Chapter 18: Magnetic Properties. ISSUES TO ADDRESS. • What are the important magnetic properties ?. • How do we explain magnetic phenomena?. • How are magnetic materials classified?. • How does magnetic memory storage work?.
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ISSUES TO ADDRESS...
• What are the important magnetic properties?
• How do we explain magnetic phenomena?
• How are magnetic materials classified?
• How does magnetic memory storage work?
• What is superconductivity and how do magnetic fields effect the behavior of superconductors?
• Computation of the applied magnetic field, H:
• Created by current through a coil:
N = total number of turns
= length of each turn (m)
I= current (ampere)
H= applied magnetic field (ampere-turns/m)
B0 = magnetic flux density in a vacuum (tesla)
• Computation of the magnetic flux density in a vacuum, B0:
permeability of a vacuum(1.257 x 10-6 Henry/m)
• Relative permeability (dimensionless)
• A magnetic field is induced in the material
B = Magnetic Induction (tesla) inside the material
magnetic field H
permeability of a solid
• B in terms of Hand M
vacuum cm = 0
Magnetic susceptibility (dimensionless)
• Combining the above two equations:
B=0H +0 mH
=(1 + m)0H
permeability of a vacuum:
(1.26 x 10-6 Henry/m)
cm is a measure of a material’s magnetic response relative to a vacuum
• Magnetic moments arise from electron motions and the spins on electrons.
Adapted from Fig. 18.4, Callister & Rethwisch 3e.
• Net atomic magnetic moment:
-- sum of moments from all electrons.
• Four types of response...
(3) ferromagnetic e.g. Fe3O4, NiFe2O4
(4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd
(2) paramagnetic (
e.g., Al, Cr, Mo, Na, Ti, Zr
as large as
(1) diamagnetic (
e.g., Al2O3, Cu, Au, Si, Ag, Zn
Plot adapted from Fig. 18.6, Callister & Rethwisch 3e. Values and materials from Table 18.2 and discussion in Section 18.4, Callister & Rethwisch 3e.
Adapted from Fig. 18.5(b), Callister & Rethwisch 3e.
Adapted from Fig. 18.7, Callister & Rethwisch 3e.
Magnetic Field (H = 0)
Magnetic Field (H)
Adapted from Fig. 18.5(a), Callister & Rethwisch 3e.
• “Domains” with
moment grow at
expense of poorly
H = 0
• As the applied field (H) increasesthe magnetic domains change shape and size by movement of domain boundaries.
Adapted from Fig. 18.13, Callister & Rethwisch 3e. (Fig. 18.13 adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.)
Applied Magnetic Field (H)
Stage 2. Apply H, align domains
Stage 3. Remove H, alignment
remains! => permanent magnet!
Stage 4. Coercivity, HCNegative H needed to demagnitize!
Stage 6. Close the hysteresis loop
Stage 5. Apply -H, align domains
• The magnetic hysteresis phenomenon
Adapted from Fig. 18.14, Callister & Rethwisch 3e.
Stage 1. Initial (unmagnetized state)
Hard magnetic materials:
-- large coercivities
-- used for permanent magnets
-- add particles/voids to
inhibit domain wall motion
-- example: tungsten steel --
Hc = 5900 amp-turn/m)
Soft magnetic materials:
-- small coercivities
-- used for electric motors
-- example: commercial iron 99.95 Fe
Adapted from Fig. 18.19, Callister & Rethwisch 3e. (Fig. 18.19 from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.)
Adapted from Fig. 18.23, Callister & Rethwisch 3e. (Fig. 18.23 from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.)
-- Particulate: needle-shaped
g-Fe2O3. magnetic moment
aligned with axis (e.g., tape).
--Thin film: CoPtCr or CoCrTa
alloy. Domains are ~ 10-30 nm
wide (e.g., hard drive).
Adapted from Fig. 18.25(a), Callister & Rethwisch 3e. (Fig. 18.25(a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, 1993. )
Adapted from Fig. 18.24, Callister & Rethwisch 3e. (Fig. 18.24 courtesy P. Rayner and N.L. Head, IBM Corporation.)
• Information can be stored using magnetic materials.
• Recording head can...
-- “write” - i.e., apply a magnetic field and align domains in small regions of the recording medium
-- “read” - i.e., detect a change in the magnetization in small regions of the recording medium
• Two media types:
Found in 26 metals and hundreds of alloys & compounds
= temperature below which material is superconductive
Fig. 18.26, Callister & Rethwisch 3e.
TC = critical temperature - if T > TC not superconductingJC = critical current density - if J > JC not superconductingHC= critical magnetic field - if H > HCnot superconducting
Fig. 18.27, Callister & Rethwisch 3e.
Fig. 18.28, Callister & Rethwisch 3e.
• A magnetic field is produced when a current flows through a wire coil.
• Magnetic induction (B):
-- an internal magnetic field is induced in a material that is situated within an external magnetic field (H).
-- magnetic moments result from electron interactions with the applied magnetic field
• Types of material responses to magnetic fields are:
-- ferrimagnetic and ferromagnetic (large magnetic susceptibilities)
-- paramagnetic (small and positive magnetic susceptibilities)
-- diamagnetic (small and negative magnetic susceptibilities)
• Types of ferrimagnetic and ferromagnetic materials:
-- Hard: large coercivities
-- Soft: small coercivities
• Magnetic storage media:
-- particulate g-Fe2O3 in polymeric film (tape)
-- thin film CoPtCr or CoCrTa (hard drive)