First-principles studies on graphene-supported transition metal clusters.

Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the binding strength of the TM cluster on to the graphene substrate. Among TM13 clusters, Co13 is absorbed relatively more strongly on pristine and defective graphene as compared to Fe13 and Ni13 clusters. The adsorbed clusters show reduced magnetic moment compared to the free clusters.

Magnetic nanostructures by adaptive twinning in strained epitaxial films.

We exploit the intrinsic structural instability of the Fe(70)Pd(30) magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials.

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.

Structural and magnetic properties of ternary Fe(1-)MnPt nanoalloys from first principles.

BACKGROUND: Structural and magnetic properties of binary Mn-Pt and ternary Fe(1-) (x)Mn(x)Pt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared. RESULTS: The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices. CONCLUSION: Addition of Mn effectively increases the stability of single crystalline L1(0) particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications.

Multiply twinned morphologies of FePt and CoPt nanoparticles.

Based on large-scale density functional theory calculations we provide a systematic overview of the size dependence of the energetic order and magnetic properties of various morphologies of FePt and CoPt clusters with diameters of up to 2.5 nm. For FePt, ordered multiply twinned icosahedra and decahedra are more favorable than the L1_(0) phase throughout the investigated size range. For CoPt, segregated morphologies predominate with considerably increased energy differences to the L1_(0) structure. The compositional trends are traced back to differences between the morphologies in the partial electronic density of states associated with the 3d element.

Shellwise Mackay transformation in iron nanoclusters.

Structure and magnetism of iron clusters with up to 641 atoms have been investigated by means of density functional theory calculations including full geometric optimizations. Body-centered cubic (bcc) isomers are found to be lowest in energy when the clusters contain more than about 100 atoms. In addition, another stable conformation has been identified for magic-number clusters, which lies well within the range of thermal energies as compared to the bcc isomers. Its structure is characterized by a close-packed particle core and an icosahedral surface, while intermediate shells are partially transformed along the Mackay path between icosahedral and cuboctahedral geometry. The gradual transformation results in a favorable bcc environment for the subsurface atoms. For Fe55, the shellwise Mackay-transformed morphology is a promising candidate for the ground state.