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Chapter 20: Magnetic Properties

Chapter 20: Magnetic Properties. ISSUES TO ADDRESS. • How do we measure magnetic properties ?. • What are the atomic reasons for magnetism?. • How are magnetic materials classified?. • Materials design for magnetic storage. • What is the importance of superconducting magnets?.

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Chapter 20: Magnetic Properties

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  1. Chapter 20: Magnetic Properties ISSUES TO ADDRESS... • How do we measure magnetic properties? • What are the atomic reasons for magnetism? • How are magnetic materials classified? • Materials design for magnetic storage. • What is the importance of superconducting magnets?

  2. Applied Magnetic Field Applied magnetic field H N = total number of turns L = length of each turn current I current applied magnetic field units = (ampere-turns/m) • Created by current through a coil: • Relation for the applied magnetic field, H:

  3. Response to a Magnetic Field B = Magnetic Induction (tesla) inside the material current I c > 0 B c measures the material response relative to a vacuum. c vacuum = 0 c < 0 H • Magnetic induction results in the material • Magnetic susceptibility, c (dimensionless)

  4. Magnetic Susceptibility magnetic moments electron electron spin nucleus Adapted from Fig. 20.4, Callister 7e. • Measures the response of electrons to a magnetic field. • Electrons produce magnetic moments: • Net magnetic moment: --sum of moments from all electrons. • Three types of response...

  5. 3 Types of Magnetism permeability of a vacuum: (1.26 x 10-6 Henries/m) (3) ferromagnetic e.g. Fe3O4, NiFe2O4 ferrimagnetic e.g. ferrite(), Co, Ni, Gd 6 c ( as large as 10 !) -4 c (2) ( ~ 10 ) paramagnetic e.g., Al, Cr, Mo, Na, Ti, Zr c vacuum ( = 0) -5 (1) c diamagnetic ( ~ -10 ) e.g., Al O , Cu, Au, Si, Ag, Zn 2 3 Magnetic induction B (tesla) Strength of applied magnetic field (H) (ampere-turns/m) Plot adapted from Fig. 20.6, Callister 7e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister 7e.

  6. (2) paramagnetic random aligned Adapted from Fig. 20.5(b), Callister 7e. (3) ferromagnetic ferrimagnetic Adapted from Fig. 20.7, Callister 7e. aligned aligned Magnetic Moments for 3 Types No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) (1) diamagnetic opposing Adapted from Fig. 20.5(a), Callister 7e. none

  7. Ferro- & Ferri-Magnetic Materials H H H H H H = 0 • As the applied field (H) increases... --the magnetic moment aligns with H. B sat Adapted from Fig. 20.13, Callister 7e. (Fig. 20.13 adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.) • “Domains” with aligned magnetic induction (B) Magnetic moment grow at expense of poorly aligned ones! 0 Applied Magnetic Field (H)

  8. Permanent Magnets 2. apply H, cause 3. remove H, alignment stays! alignment => permanent magnet! 4 . Coercivity, HC Negative H needed to demagnitize! B Adapted from Fig. 20.19, Callister 7e. (Fig. 20.19 from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.) • Hard vs Soft Magnets large coercivity --good for perm magnets --add particles/voids to make domain walls hard to move (e.g., tungsten steel: Hc = 5900 amp-turn/m) Hard Hard Applied Magnetic Soft Field (H) small coercivity--good for elec. motors (e.g., commercial iron 99.95 Fe) B • Process: Adapted from Fig. 20.14, Callister 7e. Applied Magnetic Field (H) 1. initial (unmagnetized state)

  9. Magnetic Storage recording medium recording head Adapted from Fig. 20.23, Callister 7e. (Fig. 20.23 from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.) -- Particulate: needle-shaped g-Fe2O3. +/- mag. moment along axis. (tape, floppy) --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10 - 30 nm! (hard drive) Adapted from Fig. 20.25(a), Callister 7e. (Fig. 20.25(a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, 1993. ) Adapted from Fig. 20.24, Callister 7e. (Fig. 20.24 courtesy P. Rayner and N.L. Head, IBM Corporation.) ~2.5mm ~120nm • Information is stored by magnetizing material. • Head can... -- apply magnetic field H & align domains (i.e., magnetize the medium). -- detect a change in the magnetization of the medium. Image of hard drive courtesy Martin Chen. Reprinted with permission from International Business Machines Corporation. • Two media types:

  10. Superconductivity • Tc= temperature below which material is superconductive = critical temperature Hg Copper (normal) 4.2 K Adapted from Fig. 20.26, Callister 7e.

  11. Limits of Superconductivity • 26 metals + 100’s of alloys & compounds • Unfortunately, not this simple: Jc = critical current density if J > Jc not superconducting Hc= critical magnetic field if H > Hcnot superconducting Hc= Ho (1- (T/Tc)2) Adapted from Fig. 20.27, Callister 7e.

  12. Advances in Superconductivity • This research area was stagnant for many years. • Everyone assumed Tc,max was about 23 K • Many theories said you couldn’t go higher • 1987- new results published for Tc > 30 K • ceramics of form Ba1-x Kx BiO3-y • Started enormous race. • Y Ba2Cu3O7-x Tc = 90 K • Tl2Ba2Ca2Cu3Ox Tc = 122 K • tricky to make since oxidation state is quite important • Values now stabilized at ca. 120 K

  13. Meissner Effect • Superconductors expel magnetic fields • This is why a superconductor will float above a magnet normal superconductor Adapted from Fig. 20.28, Callister 7e.

  14. Type I M Type II HC HC1 HC2 H complete diamagnetism normal mixed state Current Flow in Superconductors • Type I current only in outer skin - so amount of current limited • Type II current flows within wire

  15. X X X X X X X X Superconducting Materials CuO2 planes Vacancies (X) provide electron coupling between CuO2 planes. linear chains Ba Ba Y (001) planes YBa2Cu3O7

  16. Summary • A magnetic field can be produced by: -- putting a current through a coil. • Magnetic induction: -- occurs when a material is subjected to a magnetic field. -- is a change in magnetic moment from electrons. • Types of material response to a field are: -- ferri- or ferro-magnetic (large magnetic induction) -- paramagnetic (poor magnetic induction) -- diamagnetic (opposing magnetic moment) • Hard magnets: large coercivity. • Soft magnets: small coercivity. • Magnetic storage media: -- particulate g-Fe2O3 in polymeric film (tape or floppy) -- thin film CoPtCr or CoCrTa on glass disk (hard drive)

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

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