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Atoms, molecule, electron spin and magnetic media

  1. Oct 20, 2008 #1
    I read much about atoms, molecules and electrons but still i am not able to get answers for my study towards magnetic media. Please help me with the following questions in anyway you can, i would be really really grateful as it is going to help me a lot with my study towards magnetic media.

    My knowledge base considering the following questions: Atomic structure, how electrons fill into electron shells following hands rule, only two electrons can exist in an orbital (I am clear with the concept of orbitals as what they are their shapes energy level etc). Only two electrins can exist in an orbital and each of them should have opposite spin. Working of hard disk drives, in terms of its hardware and how the control the read / write head which consists of a coil, BUT i am more confused with what happens to the magnetic media (the magnetic material on the magnetic media) related to electron spins, electrons, flux reversals, etc. I have been coming across the new technology research to control individual electron spins and i have to clear my concepts before i move further.

    1. Are electrons freely flaoting in nature / space? Or as soon as they are generated, they try to form atoms, meaning that though there are freely floating electrons in space we can never find out their existence without being in an atom? From where do electrons come from?

    2. According to Pauli Exclusion Principle, no two electrons having same spin can exist in the same orbital. If that is the case, then how exactly are molecules formed? Example, hydrogen atom on alone has one electron. What is its electron spin? is it -1/2 for all independent hydrogen atoms or +1/2 for all independent hydrogen atoms or it can be anything by default for each individual hydrogen atom? When a hydrogem atom wants to combine with other hydrogen atom to form a molecule, does it keep searching for another hydrogen atom where the spin of the electron is antiparallel to its existing electron spin or it just searches for another hydrogen atom and automatically reverses the other hydrogen atom's electron spin and gets covalently bonded or it reverses its own electron spin and gets covalently bonded or none of the spin is reversed?

    On forming covalent bond, how do electrons behave (considering i will understand the spin behavior with above question)? Do do keep changing their positions (in terms of moving AROUND the nucleus), and thereby being in their original individual atomic prbitals sometimes and sometimes being in the hybrid orbital. how do electrons behave in each of these three orbitals (two original and one hybrid)???

    3. Can anyone tell me sharing, pairing, donating, accepting of electrons is related to what kind of bonding? I mean which (or which all) is (are) relating to covalent bonding (i.e. bonding between atoms of same elements)? Which (or which all) is(are) related to bonding of one element with other?

    4. After understanding the above doubts spin functionality of an individual hydrogen atom and hydrogen molecule, i have doubt that will an individual hydrogen atom (if it ever exists as an individual atom), will it be affected by the magnetic field? If an hydrogen atom never exists as an individual atom, does the magnetic field have an effect on the molecule of hydrogen?

    5. This was all about a hydrogen atom with one valence eletron. What happens in case of other elements, i mean how is electron spin handled when an element's atom has more than one valence electrons? (As i had already asked about how electron spin is handled in case of hydrogen, i will be able to understand how covalent bonding takes place for other atoms) However i dont know if it is separate for other elements compared to hydrogen or covalent bonding takes place in the same way?

    6. Why some elements which have valence electrons are magnetic while some are not (even though they also have valence electrons). Does it depend on the number of valence electrons or net valence electron spin or what is it?

    7. I know valence electrons in an atom all align in the same direction. How does an external magnetic field affect this alignment? Does an external magnetic field have an effect on individual atom or a molecule or a group of molecules or on what?

    8.Again once i now how covalent bonds are formed and how they handle electron spin, I am confused that if during formation of covalent bond the electrons complete the valency deficiency (either by merging with other atoms by modifying the other atoms spin or modifying its own spin or modifying the spin of none), how magnetic field will really affect? If the atoms had covalently bonded by alterating either one atom's spin, are they still aligned all in parallel to the external madgnetic field? By all i mean does this alignment happen for an individual atom or for molecules or how exactly does it happen? If the electrons had altered the spins while covalently bonding, will covalency break when they are again aligned with respect to the external magnetic field?

