Magnetic-Properties.pdf

Chapter 20: 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 beha...

Chapter 20: 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? Chapter 20 - 1  Generation of a Magnetic Field -- Vacuum 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 I (tesla) Computation of the applied magnetic field, H: NI H Computation of the magnetic flux density in a vacuum, B0: B0 = 0H  permeability of a vacuum (1.257 x 10-6 Henry/m) Chapter 20 - 2  Generation of a Magnetic Field -- within a Solid Material A magnetic field is induced in the material B B = Magnetic Induction (tesla) applied inside the material magnetic field H B = H permeability of a solid current I  Relative permeability (dimensionless) r  0 Chapter 20 - 3  Generation of a Magnetic Field -- within a Solid Material (cont.) Magnetization M = cmH Magnetic susceptibility (dimensionless) B in terms of H and M B = 0H + 0M Combining the above two equations: B = 0H + 0 cmH B cm > 0 = (1 + cm)0H vacuum cm = 0 permeability of a vacuum: (1.26 x 10-6 Henry/m) cm < 0 cm is a measure of a material’s magnetic response relative to a H vacuum Chapter 20 -  Origins of Magnetic Moments Magnetic moments arise from electron motions and the spins on electrons. magnetic moments electron electron nucleus spin Adapted from Fig. 20.4, Callister & Rethwisch 8e. electron orbital electron motion spin Net atomic magnetic moment: -- sum of moments from all electrons. Four types of response... Chapter 20 - 5  Types of Magnetism (3) ferromagnetic e.g. Fe3O4, NiFe2O4 (4) ferrimagnetic e.g. ferrite(), Co, Ni, Gd ( cm as large as 106 !) B (tesla) (2) paramagnetic ( cm ~ 10-4) e.g., Al, Cr, Mo, Na, Ti, Zr vacuum (cm = 0) (1) diamagnetic (cm ~ -10-5) e.g., Al2O3, Cu, Au, Si, Ag, Zn H (ampere-turns/m) Plot adapted from Fig. 20.6, Callister & Rethwisch 8e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister & Rethwisch 8e. Chapter 20 - 6  Magnetic Responses for 4 Types No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) opposing none (1) diamagnetic Adapted from Fig. 20.5(a), Callister & Rethwisch 8e. random aligned (2) paramagnetic Adapted from Fig. 20.5(b), Callister & aligned Rethwisch 8e. aligned (3) ferromagnetic Adapted from Fig. 20.7, Callister & (4) ferrimagnetic Rethwisch 8e. Chapter 20 - 7  Domains in Ferromagnetic & Ferrimagnetic Materials As the applied field (H) increases the magnetic domains change shape and size by movement of domain boundaries. B sat H H Adapted from Fig. 20.13, induction (B) Callister & Rethwisch H “Domains” with 8e. (Fig. 20.13 adapted Magnetic from O.H. Wyatt and D. aligned magnetic Dew-Hughes, Metals, Ceramics, and H moment grow at Polymers, Cambridge University Press, 1974.) expense of poorly aligned ones! H 0 Applied Magnetic Field (H) H=0 Chapter 20 - 8  Hysteresis and Permanent Magnetization The magnetic hysteresis phenomenon B Stage 2. Apply H, Stage 3. Remove H, alignment align domains remains! => permanent magnet! Adapted from Fig. 20.14, Callister & Rethwisch 8e. H Stage 4. Coercivity, HC Negative H needed to Stage 1. Initial (unmagnetized state) demagnitize! Stage 6. Close the Stage 5. Apply -H, hysteresis loop align domains Chapter 20 - 9  Hard and Soft Magnetic Materials Hard magnetic materials: B -- large coercivities -- used for permanent magnets -- add particles/voids to inhibit domain wall motion Soft -- example: tungsten steel -- Hc = 5900 amp-turn/m) H Soft magnetic materials: -- small coercivities -- used for electric motors -- example: commercial iron 99.95 Fe Adapted from Fig. 20.19, Callister & Rethwisch 8e. (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.) Chapter 20 - 10  Magnetic Storage Digitized data in the form of electrical signals are transferred to and recorded digitally on a magnetic medium (tape or disk) This transference is accomplished by a recording system that consists of a read/write head -- “write” or record data by applying a magnetic field that aligns domains in small regions of the recording medium -- “read” or retrieve data from medium by sensing changes in magnetization Fig. 20.23, Callister & Rethwisch 8e. Chapter 20 - 11  Magnetic Storage Media Types Hard disk drives (granular/perpendicular media): -- CoCr alloy grains (darker regions) separated by oxide grain boundary Fig. 20.25, Callister segregant layer (lighter regions) 80 nm & Rethwisch 8e. (Fig. 20.25 from -- Magnetization direction of each Seagate Recording grain is perpendicular to plane of Media) disk Recording tape (particulate media): Fig. 20.24, Callister ~ 500 nm ~ 500 nm & Rethwisch 8e. (Fig. 20.24 courtesy Fuji Film Inc., Recording Media Division) -- Acicular (needle-shaped) -- Tabular (plate-shaped) ferromagnetic metal alloy ferrimagnetic barium-ferrite particles particles Chapter 20 - 12  Superconductivity Found in 26 metals and hundreds of alloys & compounds Mercury Copper (normal) Fig. 20.26, Callister & 4.2 K Rethwisch 8e. TC = critical temperature = temperature below which material is superconductive Chapter 20 - 13  Critical Properties of Superconductive Materials TC = critical temperature - if T > TC not superconducting JC = critical current density - if J > JC not superconducting HC = critical magnetic field - if H > HC not superconducting  T 2  HC (T )  HC (0)1  2   TC   Fig. 20.27, Callister & Rethwisch 8e. Chapter 20 - 14  Meissner Effect Superconductors expel magnetic fields normal superconductor Fig. 20.28, Callister & Rethwisch 8e. This is why a superconductor will float above a magnet Chapter 20 - 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. Chapter 20 - 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 barium-ferrite in polymeric film (tape) -- thin film Co-Cr alloy (hard drive) Chapter 20 - 17  ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 20 - 18