Ferro Magnets

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Details of Ferromagnets

Transcript of Ferro Magnets

  • Magnetic Domains in Soft Ferromagnets

    A ferromagnet

    consisting of a single domain creates a large external field, which costs energy ( B2). This stray field

    is reduced

    by forming several domains. It is like cutting up the magnet and letting the pieces

    orient them-

    selves optimally (i.e., with North and South poles next to each other).

    This occurs in pure and defect-free

    materials (soft magnets). An external field is needed to magnetize them by aligning the domains (electromagnet).

  • Imaging Magnetic Domains with Iron Powder

    Small iron particles are attracted to the high magnetic stray field at the transition between domains. They scatter light in a dark field microscope. The orientation of the domains is determined by adding an external field and watching them grow or shrink.

  • Magnetic Domain Walls

    0.1 m

  • MSaturation Magnetization MsRemanence Mr


    Coercivity Hc

    Magnetic Hysteresis Curve: M(H)

    Small Coercivity

    : Soft Ferromagnets

  • Simple Magnetic Hysteresis in nm


    Sized Particles and Layers

    (Smaller than a Domain Wall

    Single Domain)

  • Permanent MagnetsThe strongest permanent magnets (hard magnets) are alloys

    of 3d transition metals

    (Fe, Co) with rare earths. They exhibit the highest energy product BH.

    Examples are neodymium-iron-boron



    B) and samarium-cobalt (SmCo5) .

    Such magnets are used for electric motors and for generators in wind turbines, both important ingredients in becoming more energy-efficient. A Prius

    contains 1 kg of neodymium. That has caused increasing demand for rare earths.

    To prevent the formation of domains in a permanent magnet, one uses materials with a high defect density. Defects

    prevent domains from forming and thereby pin

    the magnetization.

    Two neodymium-iron-boron magnets that crunched a fingertip.

  • B, H, and M

    in a Bar Magnet

    The magnetization M

    is constant in a permanent magnet (by definition). The two other fields need to satisfy:B

    = H

    + 4M


    = H

    outsideH|| is continuous at a boundaryB

    is continuous at a boundary

    The H-field inside the magnet is the demagnetizing field Hd

    . It is related to the stray field outside the magnet by the continuity of H|| at the side of the magnet. Since the outside field lines bend 180o

    from the side to the end and since B

    is continuous at the end

    face, Hd

    opposes B





    the field lines, start with M, then jump to B, and do H last.




  • Magnetostriction

    and Piezoelectricity


    has a large influence on magnetic nanostructures, such as hard disk reading heads. Small structures tend to be strained by the surrounding material, and the strain magnetizes them permanently. Their magnetization cannot be switched anymore. In permalloy



    ) the magnetostriction

    vanishes, which makes it a key material for spintronics.


    makes it possible to position a STM tip with sub-atomic precision. A piezoelectric ceramic changes its length proportional to an applied voltage. A common design is a tube-scanner which can move in all three directions (x,y,z).

    Change in B, E

    Length Change

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