    9.How is the ferromagnetic coating applied on the magnetic media actually working. What is done on this material by the external magnetic field arising from the read write coil head? Is the ferromagnetic material already magnetized when we get a brand new magnetic media. When we say brand new magnetic media, it does not have any data. what do we mean when we say it does not have any data, related to the electron spin of an atom or molecule or what exactly is the condition of the brand new magnetic media?

    10. Why indivual magnetic elements are not used as magnetic media, rather alloys are used?

    I have been reading a lot through magnetic media finally relating all of them to electron spins and / or alignment, going through terms as flux reversals generation / detection but i have not been able to correlate them exactly and it is really very important for me to understand to proceed with my study with magnetic media.

    11. What exactly is done in relation to the electrons or electron spin alignment or what
    exactly is done while writing data on magnetic media? (not the mechanical part, only the flux field or flux generation / reversal, read or made and how is all this related to electron spins and their alignment (whether in an atom or molecule or what))?

    12. What exactly is done in relation to the electrons or electron spin alignment or what exactly is done while reading data from magnetic media? Is something sensed in relation to electrons or electron spin or their alignment or what is done? How is it done (not the mechanical part, only the flux field or flux generation / reversal, read or made and how is all this related to electron spins and their alignment (whether in an atom or molecule or what))?
  2. jcsd
  3. Nov 11, 2008 #2
    1- Electrons can stay alone for a long time in a big deep vacuum (outer Space, or LEP on Earth) as long as they don't meet a proton or an ion or even a molecule. If they meet one, they will leave it if it's really hot, like in a star.

    Under inhabitable conditions, electrons are all bound.

    2- Beware that "spin" is often misused. Two electrons can share the same orbital if their angular momentums are opposed.

    If two atoms have incompatible electron angular momenta? I guess they rearrange them, because it costs few meV and the chemical bond brings several eV.

    Chemical bonds are strong enough that individual atoms are very scarce (excepted noble gases) where Mankind lives. All chemical reactions just rearrange existing molecules.

    2b - Both electrons are on the same orbital which is the best for them and encloses both nuclei (forget "rotating", they don't rotate nor move). It is a completely new orbital, which is able of deforming itself if external fields act on it. However, mathematical theories allow to express this new orbital - as any one in fact - as a combination of all the previous ones, and approximately of just the former 1s ones in the case of H2.

    4- (See also 2b) Individual hydrogen atoms exist in outer Space but molecules are more common. Magnetic fields act more on an atom than a hydrogen molecule. Once electrons are paired, magnetic fields act less, so you need special materials to have magnetic properties.

    5- Almost always, electrons are paired in matter under inhabitable conditions. Use transition elements, with incomplete layers, to get special effects.

    6- Ferromagnetism and permanent magnets are a molecular property, not an atomic one.
    - Fe, Ni are ferromagnetic but austenitic stainless steel is not. Though, 50-50 FeNi is both austenitic and ferromagnetic (mumetal).
    - Mn, Zn are not but build together "ferrites" used as magnetic cores in electronics.
    - Cr and O are nonmagnetic but CrO2 was used as a magnetic tape
    - Some plastics are magnetic... Not by a charge, but by their bonds.

    7- Do they?

    [I go on later]
  4. Nov 11, 2008 #3
    9- A magnet consists of Weiss domains which are spontaneously completely oriented but distributed randomly. Magnetizing it means orienting nearly all domains in the desired direction. The limits of these domains move during the process.

    A hard disk has a magnetic coating, which has already been written but with no useful data. Since all disks have been IDE (yes, even S-Ata, even Scsi) for over 20 years, they are formatted at the factory, so all tracks are already defined, as well as synchronization bits. So-called "low-level formatting" doesn't exist any more, now it just consists in overwriting data with zeroes and detecting bad sectors, but all tracks remain.

    Exception: Scsi disks permit to alter the sector size.

    So a "new" hard disk isn't very new in fact. Your data is absent, everything else is ready.

