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9th MMM-Intermag, Jan 5-9, 2004.

Anaheim, California

Improved Synthesis of FePt (CoPt) Nanoparticles Min Chen, Hao Zeng, Shouheng Sun IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA In collaboration with C. Murray, H. Hamann, G. Held (IBM Watson) S. Raoux, M. Toney, J. Baglin, T. Thomson, B. Terris (IBM Almaden) J. Li, Z. L. Wang (Georgia Institute of Technology) J. P. Liu (Univ. of Texas, Arlington) S. Wang, R. White (Stanford Univ.) Support in part by DARPA/ARO 19-03-1-0038 and DARPA/ONR N00014-01-1-0885

¾Motivation ¾Current synthesis. ¾Improved synthesis. ¾Application potential.

FePt: Interesting bi-component materials FePt structure is composition dependent, Fe3Pt(fcc) FePt (fct) FePt3(fcc). Face centered tetragonal (fct) structure, also called L10 structure

Fct structured FePt has large anisotropy constant, K, and is magnetically hard, meaning smaller FePt particles can still be ferromagnetic with large coercivity.

∆E ~KV/kT 2     kB T  tP f O   3  ⋅ ln    H C = H O ⋅ 1 −  ln 2 V K      U   - Sharrock

FePt: High anisotropy constant

CoSm FePt

100

anisotropy (106erg/cc)

CoPt

10

1 0

2

4

6 2

8

10

12

CoCr15Pt12. CoCrxPt12 CoCr15Ptx CoCr20Pt10B6 CoCrxTa4 FePt CoPt FePd Co5Sm Co3Pt Charap

∆E ~KV/kT

Relaxation time

τ = τ0eKV/2kT

14

5

MS (10 erg/cc) D. Weller, A. Moser, IEEE Trans. Mag. 2000, 35, 4423.

Ku V >> kT

KuV < kT

FePt: Interesting chemistry

Fe Pt Pt Fe

The binding of Fe with Pt makes Fe much more stable against deep oxidation. FePt is chemically more stable than other well-known hard magnetic materials, e.g. Sm-Co & Nd-Fe-B.

Fe Pt Pt Fe Pt Fe

Surface Pt can bind to S strongly to form Pt-S bond, facilitating site specific bonding of the particles to biomolecules for highly sensitive bioseparation and detection. Gu et al, Chem. Commun. 2003, 1966.

Potential applications (3 examples) ¾Data storage media (large Hc > 3000 Oe)

¾Permanent magnet (Large Hc + high moment) H

N

H

H

¾Bio-separation & detection. Mr

H Hc

S H

H

Ms

FePt synthesis via decomposition/reduction CO OC

CO

Fe

[Fe]

CO CO

CO

CH3 O O

CH3

O

2+

Pt

O CH3

CH3

[Fe-Pt] reduction

[Pt]

acac

Fe(CO)5 R-COOH

Fe R-NH2 Pt FePt

Sun et al, Science 2000, 287, 1889.

Pt(acac)2 R-NH2 Reduction

FePt composition control Fe

Pt Pt

Pt

Fe Pt

Fe Pt

Fe

80

Fe

70

Pt Fe

x in FexPt(100-x)

Fe

60 50 40 30 0.5

1.0

1.5

2.0

2.5

3.0

mmoles of Fe(CO)5

Fe(CO)5+ Pt(acac)2 + Oleic acid + Oleylamine ? mmol

0.5 mmol

0.5 mmol

0.5 mmol

Oleic acid: CH3(CH2)7CH=CH(CH2)7-COOH Oleyl amine: CH3(CH2)7CH=CH(CH2)7-CH2-NH2 Sun et al, IEEE Trans. Magn., 37, 1239 (2001).

Surfactant bonding to FePt surface –FTIR studies N. Shukla, et al, J. Magn. Magn. Mater. 2003, 266, 178

O

Monodendate form

O

Fe

H2N

Pt

NH2 bonds to Pt via electron donation

O

Fe

C O

Pt Fe

H2N

H

Bidendate form

Pt

N R

H

FePt Nanoparticle Structure As-syn particle thin film Fe/Pt

Annealing

C C CC C C C CC C C C C C CC C C C CC CCC C C C C C C C C CC C C C C C C C CC CC CC C CC C CCC CCC CCC CC C CC CC CC C C CC C C CC C C C C C C C C C C C C C CC C C C CC CC C C CCC C C C C C C C C C

Disordered FCC structure

Fe

Pt

N2, 580C, 30 min

Nanocrystalline thin film Ordered FCT intermetallic structure

Formation of fct structure depends on annealing conditions and Fe/Pt composition.

