Revealing Magnetic Morphologies and Spin Disorder in Ferrite Nanoparticles using polarized SANS

Abstract

Magnetic nanoparticles reveal unique magnetic properties making them relevant for data storage, electronic and mechanical engineering, and biomedical applications.Despite the considerable technological relevance and fundamental importance, a key challenge in magnetic nanoparticle research remains in the quantitative interpretation of the three-dimensional magnetic configuration and the nanoscale distribution of magnetization and spin disorder. Disorder effects - being intrinsic to nanomaterials - crucially determine the magnetization properties such as the heating performance of magnetic nanoparticles [1-4]. In this regard, the main aspects of fundamental interest are the effective magnetic anisotropy and the magnetization distribution within individual nanoparticles. Polarized, magnetic contrast-enhanced small-angle neutron scattering (SANS) is a versatile technique to investigate the chemical morphology and magnetization with nanoscale spatial resolution [5]. Addressing structural and magnetic inhomogeneities on the nanoscopic length scales of 1-300 nm, magnetic SANS is highly relevant for a variety of materials classes ranging from permanent magnets, magnetic steels and alloys to vortex lattices, skyrmions, and ferrofluids. For monodisperse nanoparticles, magnetic SANS has the unique potential to isolate core and surface-related effects of magnetization and spin disorder.In this contribution, we will present our recent results on the spin disorder in ferrite nanoparticles induced during synthesis.For the synthesis of iron oxide nanoparticles via thermal decomposition of iron oleates [7], the initial formation of FeO nanoparticles followed by topotaxial oxidation to magnetite and maghemite is well established [8,9]. Polarized SANS offers a great potential for the understanding of the microstructure in such heterostructured nanoparticles and allows to disentangle the magnetic field response of core and shell magnetization. For cobalt ferrite nanoparticles, our polarized SANS study reveals even in the wuestite-like nanoparticle core a significant magnetization contribution at ambient temperature (Fig. 1), in contrast to the conventionally assumed paramagnetic or AFM behavior. This FiM@FiM core-shell morphology explains the origin of the low-temperature exchange-bias phenomenon.Despite the overall homogeneous Co:Fe distribution within the nanoparticles, the sensitivity of SANS to nanoscale density variations enables us to verify the chemical core-shell morphology for as-prepared cobalt ferrite nanoparticles. We analyze the progressive oxidation of the core and the aging process into single-phase ferrite nanoparticles with nearly homogeneous magnetization distribution. This process occurs within years and does not compromise the size distribution or stability of the dispersion.In an idealized picture, one would consider such single-crystalline magnetic nanoparticles as defect-free nanocrystals with a collinearly magnetized core. In real systems, however, the internal spin structure exhibits a greater complexity, e.g., at the surface where compositional and structural defects are ubiquitous. As a result, the surface region is described as a shell of magnetically disordered surface spins. Polarized SANS has the necessary sensitivity to resolve the spatial distribution of spin disorder in fine detail.We have recently shown that the magnetization configuration inside magnetic nanoparticles may not be static – as often presumed – but rather susceptible to magnetic fields with the occurrence of field-dependent magnetization processes [6]. We have established a significant field-induced growth of the total particle moment by a magnetic ordering transition at the structurally disordered surface (Fig. 2). In our study, polarized SANS extends the traditional macroscopic magnetic characterization and reveals the local magnetization response. This approach allows us to quantitatively separate surface-spin disorder from intra-particle disorder contributions. Finally, we have elucidated the intra-particle spin-disorder energy, giving indirect insight into the structural defect profile in magnetic nanoparticles. **