3. Temperature dependence T C or T n Above a critical temperature called the Curie point ( TC ), ferro- and ferrimagnetic materials no longer possess a spontaneous magnetization. They become PARAMAGNETIC . So do anti-ferromagnetic materials. ferromagnetic anti-ferromagnetic ferrimagnetic T=0K paramagnetic

4. Lots and lots of domains… Domains form for a reason in ferro- and ferrimagnetic materials. They are not random structures.

5. Brown’s Paradox; lots of questions??? Take a simple iron needle. Iron is ferromagnetic , it should possess a spontaneous magnetization. The name ferromagnetic means magnetic like iron . It should attract another iron needle depending on the orientation of the magnetization vector. But it does not; Brown’s Paradox . Why??? Only if the “magnetic” iron is magnetized by a permanent magnet or an electromagnet, it will attract other pieces of iron . But this attraction disappears in a short while. Why??? How come lodestone (Fe 3 O 4 ) can stay “magnetic” for much longer times?

6. Why do Domains Form? H D H D M S Domains form to minimize (and in some cases to completely eliminate) demagnetization fields ( H D ). They are not random structures.

8. Domain Walls Bloch Wall Bloch Wall and N è el Wall

20. Magnetic Storage Media Bits on magneto-optical disk. Topography reveals grooves that delineate tracks. MFM shows written bits as well as finer domain structure in un-aligned grooves. 5µm scan. Digital Instruments 25 µm scan of magnetic domains in three topographically identical regions of 50 nm thick Permalloy film (used for read heads). William Challener, 3M Corporation

21. More magnetic domains http://www.veeco.com/nanotheatre/nano_view.asp?CatID=3&page=2&recs=20&CP=# antiferromagnetically coupled [Co/Pt/Ru] multilayer Magneto-optical: DVD-RW terfenol

22. Many thanks to Prof. Barry Wells, UConn-Physics.

23. WHAT IS SUPERCONDUCTIVITY?? For some materials, the resistivity vanishes at some low temperature: they become superconducting . Superconductivity is the ability of certain materials to conduct electrical current with no resistance. Thus, superconductors can carry large amounts of current with little or no loss of energy. Type I superconductors: pure metals, have low critical field, sudden transition from super to normal conductivity. Type II superconductors: primarily of alloys or intermetallic compounds, gradual transition from super to normal.

24. HISTORY

26. APPLICATIONS: Power Superconducting Transmission Cable From American Superconductor. The cable configuration features a conductor made from HTS wires wound around a flexible hollow core. Liquid nitrogen flows through the core, cooling the HTS wire to the zero resistance state. The conductor is surrounded by conventional dielectric insulation. The efficiency of this design reduces losses.

28. MEISSNER EFFECT B T >T c T < T c B When you place a superconductor in a magnetic field, the field is expelled below T C . Magnet Superconductor Below T C , the superconductor is diamagnetic, so fields within it are opposite to that of the magnetic field to which it is exposed.

29. A superconductor displaying the MEISSNER EFFECT If the temperature increases the sample will lose its superconductivity and the magnet cannot float on the superconductor .

30. APPLICATIONS: Superconducting Magnetic Levitation The Yamanashi MLX01MagLev Train The track are walls with a continuous series of vertical coils of wire mounted inside. The wire in these coils is not a superconductor. As the train passes each coil, the motion of the superconducting magnet on the train induces a current in these coils, making them electromagnets. The electromagnets on the train and outside produce forces that levitate the train and keep it centered above the track. In addition, a wave of electric current sweeps down these outside coils and propels the train forward.

31. 1-2-3 Superconductors (YBa 2 Cu 3 O 7-x )

33. Magnet type review large coercivity --good for perm magnets --add particles/voids to make domain walls hard to move (e.g., tungsten steel: H c = 5900 amp-turn/m) • Hard vs Soft Ferro or Ferri-magnets small coercivity--good for elec. motors (e.g., commercial iron 99.95 Fe) and hard drive media. --remove defects to make domain wall motion as easy as possible. Adapted from Fig. 20.16, Callister 6e . (Fig. 20.16 from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering , John Wiley and Sons, Inc., 1976.)