Intensity (normalized)

Fe70Pt30

Sun et al, Science 2000, 287, 1889; IEEE Trans. Magn. 2001, 37, 1239. Klemmer et al, Appl. Phys. Lett. 2002, 81, 2220.

Fe56Pt44

Fe48Pt42 Fe38Pt62 20

30

40

50



60

70

80

90

10000

Hc of FePt Nanoparticles

Annealed FePt N2, 580C 30 min 300 K Hc

8000

5000

4000

As-syn FePt 5 K Hc

Hc (Oe)

3500 3000

6000

Hc (Oe)

4500

4000 2000

2500

0

2000 1500

0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75

1000

X in FexPt(1-x)

500 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75

IEEE Trans. Magn., 37, 1239 (2001).

X in Fe x Pt(1-x)

500C annealed

550C annealed Gong, et al, JAP, 84, 4403 (1998). Weller, et al, IEEE Trans. Magn., 36, 10 (2000). 580C annealed

‰ Coercivity of FePt NPs depends on Fe/Pt composition and annealing conditions. ‰ Fe-rich Fe55Pt45 films show the largest HC in evaporated, sputtered and chemically synthesized films.

Current concerns ‰ It is difficult to make FePt nanoparticles larger than 4 nm, preventing one from studying size-dependent structure transformation (from fcc to fct) and magnetic properties. Seed-mediated growth can be used to make bigger particles (up to 10 nm), but FePt compositions from one size range to another varies, making direct comparison of sizedependent properties impossible. ‰ To make fully ordered fct FePt needs high temperature annealing, but this high temperature annealing also results in particle sintering.

A: Fe, Pt order parameter B: (111) coherence length

He, 725C

J. Phys. Chem. B 2003, 107, 5419

===========================================================

Alternative efforts in making isolated ferromagnetic FePt particle arrays: 1) Making MFePt with M = Cu, Ag, Au to lower L10 phase formation temperature. e.g. Kang et al, Nano Lett. 2002, 2, 1033; JAP 2003, 93, 7178. Sun et al, JAP 2003, 93, 7337. 2) By doping more organic surfactant, the array can stand up to 800C, Momose et al, Jpn. J. Appl. Phys. 2003, 42, L1252.

=========================================================== ‰ There is no known method to align magnetic easy axis of the particles in an array.

Size/Temperature effect on Hc – Sharrock’s law 2     kB T  tP f O   3  ⋅ ln    H C = H O ⋅ 1 −   ln 2      KU V  

For a magnetically isolated and random oriented particle system

¾ A larger particle is thermally more stable than the smaller one (KuV ~ kT).

¾ For the same value of Hc at

room temperature, larger particle can have smaller Ku.

¾ Smaller Ku means lower

annealing temperature, thus less aggregation problems.

Improved synthesis ‰ Co-reduction of metal salts Composition control

= FeCl2 + Pt(acac)2 + LiBEt3H Sun et al, J. Phys. Chem. B 2003, 107, 5419. (~4 nm FePt)

= Fe(acac)3 + Pt(acac)2 + 1,2-C16H32(OH)2 Elkins et al, Nano Lett. 2003, 3, 1647. (~2 nm FePt)

= Fe(acac)3 + Pt(acac)2 + ethylene glycol

FeCl2 + Pt(acac)2

LiBEt3H

0.5mmol 0.5mmol 0.6mmol 0.4mmol

FexPt(1-x) Fe50Pt50 Fe60Pt40

Sun et al, J. Phys. Chem. B 2003, 107, 5419.

Jeyadevan et al, J. Appl. Phys. 2003, 93, 7547. Direct synthesis of partially ordered fct-FePt, but needs improvements in dispersion stability.

‰ From core/shell Pt/Fe2O3 to FePt Pt(acac)2

Pt

Coating with Fe2O3

Pt

Reductive annealing

FePt

Teng et al, J. Am. Chem. Soc. 2003, 125, 14559. Potentially a good method to prepare large FePt nanoparticles, but it needs to solve the sintering problem under high temp (>550C) reductive annealing condition.

