probing magnetic structure with 3d sensitivity using ......probing magnetic structure with 3d...
TRANSCRIPT
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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
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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
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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
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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
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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
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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
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THANK YOU
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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