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Magnetism - · PDF file... (gauss/oersted) H(A/m), B(weber/m2 – tesla), ... B Magnetic Induction (Tesla or kg/A-s 2or Wb/m ) H Magnetic field (amp-turn/m or C/m-s) M Magnetization

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  • MagnetismMagnetism, the phenomenon by which materials assert an attractive or repulsive force or influence on other materials.

    Basic ConceptsAn electrical current in a loop

    generates a magnetic field.Magnetic fields are generated by

    moving electrically charged particles.

    Magnetic field strength - H

    Earth North Pole is actually the South magnetic pole.

    Field lines come out from the north towards the south

  • Magnetization, Permeability, and the Magnetic Field

    (A/m) LnIH =

    A current passing through a coil sets up a magnetic field (H). Where n=number of turns; L=length of the coil and I=current. Units for H (A/m) and oersted 31 104.11 = mAoersted

    )(weber/m 2HB oo =

    When H is applied in vacuum, lines of magnetic flux are inducted. The number of lines of flux, called flux density or inductance B, is related to the applied magnetic filed by the equation

    o magnetic permeability of vacuum.

    Units:H(oersted), B(gauss), o(gauss/oersted)H(A/m), B(weber/m2 tesla), o(4x10-7weber/A.m henry/m)

  • )(weber/m 2HB =When we place a material within the magnetic field, B is determined by the manner in which induced and permanent dipoles interact.

    = permeability of the material in the field>o magnetic dipole moments reinforce the field

  • Magnetization of a SolidB = oH + oM

    field-vacuum dipoles-material

    o = permeability under vacuum

    M = magnetization of the material

    Review of TermsB Magnetic Induction (Tesla or kg/A-s2 or Wb/m2)H Magnetic field (amp-turn/m or C/m-s)M Magnetization (same as magnetic field)o Permeability (henry/m or kg-m/C2)

    Magnetic susceptibility (m) 1=

    =

    rm

    m HM

    17104 = mhenryO .

    M is defined as the magnetic moment per unit volume. It is a property of the material and depends on both the individual magnetic moments of the constituent ions, atoms or molecules and how these dipole moments interact with each other. )1( mo +=

  • 17104 = mhenryO .

  • Magnetic MaterialsMagnetic behavior is determined primarily by the electronic structure of a material, which provides magnetic dipoles.

    Magnetic Dipoles: (Analogous to electric dipoles) They are the result of (a) electrons orbiting around the nucleus and (b) spin of theelectron around its axis.

  • These two motions (i.e. orbital and spin) contribute to the magnetic behavior of the material. The interaction between these dipoles determine the type of magnetic behavior of the material.The magnetic behavior can be controlled by composition, microstructure and processing.The magnetic moment of an electron due to its spin is known as the Bohr Magneton (MB Fundamental Constant)q=charge of electron,h=Planck constantme=mass of the electronThen, we can view electrons as small elementary magnets.However, the magnetic moments due to electron do not all line up in the same direction.

    224 .10274.94

    mAxm

    qhMe

    B==

  • Two mechanisms to cancel magnetic dipole moments:(1) Electron pairs have opposite spins they cancel each other.(2) Orbital moments of the electrons also cancel outThus:Atoms having completely filled electron shells (He, Ne, Ar, etc) are not

    capable of being permanently magnetized.Some elements, such as transition elements (3d, 4d, 5d partially filled) have a net magnetic moment since some of their levels have unpaired electrons. Example (Sc to Cu) the electrons in the 3d level do not enter the shells in pairs. Mn has five electrons with the same spin. Transition metals have a permanent magnetic moment, which is related to the number of unpaired electrons.

  • Types of magnetism: Ferromagnetism. Property of iron, nickel, neodymium

    Strongest type of magnetism. Uncancelled electron spins as a consequence of the electron structure.

    Paramagnetism. Exhibited by materials containing transition, rare earth or actinide elements

    Diamagnetism Exhibited by all common materials but masked if other two types of magnetism are present

    Ferrimagnetism Source of magnetic moment different as ferromagnetic. Incomplete cancellation of spin moments due to atomic position and surrounding. Ceramics (insulators).

    Antiferromagnetism Alignment of the spin moments of neighboring atoms or ions in exactly opposite direction.

  • DiamagnetismCompletely filled shells or subshells.Total cancellation of orbital and spin momentsCannot be permanently magnetized. Very weakIt is induced by change in orbital motion due to applied fieldThe dipoles induced by the field are aligned opposite to the field direction.Only exists while a field is on. It is found in all materialsVery hard to observe. It is of no practical purposer < 1 (~0.99) H = 0

    H

    Superconductors:

    r=0

  • External magnetic field acting on the atoms slightly unbalances their orbiting electrons and creates small magnetic dipoles within the atoms, which oppose the applied field, and this action produces a negative magnetic effect to the applied field.

