V. Soft & Hard Magnetic Materialsocw.snu.ac.kr/sites/default/files/NOTE/Lecture #10.pdf · 2018. 4....

Electronic Materials & Devices Laboratory Seoul National UniversityDepartment of Material Science & Engineering

V. Soft & Hard Magnetic Materials

(1) Introduction

(2) Soft Magnetic Materials

(3) Hard Magnetic Materials


(1) INTRODUCTION

Classification

Hard magnets : Hc > ~10 kA/m (~100 Oe), Soft magnets : Hc < ~1 kA/m (~10 Oe)

Originally for iron & steel (mechanically hard -> high Hc, mechanically soft -> low Hc )

Applications

Soft magnets : Mostly for electrical circuits for an amplification of the magnetic flux

General requirements : high permeability(μ), low coercivity(Hc), low hysteresis loss (WH)

Hard magnets : An energy storage device as permanent magnets generating a magnetic

field

General requirements : high coercivity(Hc), high remanence(Mr), high magnetic energy

{(BH)max, WH}


AC Losses

Wtot = WH + Wec + Wa

Where, WH = a hystereses loss, Wec = an eddy current loss, and Wa = an anomalous loss

WH : the energy loss per unit volume in one cycle = the area inside a B-H loop

Energy W dissipated in a toroidal core over one cycle = the integral of the power loss over a period:

WH =

Ampere's law and Faraday's law, V(t) = – dφ/dt = – AdB/dt

WH = iA∮H(t)dB : DC hysteresis loss

Wec : the power loss due to a current induced by ac field (dB/dt )

Classical eddy-current loss : the power loss per unit volume at low frequency for a uniform

magnetization

Wec Bmax2d2v2-1

Eddy-current loss due to wall motion : Wec WAB/d, where WAB is wall traveling distance during a

half-cycle

Wa : excessive measured loss over the classical loss due to a inhomogeneous magnetization change

near domain wall

Tt

tdttVti

0)()(

(1) INTRODUCTION


Operation of Permanent Magnets

- Demagnetization curve : useful to decide the suitability of a permanent magnet for particular applications

- Energy product = BH = μ0MH (see Fig. 13.2, 13.3 in Jiles)

Maximum energy product (BH)max: maximum amount

of useful work that can be performed by the magnet

- Load line: a line due to demagnetizing effect

Hd = – {Nd/μ0(1 – Nd)}B

- Optimum operation condition

For a given application (i.e., a given shape),

the most appropriate material is the one

with the largest value of BH along the load line.

For a given material, its shape should be made

for its load line to pass through (BH)max to optimize

its performance.

(1) INTRODUCTION


- Since the flux density B is dominated

by the contribution from M, permeability μ

is expected to be maximized

when anisotropic (Ki ≠ 0)

polycrystalline materials are textured.

B = μo(H + M) ≈ μoM : soft magnets

- The magnetization process

for a Fe single crystal

(see Fig. 10.1 in O’Handley)




Applications

i) DC Applications

① Electromagnets (Typically ~2.0 Tesla) ② Relays : Magnetic switches & control devices- Requirements : high μ, low Hc, high Ms(or Bs) - Requirements : low Hc, high μ, low Br- Typical Materials - Typical Materials

Soft iron (low-carbon steels, 35%

μr = 10,000, Hc ~ 80 A/m (1 Oe), Ms = 1.7×106 A/m 2V- Permendur

Major impurities(wt%)

0.02%C, 0.035%Mn, 0.025%S, 0.015% P, 0.002%Si

Removal of impurities in hydrogen atmosphere

μr ~ 100,000, Hc ~ 4 A/m (0.05 Oe),

Co-Fe alloys

35%Co-Fe: large Ms at room temp. for pole tips

Ms ~ 1.95×106 A/m

50%Co-Fe: larger Ms (Pemendur alloy)

2%V-49%Co-49%Fe (2V- Permendur)

If melted & magnetically annealed, called, "supermendur"


Applications

ii) AC Applications

① Transformers ③ Magnetic Shielding Materials- Requirements : low WH, high μ, high Ms (or Bs), (Shielding apparatus from stray magnetic fields)

low conductivity (σ): to reduce eddy current loss (Wec) - Typical Materials

- Typical Materials Permalloy (Fe-Ni alloys)

