Lecture by Dr. D. Zákutná

1255

IEE SAS invite you to a lecture by Dr. D. Zákutná  (Charles Univ., Prague)
April. 14, 2023, at 10,00 a.m., in IEE SAS meeting room 101

  • Dr. D. Zákutná: Polarized small-angle neutron scattering as a powerful tool for magnetic nanoparticle characterization

Abstract:
Tailoring magnetic nanoparticles (MNPs) by choosing a suitable combination of size, shape, and material is the basis for realizing various technological, biomedical, or environmental applications. For optimal performance in a specific application, it is crucial to interrelate their macroscopic characteristics with the structural and magnetic properties of MNPs and their ensembles. For example, disorder effects -ubiquitous in nanomaterials – crucially determine the magnetic heating performance of MNPs used for hyperthermia, magnetic particle imaging, and catalysis applications [1].

In this contribution, I will present the advantages of small-angle neutron scattering (SANS) for investigating the nanoscale distribution of spin disorder in the relevant mesoscopic size range from 1 to a few hundred nanometres and with nm resolution [2]. Firstly, I will show that the MNPs‘ morphology and chemical homogeneity can be revealed in great detail with SANS, which is not evident from transmission electron microscopy. Then, I will focus on the magnetic morphology of the single-phase MNPs. I will show that the classical picture that considers the single-phase MNPs as a collinearly magnetized core with a structurally and magnetically disordered surface region falls short as the more complex field-dependent magnetization processes near the surface of the MNPs were unveiled [3]. Furthermore, the spatially resolved changes in the magnetized volume allow access to the intra-particle spin-disorder energy, giving indirect insight into the structural defect profile in MNPs. Additionally, I will demonstrate that the sensitivity of SANS to nanoscale density variations has the advantage of revealing the chemical morphology of core-shell MNPs consisting of a wüstite-like core and a spinel-type shell [4]. Finally, I will show that even different local magnetizations within core-shell MNPs can be fully disentangled.

[1] A. Lak, S. Disch, P. Bender Adv. Science 8 (2021) 2002682.
[2] D. Honecker et al., Nanoscale Adv. 4, 1026 (2022).
[3] D. Zákutná et al. Phys. Rev. X 10 (2020) 031019.
[4] D. Zákutná et al. ACS Chem. Mater. (2023) accepted.

 

  • MSc. M. Gerina, D. Zákutná: Size dependence of surface spin disorder in ferrite nanoparticles

Abstract:

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.

[1] E. Tronc et al., Journal of Magnetism and Magnetic Materials 221 (2000) 6379
[2] A. Omelyanchik et al., Nanomaterials. 10 (2020) 1288
[3] B. N. Pianciola et al., J. Magn. Magn.Mater. 377 (2015) 44
[4] V. Bonanni et al., Appl. Phys. Lett. 112 (2018) 022404
[5] D. S. Negi et al., Phys. Rev. B 95 (2017) 174444
[6] S. Mühlbauer et al., Rev Mod Phys. 91 (2019) 015004
[7] D. Zákutná et al., Phys Rev X. 10 (2020) 031019
[8] M. Sanna Angotzi et al., J. Nanosci. Nanotechnol. 19 (2019) 4954