ENGR-45_Lec-12_Magnetic_Prop.ppt 1 Bruce Mayer, PE Engineering-45: Materials of Engineering Bruce Mayer, PE Licensed Electrical & Mechanical Engineer Engineering 45 Magnetic Properties

ENGR-45_Lec-12_Magnetic_Prop.ppt 2 Bruce Mayer, PE Engineering-45: Materials of Engineering Learning Goals – Magnetic Props  How to measure magnetic properties  Atom-scale sources of magnetism  How to Classify Magnetic Materials

ENGR-45_Lec-12_Magnetic_Prop.ppt 3 Bruce Mayer, PE Engineering-45: Materials of Engineering Properties of Solid Materials  Mechanical: Characteristics of materials displayed when forces and or torques are applied to them.  Physical: Characteristics of materials that relate to the interaction of materials with various forms of energy.  Chemical: Material characteristics that relate to the structure of a material.  Dimensional: Size, shape, and finish

ENGR-45_Lec-12_Magnetic_Prop.ppt 4 Bruce Mayer, PE Engineering-45: Materials of Engineering Material Properties Chemical Physical Mechanical Dimensional Composition Melting Point Tensile propertiesStandard Shapes Microstructure Thermal Toughness Standard Sizes Phases Magnetic DuctilitySurface Texture Grain Size Electrical FatigueStability Corrosion Optical HardnessMfg. Tolerances Crystallinity Acoustic Creep Molecular Weight Gravimetric Compression Flammability

ENGR-45_Lec-12_Magnetic_Prop.ppt 5 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetic Field Strength –N  Number of Coils (turns) –I  Current (Amps) –L  Coil Length (m) I B where L  Consider a Tightly Coiled Wire Carrying an Electric Current, I  This SOLENOID Configuration Generates a Magnetic Field with Strength, H

ENGR-45_Lec-12_Magnetic_Prop.ppt 6 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetic Field Strength cont  Units for B = Tesla 1 Tesla = 1 Amp-Henry I B where –µ  the Magnetic PERMEABILITY (Henry per meter, or H/m) L Think: C = ε(A/L)  H has the unusual Units of Amp-Turns per Meter (A/m)  H induces Magnetic Flux, B as

ENGR-45_Lec-12_Magnetic_Prop.ppt 7 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetization  µ 0 is a Universal Constant µ 0 = 1.257x10 -6 H/m I B L  B = µH applies to the General Case where some Material Occupies the Center of the Coil  BaseLine Case → Coil in a VACUUM

ENGR-45_Lec-12_Magnetic_Prop.ppt 8 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetization cont.  The Presence of Material in the Coil-Core Changes the Magnetic Flux The Intensity of the Materal-Filled Coil Relative to the Baseline current I B = Magnetic Induction inside the material

ENGR-45_Lec-12_Magnetic_Prop.ppt 9 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetization cont.2  Alternatively Describe the Magnetic Field Strengthening by  m is A Unitless Material Property  So B Where M  MAGNETIZATION –A Material Property –Units → Amp/m  Define Magnetic SUSCEPTIBILITY  Then µ r vs  m

ENGR-45_Lec-12_Magnetic_Prop.ppt 10 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetic Susceptibility Origins  Measures the response of electrons to an Electric field  Electrons Produce Electric Fields Due to Orbit about a the Nucleus (a tiny current) Electron Spin (recall spin-↑ & spin-↓) magnetic moments electron nucleus electron spin  The Magnetic Analog to the Electronic charge, q, is the Bohr Magneton: µ B = 9.27x J/Tesla

ENGR-45_Lec-12_Magnetic_Prop.ppt 11 Bruce Mayer, PE Engineering-45: Materials of Engineering Types of Magnetism  Magnetism arises from e - Orbit & Spin NET MAGNETIC MOMENT = Sum of all individual Magnetic moments from both Orbit & Spin  The Form of the Magnetic Sums Yields Three Types of Magnetism DiaMagnetism –DECREASES B ParaMagnetism –Weakly Enhances B FerroMagnetism –STONGLY Enhances B

ENGR-45_Lec-12_Magnetic_Prop.ppt 12 Bruce Mayer, PE Engineering-45: Materials of Engineering The 3 Types of Magnetism Magnetic induction B (tesla) Strength of applied magnetic field (H) (ampere-turns/m) vacuum (  = 0) -5 diamagnetic (  ~ − 10 ) (1) e.g.,Al 2 O 3, Cu, Au, Si, Ag, Zn ferromagnetic e.g. Fe 3 O 4, NiFe 2 O 4 ferrimagnetic e.g. ferrite(  ), Co, Ni, Gd (3) (  as large as 10 6 ) (2) paramagnetic e.g., Al, Cr, Mo, Na, Ti, Zr (  ~ ) permeability of a vacuum: (1.26 x Henrys/m)

