CORE-SHELL MAGNETIC MORPHOLOGY WITHIN Fe3O4 NANOPARTICLES

 

Kathryn Krycka, Julie Borchers, Shannon Watson, Wangchun Chen, Mark Laver

NIST Center for Neutron Research, Gaithersburg, MD, USA

 

Yumi Ijiri, Liv Dedon, Sydney Harris

Oberlin College, Oberlin, OH, USA

 

Ryan Booth, Chip Hogg, Sara Majetich

Carnegie Mellon University, Pittsburgh, PA, USA

 

James Rhyne

Los Alamos National Laboratory, Los Alamos, NM, USA

 

 

Magnetic nanoparticles have attracted enormous attention recently because of the great promise they hold for use in a broad range of applications spanning magnetic data storage, hyperthermic cancer treatment, and magnetically-directed drug delivery, the performance of which are all intimately tied to interparticle magnetic coupling.  These interparticle correlations depend not only on the spatial distribution of nanoparticles, but also on the internal magnetic structure of the nanoparticle for which many theories, but no consensus, exist.  Indeed, the ability to discriminate between models has long been severely hampered by the fact that most experimental information is inferential, having been obtained from bulk probes such as magnetometry.  Here we report unambiguous determination of the internal magnetic structure of ferrimagnetic iron oxide (magnetite) nanoparticles.  Using neutron scattering methods with enhanced polarization analysis [1], we were able to construct a 3D picture of the magnetic structure with directional sensitivity within a high field environment.  The method revealed that highly crystalline, monodisperse nanoparticles 9 nm in diameter exhibit an unexpected magnetic shell structure (1.0 + 0.2 nm thick) with a moment that is canted 90o perpendicular to that of the ferrimagnetic cores in a nominally saturating field of more than a Tesla.  This discovery dispels a widely held belief, based on the observation of a reduced moment in nanoparticle form compared with bulk, that such nanoparticles possess a disordered, magnetically “dead” layer [2].  Furthermore, we have determined that the average canted shell thickness at 200 K increases in going to 300 K, but disappears upon zero-field cooling to 9 K.  The implications of a tunable, magnetic core-shell morphology for nanoparticle and nanograined material applications are tremendous because it suggests a process for engineered, systematic control of interparticle coupling via magnetic shell modification.

 

[1] K.L. Krycka et al., Physica B 404, 2561-2564 (2009)

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