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.

With oxygen and hydrogen plasma the resistivity of transparent MXene electrodes can be controlled.

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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.

The assembly of tiny magnetic particles in external magnetic fields is important for many applications ranging from data storage to medical technologies. The development of ever smaller magnetic structures is restricted by a size limit, where the particles are just barely magnetic. For such particles we report the discovery of a kind of solution assembly hitherto unobserved, to our knowledge. The fact that the assembly occurs in solution is very relevant for applications, where magnetic nanoparticles are either solutionprocessed or are used in liquid biological environments. Induced by an external magnetic field, nanocubes spontaneously assemble into 1D chains, 2D monolayer sheets, and large 3D cuboids with almost perfect internal ordering. The self-assembly of the nanocubes can be elucidated considering the dipole-dipole interaction of small superparamagnetic particles. Complex 3D geometrical arrangements of the nanodipoles are obtained under the assumption that the orientation of magnetization is freely adjustable within the superlattice and tends to minimize the binding energy. On that basis the magnetic moment of the cuboids can be explained.M agnetic particles show an intriguing self-assembly behavior, due to mutual magnetic interactions or interaction with external magnetic fields. This can already be experienced by playing with toy magnets, which can assemble into strings or clusters. For much smaller magnetic particles, the knowledge about the magnetic assembly is crucial for technical applications involving magnetorheological fluids (1), high-density magnetic storage devices, hyperthermal cancer therapy, and magnetic resonance imaging (2). These applications use small magnetic particles, in the micrometer size range for magnetorheological fluids and down to 10 nm for magnetic resonance imaging or magnetothermal cancer therapy. Decreasing the size of the particles further decreases their magnetic moment to an extent that they are not considered in those applications anymore. In consequence, the assembly of sub-15-nm magnetic nanoparticles has been scarcely explored (3). Only recently, it was reported that cube-shaped magnetic nanoparticles of 13 nm showed a surprising magnetic-field-induced assembly into helices at the air-liquid interface (4) and 9-nm magnetic nanoparticles in the presence of a magnetic field uniquely assembled into very large, nearly defectfree monolayers and 3D cubic assemblies on solid substrates (5). This triggers the question about the arrangement of the magnetic dipoles in such assemblies where an amazing answer was recently found in the case of only eight dipoles (6), and where for larger magnetic nanoparticles and their assemblies considerable complexity was observed (7, 8).Here we report very small (sub-15-nm diameter) spherical and cube-shaped iron-oxide nanoparticles with respect to their magnetic assembly behavior. The nanoparticles are sterically stabilized by oleic acid, have very narrow size distributions, and very regular shapes (SI Appendix). The controlled synthesis of...

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The composition dependence of the magnetic as well as structural properties of epitaxial 4 -40 nm Fe 3 Si thin films on MgO͑001͒ have been investigated using ferromagnetic resonance, superconducting quantum interference device magnetometry, and magneto-optical Kerr effect. Magnetic anisotropy energy, g factor, and magnetization were determined for different samples with Si concentrations of 20%, 25%, or 30% at room temperature. Additionally, different annealing procedures were applied. The magnetization was determined to be on the order of 0 M Ϸ 1 T. It was found that the films have a dominating cubic anisotropy K 4 Ϸ 3 ϫ 10 3 J / m 3 which depends on the thermal treatment of the film and is about 1 order of magnitude smaller than the one of bulk Fe. A small uniaxial in-plane anisotropy of interfacial nature was detected. The perpendicular uniaxial anisotropy term, which is dominated by an interface contribution, favors a perpendicular easy axis. From frequency-dependent ferromagnetic resonance measurements an isotropic g factor was extracted g = 2.075͑5͒ for 8 and 40 nm samples and g = 2.080͑5͒ for the 4 nm one. Different thermal treatments of the sample showed no influence on the g factor. The magnetic anisotropy fields and g factor decrease linearly as the Si concentration increases within the D0 3 regime.

Magnetic domain walls are created in a controllable way in transversally magnetized epitaxial Fe wires on GaAs(110) by approaching a magnetic force microscope (MFM) tip. The electrical resistance-change due to the addition of these domain walls is measured. The anisotropic magnetoresistance as well as the intrinsic domain wall resistance contribute to the resistance-change. The efficiency of this procedure is proven by MFM images, which are obtained subsequent to the domain wall creation at a larger sample-to-probe distance. The contribution of the anisotropic magnetoresistance is calculated using micromagnetic calculations, thus making it possible to quantify the intrinsic domain wall resistance.