Improved synthesis - One step synthesis of larger FePt nanoparticles CO OC

Fe

CO CO

CO CH 3 O

O O

Pt

RCOO H/R-NH 2 CH 3

2+

O

CH 3

CH 3



No polyol reduction (to slow down FePt nuclei formation rate).



Size control – By adjusting concentration of oleic acid and Pt(acac)2. – By adjusting the reaction time at 140oC.



Shape control – Cubic FePt nanoparticles were obtained by adding oleic acid at room temperature and adding oleylamine immediately before the increase of temperature from 140 to 220 oC



Composition control – Molar ratio of Fe(CO)5/Pt(acac)2 – Molar ratio of oleic acid/oleylamine (or acidity of the solution).

Spherical FePt nanoparticles

6 nm spherical nanoparticles

8 nm spherical nanoparticles

Cubic-like 7 nm FePt nanoparticles

Discreet fct ordered 8 nm FePt nanoparticles

8 nm FePt, 560C 30 min, N2

Thermally annealed 8 nm FePt assemblies 4500 4000

N2, 600C 1h

(111)

4000

3000

Almost no sintering

Scherrer’s equation 2500 Lhkl = Kλ/βcosθ 2000 to get (111) coherence 1500 length. Intensity

Intensity

d ~ 8.5 nm

Lorentz fit to get β

3000

(111)

3500

2000

1000

1000 500

0 30

40

50

60

44



46

48

50

52



16000

N2, 700C 1h

(111)

12000

d ~ 32 nm sintering

Lorentz fit10000 to get β

8000

Intensity

Intensity

12000

14000

(111)

8000

Scherrer’s equation 6000 θ Lhkl = Kλ/βcos to get (111) coherence 4000 length. 2000

4000

0

0 30

40

50



60

46

47

48



49

50

Improved synthesis - Core/shell nanoparticles Fe(CO)5, air

Pt(acac)2 + Fe(CO)5 ------ FePt ------------- FePt/Fe2O3

6 nm FePt core with 1.2 nm Fe2O3 shell

7 nm FePt core with 1.2 nm Fe2O3 shell

Anti-sintering effect of the Fe2O3 shell FePt/Fe2O3 (7nm/1.2nm) w1/2 peak: 1.01883 Size: 10 nm

2000

10000

1000

0 25

30

35

40

45

50

55

60

2000

original Lorentz fit

48.1

6000 4000

55.4

38.3

27.9

2000

Co Kα 2 θ (°)

57.7

0 10000

48

25

30

1000

35

40

45

50

55

60

Original Lorentz fit

8000

Intensity

Intensity

w1/2 peak: 0.77231 Size: 13.0 nm

8000

Intensity

Intensity

FePt (7nm)

6000 4000 2000

0

0

45

46

47

48

49

50

51

46

47

48 kα 2θ (°) 49 Co

50

Co Kα 2 θ (°)

X Axis Title

Annealing under N2 at 650 oC for 1 hour. Fct FePt grain size is about 10 nm, no obvious sintering.

Annealing under N2 at 600 oC for 1 hour. Fct FePt grain size is increased to ~13 nm.

Hc of annealed FePt/Fe2O3 nanoparticles (Moment is not normalized)

Magnetic Moment (emu)

0.040

Annealed under forming gas at 650C for 1h, Hc ~11 kOe.

0.020

0.000

Annealed under N2 at 650C for 1h, Hc ~16 kOe.

-0.020

-0.040 -40000

-20000

0

20000

40000

Magnetic Field (Oe)

Important conclusion: Nanoparticles annealed under N2 show less degree of aggregation and large coercivity. They can be used to study single particle magnetism.

CoPt nanoparticles Co(acac)3 + Pt(acac)2 + Polyol/Hydrazine

4 nm CoPt nanoparticles

5 nm CoPt nanoparticles

Effect of composition and annealing temperature on coercivity Hc (Fe at.%=45.7)

20000

20000

15000

15000 Coercivity (Oe)

Coercivity (Oe)

Hc (at 675 oC)

10000

5000

10000

5000

0 20

30

40

50

60

70

Fe at. % in FePt

Optimum composition is Co:Pt=1:1

0 580

600

620

640

660

680

700

720

Annealing Tem perature (o)

Optimum annealing temperature (under N2 gas) is 675 oC

Hysteresis loop of CoPt nanoparticles

Magnetic Moment (emu)

0.05

0.03

0.00

-0.03

-0.05 -60000

-30000

0

30000

60000

Magnetic Field (Oe)

