration. I hope that this is right.
"The mechanism you are mentioning (i.e. interaction of the dipoles via their magnetic fields) is hardly ever of importance. Rather, the exchange forces between the dipoles are mostly due to the Pauli effect, which penalizes two electrons of the same spin coming close together."
So I was on the wrong track. I really didn't know this. I thought that the domains are made because of the interacting of the dipoles. Could you give me some reference about the Pauli effect, because I found on the net something different when typing it.

M Quack, I will remember those methods you said. One issue: "Paramagnets never have spontaneous magnetization." Is it because of the dipoles chaotic orientation so they cancel each other in a material?

Thanks to both of you for helping.

 

[M Quack]

"The mechanism you are mentioning (i.e. interaction of the dipoles via their magnetic fields) is hardly ever of importance. Rather, the exchange forces between the dipoles are mostly due to the Pauli effect, which penalizes two electrons of the same spin coming close together."
So I was on the wrong track. I really didn't know this. I thought that the domains are made because of the interacting of the dipoles. Could you give me some reference about the Pauli effect, because I found on the net something different when typing it.

Domains are a second order effect. Domains exist because the material is ferromagnetic and generates a magnetic field to begin with.

For the existence of ferromagnetism the dipole interaction is negligible. Exchange is the name of the game.

M Quack, I will remember those methods you said. One issue: "Paramagnets never have spontaneous magnetization." Is it because of the dipoles chaotic orientation so they cancel each other in a material?

Yes. In a paramagnet the magnetic moments ¨don't talk to each other". Each one only follows the external field. In a ferromagnet moments interact strongly, and moderate magnetic fields only align domains that are already ferromagnetic.

 

[DrDu]

Interesting. Do you have a reference?

Google had one:
http://elib.dlr.de/15791/

 

[M Quack]

Thanks DrDu.

This seems to be the published work:

http://onlinelibrary.wiley.com/doi/10.1002/1527-2648(200101)3:1/2<65::AID-ADEM65>3.0.CO;2-0/citedby

 

[Chemist@]

M Quack, I don't understand how the domains are an effect of ferromagnetism. Isn't it the opposite?
The spin exclude principle is responsible for the interaction between electrons in a domain. How do we say then, that for a paramagnetic, the dipoles are oriented chaotically when it's all about the spin, isn't this expression wrong then?

 

[M Quack]

Nope. Domains are caused by the magnetic field of the ferromagnetic material. The domains will arrange themselves to minimize the overall magnetic field as this reduces the energy of the system.

The Pauli exclusion principle is the origin of the exchange interaction. It is something that is important over very short (inter-atomic) distances.

 

[Chemist@]

I am still struggling to find a pattern here. I will try to write a short review:
-Paramagnetics don't have an interaction between the electrons spins so only a few from all magnetic dipoles will orient to the direction of an external magnetic field. When taken away from the field the dipoles orient in all directions so the magnetic property faded.
-Ferromagnetics- there are interactions between the electrons spins (I think that this could only be present in some solid or supercooled materials where the electrons move less chaotically) so domains are made. When taken into an magnetic field almost all of the dipoles will orient to its direction (because many of them are parallel and affecting each other) making the field very stronger. After leaving the field, the dipoles will remain in their position, so a permanent magnet is established.

Is this true?

 

[nasu]

I don't see a way how to distinguish para and ferromagnetics. For example, if I was asked to determine the magnetic properties of the substances: C2, Fe, O2, Al... I could only say (by looking at the electronic structure) that they are para or diamagnetic.Could the ferromagnetics really be called as a type of paramagnetics?

You are right, you cannot tell if a specific element will have ferromagnetic behavior just from looking at the electron structure of the atom. (at least not in an easy way).
And this is due to the fact that whereas the paramagnetic character can be associated with the atom (or ion) the ferro-magnetism is a collective behavior. It depends on the structure containing the atom. Because iron metal is ferromagnetic you can not say that the iron atom is ferromagnetic.

An iron ion in the crystal of FeCl3 (for example) is paramagnetic and the compound has paramagnetic behavior. The iron in the iron(metal) crystal interacts with the other ions to produce a ferromagnetic behavior. The iron ions in the iron ferrite interact with the lattice so that the resulting behavior is anti ferromagnetic.
At high temperatures these collective structures are destroyed and only the individual, paramagnetic character remains.

Edit. Sorry, I did not see the second page.
Some stuff was already said.

 

[JohnRC]

There is no way you can tell whether a substance is ferromagnetic or simply paramagnetic by looking at its molecular or composition formula. At the very least you would also need the detail of the crystal structure and the temperature. In a very general sense, you are more likely to find ferromagnetic substances among those with large numbers of parallel spin unpaired electrons in their atoms rather than just a few. For example, you will find ferromagnetic substances among high spin iron complexes, but not among low spin ones.

I think that ferromagnetic substances are best seen as a subset of paramagnetic substances. The terms are sometimes defined this way and sometimes not. One advantage of seeing ferromagnetic substances as also paramagnetic is that their paramagnetism persists, even when they are incorporated in a phase that is not ferromagnetic.

 

[Chemist@]

Okay, I see it more logical that ferro is a subgroup of paramagnetism, so I will remember it as this, because atoms,molecules or ions by themselves can't be ferromagnetic.
Thanks to everyone who helped me with this topic.