Fe, Ni, Co + (P, Si, B) Mumetal (Cu-addition)

Grain-oriented Si-Fe alloys: reduce σ to σ/4 5%Cu-75%Ni-Fe

→ ~10% decrease in Ms is not serious 2%Cr-5%Cu-77%Ni-Fe

Ni-Fe alloy (Permalloy)

Amorphous metal: rapid quenching ④ High Frequency ApplicationsMetglas alloys: Metglas 2605: Fe80B20 - Typical Materials

Metglas 2615: Fe80P16C3B Soft ferrites

Metglas 2826: Fe40Ni40P14B6 Mn-Zn ferrite (up to 500 kHz)

Metglas 2826A: Fe32Ni36Cr14P12B6 μi = 1,000-2,000, Hc< 1Oe, ρ = 20~100Ωcm

Metglas 2826B: Fe29Ni49P14B6Si2 Ni-Zn ferrite (up to 100 MHz)

(FexCo1-x)75Si15B10 μi = 10-1,000, Hc ~ several Oe, ρ = 105Ωcm

(FexNi1-x)78Si8B14 Garnets for microwave devices

② Motor & Generators YIG, RIG (R : Rare Earth elements)- Typical Materials

Non-oriented Si-Fe alloys, called, "electrical steel"



- Magnetic steels: the most common application as cores in power and distribution transformers

- Energy loss in these transformers : magnetic core loss + pure Joule heating loss in the copper

coils

- Sources of the core loss

(1) loss due to eddy currents induced in the core by the uniform changes in B

(2) microscopic eddy currents localized near moving domain walls

(3) acoustic losses due to magnetostrictive deformation of the core under changing flux in the so-

called supplementary domain structure (90o walls, closure domains)

- To minimize core loss

decreasing lamination thickness

increasing resistivity

decreasing domain size

decreasing magnetostriction

- To minimize the coil loss (= i2R)

higher remanence

lower magnetic anisotropy

better crystallographic alignment

low coil resistance

Iron and Magnetic Steels



Pure Fe (Major impurities (see Table 10.1))

- very high Bs (= 2.2 T)

- relatively small magnetocrystalline anisotropy, K1 = + 4.8×104 J/m3

- small magnetostriction constants, λ100 = + 21×10-6, λ111 = – 20×10

-6

To increase resistivity

- The addition of selected impurities to Fe : Si, Al,...

(see Fig. 10.2)

- Fe-Si phase diagram (see Fig 10.3)

- Variation of physical properties of Fe with Si content

(see Fig. 10.4)

- Comparison of physical properties of pure Fe with those

of 3%Si-Fe alloys (See Table 10.2)

To lower the core loss for motors and generators

- Si addition to Fe (see Fig. 10.5, 10. 6)

- The effects on core loss of increasing texture and permeability (Fig. 10.7)

- Control of grain size (see Fig. 10.8)

- Refinement of domain structure in textured Si-Fe

Achieving a small out-of-plane orientation of the [001] easy axis. (see Fig 10.9)

Increasing the number of domain walls by laser scribing the surface of steel

Iron and Silicon Steels



Variation of physical properties of Fe with Si content (see Fig. 10.4)

The core loss : Si addition to Fe (see Fig. 10.6)

Iron and Silicon Steels (continued)



The effects on core loss of increasing texture and permeability (Fig. 10.7)

Goss or cube-on-edge texture: rolled sheet containing a random number of {110} planes and the [001] direction

predominantly along the roll direction after annealing at 800oC, resulting in a low field magnetization along the field (roll)

direction

Control of grain size (see Fig. 10.8)

If too large, there are fewer domain walls and micro-eddy-current loss is high.

If too small, the internal stresses and abundant grain boundary pinning sites increase the loss.