    10- Because alloys are better. But Fe, Co, Ni, Gd (Pm?) would be ferromagnetic; almost pure Fe is used.
    I don't know any pure element making a permanent magnet. Maybe Bloch walls (=limits of Weiss domains) are too mobile in pure elements.

    11- See 9.

    12- Reading a hard disk doesn't change its state. Bits influence the resistivity of the sensor in the read head.

    Read heads have a giant magnetoresistive head (see GMR) where electrons tunnel from one metal layer to another more easily when both have the same magnetic orientation. Much more sensitive than a coil.
  5. Nov 12, 2008 #4
    Although almost everything is answered, I would like to remark few points.
    Actually, ferromagnetism (also antiferromagnetism, as Cr, and ferrimagnetism, as magnetite) are colective properties. The intrinsic origin of all this properties is the exchange, in brief, the energy of a system of electrons are lowest when the spins have the same spin and they don't share orbitals. This energy can explain very easily all your questions regarding molecules, atoms and simple systems. In the case of the ferro, antiferro and ferri it is not as easy as in the former cases.

    In individual atoms or molecules the effect of a magnetic field in the electronic levels is known as Zeeman effect. I am not expert on that, but electronic levels are splitted in two levels with an energy proportional to the applied magnetic field and the total angular moment. This kind of splitting is present in all materials, independently of the magnetic ground state. In fact Weiss observed that in ferromagnets seems to exist an spontaneus splitting, which corresponds to several tesla. Weiss couldn't explain that, but he introduced this splittin as a parameter, named as an internal field, and could understand many things of the ferromagnetism. Later on this internal field was indetified with exchange.

    Hope this helps
  6. Nov 13, 2008 #5


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    Gold Member

    Lots of questions

    A slightly different style of answer to consider using organic chemistry style of molecules as visuals. Protons are shells and electrons are points holding them together in these visuals. The electrons are not exactly there, but are "likely" to be close.

    Regarding bonding: Consider 3 simple molecules H-, H2, and H2+.

    H- (1 proton 2 electrons) - The 2 electrons are most likely on opposite sides of the proton, caught in the first orbital shell, but pushed apart as far as they can be. The electrons cannot break out as the do not have enough energy to get out of the shell even with the other electron pushing against it.

    H2+ (2 protons 1 electrons) - The 1 electron is caught in the first orbital shell of both hydrogen protons, right in the middle holding the 2 protons together. The protons are pushing themselves apart, but they cannot seperate as it would involve the electron popping out of at least one of the shells.

    H2 (2 protons 2 electrons) - There are 2 forms

    H2 paraform - bonded like H2+ with one electron in both shells between the 2 hydrogen protons. The other electron is in only one of the hydrogen shells.

    H2 orthoform - bonded with both electrons caught in both hydrogen shells (what would be described as a covalent of 2 electron bond). Both electrons are between the 2 protons and are pushed as far apart as possible. The protons are pushed as far as possible from each other (74picometers) but cannot break the bond without at least one electron popping out of its energy level.


    H2 Orthoform is the classic covalent bond. Notice how the electrons have to orbit in the same direction (visualize the 3d) or they will run into each other.

    Hope this is of some help.
  7. Nov 22, 2008 #6
    Regarding question 10.

    This is a topic of interest to me, as I look at hybrid systems consisting of semiconductors and thin film ferromagnetic materials.

    There exist in nature just three elements that are naturally ferromagnetic: Ni, Fe and Co. Fe is a hard ferromagnet and is used quite frequently in industry as it is cheap yet has a reasonable saturation magnetisation. It isn't used on its own in technology because it reacts with most things. This can be hugely problematic. For most devices, you want a well defined interface between material A and material B, so having anything resembling a mush of atoms at the interface is not ideal. Secondly, magenetism is a co-operative effective; Fe may well be magnetic, but what is the magnetic state of the mush at the interface? Will that mush seriously affect or degrade the magnetic properties or electronic effect(s) you are trying to harness? Most likely, yes.

    Compound ferromagnets like CrO2, MnSb, MnAs and NiMnSb actually grow quite nicely on tehcnologically important materials like GaAs and hence are much more attractive materials.
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