Permanent Magnets based on Fe-Pt Alloys

P.D. Thang, E. Brück, K.H.J. Buschow, F.R. de Boer

Financial support by STW

2

Contents

Introduction

Permanent magnets, motivation

Experimental

Sample preparation, analysis techniques

Results

Structure, magnetic and mechanical properties

Conclusions

3

Introduction. What’s permanent magnet ?

Magnetic materials

Large Mr, high Hc

High (BH)max

e.g:

SmCo5 (1969),

Sm2Co17,

Nd2Fe14B (1983)

• B = µ0(M+H)

BM

4

Applications of permanent magnets

Automobile industry.

Computer industry.

Scientific research.

Biomedical treatment.

and many other applications.

5

Dental applications

Denture retention system

Soft magnetic: Pd-Co

Magnet: NdFeB(DYNA Dental Engineering B.V.)

Aim: to find a magnet and a soft magnetic material with

Good mechanical properties

High corrosion resistance

Magnet problems

Very brittle

Low corrosion resistance

6

Introduction. Fe-Pt alloys

Good mechanical strength and high corrosion resistance

fcc statistical distribution high magnetisation

fct layer structure high magnetic anisotropy

fcc - fct phase transition

7

Experimental

Sample processing

Preparation FexPt100-x and (Fe0.6Pt0.4)100-xMx based alloys

- Arc-melting the pure elements (3N) in Ar

- Casting to cylinder, disc

Heat treatment:

- Homogenisation (as-quenched sample):

1325C/1h, under Ar + quenching in water fcc phase.

- Ageing (aged sample):

500-700C + quench in water fct phase.

8

Analysis techniques

Tensile strength: tensometer

Magnetic properties: hysteresis-loop

Microstructure analysis: TEM ... and … SANS

Applied force

9

Results. Crystallographic structure

As-quenched: presence of the fct phase.

Ageing: fcc-fct transformation.

Fe60Pt40

20 30 40 50 60 70 80 90

10h

5h

2h

625°C, 1h

as-quenched

(002)(202)

(222)(001)

fct

(311)(200)(111)

(110)

(113)

(202)(112)(201)

(222)

(311)fctfct

fct

(220)

(200)(111)

Inte

nsity (arb

. unit)

2 (deg.)

10

Optimal hard-magnetic properties for Fe60Pt40 aged at 625C, 1h:

Br = 0.97 T, BHc = 294 kA/m, (BH)max = 118 kJ/m3

Permanent-magnet properties: FexPt100-x

0.4

0.6

0.8

1.0

1.2

BH

c (kA/m)

600OC 625

OC

650OC

Br (T)

80

160

240

320

0 2 4 6 8 100

30

60

90

120

Ageing time (h)

(BH)max

(kJ/m3)

0.00

0.25

0.50

0.75

1.00

Br (T)

0

80

160

240

320

BHc (kA/m)

50 55 60 65 700

30

60

90

120

(BH)max (kJ/m3)

Fe content x (at. %)

11

Microstructure: Fe60Pt40

Dark field image: Fct particle size increases during the ageing.

Fct nano-size observed.

Selected area diffraction pattern: Degree of atomic order increases during the ageing.

625°C, 1h: 2-5 nmas-quenched: 1-3 nm

12

Magnetic properties: (Fe0.6Pt0.4)100-xMx (M = Nb, Al)

0.5% Nb aged at 625C, 24h: Br= 0.98 T, BHc= 302 kA/m, (BH)max= 125 kJ/m3

0.25% Al aged at 525C, 24h : Br= 1.02 T, BHc= 300 kA/m, (BH)max= 132 kJ/m3

0.8

0.9

1.0

(Fe0.6Pt0.4)100-xNbx

Br (

T)

100

200

300

BH

c (k

A/m

)

0 1 2 3 4 50

40

80

120

(BH

) max

(kJ

/m3 )

Nb content (x)

0.8

0.9

1.0

(Fe0.6

Pt0.4

)100-x

Alx

Br (

T)

200

250

300

BH

c (k

A/m

)

0.00 0.25 0.50 0.75 1.000

40

80

120

(BH

) max

(kJ

/m3 )

Al content (x)

13

Thermomagnetic analysis: (Fe0.6Pt0.4)100-xMx

0.5 at. % Nb:

• Tc (as-quenched) 340C

• Tc (aged) 400C

0.25 at. % Al:

• Tc (as-quenched) 380C

• Tc (aged) 400C

0 100 200 300 400 500

(Fe0.6

Pt0.4

)100-x

Nbx

x=0.5, aged

x=5x=3x=1

x=0.5

x=0.25

M

(arb

. unit)

T (°C)

0 100 200 300 400 500

(Fe0.6Pt0.4)100-xAlx

x=0.25, aged

x=1

x=0.5

x=0.25

x=0.1

M

(ar

b. u

nit)

T (°C)

14

Temperature dependence: (Fe0.6Pt0.4)100-xMx

• Nb: = -0.04 %/K for Br, = -0.12 %/K for BHc and = -0.12 %/K for (BH)max.

