Ch18 - Magnetic Materials

CHAPTER18 Magnetic Materials
Each of these steel billets is being heated uni-
formly in a cost-effective way by two adjacent
coils which utilize an oscillating electrical cur-
rent to produce an oscillating magneticflux and
the resulting heating due to the“hysteresis”of
a ferromagnetic core material.(Courtesy of
CoreFlux Heating Systems.)
Figure18-1A simple illustration of magnetism shows the magneticfield (seen as magneticflux lines)gen-erated around an electrical current
loop.
Figure18-2A magnetic material can generate a magneticfield without an electrical current.This simple
bar magnet is an example.
Figure18-3Attraction of two adja-cent bar magnets.
Figure18-4Comparison of diamagnetism and paramagnetism on a plot of induction(B)versus magneticfield strength(H).Neither of these phe-nomena is of practical engineering importance due to the modest level of
induction that can be generated.
Figure18-5In contrast to Figure18–4,the B-H plot for a ferromagnetic material indicates substantial utility for engineering applications.A large rise in B oc-curs during initial magnetization(shown by the dashed line).The induction reaches a large,“saturation”value(B s)upon application of sufficientfield strength. Much of that induction is retained upon removal of thefield(B r=remanent induction).A coercivefield(H c)is required to reduce the induction to zero.
By cycling thefield strength through the range indicated,the B-H plot continu-
ously follows the path shown as a solid line.This is known as a hysteresis loop.
Element Sc Ti V Cr Mn Fe Co Ni Cu
29
2827262524232221123554320
= electronic spin orientation
Atomic number
Figure 18-6The electronic structure of the 3d orbital for transition metals.Un-paired electrons contribute to the magnetic nature of these metals.
Figure18-7The alignment of magnetic moments for adjacent atoms leads to the large net magnetic moment(and B s on a B-H plot)for the bulk solid. The example here is pure bcc iron at
room temperature.
Figure18-8The domain structure of an unmagnetized iron crystal gives a net B=0even though individual domains have the large magnetic
moment indicated by Figure18–7.
H
Figure18-9The sharp rise in B during initial magnetization is due to do-main growth.
Figure18-10The domain,or Bloch,wall is a narrow region in which atomic moments change orientation by180◦.Domain wall motion (implied in Figures18–8and18–9)simply involves a shift in this
reorientation region.No atomic migration is required.
Figure18-11Summary of domain microstructures during the course of a ferromag-netic hysteresis loop.
H (104
A/m)
B (web/m 2
)
magnets.
B , w e b e r /m e t e r 2
Figure 18-14A comparison of initial magnetization for three ferrous alloys.The silicon addition increases magnetic permeability and,consequently,B s .Preferred orienta-tion,or texturing,increases initial magnetization substantially (see Figure 18–13).(From R.M.Rose,L.A.Shepard,and J.Wulff,The Structure and Properties of Materials ,Vol.4:Electronic Properties ,John Wiley &Sons,Inc.,New York,1966.)
H (BH
Figure18-15Replotting data in the“demagnetization quadrant”of the hys-teresis loop demonstrates a maximum value of the|BH|product,(BH)max. This quantity is a convenient measure of the“power”of permanent mag-
nets.Table18.3gives(BH)max for various hard magnets.
B (web/m 2)
|BH |
10
5
00.30.6
kJ m 3
΂΃
Figure18-16Schematic illustration of magnetic storage and retrieval using a recording medium composed of needlelike particles ofγ-Fe2O3.(From J.
U.Lemke,MRS Bulletin,15,31[1990].)
Figure18-17Schematic illustration of a thinfilm of a metal alloy,Sendust (FeAlSi),on the pole faces of the ferrite recording head in a metal-in-gap or MiG design.(From A.S.Hoagland and J.E.Monson,Digital
Magnetic Recording,2nd ed.,Wiley-Interscience,New York,1991.)
0400
30020010002040
T (K)60C r i t i c a l f i e l d (A -t u r n s /m )80YBa 2Cu 3O 7(La, Sr)2CuO 4
Nb 3Ge
100
Figure 18-18Comparison of the critical magnetic field versus temperature for a metallic superconductor (Nb 3Ge)and two ceramic superconductors.。