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CHAPTER 4: MAGNETIC PROPERTIES OF MATERIAL

CHAPTER 4: MAGNETIC PROPERTIES OF MATERIAL. Magnetic Resonance Imaging (MRI). http:// www.youtube.com/watch?v=1EuyZ5Lml4k https:// www.youtube.com/watch?v=OmMuGr0UFbs. (4.1) Basic Concept. Electric Dipoles. Magnetic Dipoles. Magnetic dipole (south & North).

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CHAPTER 4: MAGNETIC PROPERTIES OF MATERIAL

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  1. CHAPTER 4: MAGNETIC PROPERTIES OF MATERIAL

  2. Magnetic Resonance Imaging (MRI)

  3. http://www.youtube.com/watch?v=1EuyZ5Lml4k https://www.youtube.com/watch?v=OmMuGr0UFbs

  4. (4.1) Basic Concept Electric Dipoles Magnetic Dipoles Magnetic dipole (south & North). • Electric dipole (+ve & -ve charge).

  5. Comparison Dielectric vs Magnetic

  6. MAGNETIC FIELD • Magnetic field lines of forcearound: • a current loop and • a bar magnet

  7. MAGNETIC MOMENT

  8. ORIGINS OF MAGNETIC MOMENTS • The macroscopic magnetic properties of materials are a consequence of magnetic moments associated with individual electrons. • Electrons produce magnetic moments : -- Orbital motion of the electron (orbital magnetic moment) -- Spin of electron (spin magnetic moment) • Net magnetic moment: -- sum of moments from all electrons.

  9. Magnetic elements : == nickel, chromium, iron, cobalt, manganese, and etc. • The origin of atomic magnetic moments is the incomplete cancellation of electronic magnetic moments. • Electron spin and orbital motion both as source of magnetic moments. • If the cancellation of electronic moments is incomplete then the atom has a net magnetic moment.

  10. But in most atoms the electronic moments are oriented so that they cancel giving no net atomic magnetic moment, leading to diamagnetism. • The spin moment of an electron with spin up will cancel that of one with spin down. Material that has zero magnetic moment : • materials composed of atoms that having completely filled electron subshell. • Inert atom (He, Ne, Ar, etc). • Ionic material (having filled subshell).

  11. MAGNETIC FIELD STRENGTH, H • Magnetic field strength (H) is produced by current-carrying conductor. • Unit = Amp-turn/m • Magnetic Flux density (B) is new additive magnetic field once the bar inside solenoid get magnetized. • Unit = Tesla

  12. A cylindrical coil/solenoid consists of N turns, length (l) with magnitude current (I ) generate the magnetic field, then: • Magnetic field strength, H within a solenoid: H = applied magnetic field units = (ampere-turns/m). I = current. L = length of solenoid. N = number of turns.

  13. Vacuum condition Non-Vacuum condition Magnetic flux density, Magnetic flux density, μ = permeability (property of the specific medium). μo = permeability of vacuum (4π x 10 -7 H/m)

  14. RELATIVE PERMEABILITY, μr • μr is relative permeability. • μrof a material is a measure of the degree to which the material can be magnetized / or how ease the B field can be induced in the presence of an external H field. • μ is magnetic permeability. • μis the measure of the ability of a material to support the formation of a magnetic field within itself.

  15. MAGNETIZATION, M (Magnetization) • M is the induced magnetic moment per unit volume of a material when placed in a magnetic field. • It is used to quantify how "magnetized" is a material Xm = magnetic susceptibility

  16. MAGNETIC SUSCEPTIBILITY, Xm Magnetic Susceptibility • is a dimensionless proportionality constant . • indicates the degree of magnetization of a material in response to an applied magnetic field. • describes how “easily to being magnetized” that material is when placed in a magnetic field. • Xm < 0 : diamagnetic response Xm > 0 : paramagnetic response

  17. Room-Temperature Magnetic Susceptibilities for Diamagnetic and Paramagnetic Materials

  18. (4.2) TYPES OF MAGNETISM (3) ferromagnetic e.g. Fe3O4, NiFe2O4 ferrimagnetic e.g. ferrite(), Co, Ni, Gd 6 c ( mas large as 10 ) -4 c (2) ( m ~ 10 ) paramagnetic e.g., Al, Cr, Mo, Na, Ti, Zr c vacuum ( = 0) -5 (1) diamagnetic ( m ~ -10 ) e.g., Cu, Au, Si, Ag, Zn Magnetic induction B (tesla) c Strength of applied magnetic field (H) (ampere-turns/m)

