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Chapter 18: Magnetic Properties

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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Chapter 18: Magnetic Properties

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  1. 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? • What is superconductivity and how do magnetic fields effect the behavior of superconductors?

  2. Generation of a Magnetic Field -- Vacuum • Computation of the applied magnetic field, H: • Created by current through a coil: B0 N = total number of turns  = length of each turn (m) I= current (ampere) H H= applied magnetic field (ampere-turns/m) B0 = magnetic flux density in a vacuum (tesla) I • Computation of the magnetic flux density in a vacuum, B0: B0=0H permeability of a vacuum(1.257 x 10-6 Henry/m)

  3. Generation of a Magnetic Field -- within a Solid Material • Relative permeability (dimensionless) • A magnetic field is induced in the material B B = Magnetic Induction (tesla) inside the material applied magnetic field H B=H permeability of a solid current I

  4. Generation of a Magnetic Field -- within a Solid Material (cont.) • B in terms of Hand M B=0H +0M B > 0 cm vacuum cm = 0 < 0 cm H • Magnetization M=mH 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

  5. Origins of Magnetic Moments • Magnetic moments arise from electron motions and the spins on electrons. magnetic moments electron electron spin nucleus Adapted from Fig. 18.4, Callister & Rethwisch 3e. electron orbitalmotion electron spin • Net atomic magnetic moment: -- sum of moments from all electrons. • Four types of response...

  6. Types of Magnetism (3) ferromagnetic e.g. Fe3O4, NiFe2O4 (4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd cm (2) paramagnetic ( e.g., Al, Cr, Mo, Na, Ti, Zr ~ 10-4) cm ( as large as 106 !) cm (1) diamagnetic ( ~ -10-5) cm vacuum ( = 0) e.g., Al2O3, Cu, Au, Si, Ag, Zn B (tesla) H (ampere-turns/m) 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.

  7. (2) paramagnetic random aligned Adapted from Fig. 18.5(b), Callister & Rethwisch 3e. (3) ferromagnetic (4)ferrimagnetic Adapted from Fig. 18.7, Callister & Rethwisch 3e. aligned aligned Magnetic Responses for 4 Types No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) (1) diamagnetic opposing Adapted from Fig. 18.5(a), Callister & Rethwisch 3e. none

  8. Domains in Ferromagnetic & Ferrimagnetic Materials H H H • “Domains” with aligned magnetic H moment grow at expense of poorly aligned ones! H H = 0 • As the applied field (H) increasesthe magnetic domains change shape and size by movement of domain boundaries. B sat 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.) induction (B) Magnetic 0 Applied Magnetic Field (H)

  9. Hysteresis and Permanent Magnetization 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 B Adapted from Fig. 18.14, Callister & Rethwisch 3e. H Stage 1. Initial (unmagnetized state)

  10. Hard and Soft Magnetic Materials Hard Soft 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) B H 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.) 10

  11. Magnetic Storage recording medium recording head 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.) ~2.5mm ~120nm • 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:

  12. Superconductivity Found in 26 metals and hundreds of alloys & compounds • TC= critical temperature = temperature below which material is superconductive Mercury Copper (normal) Fig. 18.26, Callister & Rethwisch 3e. 4.2 K

  13. Critical Properties of Superconductive Materials 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.

  14. Meissner Effect • Superconductors expel magnetic fields • This is why a superconductor will float above a magnet normal superconductor Fig. 18.28, Callister & Rethwisch 3e.

  15. Advances in Superconductivity • Research in superconductive materials was stagnant for many years. • Everyone assumed TC,max was about 23 K • Many theories said it was impossible to increase TC beyond this value • 1987- new materials were discovered with TC > 30 K • ceramics of form Ba1-x Kx BiO3-y • Started enormous race • Y Ba2Cu3O7-x TC = 90 K • Tl2Ba2Ca2Cu3Ox TC = 122 K • difficult to make since oxidation state is very important • The major problem is that these ceramic materials are inherently brittle.

  16. Summary • 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)

  17. ANNOUNCEMENTS Reading: Core Problems: Self-help Problems:

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