From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Physics (Rated C-class, High-importance)
WikiProject icon This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
C-Class article C  This article has been rated as C-Class on the project's quality scale.
 High  This article has been rated as High-importance on the project's importance scale.

How do rare earth magnets work?[edit]

Isn't Neodymium ferromagnetic? It is used in neodymium magnets, some of the most powerful permanent magnets ever made, and according to the page on Neodymium, it is a ferromagnetic. 19:09, 6 April 2007 (UTC)

Yes, I was wondering if someone could add a simplified explanation of how rare earth magnets differ from previous permanent magnets. What makes them so much stronger? I assume they work by the same 'exchange energy' mechanism as other ferromagnets. And who invented them? This doesn't seem to be addressed in Rare earth magnet or Neodymium magnet or elsewhere. --Chetvorno 21:59, 11 August 2007 (UTC)

I think it may be because rare earths can have more unpaired electrons than transition metals, as there are more orbitals that can be partially filled (7 f orbitals vs 5 d orbitals). --Itub 13:39, 10 October 2007 (UTC)
For the rare earth magnets two things come together: a high moment for the ions and large magnetic anisotropy. This translates to high saturation and high coercivity.--Ulrich67 (talk) 22:49, 2 January 2012 (UTC)
There is a little material on rare earth magnets in Magnetism. RockMagnetist (talk) 19:14, 3 January 2012 (UTC)

Interacting magnetic fields[edit]

There is a lot of discussion of the properties of various types of magnetic materials as to how to maximize the internal properties or the materials. However there is practically no discussion of what controls the intensity of magnetic force activity in the interacting field spacial volume. If a suspended magnet is swung like a pendulum over an opposing polarity magnetic the interaction of the fields will control the physical activity (motion) of the moving magnet at some distance. The question is as to how this magnetic force is capable of being extended so as to continue to act is not discussed.WFPM (talk) 23:54, 16 February 2012 (UTC)

Diagram request[edit]

I think this article could benefit from a hysteresis diagram like found in standard textbooks and like can be found in the ferroelectricity article: (talk) 15:59, 19 November 2012 (UTC)

And then you can write something like my removed sentence, but maybe more clarified: "The most important parameters of the ferromagnetic material constitute its magnetic hysteresis loop." Because on that diagram one can see magnetic parameters of a given ferromagnetic material, determining its possible utilization: coercivity and so on. Niuthon (talk) 07:33, 24 July 2014 (UTC)

A section on hysteresis would be good, with a diagram and a discussion of some of the hysteresis parameters. Even if such a section is created, though, there remain a couple of problems with your statement . First, the "most important" label is arguable: one can also talk about more fundamental parameters like the magnetocrystalline anisotropy constant. Also, you're really talking about a particular hysteresis loop, the main loop. Clearly that must be clarified in the body of the text before adding a statement to the lead. Note also that, per MOS:LEAD, the lead should summarize the content, not add significant new information. RockMagnetist (talk) 14:55, 24 July 2014 (UTC)

Domain images[edit]

We now have a superabundance of domain images, spilling down past the next two sections and into References. Do we really need all of them? I like the first and fourth images, but the second and third are hard to interpret. RockMagnetist (talk) 17:23, 19 December 2012 (UTC)

I agree, the second and third should be removed. -ChetvornoTALK
There being no objection in two years, I have removed the images. RockMagnetist (talk) 14:58, 24 July 2014 (UTC)

Exchange interaction[edit]

First line in the section is utterly wrong. Nearby dipole magnets prefer to align their poles. The paragraph even cites the dipole-dipole interaction page which includes the Hamiltonian for the interaction. Taking a look at that Hamiltonian clearly indicates that two dipoles prefer to have their moments aligned along the separation vector. That gives the lowest energy configuration. — Preceding unsigned comment added by 2601:D:CA00:207:608B:6A12:87DB:AF1E (talk) 06:40, 6 March 2015 (UTC)

