AMORPHOUS AND NANOCRYSTALLINE MATERIALS FOR APPLICATIONS AS HARD AND SOFT MAGNETS Reiko Sato Technische Universität Wien, Vienna, Austria Collaborators: Roland Grössinger Djoko Triyono Herbert Sassik Markus Schönhart Gerald Badurek Joao Paulo Sinnecker
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AMORPHOUS AND NANOCRYSTALLINE
MATERIALS FOR APPLICATIONS AS HARD
AND SOFT MAGNETS
Reiko Sato Technische Universität Wien, Vienna, Austria
Collaborators: Roland Grössinger Djoko Triyono Herbert Sassik Markus Schönhart Gerald Badurek Joao Paulo Sinnecker
Outline
1) Comparison soft- and hard magnetic materials
2) Basic properties of magnetic materials
3) Production methods of amorphous and nanocrystallline materials
4) Modeling
5) Applications
♥ Soft magnetic materials
♦ Hard magnetic materials
Magnetic characterization: hysteresis loop
-Hc
Range of irreversiblemagnetization
Range of rotationmagnetization
Range of approachto saturation
Initial permeability range
Ms
Mr
M
HHc
Coercivity: reverse field neededto drive the magnetization to zero after being saturation
Remaining magnetization whenthe driving field is dropped tozero
Soft magnetic material Hard magnetic material
Coercive force low High ( higher than 80 kA/m)
0
0
Mag
netiz
atio
n
Applied field
Soft magnetic material Hard magnetic material
Saturation
magnetisation
As high as possible (0.8 – 2T) As high as possible (0.2 – 1.5T)
Coercive force As low as possible (< 100 A/m) As high as possible (80 – 400 kA/m)
Permeability As high as possible 10000 – 200000
Not important
Losses As low as possible, frequency dependence
Area of loop ≅ stored energy as high as possible
Shape of loop Important because determines application
Important and should be rectangular
Remanence Not important as high as possible
Conductivity Determines ac-losses Important for magnetising procedure
Development of NEW magnetic materials for applications:
Herzer, G., in Handbook of Magnetic Materials, ed.Buschow, K.H.J., Vol. 10, Chap. 3, 1997, p. 415.
G.C.Hadjipanayis, J. Magn. Magn.Mater. 200 (1999) 373
Amorphous alloys ⇒ not any long-range atomic order ⇒atomic positions do not have crystalline periodic order (frozen liquid).
Nanocrystalline alloys:
term ‘nanocrystalline alloy’⇒ grain diameters range from 1±50 nm
Schwarz, ANMM 2003, IASI, Rumenia
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Mr (
T)
i HC (MA/m)
Nd-Fe-B sintered
Sm-Co bonded
Nd-Fe-B bonded
Ferrite sintered
R-Fe-B nanocomposite
nanocompositeSm-(Co,Fe) nanocomposite
AlNiCo
Comparison of the magnetic properties for different hard magnetic materials
Hard magnetic materials: Nd-Fe-B - energy product above 450 kJ/m3 achieved!
Further improvement - new compounds!
Hard magnetic materials: nanocrystalline, nanocomposite materialsSoft magnetic materials: Amorphous materials: ribbon, wire and bulk materialsNanocrystalline materials
Soft magnetic materials: Fe-Si (about 3% Si) still most important.
Soft magnetic amorphous materials:
Composition: TM1-x(M,NM, T)x ; where x is around 0,2TM = Co, Ni, or Fe; M = B, P, Si, etc NM = Cu, Ag, Au, etc; T = Zr, Nb, Hf, Ta, etc.
It exists hard magnetic amorphous materials?
Bulk: Nd60Fe30Al10; Nd60Fe20Co10Al10 amorphous?
Sof magnetic nanocrystalline materialsnanocrystals + a residual amorphous phase
exchange coupling between magnetic nanograins through amorphous matrix).
Coercive field as function of the currentdensity of Fe73Al5Ga2P11-xC5B4Six(x = 0, 1, 3) samples obtained at roomtemperature.
Relative resistance measured in situon the ribbon as a function of timeduring JH experiment as obtainedwith different current densities
Production methods of magnetic nanocrystalline materials:
Generally, initially one obtains material in the
amorphous state and subsequently crystallised by
annealing.
.
