Talk:Micromagnetics

Which version has been rated?

It is obvious that the quality rating of this article - and many other ones - does not refer to the actual version, which has been much improved in the preceding months.

Generally, to any quality rating the version, at least the date of the article underlying the rating, should be mentioned. (In contrast, for the importance-rating a date is not necessary, although in this case at least the year of the rating would be useful.) - A happy new year! Meier99 (talk) 11:41, 1 January 2010 (UTC)

Micromagnetism or micromagnetics?

Why is the primary title for this page micromagnetism instead of micromagnetics? Brown's original name for the field was micromagnetics, and most of the references at the bottom of this article refer to micromagnetics. RockMagnetist (talk) 19:28, 15 September 2010 (UTC)

Thank you, Sheepe2004. RockMagnetist (talk) 02:19, 9 August 2011 (UTC)

Merge LLG equation

I propose that Landau–Lifshitz–Gilbert equation be merged into Micromagnetics. The target page already has a section on the LLG equation that gives both the Landau–Lifshitz form and the Gilbert form of the equation. It also provides the necessary context to understand the equation, including a description of the effective field. Actually very little is lacking in the Micromagnetics article about the LLG equation, maybe just the references. --Edgar.bonet (talk) 12:02, 27 September 2013 (UTC)

• Oppose - I agree that the treatments of the equation are pretty similar so far. However, the LL equation predates micromagnetics by three decades, and there is a lot more that could be said about it. For example, there could be a section on domain wall motion, which would be marginally relevant in the micromagnetics article. And so far, there are no applications. RockMagnetist (talk) 15:37, 27 September 2013 (UTC)

I admit I may not have the historical background to understand your point, but mainly I do not understand how domain wall motion could be more relevant to the LLG equation than to micromagnetics. Maybe I have a misconception about what “micromagnetics” actually means. As I understand it, micromagnetics is the art and technique of predicting magnetic configurations either statically, by minimizing the magnetic energy, or dynamically, by solving the LLG equation. Thus, the computation of the magnetization profile of a static domain wall falls into the realm of static micromagnetics, while the description of its motion is dynamic micromagnetics. The LLG equation can then be seen as the fundamental equation of dynamic micromagnetics, and any time you solve this equation you are solving a micromagnetics problem. I think the LLG equation would deserve a separate article only if either:

• we have substantial content to add about the equation that is not relevant to actually solving it, or
• the micromagnetics article is getting too large.

To counter my own argument, the historical origin of LLG, they ways it can be derived and the proof that the LL form and the G form are equivalent could belong to the LLG article. ––Edgar.bonet (talk) 08:21, 2 October 2013 (UTC)

A little about the history: Landau and Lifshitz did two things in their classic paper. First, they deduced the domain state of a material with uniaxial anisotropy by orienting walls so that the normal component of the magnetization was the same on each side (so that the demagnetizing field was zero). Then they formulated their dynamic equation for the motion of a domain wall. Although their equation is much the same when applied more generally, they applied it to a one-dimensional situation where magnetostatic interactions can be ignored. Micromagnetics uses similar physics, but is formulated in a general way so the equations can be solved self-consistently without any simplifying assumptions. RockMagnetist (talk) 16:34, 2 October 2013 (UTC)

Hi again, and thanks for this interesting background! I am removing the merger proposal due to lack of interest by the community, but I would like to keep the discussion open, thus the new topic below. —Edgar.bonet (talk) 08:41, 15 October 2013 (UTC)

Reorganize the article

Would it be reasonable to define micromagnetics as "the art and technique of predicting magnetic configurations, either statically, by minimizing the magnetic energy, or dynamically, by solving the LLG equation"? If so, we could say that Landau and Lifshitz solved a micromagnetics problem in their classic paper, but that it was not called "micromagnetics" at the time because it took some time (and numerical tools) for micromagnetics to develop as a specific sub-field of magnetism. Then we are entitled to put the "micromagnetics" label on this problem retroactively. The alternative would be to define micromagnetics more restrictively to mean only numerical micromagnetics. It makes sense in practice, because the practitioners always do number crunching. I find this alternative disturbing though, because a field (or sub-field) of physics is usually defined by the kind of problems it tackles, rather than the tools it uses to solve them. And a problem being hard is rarely a requirement for being part of a specific field. Thus, if a micromagnetics problem happens to be simple because the demagnetizing field is killed by some symmetry, so simple that you can solve it analytically, shouldn't it still be called a micromagnetics problem?

What I have in mind is a reorganization (practically a rewrite) of the article along the following lines:

• Define "micromagnetics" as above in the opening, state that few problems can be tackled analytically and thus in actual practice micromagnetics is always numerical
• Static micromagnetics: the micromagnetic energy terms and their meanings, the energy minima are the equilibria, there can be multiple minima, link to Stoner–Wohlfarth model for the simplest case
• Dynamic micromagnetics: the effective field, the LLG equation (link to the LLG page), the behavior of the solution in the simplest case (only applied field: the magnetization spirals towards the field)
• Applications, See also, Notes and references, etc...

Does it seem reasonable to organize the article along these lines? —Edgar.bonet (talk) 08:41, 15 October 2013 (UTC)

I agree that this article needs to be expanded significantly, and you have a pretty good outline. However, your definition misses the key defining feature of micromagnetics. Here is how Brown defines it in the preface to his book Micromagnetics:

To understand ferromagnetic materials, we must examine them on a smaller scale than that of ordinary observations. On one such scale we speak of domains; on another, of lattice sites. This tract analyzes them on an intermediate scale: small enough to reveal details of the transition regions between domains, yet large enough to permit the use of a continuous magnetization vector rather than of individual atomic spins. This approach to ferromagnetic theory - "micromagnetics" - began with the well known "wall" calculations of Landau and Lifshitz in 1935.

So I was mistaken about the Landau and Lifshitz calculation. Brown himself, the coiner of the term, considered it micromagnetics. I stand corrected!

Yes, the great majority of micromagnetic problems must be solved numerically, but that's true of just about any physical problem. In micromagnetics, the main class of problems that can be solved analytically are nucleation problems (basically, perturbation theory). RockMagnetist (talk) 15:41, 15 October 2013 (UTC)

OK, thanks. That's right, the length scales are an essential defining feature of micromagnetics. I will try to work on an expanded version of this article, keeping in mind the importance of the length scales. —Edgar.bonet (talk) 16:13, 15 October 2013 (UTC)

Done

I would say the reorganization is mostly done. The article is still lacking examples and illustrations though. —Edgar.bonet (talk) 16:06, 8 November 2013 (UTC)

It looks greatly improved. Thanks for doing that! It could use a few citations, though. RockMagnetist (talk) 17:03, 8 November 2013 (UTC)