Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe3O4/MnxFe3−xO4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface.

This work reports on the effect of the oleic acid concentration on the magnetic and structural properties of Fe3-xO4 nanoparticles synthesized by thermal decomposition of Fe(acac)3 in benzyl-ether. This method allows the synthesis of highly monodisperse particles ranging from 7 to 100 nm in size by only varying the concentration of oleic acid in the reaction mixture. The structural and magnetic characterization reveal homogeneous particles in composition, with narrow particle size distribution, which are single-phase magnetite with almost bulk-like values of the saturation magnetization of about 90-99 emu g(-1) at low temperatures and show the characteristic anomaly in the zero field-cooling magnetization curves associated with the Verwey transition for nanoparticles bigger than about 7 nm. In addition, the analyses of aliquots of the reaction mixtures by Fourier transform infrared spectroscopy at various stages shed light on the nucleation and growth processes of the particles.

A general method for the quantification of dipolar interactions in assemblies of nanoparticles has been developed from a model sample constituted by magnetite nanoparticles of 5 nm in diameter, in powder form with oleic acid as a surfactant so that the particles were solely separated from each other through an organic layer of about 1 nm in thickness. This quantification is based on the comparison of the distribution of energy barriers for magnetization reversal obtained from time-dependent relaxation measurements starting from either (i) an almost random orientation of the particles' magnetizations or (ii) a collinear arrangement of them prepared by previously field cooling the sample. Experimental results and numerical simulations show that the mean dipolar field acting on each single particle is significantly reduced when particles' magnetizations are collinearly aligned. Besides, the intrinsic distribution of the energy barriers of anisotropy for the non-interacting case was evaluated from a reference sample where the same magnetic particles were individually coated with a thick silica shell in order to make dipolar interactions negligible. Interestingly, the results of the numerical simulations account for the relative energy shift of the experimental energy barrier distributions corresponding to the interacting and non-interacting cases, thus supporting the validity of the proposed method for the quantification of dipolar interactions.