ENGR-45_Lec-12_Magnetic_Prop.ppt 13 Bruce Mayer, PE Engineering-45: Materials of Engineering DiaMagnetism  NonPermanent and Weak  m ~–10 -5 (recall Ni  600)  Exists Only When H-Field Applied Atoms have NO permanent Magnetic DiPoles When Field Applied, the Generated Dipoles COUNTER the Field → B<B 0 Thus – u r <1 (i.e., a %-age) –  m <0m (i.e., NEGATIVE)

ENGR-45_Lec-12_Magnetic_Prop.ppt 14 Bruce Mayer, PE Engineering-45: Materials of Engineering ParaMagnetism  Each Atom DOES Possess a Permanent DiPole  Atomic Dipoles are RANDOMLY Arranged  A “Chunk” of Material has NO Net Macroscopic Magnetism  However, Dipoles Do Align to an Applied Field, Strengthening it Yields a Small & Positive  m :

ENGR-45_Lec-12_Magnetic_Prop.ppt 15 Bruce Mayer, PE Engineering-45: Materials of Engineering FerroMagnetism  Material is Magnetic, Even withOUT an Applied H-Field Relatively Rare in Nature Large Susceptibility Caused by Parallel Alignment of Domains due to Coupled Spin Moments of UnPaired Electrons Yeilds Large & Positive  m : –Recall Field Strength Eqn

ENGR-45_Lec-12_Magnetic_Prop.ppt 16 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetization vs H  M for Ferromagnetics is a Function of the Applied Field, H  As H increases, B approaches a Maximum Value i.e., the Magnetization SATURATES  M sat for FerroMags Where –n B  Bohr Magnetons per atom –µ B  Bohr Mageton Value –N  atomic Density

ENGR-45_Lec-12_Magnetic_Prop.ppt 17 Bruce Mayer, PE Engineering-45: Materials of Engineering AntiFerroMagnetism  Atoms Contain a Permanent DiPole, but There are equal Quantities of Oppositely Directed Dipoles Therefore, the magnetic field cancels out and the material appears to behave in the same way as a paramagnetic material  Dipoles will Align somewhat to an Applied Field thus Yields a Small & Positive  m : –Similar to Paramagnetics

ENGR-45_Lec-12_Magnetic_Prop.ppt 18 Bruce Mayer, PE Engineering-45: Materials of Engineering FerriMagnetism  Arises in Ceramics Where The Oxidant (the metal) Exists in More than One Valence State  Example = Ferrite (Lodestone), Fe 3 O 4 Fe Exists in Two Valence States: +2 & +3 –The Fe DiValent:TriValent Ratio = 1:2 Note that The Divalent & TriValent Iron Ions have Bohr Magneton Ratios of 4 & 5 respectively O 2- ions are Magnetically Neutral

ENGR-45_Lec-12_Magnetic_Prop.ppt 19 Bruce Mayer, PE Engineering-45: Materials of Engineering FerriMagnetism cont  The Atoms in Lodestone Arrange in A Xtal Structure that can be Represented as  Note that Magnetic Spin Moments for the TRIVALENT ions Cancel X X X X X X  This Leaves a Net Ferrimagnetic Form due to the DiValent Fe 2+ ions

ENGR-45_Lec-12_Magnetic_Prop.ppt 20 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnet Types Summarized

ENGR-45_Lec-12_Magnetic_Prop.ppt 21 Bruce Mayer, PE Engineering-45: Materials of Engineering Temperature Affects  Even though electronic exchange forces in ferromagnets are very large, thermal energy eventually overcomes the exchange and produces a randomizing effect.  This occurs at a particular temperature called the Curie temperature (T C ).  Below the Curie temperature, the ferromagnet is ordered  Above T C the FerroMagnet is DISordered.  The saturation magnetization goes to zero at the Curie temperature. (ºC)

ENGR-45_Lec-12_Magnetic_Prop.ppt 22 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetic Domains  In Ferr(o/i)Magnetic Materials There Exist Small PHYSICAL volumes of Well Aligned Magnetic Dipoles called DOMAINS  The Domains are MicroScopic, and, in polyXtal Materials a Grain May contain More than one Domain  The Magnitude of M for a Macroscopic Piece of Material is the VECTOR sum of all Magnetization for all Domains This is a VOLUME-Weighted Integration