• Al: = -0.06 %/K for Br, = -0.15 %/K for BHc and = -0.17 %/K for (BH)max.

0.3

0.6

0.9

1.2

M = Nbx = 0.5

Br (T)

100

200

300

400

500

BHc(kA/m)

0 100 200 300 4000

50

100

150

200

T (K)

(BH)max(kJ/m3)

0.3

0.6

0.9

1.2

M = Alx = 0.25

Br (T)

100

200

300

400

500

BH

c(kA/m)

100 200 300 4000

50

100

150

200

T (K)

(BH)max(kJ/m3)

15

Mechanical properties: (Fe0.6Pt0.4)100-xNbx

• Hardness

• Tensile strength of 0.5% Nb

0.0 0.5 1.0 1.5 2.0 2.5 3.0250

300

350

400

450

500

550(Fe

0.6Pt

0.4)100-x

Nbx

Nb content x (at. %)

Har

dnes

s (H

V)

as-quenched aged

0 50 100 150 200 250 300 350

0

100

200

300

400

----- as-quenched sample----- aged sample

Str

ess

(MP

a)

Load time (s)

16

Mechanical properties: comparison

Hardness: comparable to file band or of cutting tools.

Tensile strength:

0

100

200

300

400

500

600

700

800

900Ni-Cr

64%Au-Ag-Cu

CuAg0.08%

Fe-Pt aged

Au-Pt-Pd

Fe-Pt aq

Alumina

790

550

400350

260

150120

Ten

sile

stren

gth

(MP

a)

Alloys

17

Microstructure: (Fe0.6Pt0.4)99.5Nb0.5

as-quenched: 1 nm 625°C,12h: 1-3 nm

625°C,24h: 3-8 nm 625°C,48h: 8-16 nm

18

Microstructure: (Fe0.6Pt0.4)99.75Al0.25

as-quenched:

2 nm

525°C, 24h:

3-7 nm

100 nm

High coercivity: correlated with the magnetic anisotropy, i.e. the atomic order in the fcc/fct and fct grain growth.High remanence: originated by the exchange coupling of the soft fcc phase with the nano-sized hard fct phase.

19

What’s neutron and why SANS ?

Properties mn = 1.674710-24 g

n = 9.6628610-27 J/T

q = 0, 1/2 = 624 s

Interaction with matter - Scattering from the atomic nucleus

- Magnetic scattering

SANS: Small angle neutron scattering

- 10 Å 102 Å

- Domain and particle size

q = kf - ki

I(q,) = A(q) + B(q)sin2(/2)

nuclear scatterin

g

magnetic scattering

20

2D SANS images: Fe59.7Pt39.8Nb0.5

B = 0 (virgin) B = 1.8 T (in field) B removed (remanent)

As-

que

nch

edA

ged

At 5m

21

Reduced SANS data: Fe59.7Pt39.8Nb0.5

I(Q): difference in the virgin, field and remanent states

I(Q): difference for the as-quenched and aged samples

As-quenched Aged

1E-3 0.01 0.1 110

0

101

102

103

104

105

106

virgin

in field

remanence

I (c

m-1)

Q (nm-1)

1E-3 0.01 0.1 110

0

101

102

103

104

105

106

virgin

in field

remanence

I (c

m-1)

Q (nm-1

)

22

SANS analysis

Model fitting: monodisperse or polydisperse model ?

Virgin state Field state

SANS dominated by particles with different magnetisation:

polydisperse model

SANS dominated by randomly oriented magnetic domains:

monodisperse model

f c t f c c

Magnetic domains

B

Particles

23

Model fitting: Fe59.7Pt39.8Nb0.5 aged

Mono.: domain size ~ 100 nm.

Poly.: fct particles R = 6 nm.

(TEM: fct particles 3-8 nm)

1E-3 0.01 0.1

101

102

103

104

105

virgin & mono. cal. field & poly. cal. rem. & poly. cal.

I (cm

-1)

Q (nm-1)

0 50 100 150

0

20

40

60

80

100

p

r (nm)

monodisperse

Correlation length 105 nm

0 10 20 30 40 50 600.00

0.01

0.02

0.03

0.04

(x2): field remanence

D

R (nm)

6 nm polydisperse

24

Conclusions

Best permanent magnets obtained with (Fe0.6Pt0.4)100-xMx:

M = 0.5 at. % Nb and 0.25 at. % Al.

Good thermal stabilisation.

Fe67Pt33: soft magnetic properties.

Good mechanical properties.

Suitable for biomedical applications, e.g. denture retention.

Coexistent nanostructure observed by TEM and SANS:

High coercivity: correlated with the atomic order in the fcc/fct

structures and the fct grain growth.

High remanence: originated by the exchange coupling of the

soft fcc phase with the nano-sized hard fct phase.