Callister - An introduction - 8th edition

834 • Chapter 20 / Magnetic Properties
Magnetic Storage
• Information storage is accomplished using magnetic materials; the two principal
types of magnetic media are hard disk drives and magnetic tapes.
• The storage medium for hard disk drives is composed of nanometer-size grains
of an HCP cobalt–chromium alloy. These grains are oriented such that their direction
of easy magnetization (i.e., [0001]) is perpendicular to the plane of the
disk.
• For tape-memory storage, either needle-shaped ferromagnetic metal particles
or plate-shaped ferromagnetic barium–ferrite particles are employed. Particle size
is on the order of tens of nanometers.
Superconductivity
• Superconductivity has been observed in a number of materials; upon cooling and
in the vicinity of absolute zero temperature, the electrical resistivity vanishes (Figure
20.26).
• The superconducting state ceases to exist if temperature, magnetic field, or current
density exceeds a critical value.
• For type I superconductors, magnetic field exclusion is complete below a critical
field, and field penetration is complete once H C is exceeded. This penetration is
gradual with increasing magnetic field for type II materials.
• New complex oxide ceramics are being developed with relatively high critical
temperatures, which allow inexpensive liquid nitrogen to be used as a
coolant.
Equation Summary
Equation
Page
Number Equation Solving For Number
20.1 H NI
l
Magnetic field strength within a coil 802
20.2 B mH
Magnetic flux density in a material 803
20.3 B 0 m 0 H
Magnetic flux density in a vacuum 803
20.4 m r m m 0
Relative permeability 803
20.5 B m 0 H m 0 M
Magnetic flux density, in terms of magnetization 803
20.6 M x m H
Magnetization 804
20.7 x m m r 1
Magnetic susceptibility 804
20.8 B m 0 M
Magnetic flux density for a ferromagnetic material 807
20.9 M s 0.60m B N
Saturation magnetization for Ni 808
20.11
M s N¿m B
Saturation magnetization for a ferrimagnetic
material
812