  19. (2) paramagnetic random aligned (3) ferromagnetic ferrimagnetic aligned aligned MAGNETIC MOMENTS FOR 3 TYPES No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) (1) diamagnetic opposing none

  20. (1). DIAMAGNETISM - Weak magnetism that is non permanent moment and persist only while an external field applied.- Magnetic moment is extremely small, and in a direction opposite to that of the applied field.- Relative permeability : µr < 1 (less than vacuum)- Magnetic susceptibility : Xm < 0 (negative value) No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) (1) diamagnetic opposing none

  21. (2). PARAMAGNETISM No Applied Applied Magnetic Field (H = 0) Magnetic Field (H) - When H = 0, each atomic possesses a permanent moment, randomly oriented.- while an external field applied, these magnetic moments preferentially align by rotation.- Relative permeability :µr > 1 (greater than vacuum) .- Magnetic susceptibility :Xm > 0 (positive value) (1) diamagnetic opposing none

  22. DIAMAGNETISM AND PARAMAGNETISM • Schematic representation of the flux density B versus the magnetic field strength H for diamagnetic and paramagnetic materials. • Diamagnetism and paramagnetism materials are considers to non magnetic because they exhibit magnetization only when in the presence of an external field.

  23. (3) ferromagnetic ferrimagnetic aligned aligned (3). FERROMAGNETISM • - Possess a permanent magnetic moment in the absence of an internal field due to electron spin – uncancelled electron spin and small contribution of orbital magnetic moment. • Manifest very large and permanent magnetization. • Magnetic susceptibilities (Xm) : much higher.

  24. (4.3) DOMAINS AND HYSTERESIS • Schematic depiction of domains in a ferromagnetic or ferrimagnetic material. • Arrows represent atomic magnetic moment. • Within each domain, all moments are aligned, whereas the direction of alignment varies from one domain to another.

  25. DOMAINS AND HYSTERESIS The gradual change in magnetic moment orientation across a domain wall

  26. H H H H H H = 0 DOMAINS AND HYSTERESIS • As the applied field (H) increases, the domains that are oriented in directions favorable to (nearly aligned with) the H grow at expense of poorly oriented. B sat • “Domains” with aligned magnetic induction (B) Magnetic moment grow at expense of poorly aligned ones! 0 Applied Magnetic Field (H)

  27. DOMAINS AND HYSTERESIS Magnetic flux density (B) versus the magnetic field strength (H) for a ferromagnetic material that is subjected to forward and reverse saturations (points S and S’). The hysteresis loop is represented by the solid red curve; the dashed blue curveindicates the initial magnetization. The remanenceBr and the coercive force Hc are also shown.

  28. DOMAINS AND HYSTERESIS • Hysteresis effect is produced in which the B fields lags behind the applied H field, or decreases at a lower rate. • At field H =0 (point R on the curve), there is a residual B field called the remanenceor remanent flux density, Br. • At Br , the material remains magnetized in the absence of an external H field.

  29. DOMAINS AND HYSTERESIS • To reduce the B field , an H field of magnitude -Hc must be applied opposite to the original field; Hc = coercivity or coercive force. • Large coercivity, good for perm magnets -- add particles/voids to make domain walls hard to move. • Small coercivity -- good for elec. Motors.

  30. (4.4) SOFT VS HARD MAGNETS

  31. SOFT MAGNETS • Easily magnetized and demagnetized. • Free of structural defects, produce small coersive force (Hc). • Due to the easy movement of domain wall as the magnetic field change magnitude /direction. • High initial permeability (μ) • Low coercive (Hc). • Low hysteresis energy losses.

  32. Soft magnetic materials is easily magnetized and demagnetized. • Suitable used in devices that are subjected to alternating magnetic fields. Application: • Core for distribution power transformers. • Small electronic transformers • Stator and rotor materials for motors/ generators. • Dynamos, and switching circuits

  33. HARD MAGNETS • Difficult demagnetized. • Has structural defects, produce large coercive (Hc). • Structural defects (particles of non-magnetic phase, voids in the magnetic material) tend to restrict the motion of domain walls, and thus increase the coersive (Hc). • Low initial permeability (μ). • High coercive (Hc). • High Remanence (Br ). • High hysteresis energy losses.

  34. Hard magnet is difficult demagnetized. • Suitable used for applications requiring permanent magnet Application: • Permanent magnet in laud speaker/ telephone receivers/ synchronous/ brushless motors/ automotive starting motors.

  35. 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.

  36. MAGNETIC FIELD VECTORS Magnetic Units and Conversion Factors for the SI and cgs–emu Systems

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