Brown s Paradox; lots of questions

1. Brown�s Paradox; lots of questions???

2. Lots and lots of domains in Ferro- (or Ferri-) Magnets

4. Why do Domains Form?

5. Magnetic Domains In reality, a ferro- or ferrimagnet is comprised of many regions (�domains�) with mutual alignment of the individual atomic magnetic dipole moments. These domains are not necessarily aligned with respect to each other. Domain walls between the domains are characterized by a gradual transition from one orientation to the next. The overall magnetization of the material (M) is the vector sum of the magnetization vectors for all of the individual domains. If not magnetized, the overall magnetization is simply zero.

6. Domain Walls

7. Domain orientation (poling) Ferromagnets are simply considered to have extremely high and linear permeabilities (the same is true for susceptibilities). But, this simple picture ignores the domain structure of magnetic materials.

8. Magnetic Hysteresis Once a magnetic material is saturated, decreasing H again does not return M (or B) to the same position. This hysteresis in the magnetic response is related to a) the mechanism (the last domain switched may not be the first to switch back the other direction) b) drag of domain wall motion For no external magnetic field, a remanent induction (�Br) will remain. Some domains remain aligned in the �old� direction. A �negative� field, the �Coercive Field (�Hc),� must be applied to eliminate all Br. The opposite mechanism occurs for increasing the external field after total saturation in the reverse direction.

9. Partial hysteresis If the applied external field sweeps through a portion of the hysteresis loop, there will be some finite hysteresis in the B response even if the field does not reach the coercive field: due to the same mechanisms as cause hysteresis in general domain wall drag, and the order of domain reorientation.

10. Unmagnetized vs. magnetized H=External magnetic field (magnetic field strength). B=magnetic induction (magnetic flux density). �=permeability (depends on the material, often referred to in terms of the relative permeability or the susceptibility) This equation for the magnetic induction is explicitly for an unmagnetized ferromagnet. M=magnetization, representing the magnetic moments within a material in the presence of a magnetic field of strength H. Once the material has been poled, though, the equation must be modified. Br accounts for any remanent magnetic induction (domain orientation).

11. Magnet types Magnets are categorized depending on the shape of the magnetic hysteresis loop. Soft magnet = narrow in H Hard magnet = broad in H The area of the loop represents energy lost in moving the domain walls as the magnet is poled from one extreme to the other and back again. Energy may also be lost due to local electric currents generated within the material caused by the external field. AC electric field causes a magnetic field, and vice versa.

12. Soft magnets Strong induction for a relatively weak external field. High saturation field (Bs), High permeability (�), low coercive field (Hc) Therefore a low energy loss per poling cycle. Applied when rapid, lossless switching is required; usually subjected to ac magnetic fields: Transformer cores

13. Soft magnet optimization Saturation field is determined by the composition Coercivity is a function of structure (related to domain wall motion) For the best soft magnet, minimize defects such as particles or voids as they restrict domain wall motion. Lower energy loss per loop if non-conducting (no eddy currents). Form a solid solution such as Fe-Si or Fe-Ni to improve resistivity by a factor of 4 or 5 (from 1*10-7 to 4*10-7). Use a ceramic ferrite to improve resistivity by 10 to 14 orders of magnitude (insulators instead of metals). (MnFe2O4, ZnFe2O4: 2000), (NiFe2O4, ZnFe2O4: 107)

14. Hard magnets High saturation induction, remanence, and coercivity. High hysteresis losses It is �hard� to repole a hard magnet. Standard and high energy hard magnets. Standard are simple tungsten steel; FeNiCu alloys High energy hard magnets are 100 times �stronger.� SmCo5, Nd2Fe14B is the most common

15. Hard magnet optimization As for soft magnets, the microstructure is related to the energy required to move magnetic domains and thus how �hard� the magnet is. Now, though, we want a wide hysteresis loop so we may want to: Introduce defects such as second phase particles. Optimize size, shape, and orientation of crystallites in a polycrystalline magnet. Have a conducting material (eddy current losses).

16. Magnetic hard drives The magnetic disk has a soft magnet (easy to pole and repole with little energy loss. The read/write head is a hard magnet, or an electromagnet. Concept is the same as for an audio tape or video tape. Magnetics have thus far ruled for computer hard drives. Flash (solid state, Si based) is coming on strong Ferroelectrics are also increasingly being applied Thermo-mechanical methods may also be used in the future

17. Microstructure Magnetic recording media used to include needle shaped particles. Now, extremely flat thin films are used to diminish surface roughness.

18. Magnetic Storage Media

19. More magnetic domains