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Theory of magnetism

  1. Oct 27, 2004 #1
    I understand that no conclusive theory on magnetism has been formulated yet. But i was wondering if it did answer this question.

    If the electrons orbiting the nucleus give a magnetic material its magnetic properties. Then why of all atoms would Iron or nickel be the few which possesses magnetic properties?
  2. jcsd
  3. Oct 27, 2004 #2


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    There are several issues and miconceptions here, and this question should have been posted in the condensed matter section.

    First of all, let's make sure we are clear that the theory of magnetic FIELDS are well-known and certainly VERY conclusive. I know you did not ask about fields, but I want to make sure those who are reading this do not get confused between the concept of magnetic fields and "magnetism", which is the magnetic properties of MATTER.

    Secondly, "magnetic properties" is highly ambiguous. There are several "magnetic properties" of matter: paramagnetism, diamagnetism, ferromagnetism, antiferromagnetism, etc. What you are asking by bringing out the example of iron and nickle is ferromagnetism.

    Thirdly, we know the origin on why things have all these 'ism. It is due to the presence or absence of a "magnetic moment" of the valence shell.

    Fourthly, the issue isn't that such magnetic moment exist, but rather, why are there LONG-RANGE ordering of these moments over a substantial size of the material. Ferromagnetism requires a net and FIXED allignment of a gazillion of these magnetic moments even without an external magnetic field, whereas paramagnetism only causes such long range ordering only in the presence of an external magnetic field.

    Fifth, to answer the fourth point, you have to know many-body theory. Whenever there are any long range ordering involving a gazillion particles, it automatically means that there are a gazillion interactions that somehow produces a lowering of an energy for a favorable state, at a particular temperature. This isn't easy. In many instances, we only learn what kinds of parameters to use only after we know how the system behave, and then learn from that system when we apply it to a similar system. So in most of these, it is still very much a phenomenology.

    Sixth, look up an example of the Ising model. It will tell you why, even in a simple 2D array of spins, that by simply changing slightly the nature and range of coupling of each spin, you can end up with an entirely different magnetic ordering for the system.

    This is why, if I give you a bunch of magnetic spins, and ask you to tell me what is the ground state magnetic system you will end up with, you will not be able to answer my question - no one can, at least not for the most generalized situation. The theory of magnetism is known to the extent that we know what we still do not know yet.

  4. Oct 27, 2004 #3
    ahh, that was helpful, thanks alot.

    : )
  5. Nov 3, 2004 #4


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    There is a followup to this discussion for anyone interested in pursuing it. This month's issue of Physics World has a report on the issue of ferromagnetism in carbon. Along the way, they presented a good summary of why certain solids are ferromagnetic, and why it (and magnetism in general) is a very complex phenomena.


  6. Nov 7, 2010 #5
    As a follow up to the discussion....( i hope that will be fine...)

    can you be a little more elaborate in explaining the cause of Fixed alignments in the ferromagnetic substance ..........
    for sure it means that the parallel system of alignment is more stable ....but what are the causes behind that..?
  7. Nov 7, 2010 #6


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    Wow! You had to dig up something from 2004? This must be close to a record for necroposting.

    First of all, the alignment of various magnetic moment is not the simplest thing in the world. Quantum magnetism is the subject of whole books.

    Secondly, it isn't true that "parallel system of alignment is more stable". If it is, we won't have antiferomagnetic material, such as the parent material of high-Tc cuprates. Now this, already indicate that the ground state system isn't trivial, and can depend on (i) the proximity of neighboring spins (ii) the crystal structure, (iii) Heisenburg coupling, etc... etc.

    I would recommend the graduate text by Assa Auerbach "Interacting Electrons and Quantum Magnetism" to fully do justice on how complex this topic is.

  8. Nov 7, 2010 #7
    So why is aluminium and copper not magnetic but iron and steel are?
  9. Nov 8, 2010 #8
    Aluminum and Copper do not have the localized electrons to act as tiny magnetic moments whereas Iron has (partially filled d subshell). In case of ferromagnets, these magnetic moments are aligned in the same direction (within a domain).
  10. Nov 8, 2010 #9
    So you mean for aluminium and copper, the electron magnetic moments all cancelled out?
    Are these magnetic moments due to spin or orbital angular momentum or both?
  11. Nov 8, 2010 #10
    In all magnets, the magnetism is due to the spin of the electron. The magnetic nanoparticles are the exceptions where the orbital magnetic moments is not quenched when the coordination number decreases with the decrease in size.
  12. Nov 9, 2010 #11
    can i interrupt a bit.....

    i want to ask when do we say that presence of unpaired electron in the last orbital makes the substance para - or ferro magnetic.... what is the basis of that?
    i want to ask .... why are domains not formed in a paramagnetic substance and only in ferro magnetic substance?
  13. Nov 9, 2010 #12
    first of all , i am not a graduate student to understand such an advanced book.....
    but i will still try ....or can you suggest some simpler book on this topic....

    further, " parallel system of alignment is more stable " in case of ferromagnetic substance
    ( within the domain....) is true.......right?

    ""the ground state system isn't trivial "" what do you mean by this??

    and what is " the proximity of neighboring spins " ?and how does that effect the alignment ?

    ( i know i am asking too many questions , but you see ... curiosity......)

    thanks a lot in advance...
  14. Nov 9, 2010 #13
    When there is an exchange interaction (leading to parallel alignment) between the unpaired spins, then the material is ferromagnetic. Due to this interaction, the spins survive the thermal agitation (in absence of magnetic field).

    Since there is no exchange interaction in the paramagnetic materials, the magnetic moments do not align themselves in one direction in the absence of a magnetic field. The small magnetic interaction among the moments cannot overcome the thermal motion which causes randomness. Hence there is a lack of macroscopic magnetic moment. Since the tiny magnetic moments are random, so there is no question of any domains.

    You can try books by: A. J. Dekker; C. Kittle; Ashcroft and Mermin. You can also try "The Physics and Chemistry of Materials" by Joel I. Gersten and Frederick W. Smith. This a very readable book.
  15. Nov 9, 2010 #14
    Is the exchange interaction a QM effect?
  16. Nov 9, 2010 #15


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    Last edited: Nov 9, 2010
  17. Nov 10, 2010 #16

    according to this , exchange interaction plays a significant role .....
    can you explain how does it affect the alignment?
    ( mentioning that i have read the suggested wiki article....and also tried reading the main article about the exchange interaction ...but found the later hard to understand) please help with that.... i am just a high school student....and have studied QM extreme basic in my chemistry book....
  18. Nov 10, 2010 #17
    The exchange interaction between two spins/magnetic moments tries to align them in either the parallel or opposite direction. This tendency leads to the long range magnetic alignment. U can read a QM book, for e.g. by Landau and Lifgarbagez, to understand why this happens.
  19. Nov 11, 2010 #18
    book : Quantum mechanics: non-relativistic theory
    Lev Davidovich Landau, E. M. Lifgarbagez

    will this book be fine for my level...( high school)
  20. Nov 11, 2010 #19
    You are a highschool student? In that case, I cannot suggest you any book. I am not sure if these interactions can be explained in any details to a highschool student. Sorry dear.
  21. Nov 11, 2010 #20
    You need to learn High School physics and calculus first.
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