A. Trunova

A guideline for atomistic design and understanding of ultrahard nanomagnets.

Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments.

Element-Specific Magnetic Hysteresis of Individual 18 nm Fe Nanocubes

Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies.

Magnetic characterization of iron nanocubes

Nearly perfect single crystalline Fe core-shell nanocubes with (100) facets and 13.6 nm edge length were prepared by wet-chemical methods. While the core is metallic, the shell is composed of either Fe3O4 or γ-Fe2O3. The cubes were deposited onto GaAs substrates with monolayer coverage as proved by scanning electron microscopy. Oxygen and hydrogen plasmas were used to remove the ligand system and the oxide shell. Both types of samples were investigated by ferromagnetic resonance. While the g-factor (g=2.09) and crystalline anisotropy (K4=4.8×104 J/m3) of the pure iron cubes show up with bulk values, the saturation magnetization is reduced to (M(5K)=(1.2±0.12)×106 A/m) 70% of bulk value and the effective damping parameter (α=0.03) is increased by one order of magnitude with respect to bulk Fe.

X-ray absorption measurements on nanoparticle systems: self-assembled arrays and dispersions

X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material.As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations.

Correlation of magnetic moments and local structure of FePt nanoparticles

The influence of structural and compositional changes within FePt nanoparticles on their magnetic properties was studied by means of x-ray absorption spectroscopy in the near-edge regime and its associated magnetic circular dichroism as well as by analysis of the extended x-ray absorption fine structure. The magnetic moments at the Fe sites were found to be a sensitive monitor to changes of the local surrounding: While compositional inhomogeneities in the nanoparticles yield significantly reduced magnetic moments (by 20–30%) with respect to the corresponding bulk material, thermally induced changes in the crystal structure yields strongly enhanced orbital contributions (up to 9% of the spin magnetic moment). Also the break of crystal symmetry at the surface leads to an enhanced orbital magnetism which was confirmed by determination of the ratio of orbital-to-spin magnetic moment for FePt particles with different sizes between 3 and 6 nm in diameter.

Intrinsic Magnetism and Collective Magnetic Properties of Size-Selected Nanoparticles

Using size-selected spherical FePt nanoparticles and cubic Fe/Fe-oxide nanoparticles as examples, we discuss the recent progress in the determination of static and dynamic properties of nanomagnets. Synchroton radiation-based characterisation techniques in combination with detailed structural, chemical and morphological investigations by transmission and scanning electron microscopy allow the quantitative correlation between element-specific magnetic response and spin structure on the one hand and shape, crystal and electronic structure of the particles on the other hand. Examples of measurements of element-specific hysteresis loops of single 18 nm sized nanocubes are discussed. Magnetic anisotropy of superparamagnetic ensembles and their dynamic magnetic response are investigated by ferromagnetic resonance as a function of temperature at different microwave frequencies. Such investigations allow the determination of the magnetic relaxation and the extraction of the average magnetic anisotropy energy density of the individual particles.