CoPt (Co:Pt=49.7%:50.3%) nanoparticle assembly annealed under N2 at 675 oC for 1 hr. Hc=16950 Oe

Next step: Control magnetic easy axis direction One method: ‰ Directly synthesizing fct-FePt nanoparticle dispersion ‰ Assembling these particles under a magnetic field. Add H field H-Loop of Co Particle Assembly at 300K The particles were deposited on SiO2/Si under in-plane field of 0.5T

-5

8.0x10

-5

1.5x10

out-of-plane loop

in-plane loop -5

6.0x10

-5

M (emu)

M (emu)

1.0x10 -5

4.0x10

-6

5.0x10 -5

2.0x10

0.0 -150

0.0 -100

-50

0

H-Field (Oe)

50

100

150

-150

-100

-50

0

H-Field (Oe)

50

100

150

Solvent evaporation

Next step: Control magnetic easy axis direction Alternative methods: 1) 2)

Magnetic annealing. Shape controlled assembly Cobalt nanoparticles

annealing under H

(??)

BaCrO4 nanorods

Kim et al, J. Am. Chem. Soc. 2001, 123, 4360.

Shape induced alignment seems to be more promising.

FePt NP array for ultra-high density data storage

SA film, ~ 120 nm thick 4

500 fc/mm

2

Key challenges:

0

MR signal [mV]

-2

¾To have an exchange-decoupled array.

-4 -6 -8

¾To have a uniform 2D assembly.

-10 -12

¾To control magnetic easy axis direction.

-14 -16 -18 -20

5000 fc/mm 0

5

10

15

20

x [µm]

Sun et al, Science 2000, 287, 1989.

25

Weller & Moser, IEEE Trans. Magn. 2000, 35, 4423.

Exchange-spring nanocomposites for high density magnetic energy storage Soft

Hard

Soft

¾An exchange-spring composite contains two modulated phases that are in intimate contact with one being magnetically hard and another magnetically soft.

H

¾The system can store high density magnetic energy because it has both large coercivity and high magnetic moment.

Illustration of a modulated hardsoft exchange-coupled system

¾The key for high performance composites: The size of the soft phase is at ~<10nm; Magnetic easy axis is aligned. H

N

H ‰Kneller, E. F., Hawig, R. IEEE Trans. Magn., 27, 3588 (1991). ‰ Skomski, R., Coey, J. M. D.

H Magnetic hysteresis loops for a nonexchange coupled system (top) and an exchange-coupled system (bottom).

H Phys. Rev. B, 48, 15812 (1993).

H

S

H

‰ Schrefl, T., Kronmüller, H., Fidler, J. J. Magn. Mag. Mater., 127, L273 (1993).

Isotropic nanocomposites

10

(A) Fe58Pt42 (B) FePt:Fe3Pt

8

B (kG)

6

m (arb. unit)

0.4 4 nm:4 nm 0.2

4 2

0.0

0

-0.2 -0.4

-2

A -60 -40 -20 0 20 40 60 H (kOe)

4 nm:8 nm

10

0.1 8

0.0 -0.1 -0.2

-8

-6

(BH)max~ 14.7 MGOe (117 kJ/m3)

-4 H (kOe)

-2

0

12

B (kG)

m (arb.unit)

0.2

(BH)max ~ 20.1 MGOe (160 kJ/m3)

6

Ms = 1050 emu/cc Hc = 2.4 T (BH)max ~24 MGOe (191 kJ/m3)

4

B -60 -40 -20 0 20 40 60 H (kOe)

H-loops for the FePt-Fe3Pt composites made from self assembly and reductive annealing of 4nm-4nm and 4nm-8nm FePt-Fe3O4 binary nanoparticles. Zeng et al, Nature 2002, 420, 395.

2 0 -10

-8

-6

-4

-2

0

H (kOe)

(BH)max, energy product, reflects the ability for a composite to store magnetic energy, the larger the better.

Conclusion remarks 1) There are a lot rooms for improvements in FePt (CoPt) synthesis. 2) Large FePt (CoPt) nanoparticles can be made by one-step chemical synthesis. 3) Core/shell structure with hard inorganic shell may be necessary for anti-sintering during high temperature annealing (an alternative anti-sintering route is via doping more organic surfactant into the assembly), and rodshaped particles may induce much needed magnetic easy axis alignment. 4) The particle assemblies with controlled interparticle interactions and magnetic easy axis direction should have great potential for future ultra-high density information storage applications. (For bio-magnetic applications, see FB-03 & FB-07.)

