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Magnetic Domains

  1. Apr 14, 2006 #1
    In ferromanetic materials like steel , molecules adjacent to each other will align themselves and "pull " in the same direction.When enough domains have their North poles facing in the same direction they collectively produce a magnetic field.

    So how does that actually happen ? Does it have something to do with the spin of the electrons in the outer shell of the iron atoms ?
  2. jcsd
  3. Apr 15, 2006 #2
    Domain formation happens because it minimizes the interaction energies between the spins in the system. The uncoupled spins in atoms are responsible for the magnetic properties of materials so you're almost correct. It's not neccessarily the outmost electrons that are responsible for the magnetic moment. Iron for instance has the outmost shell filled (spins cancel eachother) but it has 5 spins pointing up and one down in the 3d shell.

    The domains cancel eachother out unless you magnetize the material btw. It's energetically favourable.
  4. Apr 15, 2006 #3
    Thank You IHNA

    I am just getting started in my study of magnetism , and I thank you for your thoughtfull reply.

    I have a follow up question for you if you should have more free time

    If I follow, the greater the spin variance the more easily and strongly a substance can be magnetized.When an iron bar is placed in a magnetic field it will become itself partially magnetized for a period of time.

    Is it not so that in order to align the domains energy must be added. If the only energy source is the permanent magnet , then the energy must be coming from there. What do we call this energy? Does it operate on the iron atom as a whole and push the atoms into the domains ? Or on the elctrons changing the spin , or the shell energy the way a photon will move an electron to a temporarily higher state?

    Thanks again
  5. Apr 16, 2006 #4
    I may not have my physics terminology correct, but you will get the jist:

    1) The application or dissipation of energy IS required for a dynamical change in a system. This can not be avoided.

    2) You wonder how a permanet magnet can magnetize an iron bar without seemingly applying energy. Think again. You had to MOVE the permanent magnet or iron bar close to each other. That movement requires energy, in this case, your own.
  6. Apr 16, 2006 #5

    I thank you for your input , and I certainly agree to a point.Moving the permanent magnet over the iron bar is work , and that work can be transformed to energy that would impact the domains and or spins of the system.

    But what would be the form of the conversion from the work of my hand to the energy that reorients the spins and or domains . Is it electric , light , microwave , magetic itself etc.
  7. Apr 17, 2006 #6
    You'd have to use a magnetic field to reorient the spins. Are you getting your intro to magnetism classically or in the context of solid state physics? If the latter I'd suggest you get a hold of Kittel's or Hook&Hall's introduction to solid state physics texts. Both cover the microscopics of magnetism pretty well.
  8. Apr 17, 2006 #7
    Hello inha

    I guess I would have to say I am getting my intro haphazardly. I became interested in some of the odd and vague claims by Tesla late in his career and almost simultaneously the desire to understand the process of converting static electricity to kinetic.

    It also seems to me like Cosmology and Quantum theory there applied get all the headlines and was just wondering what modern scholars interested in magnetism and power generation pay attention to these days
    So I decided to start at the beginning and read things like electricity for dummies , and the US Navy's training manual for electrical engineering.

    I think your suggestion is an excellent one , and I will read at least one of the texts you suggest. I am thinking abaout retaining a tutor to help me with the related math. I am located near the University of Minnesota and have some friends in the math department at the graduate instructor level.
    If you have any colleagues there , I would be grateful for an introduction of sorts
  9. Apr 17, 2006 #8


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    What do you mean by "spin variance" ? If you are not using a known terminology, it helps to explain what you mean by the terms you use.

    Ferromagnetism is a macroscopic property that results from the interactions between a large number of atoms. The spin of an individual atom does not determine whther something will form a ferromagnet. For example, Mn has a larger net spin moment than Fe, yet is not a ferromagnet. This, however, is not saying that spin does not play any role - it does. But in addition to the spin, one also needs to consider the interatomic (exchange) interactions. These are purely quantum mechanical in nature, and can not be explained classically.

    When a ferromagnet is magnetized by applying a magnetic field, there are typically two processes that occur which increase the magnetization of the magnet :

    (i) domain wall motion - Domains lined up along the direction of the external field grow in size, at the cost of domains that are not favorably aligned.

    (ii) domain rotation - Usually (and roughly speaking), when you are left with only one favorably aligned domain (which has "eaten up" all other domains), an additional increase in the applied field causes the net magnetization (spin orintation) to rotate towards the direction of the applied field. The mechanism for this rotation can be intuited from the classical effect of an electric/magnetic field on an electric/magnetic dipole. The torque experienced by the dipole is given by [itex]\tau=m \times H[/itex].

    There have been other discussions on ferromagnetic domains in PF. I suggest you search for "domain" under the Atomic, Molecular, Solid State Physics subforum.
  10. Apr 17, 2006 #9
    For inha

    By the way , in your reply you mentioned that magentic force would be the agent that created the domain alignment in the iron bar causing it too to become magnetic.

    Do the magnetic lines of force work directly to molecules in the domains , or does it affect the spins of the electrons the way NMR does ( reversing them)

  11. Apr 17, 2006 #10
    Thank you Gokul4321

    I appreciate your reply , and will follow your suggestion to read other threads in the Forum that deal with domains.

    1) My term spin variance is as you say not a known term and I think is the same as what you call spin moment

    2) I find it very interesting that Mn is not ferromagnetic and that spin does not itself drive the behavior of the material

    3) I find it intimidating that I have to learn quantum mechanics in order to understand what does drive the behavior, but it is what it is

    Question : Domains have a net spin monent and align themselves in the direction of the magnetic field applied with the two processes you mention , wall shifting and rotation. Are the net spin moments for iron always either up or down , or can they be either . I assume when up the direction of the alignment would be opposite with respect to the external feild than if down.

    Also can the nature of an external field ( such as used in NMR) be such that the up or down characteristic can be altered , and what effect would that have . Perhaps its just a different source of the same thing that induces rotation and wall shifting.

    Thanks again
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