Probing magnetic structure with 3D sensitivity using polarized SANS

Kathryn Krycka, Julie Borchers, Wangchun

Chen, Shannon Watson, Mark Laver, Thomas Gentile (NIST Center for Neutron Research)

Ryan Booth, Charles Hogg, Sara Majetich

(Carnegie Mellon University)

Yumi

Ijiri, Sydney Harris, Liv

Dedon

(Oberlin College)

Jim Rhyne

(LANSCE, Los Alamos National Laboratory)

NCNR Summer School May 13, 2010

Outline

Motivation for studying 9 nm Fe3O4 nanoparticles

Benefits of polarization-analyzed SANS

Data and modeling at high field canted magnetic shell

Altering magnetic shell thickness by:

Varying temperature at high field

Varying field at very low temperature

Removing field and interparticle correlations

Summary of results

Monodisperse, 9 nm, ferrimagnetic magnetite (Fe3O4) particles1crystallize into a face-centered cubic crystallites ≈ μm. These crystallites are randomly oriented and form a powder. Magnetite in particular is biocompatible, stable, and has a moment comparable to Ni.

Goal is to extend the polarized-analyzed SANS technique4 to probe magnetism with 3D sensitivity5 within any applied field.

Magnetic particles are promising for data storage and biomedical applications. However, precise determination of internal spin structure and interparticle coupling is elusive.

1. S. Sun et al., J. Am. Chem. Soc. 126, 273 (2004)

2. P. Dutta

et al., JAP 105, 07B510 (2009); J. Curiale

et al., Appl. Phys. Lett. 95, 043106 (2009)

3. L. Berger et al., Phys. Rev. B 77, 104431 (2008); J. Mazo-Zuluaga

et al., JAP 105, 123907 (2009)

4. T. R. Gentile et al., J. Appl. Crystallog. 33, 771 (2000); A. Wiedenmann

et al., Physica

B 356, 246 (2005); A.M. Gaspar et al., Biochim. Biophys. Acta. 1804, 76 (2010)

5. K. Krycka et al., Physica

B, 404, 2561 (2009)

Introduction

Theoretical models suggest surface disorder2 produces a magnetically dead (spin-glass) layer. Inclusion of surface anisotropy3 has lead to differing predictions of hedgehog, artichoke, or throttled configurations. Either approach could account for the reduced moment experimentally observed with bulk probes.

Disordered Hedgehog Artichoke Throttled

Small Angle Neutron Scattering (Unpolarized)

Q

Applied field, H || X

Neutrons magnetically scatter only from moments perpendicular to Q.

H

N2

N2

+ M2SAT

(ρM

/ ρN

)2 = 4%

lengthQ π2=

0.02 0.04 0.06 0.08 0.10 0.12

200

400

600

800

Rel

ativ

e In

tens

ity

Q (Å-1)

N2 + M2

N2

0.02 0.04 0.06 0.08 0.100

1000

2000

3000

4000

Nuclear Scattering

Rel

ativ

e In

tens

ity (A

.U.)

Q (Å-1)

Set-Up for Polarization Analysis

FeSi supermirror is stable and can achieve polarization of ~95%.

FeSi

Supermirror Polarized 3He Cell

3He cells cover divergent beam, but have reduced transmission.

Q

MbMbMbI ccc 2222 ±+=±∝

Fe

FeSi

Si

X X X X X X X X X X X X X X

X X X X X X XX X X X X X X

Polarized 3He allows spin-up neutrons of one orientation to pass while absorbing the opposite orientation.

3He polarization can be reversed with NMR pulse.

(↑

and ↓)

Electromagnetic flipper is used to reverse polarization at will.