Refinement of domain structure in textured Si-Fe

Achieving a small out-of-plane orientation of the [001] easy axis. (see Fig 10.9)

Increasing the number of domain walls by laser scribing the surface of steel



The paths of the zero anisotropy and magnetostriction lines in Fe1-x-ySixAly (see Fig. 10.10)

The composition of sendust : 10% Si, 5% Al, 85% Fe

The permeability peaks near this zero-anisotropy and zero-magnetostriction composition (see Fig 10.11)

Application : some magnetic recording heads (because of mechanical hardness and magnetic softness)

Sendust : named by researchers at Tohoku Univ., Japan because it can be used in powder of dust form



- Three major Fe-Ni compositions

78% Ni-Fe permalloys (e.g., Supermally, Mumetal, Hi-mu 80)

Both magnetostriction and magnetocrystalline anisotropy

pass through zero near this composition.

Application : devices for the highest initial permeability

65% Ni-Fe permalloys (e.g., A alloy, 1040 alloy)

Strong response to field annealing while maintaining K1≈ 0

50% Ni-Fe permalloy (e.g, Deltamax)

Higher flux density (Bs = 1.6 T) as well as their responsiveness

to field annealing, which gives a very square loop

- Variation of Ms, Tc, K, and λ with Ni content in the FCC

Fe-Ni alloys (see Fig. 10.13)

- Zero-magnetostriction compositions (homework)

Fe-Ni Alloys (Permalloys)



Magnetic properties of BCC Fe-Co alloys

(see Fig. 10.17)

- Very high saturation induction (Bs ≈ 24 kG)

- Relatively low magnetic anisotropy: K1 (disordered) ≈ - 1×105 J/m3 and K1 (ordered) ≈ 0

- Substantial magnetostriction: λ100 = +150×10-6, λ111 = 25×10

-6

polycrystalline average, λs ≈ 60×10-6

- The anisotropy (including stress-induced) sets

the upper limit for the permeability

the lower limit for the coercivity

- The primary factor determining the technical

magnetic properties : grain size

- Order-disorder transformation to CsCl structure

(below 730℃): the anisotropy, magnetostriction,and mechanical properties depend strongly on

annealing and on cooling rates.

- The addition of V

(V-permendur or Supermendur: Fe49Co49V2)

(see Table 10.3)

- Applications: transformers and generators

on aircraft power systems

Fe-Co Alloys (Permendur)



- Without long-range order, amorphous alloys rapidly quenched from the melt have no magnetocrystalline anisotropy; short-

range order

- Amorphous metallic alloys based on transition metals can show a very easy magnetization process.

- Sources of anisotropy in amorphous alloys: magnetoelastic, thermomagnetic, and slip-induced anisotropy

- High electrical resistivity (120-150μΩ・cm) compared to Si-Fe and Fe-Ni alloys (30-50μΩ・cm) attractive for high-frequency operation

- Metalloid atoms: B, P, Si, and so on are needed to stabilize the glassy state.

-The discovery of high permeability in ferromagnetic amorphous alloys based on Fe-P-C in 1964 by Duwez and Lin

High-Induction Amorphous Alloys- Lower magnetostriction in amorphous alloys compared to crystalline Fe-Co alloys :

(FeCo)80B20 amorphous alloys compared to crystalline Fe-Co alloys (see Fig. 10.17b)

less sensitive to stress-induced anisotropy

Inherently high values of electrical resistivity and yield stress

- Commercial high-induction amorphous Fe-B alloys (Bs ≈ 16 kG)

- Core loss of selected amorphous and crystalline alloys (see Fig 10.18)

Other Amorphous AlloysKey factors for controlling soft magnetic properties : stress and magnetostriction coefficient

- CoxFe1-xB20 amorphous alloys (see Fig. 10.20)

- Fe-Co-Ni amorphous alloys (see Fig. 10.21)

- Important λs = 0 compositions in (FeCoNi)100-xTEx (TE = Zr, Ta, Nb , or Hf)

(see Fig. 10. 22)

Amorphous Alloys



Soft Ferrites

Spinel ferrites

Mn-Zn ferrite (see Fig. 10.23(a))

- Compositional dependence of crystal anisotropy and magnetostriction constants (see Fig. 10.24)

- Polycrystalline samples : highest permeability when λ100 = 0 in a K1 > 0 field (easy axis [100])

or when K1 < 0 field (easy axis [111]) and small λ111 (see Fig. 10.25)

- Variation of magnetostriction and permeability for the (MnZnFe)-Fe2O3 system (see Fig. 10.25)