    A weak, negative, repulsive reaction of a material to an applied magnetic field.

    is around 10-6to 10-5 . Inert gases, many organic compounds, some metals (Bi, Zn, Ag) and nonmetals (S, P, Si)

    Magnetization is negative (

  • ParamagnetismIncomplete cancellation of electron spin/orbital magnetic momentsPermanent Dipoles. Randomly oriented when no field is presentParamagnetism. Permanent dipoles align with an external field No interaction between adjacent dipoles. Exists only in a magnetic fieldRandomly oriented permanent dipoles align with field. (Only present

    when field is applied)r = 1.00 to 1.01 H = 0

    H

  • Magnetization is positive (> 0)Applied field aligns the individual magnetic dipoles of the atoms or molecules and slightly increases magnetic induction, B. The magnetic susceptibility ranges from 10-6 to 10-2Temperature reduces the paramagnetic effectsDiamagnetism and paramagnetism are all induced by an applied field, when the field is removed, the effect disappears. Rare earth metals, Li, Na, K, RhA weak, positive, attractive reaction of a material to an applied fieldVery limited engineering applications.

  • DiamagneticBismuth -16.6Mercury -2.85Silver -2.38Carbon (diamond) -2.1Gold -3.44Sodium chloride -1.4Copper -1.8ParamagneticIron aluminum alum 66Uranium 40Platinum 26Aluminum 2.07Sodium 0.85Chromium 3.13

    Material Susceptibility m(x 10-5)

  • FerromagnetismElectron Spins dont cancel outCoupling interactions cause adjacent atoms to align with one anotherFerromagnetism. Permanent magnetic moment in the absence of an external field.

    large magnetizationr = up to 106

    Permanent dipoles are aligned even in the absence of a magnetic field.

    H = 0

  • Magnetic susceptibility is positive and very large (101< < 106)Very large magnetization will be created by the material.Relationship between magnetization (M) and applied field (H) is nonlinear and complicated. Repeated magnetization leads to hysteresis.Large magnetic field can be retained after the applied field removed.Most important ferromagnetic elements are: Fe, Co, Ni Great engineering importance.A rare-earth element gadolinium (Gd) is also ferromagnetic below 16oC, but has little engineering application.

  • Fe atom has four unpaired 3d electrons; Co has three unpaired 3d electrons; Ni has two unpaired 3d electrons.

    Spins of the 3d electrons of adjacent atoms align in a parallel direction by a phenomenon called spontaneous magnetization. This parallel alignment of atomic magnetic dipoles occurs in microscopic regions called magnetic domains. Most critical difference between Ferromagnetism and Paramagnetisim: The former has Spontaneous Magnetization. Randomly oriented domains No net magnetizationDomains aligned in a magnetic field Very strong magnetic induction

  • As a whole the materials magnetic domains are oriented randomly and effectively cancel each other out

    If H is applied, domains align giving a strong net H field in same direction as H

    Net H field partially exists even when Hext is removed

  • Fe, Co and Ni ferromagnetic materials

    Cr and Mnnot ferromagnetic materials, Why?

    (they all have unpaired 3d electrons)

    Magnetic exchange interaction energy Energy associated with the coupling of individual magnetic dipoles into a single magnetic domain. Only when this exchange energy is positive material be ferromagnetic. Magnetic exchange energy is related to the ratio of atomic spacing to 3d orbit, must be in the range of 1.4 to 2.7.

  • AntiferromagnetismAlignment of neighboring atomsSpin moments are opposite

    no net magnetic moment

    Antiparallel alignmentCeramic oxides, Manganese (Mn), chromium (Cr), MnO, CrO, and CoOexhibit this behavior M2+ O 2-

    A type of magnetism in which the magnetic dipoles of atoms are aligned themselves in opposite directions by an applied field so that there is no net magnetic moment.

  • If we place ferromagnetic material (e.g. iron) inside a solenoid with field B0 , increase the total B field inside coil to

    BM is magnitude of B field contributed by iron coreBM result of alignment of the domainsBM increases total B by large amount - iron core inside solenoid increases B by typically about 5000 times For the electromagnetic core we use soft iron where the magnetism is not permanent (goes away when the external field isturned off).

    The maximum possible magnetization, or saturation magnetization Msrepresents the magnetization that results when all the magnetic dipoles in a solid piece are mutually aligned with the external field. There is also a corresponding saturation flux density Bs

    MBBB += 0

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