0.02 0.04 0.06 0.08 0.10 0.12

200

400

600

800

Rel

ativ

e In

tens

ity

Q (Å-1)

N2 + M2

N2

Polarized Small Angle Neutron Scattering

Moon, Riste, and Koehler, Physical. Review 181, 920 (1969)

)cos()cos()cos()ˆˆ(ˆˆ ,,, MQAQAMAqMQQMq φφφ −=⇒⋅−=r

2222

2

,

222

2

, 2

ZZYYk

RiQmHZ

j

RiQmY

XXXXk

RiQmxX

j

RiQn

MqMqeiqeqI

NMqMqNeqeI

k

k

j

j

k

k

j

j

+=±∝

+=∝

∑∑

∑∑

⋅⋅↓↑↑↓

⋅⋅↓↓↑↑

ρρ

ρρ mm

Q

X, H

From these eqns. we can separate N2, M2PARL

, and M2PERP

.

Y

0.02 0.04 0.06 0.08 0.10 0.12-50

0

50

100

150

200

250

300

350

400

450

500

550

Rel

ativ

e In

tens

ity

Q (Å-1)

Down-Down Along Y Down-Down Along X Up-Up Along X Up-Up Along Y

θ

↓↓

N2

+ 2MN

↓↓

,↑↑

N2

↑↑

N2

- 2MN

Scattering Profiles at 1.2 Tesla, 200 K

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.110

200

400

600

800

1000

0

10

20

30

40

50 N2

N2 I

nten

sity

(cou

nts

per p

ixel

)

Q (Å-1)

M2PAR

L Intensity (counts per pixel)

M2PARL via SF

M2PARL via NSF

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

0

1

2

3

4

5

6

M2PERPM

2 PE

RP In

tens

ity (c

ount

s pe

r pix

el)

Q (Å-1)

)(2 ↓↓↑↑ + XX IIN p

3/)(

25.1,,2

2,45

2

↓↑↑↓↓↑↑↓

↓↑↑↓

+=

−=

YXPERP

PERPPARL

IIM

MIM o

222 8/)( NIIM YYPARL↓↓↓↓ −p

XX NMMN 222 ++ XX NMMN 2

22 −+

2N

22ZY MM +

2ZM )(25.1 222 ZYX MMM ++

↑↓+↓↑

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.110

200

400

600

800

1000 N2

N2 I

nten

sity

(cou

nts

per p

ixel

)

Q (Å-1)

0.02 0.04 0.06 0.08 0.10 0.12 0.14

1E-101E-91E-81E-71E-61E-51E-41E-30.010.1

1

|F|2

(A.U

.)

Q (Å-1)

Form Factor for 9.0 nm Sphere 0.02 0.04 0.06 0.08 0.10 0.12 0.14

1E-101E-91E-81E-71E-61E-51E-41E-30.010.1

1

|F|2

(A.U

.)

Q (Å-1)

Form Factor for 7.0 nmCore

Modeling Intensity by Structure Factor (|S|2) x Form Factor (|F|2)

ρN

=

6.97E-6 Å-2

ρM

=

1.46E-6 Å-2(513 emu / cc)

Modeled Diameters:

Sphere 9.0 nm

Ferrimagnetic core 7 nm

Canted shell 7 to 9 nm (+0.2 nm)

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

0

1

2

3

4

5

6

M2PERP

M2 PE

RP

Inte

nsity

(cou

nts

per p

ixel

)

Q (Å-1)

Physical Review Letters (accepted)

0.04 0.06 0.08 0.10 0.120.0

0.2

0.4

0.6

0.8

1.0

1.2

|S|2

(A.U

.)

Q (Å-1)

Structure Factor for N2 and M2PARL

0.04 0.06 0.08 0.10 0.120.0

0.2

0.4

0.6

0.8

1.0

1.2

|S|2

(A.U

.)

Q (Å-1)

Structure Factor for M2PERP

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.110

200

400

600

800

1000 N2

N2 I

nten

sity

(cou

nts

per p

ixel

)

Q (Å-1)

0

10

20

30

40

50

M2PA

RL Intensity (counts per pixel)

M2PARL via SF

M2PARL via NSF

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

0

1

2

3

4

5

6

M2PERP

M2 PE

RP

Inte

nsity

(cou

nts

per p

ixel

)

Q (Å-1)0.02 0.04 0.06 0.08 0.10 0.12 0.14

1E-101E-91E-81E-71E-61E-51E-41E-30.010.1

1

|F|2

(A.U

.)

