Magnetic Properties of Materials — Chapter 20 Summary from Callister’s Materials Science and Engineering

Chapter 20 of Materials Science and Engineering by William D. Callister, Jr. and David G. Rethwisch examines the fundamental magnetic properties of materials and their practical significance in engineering, power generation, electronics, and data storage. This summary covers the origins of magnetism, the different types of magnetic behavior, domain structure, hysteresis, magnetic anisotropy, and advanced applications like superconductors.

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Fundamentals of Magnetism

• Magnetic Dipoles & Moments: Generated by the motion of electrons (orbital and spin). The Bohr magneton (μB) is the unit of atomic magnetic moment.
• Magnetic Susceptibility (χ): Measures a material's tendency to become magnetized in an applied field, closely related to relative permeability (μr).
• Key Parameters:
  • Magnetic Field Strength (H): Intensity of the applied magnetic field.
  • Magnetic Induction (B): The total field inside the material.
  • Magnetization (M): Net magnetic moment per unit volume.

Types of Magnetism

• Diamagnetism: Weak, nonpermanent response; all materials exhibit it to some extent.
• Paramagnetism: Materials with permanent atomic dipoles that align weakly with an external field (e.g., aluminum).
• Ferromagnetism: Strong, permanent magnetization due to parallel alignment of atomic moments (e.g., iron, cobalt, nickel).
• Antiferromagnetism: Atomic moments align antiparallel, canceling each other out (e.g., manganese oxide).
• Ferrimagnetism: Incomplete cancellation of antiparallel moments, common in ferrites (magnetic ceramics).

Temperature Effects and Magnetic Domains

• Curie Temperature (Tc): Above this point, ferromagnetic and ferrimagnetic materials lose their permanent magnetization and become paramagnetic.
• Domains: Materials are composed of regions (domains) with aligned magnetic moments. Changing the direction of domains under an applied field causes hysteresis.

Hysteresis and Magnetic Anisotropy

• Hysteresis: The lag between applied field and magnetization, shown as a hysteresis loop. Key measures include remanence (Br) and coercivity (Hc), indicating how well a material retains magnetization.
• Magnetic Anisotropy: Magnetic properties vary with crystallographic direction, influencing magnetization ease and application design.

Soft and Hard Magnetic Materials

• Soft Magnetic Materials: Easily magnetized and demagnetized (low coercivity), ideal for transformer cores and electrical machinery.
• Hard Magnetic Materials: Retain strong magnetization (high coercivity), used in permanent magnets.
• High-Energy Magnets: Materials like SmCo₅ and Nd₂Fe₁₄B offer very strong, stable magnetic properties for cutting-edge applications.

Applications: Magnetic Storage and Superconductivity

• Magnetic Storage: Data stored using magnetic hard drives (perpendicular magnetic recording) and tapes (particulate media).
• Superconductivity: At critical temperature (Tc), certain materials exhibit zero electrical resistance and expel magnetic fields (Meissner effect). Type I and II superconductors differ in their behavior under magnetic fields.

Glossary of Key Terms

• Bohr Magneton (μB): Basic unit of magnetic moment.
• Curie Temperature (Tc): Transition point above which ferromagnetic materials become paramagnetic.
• Coercivity (Hc): Reverse field needed to reduce magnetization to zero.
• Remanence (Br): Remaining magnetization after removing the external field.
• Saturation Magnetization (Ms): Maximum magnetization a material can attain.
• Meissner Effect: Superconductors completely exclude magnetic fields below their critical field.
• Ferrite: Ceramic with ferrimagnetic properties.
• Magnetic Anisotropy: Directional dependence of magnetic properties.

Conclusion: The Role of Magnetism in Modern Materials

Magnetic properties are foundational for electrical engineering, energy systems, storage devices, and advanced research in superconductivity. By understanding magnetism at the atomic and material levels, engineers can design new generations of electronics, power technology, and data storage solutions. For a clear walkthrough of the principles, watch the podcast above and subscribe to Last Minute Lecture for expertly summarized chapters in materials science.

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