This confusion arises because the configuration is incompletely stated. Presumably what is meant is that "two dipoles that are free to rotate in any direction will tend to align with their dipole moments perpendicular to the vector separating them, and oppositely oriented with respect to each other". If the orientation of one dipole is constrained to being parallel with the separation vector, then second dipole will align itself with its dipole in the same direction as the constrained dipole. The sources are unclear on this, so it is worth extending the wording along the lines suggested by me here. Otherwise, we will continue to get this kind of reaction. —Quondum 15:09, 6 March 2015 (UTC)
Actually, both the parallel and perpendicular orientations are stable without the need of constraints, so it depends on initial conditions. However, over larger scales a collection of interacting dipoles will arrange themselves so their moments nearly cancel out (see Demagnetizing field). Still, books on ferromagnetism don't preface discussions of the exchange interaction with this information, so maybe we don't need to either. Maybe it would be better to contrast the effects of the exchange interaction with diamagnetism. RockMagnetist(talk) 17:06, 6 March 2015 (UTC)
to Quodom, that isnt correct. The lowest energy configuration is parallel orientation and pointing along the separation vector. This gives -2 (d1 x d2)/r^3 as the energy. The second lowest is the perpendicular to separation vector and opposite orientation. This gives -1 (d1 d2 )/r^3. D1 and d2 are the dipole moments. — Preceding unsigned comment added by (talk) 18:22, 6 March 2015 (UTC)
@IP.66: I was making an assumption based on the bare assertion in the references, but I have no objection to what you say. However, based on what RockMagnetist says, each of the two configurations mentioned are at a local minimum in the total energy, so the situation is more complicated than I stated.
@RockMagnetist: IMO, WP should not take its lead on style from texts that leave the reader scratching their head, and saying, "Hmm, this can't be right." Often enough, even such texts do make contextual assumtions that can usually be deduced by a careful reading of the preceding text (sometimes, this means reading the entire set of preceding chapters with expert foreknowledge). For example, many texts do not explicitly state whether they are dealing with natural numbers, integers, reals or complex numbers, but the expert will be able to determine from the applicability of the conclusions what the implicit assumptions are. Your own statement here about stability of both configurations suggests that the references are, to put it bluntly, incorrect in the general case. The bald statement, as quoted, seems to apply to an isolated pair of dipoles, but neither the parallel nor the antiparallel configurations extend naturally to arrays of dipoles such as occur in crystals. So, as stated, I support the "dubious" tag: the statement definitely needs work. (The diamagnetism suggestion might be a nice and noncontroversial contrast, but completely fails to address the question this text tries to deal with.) —Quondum 19:53, 6 March 2015 (UTC)
@Quondum: Depends which references you mean. If we follow the lead of texts like Chikazumi or Aharoni, I don't think there will be a problem. But I agree with the dubious tag. RockMagnetist(talk) 02:43, 7 March 2015 (UTC)
I was refering to the two sources given at the end of the sentence. These say, respectively:
  • Joy Manners, Static Fields and Potentials: "It is important to notice that the interaction which gives rise to the magnetic ordering is the electric repulsion of electrons. [...] – as may be familiar to you if you have played with a pair of magnets – magnetic interactions cause dipoles to align in opposite directions: colloquially, north pole to south pole."
  • Hans J. Kreuzer, Isaac Tamblyn, Thermodynamics: "According to classical electromagnetism, two nearby magnetic dipoles will tend to align in opposite directions."
I see nothing in the immediate vicinity in either of these to clarify what we have been speaking of, and it seems to me that these particular sources should not be used for this statement at all. Their focus is, after all, not classical physics. The first sentence that I quoted above illustrates the lack of rigour: the exchange interaction cannot be described as "electric repulsion", because it has nothing to do with electromagnetism (although the source of the energy does largely arise from electric repulsion/attraction of the different orbital configurations, but it would be improper to call this the exchange interaction). Chikazumi, Physics of Ferromagnetism makes for interesting reading, and seems pretty thorough. Pages 6–7 give a classical treatment of the potential energy of a pair of dipoles, and a statement that parallel alignment along the the axis of separation is stable and that parallel alignment transverse to the axis of separation is unstable; antiparallelism is not dealt with in as much detail, but it seems that the situation is reversed.
In all, I think that we should simply remove the statement about alignment related to classical magnetism. Chikazumi says (p. 130) that "The value [of the difference of the Coulomb energy between parallel and antiparallel unpaired electron spins] estimated from (6.42) is [say 1% of] 105 times larger than the magnetic dipolar interaction calculated from (1.17)." I think that this would be a far more sensible point to make in the article – essentially that the interaction between magnetic moments is completely swamped. —Quondum 06:01, 7 March 2015 (UTC)
I agree. RockMagnetist(talk) 16:04, 7 March 2015 (UTC)
Okay, change made. —Quondum 20:59, 7 March 2015 (UTC)

Room temeprature ferromagnetism in transition metal oxides[edit]

I was wondering if it's worth making a new section about this topic. It appears that it has only been researched recently and it is not yet well understood as it occurs only under particular conditions. Here is a link to an article that got me thinking about this:

Charbon (talk) 17:36, 27 June 2015 (UTC)

My feeling is that it doesn't merit a separate section but just a sentence or two. This article already has several sections, Actinide ferromagnets and Lithium gas that give WP:UNDUE WEIGHT to exotic ferromagnetic species. All these should probably be boiled down to a few sentences. --ChetvornoTALK 19:49, 27 June 2015 (UTC)