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Week 14: Magnetic Fields Announcements

Week 14: Magnetic Fields Announcements. MatE 153, Dr. Gleixner 1. General Concept Behind Magnetism. Circulating current sets up a magnetic moment ( m m ) perpendicular to the current

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Week 14: Magnetic Fields Announcements

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  1. Week 14: Magnetic FieldsAnnouncements MatE 153, Dr. Gleixner 1

  2. General Concept Behind Magnetism • Circulating current sets up a magnetic moment (mm) perpendicular to the current • This results in a B field (magnetic field) that must terminate back on itself (all magnets must have a north & south, can’t be alone like electrical charge) • Electrons in atoms are the same concept- circulating charge that sets up a magnetic moment and a magnetic field • The magnetic field is due to both the orbital angular momentum and the spin • Only electrons in unfilled shells contribute to a net magnetization (as those in full shells will cancel each other out) MatE 153, Dr. Gleixner 2

  3. Comapring a Magnetic Moment with a Bulk Magnet *Used with permission from Kasap MatE 153, Dr. Gleixner 3

  4. Magnetism from an Orbiting Electron *Used with permission from Kasap MatE 153, Dr. Gleixner 4

  5. Remember L?? *Used with permission from Kasap MatE 153, Dr. Gleixner 5

  6. Spin Magnetic Moment *Used with permission from Kasap MatE 153, Dr. Gleixner 6

  7. Remember S?? *Used with permission from Kasap MatE 153, Dr. Gleixner 7

  8. Net Magnetization • Net magnetization is due to magnetic moments from both forms of angular momentum • However, only electrons in un-filled shells contribute to an overall magnetic moment MatE 153, Dr. Gleixner 8

  9. Average Magnetic Moment for S shell • Consider the example of an unfilled s shell • In an applied magnetic field, m spin can not align with B because S is space quantized. • The torque that results cause the spin magnetic moment to precess about B. MatE 153, Dr. Gleixner 9

  10. Bohr Magneton • Each spin magnetic moment (ms) contributes a average magnetic moment on the z axis in the presence of a magnetic field MatE 153, Dr. Gleixner 10

  11. Important Macroscopic Magnetism Terms • Bo • mo • H MatE 153, Dr. Gleixner 11

  12. Important Macroscopic Magnetism Terms • M • B • Cm MatE 153, Dr. Gleixner 12

  13. Solenoid With and Without Magnetizable Material *Used with permission from Kasap MatE 153, Dr. Gleixner 13

  14. A Look at Where M Comes From *Used with permission from Kasap MatE 153, Dr. Gleixner 14

  15. Types of Magnets • The M that results from the applied field is a function of the material in the core • The material types can be divided into several main categories • Diamagnetism • Paramagnetism • Ferromagnetism • Antiferromagnetism • Ferrimagnetism MatE 153, Dr. Gleixner 15

  16. Diamagnetism vs Paramagnetism • Diamagnetic • Paramagnetic MatE 153, Dr. Gleixner 16

  17. Diamagnetism *Used with permission from Kasap MatE 153, Dr. Gleixner 17

  18. Paramagnetism *Used with permission from Kasap MatE 153, Dr. Gleixner 18

  19. Ferromagnetic • Posses magnetization even without the presence of an applied field • Exists up to TC the Curie temperature • Only certain materials are ferromagnetic • criteria 1 is that there is an unfilled shell • criteria 2 is that there is a positive exchange energy MatE 153, Dr. Gleixner 19

  20. Ferromagnetism *Used with permission from Kasap MatE 153, Dr. Gleixner 20

  21. Curie Temperature of Ferromagnets *Used with permission from Kasap MatE 153, Dr. Gleixner 21

  22. Exchange Energy • Eex=-2JeS1S2 • S1 and S2 are spins of electrons • Je is negative for most materials • So the exchange energy is negative (minimum) if the spins are misaligned • For Fe, Co, Ni • Je is positive • So the exchange energy Eex is negative (minimum) if the spins are aligned • Fe, Co, Ni most common examples of ferromagnets MatE 153, Dr. Gleixner 22

  23. Exchange Energies of Different Materials *Used with permission from Kasap MatE 153, Dr. Gleixner 23

  24. Antiferromagnetism • Individual atoms bond as to give no magnetic moment even in the presence of a field due to the crystal structure • Exists only below Neel Temperature: TN MatE 153, Dr. Gleixner 24

  25. Antiferromagnetism *Used with permission from Kasap MatE 153, Dr. Gleixner 25

  26. Ferrimagnetism • Results in a net magnetization even when there is no applied field (similar to ferromagnetic). • It comes from opposite magnetizations in crystal structure of differing magnitudes resulting in a net permanent magnetization in one direction . MatE 153, Dr. Gleixner 26

  27. Ferrimagnetism *Used with permission from Kasap MatE 153, Dr. Gleixner 27

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