7) Hard and Soft magnetic materials

Hard materials

high coercivity

Soft materials

low coercivity

Applications of hard magnets

permanent magnets for energy generation/recovery, electric motors

Permanent magnets

create magnetic field without external energy input; magnet that produces a stronger field also produces a strong demagnetising field; if Hd>Hc in the material then it won’t be stable in a magnetised state; soft materials have Hc<10kA/m

Properties for a good permanent magnet

high magnetic fields: high Ms/Mr; while resisting it own demagnetising field ie want high Hc

Figure of merit

2nd quadrant of B-H loop (top left); FoM is |BH|_max (the energy product); measures the energy stored in the magnetic material (approximates area under hysterisis curve)

What is a B-H loop

combines a MH loop (normal hysterisis plot) with HH plot (straight line); shape essentially the same, same Hc; remanence shifts by μ_0

How do we get high Hc and Mr

use anisotropy energy; older materials needed shape anisotropy to stabilise magnetism but newer better materials with stronger magnetocrystalline anisotropy can be made into any shape

Rare earth magnets

combine 4f RE metals (large anisotropy) with 3d metal (large magnetic moment) produces high BH_max; eg Sm-Co or Nd-Fe-B

Why do REMs show strong anisotropy

spin orbit coupling: e sees positive nucleus spinning around it producing magnetic field that couples with the electron moment; effect increases with atomic charge as Z^4; RE have large Z therefore strong spin orbit coupling = strong anisotropy

Temperature dependence

only useful operation below Tc (paramagnetic above) as higher T helps magnetic switching; Nd-Fe-B has much lower Tc than Sm-Co (312 vs 723K) limiting its applications

Making Nd-Fe-B magnets

mix single crystal powders and press into mould; apply field to align magnetisation; sinter in a vacuum at 1100C; machine and then magnetise

Structure of Nd2Fe14B

tetragonal structure with strong anisotropy along c axis; presinter field aligns c axis of each crystal giving final product strong anisotropy

Phases in Nd-Fe-B

dominant 2-14-1 phase has strong anisotropy and high magnetisation due to ferromagnetic Nd-Fe coupling; other Nd rich phases form 1nm thick layers at 2-14-1 grain boundaries and fill triple points: non magnetic and decouples the magnetic grains (reducing cooperative switching) and pins domain walls increasing coercivity

Dysprosium, Dy

often added to increase anisotropy (and therefore Hc) and increases Tc; BUT: reduces Ms due to Dy coupling antiferromagnetically with Fe; cost of Dy is also major issue

Applications of soft magnetic materials

respond quickly to magnetic field: electromagnets, magnetic shielding, transformers, sensor

Electromagnets

soft magnetic core is magnetised by solenoid increasing the field strength; magnetic field generated using current

Transformer

changing magnetic fields to produce current; voltage ratio given by V1/V2 = N1/N2

Magnetic field screening

Maxwell: B field lines have to be continuous; but high χ material can draw B field lines into it

Materials for electromagnets

want soft magnet with high Ms, high χ, low Mr and low Hc

soft iron (with low C content): high Ms (1700kA/m), low Hc and low Mr

Materials for transformers

want: high χ, low Hc, high resistivity;

Fe-3% Si has good properties: resistivity increases with Si content but above 3% becomes too brittle; laminate material with epoxy to reduce eddy currents

How does the frequency affect the material in a transformer

higher frequency increases Hc (greater hysterisis losses) and creates large dB/dt so greater eddy current loss: need higher resistivity;

use ferrite materials eg MnZn-ferrite: decreases Ms 10x (require more turns or bigger core) but increases resistance by several OoM greatly reducing eddy current losses

Materials for magnetic field screening

want highest suscepitibility (particularly initial χ); need to minimise sources of anisotropy, low K and low saturation constant λ_s to prevent strain induced anisotropy; Ni80Fe20 very popular

How does nanostructure affect Hc

Hc decreases with larger grain size as there is less domain wall pinning at GBs (fewer defects)

Why does Hc suddenly drop for very small grains

exchange coupling can overpower intrinsic magnetocrystalline anisotropy of individual grains; coupling dominates over lengths smaller than the exchange length

Nanocrystallin/amorphous alloys

K averaged out over exchange length proves sharp crash in average K as grain size decreases so Hc drops; very low coercivity in these materials (softest materials we can make); eg Metglas and Finemet; created via rapid solidification processes