Polarized Analyzed Small Angle Neutron Scattering of MnFe2O4 Nanoparticles

Hillary Pan, Ian Hunt-Isaak, and Yumi Ijiri, Oberlin College, Oberlin OHSam Oberdick, Ahmed Abdelgawad, and Sara Majetich, Carnegie Mellon University, Pittsburgh, PA

Julie Borchers and Kathryn Krycka, NIST Center for Neutron Research, Gaithersburg, MDJim Rhyne, Department of Energy, Gaithersburg, MD

Why Magnetic Nanoparticles?• Ultradense data storage

– Binary code, magnetic moment is either 1 or 0– Store a lot of data in a small space

• Biomedical Applications– Targeted drug delivery system – Hyperthermic cancer treatment

• However… – Materials can act differently on nano-length scale– Need techniques to measure

magnetic properties directly on this scale

Y. Ijiri, “Magnetic nanoparticles for storage,mechanical, and biomedical devices”

Small Angle Neutron Scattering• NIST Center for Neutron Research, MD• 2D detector registers the scattering• Work in Q-space:

• Temperatures between 10K - 400K• Applied magnetic fields of 0T – 1.5T

PASANS

• Supermirror polarizes incoming neutron beam• Flipper selects spin state before sample• 3He Cell with in situ NMR flipper selects state after sample• Four possible neutron spin states:

• Can isolate the component of magnetic scattering parallel and perpendicular to the applied field from these states

UU UD DU DD

Previous Work• Moments not necessarily parallel to applied magnetic field• 9 nm diameter Fe3O4

– Magnetic core-shell structure

• 11 nm diameter CoFe2O4

– Uniform canted magnetization– No correlation of the cant between particles

CoFe2O4Fe3O4

Details: Phys. Rev. Lett. 104, 207203 (2010). Phys. Rev. B 90, 180405(R) (2014).

MnFe2O4 Synthesis• Synthesized by solution chemistry methods

– Mn(acac)3 and Fe(acac)3 with oleic acid

• Particle size from TEM: 7.5 nm ± 1 nm• Organized into crystalline assemblies• Analytical tools

– SQUID magnetometry, SAXS, Mössbauer spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism

7.5 nm MnFe2O4 nanoparticles

Data Analysis• Take sector cuts of 2-D scattering to extract relevant behavior• For our sample, structural scattering is isotropic • Simplified equations detailed below where

N : structural scattering elementMPERP : magnetic scattering perpendicular to applied fieldMPARL : magnetic scattering parallel to applied field

Details: Phys. Rev. Lett. 104, 207203 (2010). J. Appl. Cryst. 45, 554 (2012).

MnFe2O4 PASANS Results• Interparticle diffraction

peak centered at 0.085Å-1 • N2 fit with 7.5 nm ± 1 nm

spheres in FCC arrangement• M2

PARL is weaker, but mimics structural scattering

• In high field, moment ordering persists from nanoparticle to nanoparticle

• In remanent field, coherence is reduced, especially at elevated temperatures

N2 In

tens

ity (a

rb u

nits

)X2 PA

RL In

tens

ity (a

rb u

nits

)

M PERP

• Both FCC and Single particle arrangements• Interparticle peak height decreases as temperature increases

– Suppressed at 400K• Hard to determine field effect

– Both peak and sphere form factor are present in some cases

Unusual Properties Contd.

• DU≠UD! • Positive peaks at 90° and 270°, DU-UD=0 at 0° and 180°• Variation of peak size with temperature and field strength

UD – DU angular averaged at q=0.085Å-1

Summary and Future Work• Have observed unusual features in Mperp MnFe2O4 nanoparticle

• Characterize DU≠UD and test correlation with Mperp interparticle peak

• Look at angular dependence of spin flip scattering• Begin modeling the data

– Revisiting energy-based model for Fe3O4 and CoFe2O4 and updating it with MnFe2O4 parameters• Zeeman, magnetic anisotropy, exchange, and interparticle dipole

coupling energy considerations• Object Oriented MicroMagnetic Framework (OOMMF)

• Correlate data to Mössbauer, SAXS, XAS, XMCD work

Acknowledgements We acknowledge the support of NSF DMR-1104489, DMR-0704178,

and DMR-0922588. This work used facilities supported in part by the NSF DMR-0944772.