- Variation of permeability and magnetostriction for the (MnZnFe)-Fe2O3 system (see Fig. 10.26)

- Variation of permeability and anisotropy with temperature for Mn0.31Zn0.11Fe1.06Oy (see Fig. 10.27)

Ni-Zn ferrite (see Fig. 10.23(b))- Dependence of permeability and magnetostriction on Fe2O3 content (see Fig. 10.28)

Microwave ferrites

Garnets : YIG, RIG

Hexagonal ferrites



Soft Ferrites (continued)

Mn-Zn ferrite (see Fig. 10.23(a)) Ni-Zn ferrite (see Fig. 10.23(b))



- Requirements

A strong net magnetization : large BrA stable magnetization in the presence of external fields : large HcA high energy product BH : the materials characteristics of a permanent magnet are most efficiently used

at the point of (BH)max

- Applications

motors, generators, loudspeakers, bearings, fasteners, and actuators

- Definition of coercivity

For fields opposing the direction of magnetization of

a hard magnet, a smaller external field is required

to give B = 0 than to give M = 0.

BHc : the B-H loop coercivity

iHc : the intrinsic M-H loop coercivity

iHc > BHc(see Fig. 13.1)



- Comparison of second quadrant M-H loops

for some commercial permanent magnets (see Fig. 13.4)

1 MGOe = 1/4π erg/cm3 = 10-4/4π kJ/m3



- Three factors causing iHc to fall short of its maximum value

(1) a dispersion in easy-axis(grain) orientations

(2) the presence of mobile domain walls

(3) exchange coupling between single-domain particles

- Major factors in the design of different permanent magnet materials

(1) optimizing Ku (by crystallography, chemistry, and/or particle shape)

(2) maximizing Br by introducing texture (preferred orientation) into the grain structure

(3) eliminating domain walls (by making single-domain particles) or pinning domain wall motion (by

introducing certain defects)

(4) minimizing exchange coupling between single-domain particles (nonmagnetic grain boundaries)

- Early Permanent Magnets

lodestone : impure form of iron oxide (mostly magnetite Fe3O4 with fine regions of γ-Fe2O3)

Impure metallic iron : C (see Fig 13.5)

Tungsten steel (7-8% W), Co-Mo and Co-Cr steels

Slowly cooled FeCo alloy through the order-disorder transformation temperature (800℃)



AlNICO & FeCrCo MagnetsBased on Co-addition to Fe2NiAl (intermetallic Heusler compound, called Michima alloys)

Hexagonal Ferrites and Other Oxide MagnetsBased on MO・6Fe2O3 (M = Ba, Sr, and Pb) have the magnetoplumbite structure(see Fig. 13.15-20 and Table 13.2)

Rare Earth-Transition Metal Intermetallics (see Table 13.4 and Figs. 13.21-25)Cobalt/Rare-Earth Magnets (see Fig. 13.26-32 and Table 13.5) : RCo5, R2Co17

Rare-Earth Intermetallics based on Nd2Fe14B1(1) Large uniaxial magnetic anisotropy (Ku = + 5×10

6 J/m3) of this tetragonal phase

(2) The large magnetization (Bs = 1.6 T) owing to the ferromagnetic coupling between the Fe and Nd

moments

(3) The stability of the 2-14-1 phase allowing development of a composite microstructure characterized

by the 2-14-1 grains separated by nonmagnetic B-rich and Nd-rich phases, which tend to decouple the

magnetic grains

Other Permanent MagnetsCoPt

FePt and FePd alloys

MnAlC magnets

Spinel oxides : CoO・Fe2O3



AlNICO Magnets



Hexagonal Ferrites and Other Oxide Magnets



Rare Earth-Transition Metal Intermetallics(see Table 13.4 and Figs. 13.21-25)

Cobalt/Rare-Earth Magnets(see Fig. 13.26-32 and Table 13.5) : RCo5, R2Co17



Rare Earth-Transition Metal IntermetallicsRare-Earth Intermetallics based on Nd2Fe14B1 (see Figs. 13.33-38)



Summary


V. Soft & Hard Magnetic Materialsocw.snu.ac.kr/sites/default/files/NOTE/Lecture #10.pdf · 2018. 4....

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