Magnetic Properties of Materials Notes

Types of Magnetic Materials

  • Diamagnetic Materials

    • Equal numbers of electron spins, randomly oriented leading to a net magnetic moment of zero.
    • When placed in a magnetic field, they repel magnetic flux lines.
    • Example materials: Gold, Antimony, Bismuth, Water, Silicon, Hydrogen, Germanium.

  • Paramagnetic Materials

    • Unequal number of electron spins leading to a permanent magnetic moment.
    • When placed in a magnetic field, they align parallel to the field direction.
    • Susceptibility varies inversely with absolute temperature.
    • Example materials: Platinum, Chromium, Aluminium, Copper sulfate.

  • Ferromagnetic Materials

    • Large number of unequal electron spins, leading to a significant permanent magnetic moment.
    • When placed in a magnetic field, they are highly attracted to the magnetic flux lines and can retain magnetic properties even after the field is removed.
    • Example materials: Iron, Nickel, Cobalt, Steel/</li>

Domain Theory of Ferromagnetism

  • According to Weiss hypothesis:
    • Ferromagnetic materials consist of small regions called domains, each with spontaneous magnetization.
    • The direction of magnetization varies among domains, resulting in a net magnetization of zero.

Magnetization Process

  • **Two methods of magnetization:
    1. Movement of domain walls**
    • Occurs in weak magnetic fields where domain boundaries displace, increasing magnetic moment.
    1. Rotation of domain walls
    • Takes place in strong magnetic fields where the direction of magnetization is adjusted to align with the field.

Types of Energy in Ferromagnetic Domains

  1. Exchange Energy:
    • Energy required to align adjacent dipoles, resulting in potential energy stored when assembling atomic magnets into a domain.
  2. Anisotropy Energy:
    • Energy variation based on easy and hard magnetization directions within the crystal structure.
  3. Domain Wall Energy:
    • Energy associated with the transition layer (Bloch wall) that separates adjacent domains, which can be classified as thick or thin walls.
  4. Magnetostrictive Energy:
    • Caused by slight length changes in a magnetized ferromagnetic material due to domain rearrangement.

Hysteresis in Ferromagnetic Materials

  • Hysteresis refers to the lagging of magnetic induction behind the magnetizing field, depicted via a closed loop / hysteresis curve.
  • When the magnetizing field is removed, residual magnetization occurs, indicated by retentivity.
  • Coercivity is the field strength required to remove residual magnetization.

Explanation of Hysteresis based on Domains

  • Initial small magnetization occurs when domain walls start to displace.
  • Once the field is removed, reversible domains revert to original states.
  • With larger applied fields, more domains contribute to magnetization, resulting in a maximum magnetized state, characterized by residual magnetism.

Soft and Hard Magnetic Materials

Soft Magnetic Materials:

  • Easily magnetized/demagnetized.
  • High susceptibility and permeability.
  • Low hysteresis loss; used for temporary magnets and applications requiring low eddy current loss.

Hard Magnetic Materials:

  • Difficult to magnetize/demagnetize.
  • High hysteresis loss; used for permanent magnets.

Ferrimagnetism (Ferrites)

  • Ferrites: Modified iron structures without carbon; possess anti-parallel magnetic moments of different magnitudes.
  • General chemical formula: X2+Fe2+3O4 2- (where X represents divalent metal ions).
  • Types of Structures:
    1. Regular Spinel Structure: Each divalent metal in tetrahedral sites and trivalent in octahedral sites.
    2. Inverse Spinel Structure: Some octahedral sites occupied by divalent metal ions and remaining by trivalent ions.

Applications of Ferrites:

  • Digital computers, low-frequency ultrasonic waves, non-reciprocal devices, magnetic amplifiers.

Magnetic Hard Disk and Tape

Magnetic Tape:

  • Easy handling with portable capabilities, useful for long-term storage but has sequential access limitations.

Magnetic Disks:

  • Popular for direct access storage, including hard disks (sealed, high storage, prone to damage) and floppy disks.

Magnetic Hard Disk Drives with GMR Sensors

  • Principle based on reading binary data through magnetic moments using GMR effect.
  • Composite constructing consisting of thin magnetic layers to store data as zeros and ones.
  • GMR sensors read data through resistance changes depending on magnetic orientation.

Advantages/Disadvantages:

  • GMR HDD Advantages: Large storage capacity, non-diffusive, high-sensitivity reading.
  • GMR HDD Disadvantages: Bulkier, higher power consumption risks.

Spintronics

  • Manipulating electron spin for data storage and processing.
  • Tunnel Magnetoresistance (TMR): A quantum phenomenon crucial for spintronic devices, allowing electron tunneling between ferromagnets through an insulating barrier.