Soft and hardmagnetic prop
Diagram for developing a nanocrystalline soft and hard magnetic materials from an amorphous precursor (adapted from M.E. McHenry et al., Progress in Materials Science 44 (1999) 291-433)
0 100 200 300 400 500 600
0
2000
4000
6000
8000
10000
annealing 1 h
Fe73.5Cu1Nb3Si13.5B9VAC E 4229/2f = 5 kHzHac = 0.05 A/m
susceptibility
susc
eptib
ilty χ
ac
temperature T [°C]
0 100 200 300 400 500 600
0
20
40
60
80
100
FINEMET: Different QR
A B C
Coe
rciv
e fie
ld (
A/m
)
Temperature (°C)
Temperature dependence of the coercivity of FINEMET obtained from annealing the amorphous ribbons
produced with different quenching rates.
Proposed sequence of events in the nanocrystallization of FINEMET alloys [K. Hono et al. Acta Metall Mater 1992;40:2137
Grain size dependenceRandom anisotropy model for nanocrystalline
alloys [G. Herzer, Scripta Metall Mater 1995;33:1741].
Herzer considers:
a) grain size D < exchange length Lex
b) effective anisotropy is average over several
grains – reduced anisotropy
c) characteristic volume whose linear dimension
is the magnetic exchange length, Lex ~
(A/K)1/2. ( Volume ∝ Lex3).
d) N grains, with random easy axes, within a
volume of Lex3 to be exchange coupled.
In these conditions:
The effective anisotropy is : Keff = K/N1/2 and the number of grains in
this exchange coupled volume is: N = (Lex/D)³. Then:
= 3
642/3 ~~
ADK
A
KKDK eff
eff
Since Hc can be taken as proportional to Keff:
Hc ~ HK ~ D6
For sufficiently small nanocrystals ⇒ superparamagnetic
3
641 .
AJDKpH
SCC =
6410
32
DKAJp S
i µµ µ=
♣Permanent magnets: ⇒ Two types:♦ single nanocrystalline hard magnetic phase♦ nanocomposites: known as spring magnets (hard + soft magnetic phases).
⇒ exchange coupling between magnetic nano-grains through soft magnetic grains.
Mag
netis
atio
n
Applied fieldSoft particles
Hard particlesNanocomposite
Spring magnets (hard + soft magnetic phases): Ms(soft) > Ms(hard).
Enhancement of remanence due to: exchange coupling + high Ms(soft)
Remanence increases however coercivity decreases.
After Davies at al
Exchange coupling leads to a remanence enhancement!
Theoretical limit for the maximum energy product, (BH)max is:
Remanence for a polycrystalline magnet - non-interacting isotropic, uniaxial grains:
Nanocrystalline material due to exchange coupling - for isotropic material Jr/JS > 0.5 is possible!
( )0
2
max 4µsJBH ≤
ssr Jdd
ddJJ
21
sin
sincos
2
0
2
0
2
0
2
0 ==
∫ ∫
∫ ∫ππ
ππ
ϕϑϑ
ϕϑϑϑ
Nanocrystalline materials
For nanocrystalline material the way how the small grains are coupled is of great importance for the understanding of the remanence enhancement. For grains of nano-size different exchange length have to be considered:
Exchange length due to external field:
Exchange length due to crystal energy:
Exchange length due to stray fields:
Which type of “exchange length” is more important, it depends on the material.
SH MH
A..
2
0µ
=l
KA
K =l
( )20
0
.
2
Ss M
A
µ
µ=l
Hard magnetic materialsModeling: Finite element modeling for nanocomposite to obtain a remanence enhancement
Advantages of nanocomposite over conventional isotropic magnets
# Stability of the powders both in physical and chemical aspects
# Availability of relatively fine particles sizes for molding small parts and for injection molding process,
# Negligible long-term structural losses,
# Tailoring of magnetic properties
Applications of soft magnetic materials
Power devices (low losses): -power transformers - magnetic shields - acoustic delay lines - tensile stress transducers- transverse filters.
Electronics (Ms, µ, eddy current and magnetoelastic properties):- 400 Hz power transformers;- Inductive components for switched mode power supplies;- Magnetic shields;- Magneto-elastic transducers;- Magnetic heads for data storage applications;- Magnetic springs;- Acoustic-magnetic systems.
(a) 60 Hz distribution(b) Ribbon wound cruciform distribution transformer
applications ( Suzuki et al, Mat. Sci. Eng. 1994)
Acoustic-magnetic systems
Applications of nanocomposites
- as magnetic component of resin-bonded magnets
- in motor: e.g. internal permanent magnet type of rotor; multi-pole rotor used in a stepping motor.