Manuel Richter

Co dimers on hexagonal carbon rings proposed as subnanometer magnetic storage bits.

It is demonstrated by means of density functional and ab initio quantum chemical calculations, that transition-metal-carbon systems have the potential to enhance the presently available area density of magnetic recording by 3 orders of magnitude. As a model system, Co2 benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy of the order of 0.1 eV per molecule, large enough to store permanently 1 bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite.

Full tunability of strain along the fcc-bcc bain path in epitaxial films and consequences for magnetic properties.

Huge deformations of the crystal lattice can be achieved in materials with inherent structural instability by epitaxial straining. By coherent growth on seven different substrates the in-plane lattice constants of 50 nm thick Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches 8,3 % relative to the fcc lattice. The in-plane lattice strain results in a remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. This has dramatic consequences for the magnetic key properties. Magnetometry and X-ray circular dichroism (XMCD) measurements show that Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad ranges.

Magnetocrystalline anisotropy in L10 FePt and exchange coupling in FePt/Fe3Pt nanocomposites

The magnetic and structural properties of Fe–Pt nanocomposites and related idealized structures have been investigated by a combination of experimental and theoretical techniques. The dependence of magnetocrystalline anisotropy (MCA) of L10 FePt on the ratio of the tetragonal lattice parameters, c/a, has been calculated with a relativistic version of the full potential local orbital method, assuming complete chemical order and fixed unit-cell volume. It has been found that the well known tetragonal lattice distortion in this phase has a relatively small influence on the MCA (compared to the influence of chemical ordering) and even reduces the MCA. The calculated in-plane anisotropy is negligible. The structure, magnetic properties and magnetization reversal processes of Fe100−xPtx (x = 40, 45, and 50) powders produced by mechanical milling and subsequent annealing have been investigated. Structural studies reveal that upon annealing of the as-milled powders consisting of fine Fe/FePt(A1)/Pt lamellae, chemically highly ordered L10 FePt and, in the case of the Fe-rich compositions, L12 Fe3Pt are formed. The nanometre scale multilayer structure preserved after annealing gives rise to large effects of exchange interactions between the crystallites of the phases. With decreasing Pt concentration x, the remanence enhancement increases, due to the increase of the Fe3Pt fraction, whereas the coercivity and the switching fields for irreversible magnetization reversal are reduced.

Electric-field control of surface magnetic anisotropy: a density functional approach

In a recent experiment, Weisheit et al (2007 Science 315 349) demonstrated that the coercivity of thin L10 FePt and FePd films can be modified by the external electric field in an electrochemical environment. Here, this observation is confirmed by density functional calculations for the intrinsic magnetic anisotropy. The origin of the effect is clarified by means of a general and simple method to simulate charged metal surfaces. It is predicted that the coercivity of thin CoPt films is much more susceptible to electric field than that of FePt films.

Heat and charge transport properties of MgB2

Abstract A polycrystalline sample of the MgB 2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8 and 300 K in zero magnetic field. The electrical resistivity shows a superconducting transition at T c =38.7 K and, similarly to borocarbides, a T 2.4 behaviour up to 200 K. The electron diffusion thermopower and its band-structure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electron–phonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below T c is observed resulting in a T 3 behaviour below 7 K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.

Magnetic-field- and temperature-dependent Fermi surface of CeBiPt

The half-Heusler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov–de Haas (SdH) and Hall-effect measurements. Neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below TN = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field and stoichiometry. Above a certain, sample-dependent, threshold field (B>25 T), the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4f compound LaBiPt. Electronic-band-structure calculations can well explain the observed behaviour by a 4f-polarization-induced Fermi-surface modification.

Jahn–Teller-like origin of the tetragonal distortion in disordered Fe–Pd magnetic shape memory alloys

The electronic structure and magnetic properties of disordered FexPd100−x alloys (50<x<85) are investigated in the framework of density functional theory using the full potential local orbital method. Disorder is treated in the coherent potential approximation. Our calculations explain the experimental magnetization data. The origin of the tetragonal distortion in the Fe–Pd magnetic shape memory alloys is found to be a Jahn–Teller-like effect, which allows the system to reduce its band energy in a narrow composition range. Prospects for an optimization of the alloys’ properties by adding third elements are discussed.

Magnetic field dependence of the maximum adiabatic temperature change

The field dependence of the maximum magnetocaloric ΔT-effect in ferromagnets with second-order phase transitions is studied by way of direct measurements. All studied materials are found to follow the formula ΔTmax=A(H+H0)2/3-AH02/3, where A and H0 are constants and H is the internal magnetic field. It is essential to distinguish the latter from external field Hext. The dependence of ΔTmax on Hext is qualitatively distinct, the difference being particularly pronounced in the low-field region. In the field range relevant to applications (0.1–2 T), ΔTmax follows a linear dependence on H2/3. It is proposed to use the slope of this dependence as a figure of merit of magnetic refrigerants.

Transition metal dimers as potential molecular magnets: A challenge to computational chemistry

Dimers are the smallest chemical objects that show magnetic anisotropy. We focus on 3d and 4d transition metal dimers that have magnetic ground states in most cases. Some of these magnetic dimers have a considerable barrier against re‐orientation of their magnetization, the so‐called magnetic anisotropy energy, MAE. The height of this barrier is important for technological applications, as it determines, e.g., the stability of information stored in magnetic memory devices. It can be estimated by means of relativistic density functional calculations. Our approach is based on a full‐potential local‐orbital method (FPLO) in a four‐component Dirac‐Kohn‐Sham implementation. Orbital polarization corrections to the local density approximation are employed. They are discussed in the broader context of orbital dependent density functionals. Ground state properties (spin multiplicity, bond length, harmonic vibrational frequency, spin‐ and orbital magnetic moment, and MAE) of the 3d and 4d transition metal dimers are evaluated and compared with available experimental and theoretical data. We find exceptionally high values of MAE, close to 0.2 eV, for four particular dimers: Fe2, Co2, Ni2, and Rh2.

Influence of composition and order on the magnetism of Fe-Pt alloys : Neutron powder diffraction and theory

The dependence of the magnetic moments on the compositional order in Fe–Pt alloys was studied by neutron powder diffraction. For alloys with almost perfect L10-type long-range order the experimental value of the Fe magnetic moment was determined to be 2.8±0.1μB (extrapolated to zero temperature). Combined analysis of experimental and density functional data shows that the Fe moment drops with increasing Fe content, but is less sensitive to the degree of order, in contrast to the well-known behavior of Fe–Al alloys.