ENGR-45_Lec-12_Magnetic_Prop.ppt 23 Bruce Mayer, PE Engineering-45: Materials of Engineering Magnetization vs H Revisited Applied Mag Field (H) H H H H H H = 0 Magnetic induction (B) 0 B sat  c.f. cool photos on text pg W19  For Ferr(o/i)Magnetics as H↑ the Overall Dipole Alignment becomes Stronger.  In other Words, The Favorably-Aligned Domains GROW at the Expense of the less favorably Aligned Domains  Thus Domain Structure can overcome Grain (Physical) Structure

ENGR-45_Lec-12_Magnetic_Prop.ppt 24 Bruce Mayer, PE Engineering-45: Materials of Engineering Permanent Magnets  Permanent Magnets Exhibit Hysteresis in their B-H Curves Due to B-Lags in the Aligning & Unaligning processes An Outline of the Process 1.Initial (Unmagnetized) State 2.Apply H, Cause Domain Alignment & Growth 3.Bring H to Zero, some Alignment remains (Remnance, B r ). Have Permanent Magnet 4.To Reach B=0 Must Apply NEGATIVE H (Coercivity, H C ) Magnet is Still Perm. B Applied H Field

ENGR-45_Lec-12_Magnetic_Prop.ppt 25 Bruce Mayer, PE Engineering-45: Materials of Engineering Soft & Hard Magnet Materials  The Area within the B-H Curve is Proportional to Energy Absorbed by the Permanent Magnet This will be Dissipated as Heat  “Soft” Materials Have Small Hysteresis Areas, but Lower Magnetizations Good for Devices Where High H-Field Reversal Rates and hence Heat dissipation is as issue; e.g., Electric Motors  “Hard” have Large Hysteresis Areas, but Higher Remnance The High Remnance, and resistance to demagnetization, Makes Hard Materials well suited for Permanent Magnet applications

ENGR-45_Lec-12_Magnetic_Prop.ppt 26 Bruce Mayer, PE Engineering-45: Materials of Engineering B-H Energy Density  Consider a Hard Material Hysteresis Curve at right  The Area Under the Curve has units of Energy Density:  Now Mult by m 2 /m 2 :   (BH) max as measured in the 2nd Quadrant is the Industry Standard metric for Resistance by a PM to Demagnetization PM material Microstructure is adjusted to impede Domain Wall motion, enhancing (BH) max

ENGR-45_Lec-12_Magnetic_Prop.ppt 27 Bruce Mayer, PE Engineering-45: Materials of Engineering 9 Information is stored by magnetizing material. recording head Tape recording medium Simulation of hard drive courtesy Martin Chen. Reprinted with permission from International Business Machines Corporation. Head can... --apply magnetic field H & align domains (i.e., magnetize the medium). --detect a change in the magnetization of the medium. Two media types: --Particulate: needle-shaped  -Fe 2 O 3. +/- mag. moment along axis. (tape, floppy) --Thin film: CoPtCr or CoCrTa alloy. Domains are ~ 10-30nm! (hard drive) Adapted from Fig , Callister 6e. (Fig from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.) Adapted from Fig , Callister 6e. (Fig courtesy P. Rayner and N.L. Head, IBM Corporation.) Adapted from Fig (a), Callister 6e. (Fig (a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, ) Application  Magnetic Storage

ENGR-45_Lec-12_Magnetic_Prop.ppt 28 Bruce Mayer, PE Engineering-45: Materials of Engineering Summary  Magnetics  A magnetic field can be produced by Running 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 Mag-field are Ferri- Or Ferro-magnetic (Large Magnetic Induction) Paramagnetic (Poor Magnetic Induction) Diamagnetic (Opposing Magnetic Moment)

ENGR-45_Lec-12_Magnetic_Prop.ppt 29 Bruce Mayer, PE Engineering-45: Materials of Engineering Summary cont.  HARD magnets → LARGE Coercivity.  SOFT magnets → SMALL Coercivity.  Magnetic storage media: Particulate  -Fe 2 O 3 in Polymeric Film (Tape Or Floppy) Thin Film CoPtCr or CoCrTa On Glass or Aluminum Disk (Hard Drive)

ENGR-45_Lec-12_Magnetic_Prop.ppt 30 Bruce Mayer, PE Engineering-45: Materials of Engineering WhiteBoard Work - Magnetics 1.35  Remember the 1.35 Tesla Value A bar of an Fe-Si Allow has B-H characteristics shown at Left. A bar of this Material inserted into a wire coil 0.2 m long, and having 60 turns, thru which passes a current of 100 mA. For This arrangement: (a) What is the B-Field within the bar? (b) At this magnetic field find: 1) The Permeability 2) The Relative Permeability 3) The Susceptibility 4) The Magnetization