Size dependence of surface spin disorder in ferrite nanoparticles
M.Gerina1, M.S. Angotzi2, , V. Gajdošová3, M. Dopita4, D.Zákutná1*
1Faculty of Science, Charles University, Prague, Czech Republic
2Faculty of Chemical and Geological Science, University of Cagliari, Monserrato, Italy
3Polymer Morphology, Institute of Macromolecular Chemistry CAS, Prague, Czech Republic
4Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
Surface spin disorder or canting arises from the breaking of exchange bonds and the breaking symmetry of the lattice, and thus crucially determines the performance of magnetic nanoparticles (NPs) and their potential technological and biomedical applications[1,2]. Despite an enormous interest and technological relevance of magnetic NPs, there is still a lack of knowledge on the magnetic NPs spin structure. Due to the surface-to-volume ratio, surface effects will be closely related to the particle and coherent domain size. However, it is difficult to isolate the surface contribution from the bulk effects using macroscopic magnetization techniques, such as magnetization measurements, ferromagnetic resonance, Mössbauer spectroscopy[3], x-ray magnetic circular dichroism[4], and electron energy loss spectroscopy[5]. A spatially resolved magnetization is required to unveil and disentangle the surface contribution. Half-polarized small angle-neutron scattering (SANSPOL) enables us to investigate the magnetization on the nanometer scale[6]. Our previous study has proven that the magnetic volume in ferrite NPs is not fixed at the coherent domain size but increases with the applied magnetic field[7]. This implies that the applied magnetic fields polarize the disordered surface spins, leading to an increase in the magnetic size of the NPs[7].
In this contribution, we will present the size dependence of the disorder energy and the surface anisotropy in spherical CoFe2O4 NPs with different coherent domain sizes range of 3.1(1), 6.3(2), and 8.6(1) nm synthesized using the oleate-based solvothermal method[8] with narrow size distribution confirmed by transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS). Rietveld’s analysis shows that coherent domain size is smaller than mean particle size, suggesting a possible presence of a spin disorder or canting. The spatial magnetization distribution obtained from SANSPOL reveals significant magnetic field dependence of magnetized volume for each sample, but with different degrees of the total magnetized NP volume. Ultimately, we will discuss the particle and coherent size dependence of the surface anisotropy constant.
Figure 1 Schematic presentation of the size dependence of magnetized volume growth in applied magnetic field. Grey and brown particle part corresponds to the disorder and magnetized volume of nanoparticles.
We greatly acknowledge Dr. Jana Havlièková for the TGA analysis, and Dušan Roha¾ for synthetizing 1 samples. We acknowledge the Institut Laue-Langevin, Grenoble, France for the provision of beamtime at the instrument D33 and Dr. Nina J. Steinke for the technical support at the instrument.