Supraferromagnetic correlations in clusters of magnetic nanoflowers

Abstract

Magnetic nanoflowers are densely packed aggregates of superferromagnetically coupled iron oxide nanocrystallites, which excel during magnetic hyperthermia experiments. Here, we investigate the nature of the moment coupling within a powder of such nanoflowers using spin-resolved small-angle neutron scattering. Within the powder the nanoparticles are agglomerated to clusters, and we can show that the moments of neighboring nanoflowers tend to align parallel to each other. Thus, the whole system resembles a hierarchical magnetic nanostructure consisting of three distinct levels, i.e. (i) the ferrimagnetic nanocrystallites as building blocks, (ii) the superferromagnetic nanoflowers, and (iii) the supraferromagnetic clusters of nanoflowers. We surmise that such a supraferromagnetic coupling explains the enhanced magnetic hyperthermia performance in case of interacting nanoflowers.The working principle of magnetic hyperthermia (MHT) is to administer a moderate quantity of magnetic nanoparticles within tumors and to heat them up by applying alternating magnetic fields with clinically acceptable parameters (i.e. comparatively high frequencies 100 kHz but low amplitudes 20 mT 1,2 ) to kill the tumors. Additionally, a magnetomechanical actuation of the embedded particles may disrupt the cytoskeleton and lead to cell death. 3,4 In physiological environment nanoparticles usually agglomerate, which can significantly modify their magnetic properties compared to the dilute non-interacting case 5,6 and which in turn may alter their heating behavior. 7-10 Depending on the characteristics of the individual particles and the field parameters, such a clustering can either improve or impair the MHT performance. [11][12][13][14][15] In fact it was observed for so-called nanoflowers, which are densely packed aggregates of iron oxide crystallites, that they excel during MHT experiments compared to the singlecrystals 16 and other systems such as magnetosomes. 17 This intriguing result motivated numerous studies regarding synthesis and characterization of such flower-shaped particles. [18][19][20][21][22][23] It can be shown that an exchange coupling between the cores leads to a superferromagnetic magnetization state 24 within the individual nanoflowers, 25 but with a significant internal spin disorder caused by the high defect density, e.g. due to the grain boundaries. 26 It is speculated that such a disordered state enables an increased excitation of the moments 27,28 , similar to other defect-rich particles. 29 When introduced into tumors, it is safe to assume that the nanoflowers will agglomerate to clusters, and thus interparticle interactions will be relevant. 30 In Bender et al. 31 we could show for homogeneous superparamagnetic nanoparticles a predominance for antiferromagnetic-like moment correlations within particle clusters via polarized small-angle neutron scattering (SANS). In this work we use the same approach to determine the nature of the moment coupling within a powder of iron oxide a) Electronic mail: philipp.bender@uni...

Embracing Defects and Disorder in Magnetic Nanoparticles

“…[ 34 ] This can be related to a parallel alignment between neighboring particle moments within dense aggregates. [ 87 ] This behavior is in contrast to conventional interacting iron oxide nanoparticle systems (i.e., dense powder samples of single‐core particles), in which an antiparallel alignment of neighboring particle moments has been observed, [ 88,89 ] culminating in reduced susceptibility and decreased heating. [ 90 ]…”

From Single-Core Nanoparticles in Ferrofluids to Multi-Core Magnetic Nanocomposites: Assembly Strategies, Structure, and Magnetic Behavior

“…Very recently, Bender et al [255] looked at so-called "magnetic nanoflowers", which are defined as densely packed aggregates of superferromagnetically coupled iron oxide nanocrystallites. Polarized SANS was used to investigate the moment coupling within a powder of such nanoflowers.…”

Unraveling Nanostructured Spin Textures in Bulk Magnets

“…In most studies, data analysis is done by analyzing 1D sectors or radial averages, e.g., by fitting the data to a particular model in reciprocal space, [21,22] or by determining model independently the realspace 1D correlation functions. [23,24] But due to the anisotropic DOI: 10.1002/smsc.202000003…”

“…In most studies, data analysis is done by analyzing 1D sectors or radial averages, e.g., by fitting the data to a particular model in reciprocal space, [ 21,22 ] or by determining model independently the real‐space 1D correlation functions. [ 23,24 ] But due to the anisotropic nature of magnetic scattering, reducing the analysis to 1D essentially means a loss of information. Moreover, in many studies structural form‐factor models, adapted from nuclear SANS, are utilized, which fail to account for the existing spin inhomogeneity inside magnetic nanostructured systems.…”