Q (Å-1)

Form Factor for Shell (1.0 nm thick)

0

10

20

30

40

50

M2P

ARL Intensity (counts per pixel)

M2PARL via SF

M2PARL via NSF

Altering Shell with Temperature at 1.2 Tesla

•

300-320 K dip at 0.082 Å-1 comes from shell 1.5 nm thick (6 nm inner to 9 nm outer diameter)

•

160 K dip at 0.075 Å-1 comes from shell 1.0 nm thick (7 nm inner to 9 nm outer diameter)

•

Field cooling to 10 K (

Removal of field does not eliminate all magnetic scattering

Effect of Temperature on Interparticle Correlations

Non Spin-Flip

Spin-Flip

Magnetic domains range from 1000 Å

(~ 10 particles) at 50 K down to 100 Å

(~ 1 particle) at 300 KH

MX2

= MY2

= MZ2

at 0.005 T

Eliminating field and interparticle interactions at 300 K

0.02 0.04 0.06 0.08 0.100.1

1

10

100

Inte

nsity

Q (Å-1)

300 K Remanence 9.0 nm Diameter Sphere

• Nanoparticles show no shell features and behave as uniform, ferrimagnetic spheres randomly oriented in space. Thus, shell is magnetic in origin.

22PARLPERP MM =

•

Low-Q interparticle correlations virtually eliminated at 300 K, 0.005 T

Recap

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.110

200

400

600

800

1000

0

10

20

30

40

50 N2

N2 I

nten

sity

(cou

nts

per p

ixel

)

Q (Å-1)

M2P

ARL Intensity (counts per pixel)

M2PARL via SF

M2PARL via NSF

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

0

1

2

3

4

5

6

M2PERP

M2 P

ERP In

tens

ity (c

ount

s pe

r pix

el)

Q (Å-1)

•

Direct evidence of canted magnetic shell 1.0 to 1.5 nm thick under high field between 160 and 320 K

•

Zero-field cooling to 10 K

eliminates ordered shell (though a disordered shell is probable)

•

When field and interparticle interactions are removed (0.005 T, 300 K) the nanoparticles exhibit no shell morphology

Possible origin of magnetic shell

•

Discovery of canted magnetic shells exclude models involving disordered shells and those with radial symmetry at near-saturation conditions

•

Shells show correlations over one to several FCC lattice units, as seen by |S|2.

•

We speculate that the combination of surface anisotropy and interparticle coupling gives rise to the canted magnetic shells [J. Nogues

et al., PRL 97, 157203 (2006); D. Kechrakos

et al., JMMM 316, E291 (2007)]

•

High-field temperature dependence (1.5 nm thick @ 300 K, 1.0 nm thick

at 200 K, and missing or disordered at 9 K) is a mystery. Interparticle coupling changes with temperature and anisotropy may as well. There is a Verwey

transition at 122 K and a bulk blocking temperature at 65 K.

Recap

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.110

200

400

600

800

1000

0

10

20

30

40

50 N2

N2 I

nten

sity

(cou

nts

per p

ixel

)

Q (Å-1)

M2P

ARL Intensity (counts per pixel)

M2PARL via SF

M2PARL via NSF

0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11

0

1

2

3

4

5

6

M2PERP

M2 P

ERP In

tens

ity (c

ount

s pe

r pix

el)

Q (Å-1)

•

Direct evidence of canted magnetic shell 1.0 to 1.5 nm thick under high field between 160 and 320 K

•

Zero-field cooling to 10 K

eliminates ordered shell (though a disordered shell is probable)

•

When field and interparticle interactions are removed (0.005 T, 300 K) the nanoparticles exhibit no shell morphology

•

Only with polarization analyzed SANS were we able to see details of perpendicular magnetic shells

THANK YOU

Correcting for Polarization Efficiencies

“user-friendly”

polarization efficiency correction software

Probing magnetic structure with 3D sensitivity using polarized SANSSlide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17