Magnetic small-angle neutron scattering from self-assembled iron oxide nanoparticles influenced by field

Self-assembly of magnetic nanoparticles, in general, is of interest due to the broad range of applications in material science and biomedical engineering [1,2]. Parameters that affect self-assembly in nanoparticles include particle size, the applied magnetic field profile, concentration and synthesis routines [3]. A range of different sizes of iron oxide nanoparticles between 17 and 27 nm were investigated using polarized small-angle neutron scattering (SANS) at the KWS-1 instrument operated by the Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. Nanoparticles were dispersed in toluene and measured at room temperature in a range of applied fields between ±2.2 T. The observed self-assembly strongly depended on both nanoparticle size and applied field. For smaller particles (diameter ≤ 20 nm), there was no indication of self-assembly even at high concentration (1% v/v), while 27 nm nanoparticles assemble into linear chains even in low concentrations (0.42% v/v) and low field.The smallest nanoparticles (d = 17 nm) were studied by contrast variation; by altering the isotopic composition of the toluene solvent, the magnetization profile within the cores of the nanoparticles could be extracted with high-resolution when using a spin-polarized incident neutron beam [4]. For larger nanoparticle, the structural and form factors were obtained by sector analysis of the 2-D SANS patterns (Fig. 1(a) and (b)). The extracted structure factors suggest that the chains grow longer and straighter and align more closely with the field direction up until application of the maximum field (Fig. 1(c)). This is understood in terms of a minimization of the dipole energy of the nanoparticles in the presence of the applied field and neighbouring particles. The implications for the control of self-assembly of more complex nanoparticles will be discussed.[1] G. Ozina, K. Hou, B. Lotsch, L. Cademartiri, D.Puzzo, F. Scotognella, A. Ghadimi, J. Thomson, Materials Today, Vol. 12, p.12 (2009)[2] P. Tartaj, Current Nanoscience, Vol. 2, p.43 (2006) [3] Z. Fu, Y. Xiao, A. Feoktystov, V. Pipich, M. Appavou, Y. Su, E. Feng, W. Jin and T. Brückel, Nanoscale, Vol. 8, p.18541 (2016)[4] A. Wiedenmann, Journal of Applied Crystallography, Vol. 33, p.428 (2000