C. Antoniak

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.

MAGNETISM AT THE NANOSCALE: THE CASE OF FePt

FexPt1-x nanoparticles prepared by organometallic synthesis and gas-phase condensation were structurally and magnetically characterised. The effective spin magnetic moments at both the Fe and Pt sites are reduced with respect to the moments in the corresponding bulk material by up to 20% and further decrease with decreasing particle size at the Fe sites. The ratio of orbital-to-effective-spin magnetic moment at the Fe sites increases from 2.1% for 6 nm particles to 3.4% for 3.4 nm particles due to the break of symmetry at the surface. A lowering of the crystal symmetry after the transformation to the chemically ordered L10 state yields a ≈ 9% and is accompanied by an enhancement of the coercive field at 15 K from (36±5) mT to (292±8) mT indicating an increase of the anisotropy.

Enhancement of L10 phase formation in FePt nanoparticles by nitrogenization

FePt particles 6 nm in size are produced by argon/nitrogen sputtering and gas-phase condensation. Marked changes in the atomic structure and morphology of the particles occur upon addition of nitrogen to the sputter gas. Electron energy loss spectroscopy and x-ray absorption spectroscopy show that nitrogen is incorporated in the particles in molecular and compound form. In situ sintering of the particles drives out the nitrogen causing enhanced diffusion leading to the preference of the L10 structure over the multiply twinned icosahedral structure, which forms in the absence of nitrogen in the sputter gas.

Textured growth of the high moment material Gd(0 0 0 1)/Cr(0 0 1)/Fe(0 0 1)

By magnetic coupling of Fe and Gd via Cr interlayers, the large local moment of Gd can be combined with the high Curie temperature of Fe. The textured growth of a Gd film is studied here by preparing trilayer systems of Fe/Cr/Gd on MgO(1 0 0) substrates by molecular-beam epitaxy (MBE). The thickness of the Cr interlayer was varied between 3 and 5 monolayers. The structural quality of the samples was confirmed by in situ RHEED and ex situ XRD measurements. Epitaxial Cr(0 0 1)/Fe(0 0 1) growth was observed, as expected. By use of 57Fe-CEMS (conversion electron Mossbauer spectroscopy) in combination with the 57Fe tracer layer method the Fe/Cr interface could be examined on an atomic scale and well separated Fe/Gd layers for all Cr thicknesses were confirmed. The unusual Gd/Cr crystallographic relationship of Gd(0 0 0 1)∥Cr(0 0 1), with domains of the hexagonal Gd basal planes randomly oriented in the sample plane and not in registry with the underlying Cr(0 0 1) lattice, was found from combined RHEED and x-ray measurements. Annealing of the samples resulted in a remarkable improvement of the crystalline structure of the Gd layers. On the other hand, the appearance of a single line in the CEM spectrum leads to the conclusion that during annealing a small amount of Fe diffuses into the Cr layer. The electronic structure and magnetism of this system are investigated by first-principles theory.

Induced magnetic Cu moments and magnetic ordering in Cu2MnAl thin films on MgO(0 0 1) observed by XMCD

The disorder–order transition of a highly defective A2-ordered Cu2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 ± 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 ± 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations.

FeSi diffusion barriers in Fe/FeSi/Si/FeSi/Fe multilayers and oscillatory antiferromagnetic exchange coupling

We study the diffusion of 57 Fe probe atoms in Fe/FeSi/Si/FeSi/Fe multilayers on Si(111) prepared by molecular beam epitaxy by means of 57 Fe conversion electron Mossbauer spectroscopy (CEMS). We demonstrate that the application of FeSi boundary layers successfully inhibits the diffusion of 57 Fe into the Si layer. The critical thickness for the complete prevention of Fe diffusion takes place at a nominal FeSi thickness of tFeSi = 10-12 u A, which was confirmed by the evolution of the isomer shift δ of the crucial CEM subspectrum. The formation of the slightly defective c-FeSi phase for thicker FeSi boundary layers (∼20 u A)

Magnetic anisotropy in nanoscaled materials probed by ferromagnetic resonance

Ferromagnetic resonance measurements probe the dynamical response of magnetic systems due to an excitation within the microwave regime. Offering high sensitivity and energy resolution in the μeV range of ferromagnetic resonance this technique is well suited for the investigation of magnetic anisotropy in nanoscale systems. Ferromagnetic Resonance experiments give direct and quantitative access to magnetic anisotropy based on an analysis that uses the Landau--Lifshitz equation of motion. This will be demonstrated for the case of ultrathin magnetic 5--20ML thick Fe films on {4×6}GaAs(001) (2D system) which have been grown and measured in situ in ultra high vacuum, magnetic MnAs stripes (1D system) grown on GaAs(001) as well as for arrays of highly monodisperse FePt nanoparticles (quasi 0D system).

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.

X-Ray Magnetic Dichroism

An introduction is given to the X-ray magnetic dichroism focussing on X-ray magnetic circular dichroism (XMCD). The standard analysis of XMCD spectra by using the sum rules is elucidated. Additionally, aspects of the experimental realization and the data analysis are presented. By means of experimental examples of light 3d metal films, rare earth single crystals, and Fe-porphyrin molecules, the assets and drawbacks of the XMCD technique are illustrated. It is shown that the comparison of ab initio calculated spectra to the experimental results can provide the magnetic properties of the samples if the standard analysis fails.