Role of Dipolar Interactions on the Determination of the Effective Magnetic Anisotropy in Iron Oxide Nanoparticles.

Challenging magnetic hyperthermia (MH) applications of immobilized magnetic nanoparticles require detailed knowledge of the effective anisotropy constant (Keff ) to maximize heat release. Designing optimal MH experiments entails the precise determination of magnetic properties, which are, however, affected by the unavoidable concurrence of magnetic interactions in common experimental conditions. In this work, a mean-field energy barrier model (ΔE), accounting for anisotropy (EA ) and magnetic dipolar (ED ) energy, is proposed and used in combination with AC measurements to a specifically developed model system of spherical magnetic nanoparticles with well-controlled silica shells, acting as a spacer between the magnetic cores. This approach makes it possible to experimentally demonstrate the mean field dipolar interaction energy prediction with the interparticle distance, dij , ED ≈ 1/dij 3 and obtain the EA as the asymptotic limit for very large dij . In doing so, Keff uncoupled from interaction contributions is obtained for the model system (iron oxide cores with average sizes of 8.1, 10.2, and 15.3 nm) revealing to be 48, 23, and 11 kJ m-3 , respectively, close to bulk magnetite/maghemite values and independent from the specific spacing shell thicknesses selected for the study.

Significant Surface Spin Effects and Exchange Bias in Iron Oxide-Based Hollow Magnetic Nanoparticles.

Exchange bias (EB) properties have become especially important in hollow magnetic nanoparticles (MNPs) due to the versatility and reduced size of these materials. In this work, we present the synthesis and study of the EB properties of iron-oxide-based hollow MNPs and their precursors Fe/iron oxide MNPs with core/void/shell structure. The two mechanisms involved in EB generation were investigated: the frozen spins present in the nanograins that form the nanoparticles and the surface spins. The effect of external parameters on the coercivity (HC), remanence (MR), exchange bias field (HEB) and frozen spins, such as cooling field (HFC) and temperature, was investigated. Both HC and HEB present a maximum threshold above which their values begin to decrease with HFC, showing a new trend of HEB with HFC and allowing modulation on demand. The existence of surface spins, present on the outer and inner surfaces, was demonstrated, and an intrinsic EB phenomenon (HEB = 444 Oe for hollow iron oxide-based MNPs of 13.1 nm) with significant magnetization (MS~50 emu/g) was obtained. Finally, core/void/shell MNPs of 11.9 nm prior to the formation of the hollow MNPs showed a similar behavior, with non-negligible HEB, highlighting the importance of surface spins in EB generation.

Axial Ligation in Ytterbium(III) DOTAM Complexes Rationalized with Multireference and Ligand-Field ab Initio Calculations.

The nature of the axial ligand coordinated to the Yb3+ ion in [Yb(DOTAM)]3+ has profound consequences in the magnetic anisotropy and optical properties of the complex, as evidenced by 1H NMR and UV-vis spectroscopies. The pseudocontact shifts of 1H nuclei and the 2F5/2 ← 2F7/2 absorption band were found to be very sensitive to the nature of the axial ligand (MeOH, H2O, MeOH, or F-). The energy levels of the 2F5/2 and 2F7/2 manifolds in [Yb(DOTAM)(X)]3+ (X = MeOH, H2O, or dimethyl sulfoxide (DMSO)) and [Yb(DOTAM)F]2+ complexes were assigned from the analysis of the optical spectra and ab initio calculations based on CASSCF wave functions that considered dynamic correlation through perturbation theory (NEVPT2) and spin-orbit coupling effects. The magnetic anisotropies obtained with ab initio calculations are in good agreement with the experimental values derived from 1H NMR spectral data, though for the [Yb(DOTAM)(H2O)]3+ and [Yb(DOTAM)F]2+ complexes, the explicit inclusion of a few second-sphere water molecules is required to improve the calculated data. Crystal-field calculations show that the observed pseudocontact shifts do not correlate well with the crystal-field parameter B20, as predicted by Bleaney's theory. The change in the sign of the magnetic anisotropy from prolate (X = MeOH, H2O, or DMSO) to oblate in [Yb(DOTAM)F]2+ is related to the relative energies of the 4fz3 orbital and the 4fx3/4fy3 pair, which are affected by the coordination ability of the axial ligand.