Coercivity and Domains inSm2

Co17

based magnets

- dedicated to I. R.Harris -

K.-H. Müller, O. Gutfleisch and L.SchultzIFW Dresden, Germany

The year 1989⇒

Hanau, Erlangen, Stuttgart, Vienna, Birmingham, Grenoble, Dublin, Berlin,Sheffield, Madrid, Parma, Athens,

…

Nd2

Fe14

B → sintered, melt

spun, mechanically

alloyed,remanence

enhanced

Sm2

Fe17 → interstitially

modified

by

N or

C

SmCo5 → well established

Sm2

Co17 → well established

(Ba,Sr)Fe12

O19 → well established

400OC*1h

slow cooling 0.7°C/min

Isothermal aging at 750OC~850OCfor 12~24h

1100~1200OC*4~10 h

Tem

pera

ture

Time

100nm

100nm

c-axis

e.g. Sm(Co0.784

Fe0.1

Cu0.088

Zr0.028

)7.19

Precipitation hardeningof 2:17 magnets

3 μm

400OC*1h

slow cooling 0.7°C/min

Isothermal aging at 750OC~850OCfor 12~24h

1100~1200OC*4~10 h

Tem

pera

ture

Time

100nm

100nm

c-axis

e.g. Sm(Co0.784

Fe0.1

Cu0.088

Zr0.028

)7.19

Precipitation hardeningof 2:17 magnets

3 μm

●

same

microstructures

forsintered

magnets,

ingots,melt-spun

samples

●

microstructure

unchangedafter

slow

cooling

●

⇒ what

happens

during

slow

cooling

?

Overview of Sm2Co17 Compositions

0

2

4

6

8

10

0 5 10 15 20 25

Fe wt%

Cu

wt%

High Temperature

Current Industrial

EU- HITEMAG

Composition

of Sm(Co,Fe,Cu,Zr)z

magnets

A. Yan et al., J. Appl. Phys. 93 (2003) 7975 / Appl. Phys. Lett. 83 (2003) 2208

EDX profiles of Cu across the cell boundary

0,00

0,25

0,50

0,75

1,00

0,00

0,25

0,50

0,75

1,00

(a)

1:5 phase

Cu

(at%

)/Cum

ax

(b)

2:17 phase

Cu

(at%

)/Cum

ax

0 2 4 6 8 10Position (nm)

850ºC*3h, slowly

cooled

to 400ºC high Hc at RT

850ºC*3hquenched

low Hc at RT

0 2 4 6 8 100,00

0,25

0,50

0,75

1,00

Cu

(at%

)/Cum

ax

Position (nm)

subsequently850ºC*5 min, quenched

⇒ ⇒

low Hc at RT

high coercivity large gradient of Cu within the 1:5 cell boundary

Physics

of permanent magnetism●

Continuum

theory

(micromagnetism)

using

material properties

such as: Ms

(r;T), A(r;T), K(r;T), …-

nonlinear

relations

-

non-local

relation

Ms

(T;r) → H(r)-

long-range

(mgnetostatic) interactions

⇒ sample-size

and sample-shape

dependent

phenomena⇒ no thermodynamic

limit

●

metastability⇒ no equilibrium

thermodynamics

⇒ non-single

valued

relation

M(H)⇒ single

crystals

are

useless

- multiphase, texture, grain

boundaries, concentration

gradients, ….⇒ much

larger variety

of magnetic

structures

(„domains“)

⇒ hard

to observe- simplified

pictures: nucleation

type

magnets, pinning

type

magnets, …

●

non-homogeneous

materials:

Dc

≈

70 (AK)1/2/μ0

Ms2

Db

≈

15

DcWb

∼

D1/2

…D2/3

(bulk-domain

width) Ws

≈

3 Dc

(surface-domain

width) δ

≈

π

(A/K)1/2

(domain-wall

width)1.260.45Sm2

Co17

0.630.24Nd2

Fe14

B

Ws

[μm]Db

[μm]Dc

[μm]δ[nm]

10 μm

Db

< Ddomain

branching

D < Dcsingle-domain

particles

Dc

< D < Dbmultidomain

particle

2 μm0.1 μm 0.1 μm

●

homogeneous

material (single

crystals):

