Using Polarization Analyzed SANS to investigate Magnetic Nanoparticles

Magnetic nanoparticles and nanoscale structures are intriguing in part because of the exotic 

properties that can emerge compared with bulk. A reduction of net magnetic moment per atom in 

magnetite (Fe3O4) with decreasing nanoparticle size, for example, is known to occur. This decrease in 

magnetism has been hypothesized to originate from surface disordering or anisotropy-induced radial 

canting [1-3], resulting in proposed morphologies that are difficult to verify using conventional 

magnetometry. Yet, other experiments [4-6] suggest that in certain circumstances oleic acid coating may 

preserve the surface magnetism. When the nanoparticles are brought close together, additional dipolar 

interactions come into play.

Application of Polarization Analyzed Small-Angle Neutron Scattering (PASANS) to 9 nm magnetite 

nanoparticles closed-packed into face-centered crystallites of up to a micron revealed that at nominal 

saturation the missing magnetic moments unexpectedly interacted to form ordered shells 1.0 to 1.5 nm 

thick with a magnetic component canted perpendicular to their ferromagnetic cores between 160 to 320 

K [7]. These shells additionally displayed intra-particle “cross-talk”, i.e. they selected a common 

orientation over clusters of tens of nanoparticles (averaging to zero across the entire sample). However, 

the shells disappeared when the external field was removed and inter-particle magnetic interactions were 

negligible (at 300 K), confirming their magnetic origin. 

Figure 1 depicts a simplified schematic showing the uniformly canted magnetic shell (green area) with a 

component that is perpendicular to the applied field of 1.2 Tesla (shown as a light blue arrow).

The canted shell formation is thought to result from the fact that the nanoparticles’ crystalline 

axes are randomly oriented and fixed in place with oleic acid with respect to the applied magnetic field. 

Each nanoparticle has a preference to magnetically align along its own [111] crystalline axis, denoted 

magnetocrystalline anisotropy. When a magnetic field is applied, however, it tends to pull the magnetic 

spins away from this preferred crystalline axis. Since the bonds that hold the magnetic spins in place to 

are weaker (have fewer nearest neighbor spins to interact with) close to the nanoparticle surface, the 

spins are more easily canted here in response to the competition between magnetic field determined 

Zeeman energy and the preferred magnetocrystalline anisotropy direction. 
To better understand the role of magnetocrystalline anisotropy, we shall study a similar system 

of close packed 10 nm spherical particles of similar magnetism per volume but composed of cobalt ferrite 

(CoFe2O4) with has a 16 to 18 times higher internal magnetocrystalline anisotropy than Fe3O4. Using 

PASANS we will be able to:

1) Unambiguously separate the structural scattering from the magnetic scattering

2) Separate the magnetic scattering into components that are “parallel to” and “perpendicular to” an 

external applied field 

3) Map how the magnetic canting angle and magnitude varies as a function of field and/or temperature

References

[1] P. Dutta et al., J. Appl. Phys. 105, 07B501 (2009)

[2] J. Curiale et al., Appl. Phys. Lett. 95, 043106 (2009) 

[3] A. Kovacs et al., Phys. Rev. Lett. 103, 115703 (2009)

[4] J. Salafrana et al., Nano Lett 12, 2499-2503 (2012)

[5] M. Darbundi et al., J. Phys. D: Appl. Phys. 45, 195001 (2012)

[6] A. G. Roca et al., J. Appl. Phys. 105, 114309 (2009)

[7] K.L. Krycka et al., Phys. Rev. Lett. 104, 207203 (2010)