Ingo Opahle

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.

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.

Multistep approach to microscopic models for frustrated quantum magnets: the case of the natural mineral azurite.

The natural mineral azurite Cu(3)(CO(3))(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

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.

Microscopic origin of pressure-induced phase transitions in the iron pnictide superconductors A Fe 2 As 2 : An ab initio molecular dynamics study

Using {\it ab initio} molecular dynamics we investigate the electronic and lattice structure of

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

A

Effect of external pressure on the Fe magnetic moment in undoped LaFeAsO from density functional theory: Proximity to a magnetic instability

Fe

Thermally induced crystal-to-crystal transformations accompanied by changes in the magnetic properties of a CuII-p-hydroquinonate polymer

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