Magnetic

domains-

in easy-axis

materials

-

Magnetic domain structure by Kerr microscopy

domains as typical for coarse grained materials with easy-axis-type magnetic anisotropy

as-cast Sm2

Co17 Sm(Co0.784

Fe0.100

Cu0.088

Zr0.028

)7.19 -

solid-solution treated -

The

influence

of slow

cooling

900 800 700 600 500 400

0

1

2

3

4 high temperature grade

coer

civi

ty μ

0iH

c (T)

quenching temperature Tque (oC)

coercivity

at room

temperature

⇒ refinement

of the

domain

structure

400 °Cµ0i

Hc

=3.8 T600 °C

µ0i

Hc

=2.54 T700 °C

µ0i

Hc

=0.92 T850 °C

µ0i

Hc

=0.08 T800 °C

µ0i

Hc

=0.12 T750 °C

µ0i

Hc

=0.23 T

10µm

MFM: c-axis

perpendicular

to the

imaging

plane; thermally demagnetised

400OC*1h

slow cooling 0.7°C/min

Isothermal aging at 750OC~850OCfor 12~24h

1100~1200OC*4~10 h

Tem

pera

ture

Time

⇒ increase

of j

Hc

0 1 2 30

1

2

3

4

Tq=850°C800

700

725 750

600

400

C

oerc

ivity

μ0i

Hc

(T)

Domain width W (μm)

coercivity i

Hc

↔ domain

width

W

Acta

Mat. 54 (2006) 997

High-resolution MFM

⇒ Interaction domains

in both

cases

High-coercivity

2:17 magnet

coarser

than

cell

size

( ≈

100 nm)

1 μm

Die-upset

melt-spun

Nd-Fe-B

coarser

than

grain

size

( ≈

300 nm)

O. Gutfleisch , K.-H. Müller, K. Khlopkov, M. Wolf, A. Yan, R. Schäfer, T. Gemming, L. SchultzActa Materialia

54 (2006) 997

ConclusionsSlow cooling of 2:17 magnets down to 400 oC

large gradient of Cu concentration in the 1:5 boundaries

strong refinement of domain structure

no classical domains but interaction domains

decoupling of the cells by Cu

Some more details

2 μm

1 μm

400 nm

Interaction domains by MFM

2:17 well prepared Die upset

Nd-Fe-B

400 °Cµ0i

Hc

=3.8 T600 °C

µ0i

Hc

=2.54 T700 °C

µ0i

Hc

=0.92 T850 °C

µ0i

Hc

=0.08 T800 °C

µ0i

Hc

=0.12 T750 °C

µ0i

Hc

=0.23 T

10µm

KER

R

10µm

MFM

Sm(Co0.784

Fe0.1

Cu0.088

Zr0.028

)7.19-

evolution of domain structure at room temperaturein dependence on quenching temperature from the slow cooling

ramp- samples thermally demagnetised- c-axis

perpendicular

to the

image plane

Comparison

of MFM and Kerr

3-dimensional Atom Probe analysisOrthoslice

surfaceDepth ~ 7 nm

2:17 2:17

1:5

Depth ~ 7 nm

Cu

2:17 2:171:5

Cu concentration

850ºC*3h,slowly

cooled

to 400ºC

high coercivity at RT

cooperation with Prof. Hono, NIMS, TsukubaR. Gopalan et al., Scripta materialia 60 (2009) 764

40 at%

4 at%

1:52:172:17

• Cu concentration: ~20 at% in 1:5 boundary -

almost uniform; ~4 at% in 2:17

850ºC*3h, quenched low coercivity at RT

cooperation with Prof. Hono, NIMS, TsukubaR. Gopalan et al., Scripta materialia 60 (2009) 764

3-dimensional Atom Probe analysis

ÜSThe year 892:17-Start2:17-StartcompositionsCu-gradientphysicsDomains-0Kerr-0Hc(Tcool)Domain widthMFM-detailconclusionsSome more detailsMFM-det+MFM/KERRMapping-highMapping-low