60 ⋅ k B ⋅ T Minimal Stable Grain Diameter DP = K 3

alloy system

Co-alloys

L10 phases

RE-TM

material

K: anisotropy in 1e7 erg/cc

Ms: saturation magnetizati on in emu/cc

Hk: anisotropy field in kOe

Dp: minimal stable diameter in nm

CoCrPtX 0.20 Co 0.45 hex Co3Pt 2.0

200-300 1400 1100

15-20 6.4 36

8-10 8 4.8

FePd FePt

1.8 6.6-10

1100 1140

33 116

5.0 2.8-3.3

CoPt MnAl

4.9 1.7

800 560

123 69

3.6 5.1

1270 910

73 240-400

3.7 2.2-2.7

Nd2Fe14B 4.6 Co5Sm 11-20

See e.g. D. Weller and A. Moser, IEEE Trans. Magn.35, 4423(1999)

Nanocomposite High Ku Materials SmCo//Cr PrCo//Cr am. CoSm

Lambeth (1991);Liu et al.(1995) Malhotra et al.(1996) Kubota, Marinero, Toney (1999)

FePt:ZrOx CoPt:SiO2 FePt:B CoPt:M (Ag, C) CoPt:C Fe/Pt/Al2O3

Coffey et al. (1995) Ichihara et al. (1998) Li, Lairson, Kwon (1999) Stavroyiannis et al.(1998) Yu, Liu, Sellmyer (1999) Bian, Laughlin... (1999)

FePt:C

Sun, Murray, Weller, Moser, Folks (2000)

Formation of ordered L1o-FePt phase HRTEM image of an individual Fe52Pt48 nanocrystal after annealing at 530 °C for 1 hour

Fourier transform of the corresponding HRTEM image

Enlarged HRTEM image of the chemically ordered L1o-FePt structure

Corresponding simulated HRTEM image

Simulated electron diffraction pattern

Dai, Sun, Wang, NanoLett., 1, 443 (2001).

1.0

Remanence curves of self-assembled Fe58Pt42 nanoparticles annealed at different conditions.

650 C 5 min N2 700 C 60 min FG

0.0

-0.5

1.0 700 C 60 min FG 600 C 60 min FG 550 C 60 min FG

-1.0 -70

-60

-50

-40

-30

-20

-10

0

H (kOe)

δM plots for self-assembled Fe58Pt42 nanoparticles annealed at different conditions

0.5

δM

m (a.u.)

0.5

0.0

0

H. Zeng et al, APL, 80, 2583(2002)

5

10 15 20 25 30 35 40 45 50

H (kOe)

Polar Kerr angle (deg)

Hc=4.6 kOe

-20

-10

0.05 0.04 0.03 0.02 0.01 0 -0.01 0 -0.02 -0.03 -0.04 -0.05

Thin film magnetics 10

20

300 K 10 K

H (kOe)

Kerr, 3 layers, 4nm particles 580C, 30min, N2 Hc=2.4 kOe

Polar Kerr angle (deg)

0.02 0.015 0.01 0.005 0 -20

-10

-0.005 0

10

20

-0.01

-40

-30

-20

-10

0

H (KOe)

-0.015 -0.02 H (kOe)

Kerr, 2 layers, 4nm particles 580C 30 min, N2

Remanence Curves 2 layers, 4 nm Fe58Pt42 particles 650C, 5 min, N2.

Magnetic recording on 3 Layer 4nm FePt Nanoparticles Computer

V

First static read/write test on 3 layer thick sample (low density of 650 fc/mm)

I

Servo Electronics

>

Slider Sample

>

X-Y Piezo Scanning Stage

to piezos

Static read/write tester by A. Moser

S. Anders

Polymer Assisted Layer-by-Layer Assembly - Assembly Thickness Control

Sun, et al, J. Am. Chem. Soc., 124, 2884 (2002)

Mass density of a 3-layer FePt assembly from X-ray reflectivity. Mike Toney, IBM Almaden

Nanoparticle Media Perspective Lubricant

Carbon Co-alloy CrV NiAl

glass substrate

35 Gbit/in2 Media Grains: ~ 9 nm Co-M

>100 Gbit/in2 Grains: ~4 nm FePt Hc 3000-5000 Oe Thickness < 10 nm ……

Terabit/in2 Regime Grains: < 5 nm FePt Each dot as a bit ……