Karl-Hartmut Müller

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.

High trapped fields in bulk YBa2Cu3O7−δ samples at temperatures around 50 K

Bulk melt textured YBa2Cu3O7−δ samples with single grains of about 24 mm diameter were obtained by use of SmBa2Cu3O7−x seed crystals. The maximum trapped field B0 in the gap between two samples was investigated as function of temperature. B0 increased from 1 T at 77 K to 8.5 T at 51.5 K, which is the highest trapped field achieved in nonirradiated samples. At low temperatures, cracking of the samples was observed under magnetic pressure. In this temperature range, the trapped field is limited by the mechanical strength of the samples, for which a value of 25 MPa was estimated.

Ferromagnetic carbon with enhanced Curie temperature

Abstract The discovery of a ferromagnetic form of carbon (Nature 413 (2001) 716) gives a new perspective in the investigation of magnetic materials. The existence of a ferromagnetic state with the very high Curie temperature T C ≈500 K for a material with only s- and p-electrons as well as the nature of its underlying interaction are of great fundamental interest. Here we report on the observation of the ferromagnetically ordered state in a material obtained by high-pressure high-temperature treatment of the fullerene C60. It has a saturation magnetization more than four times larger than that reported previously. From our data we estimated the considerably higher value of T C ≈820 K .

High trapped fields in melt-textured YBCO samples

Abstract Bulk melt textured YBCO samples in the shape of disks were developed for use as trapped field magnets. The trapped field in single grains with diameter of 24 mm reached maximum values up to 0.56 T at 77 K and a value of 5.5 T at 30 K. Degradation of the trapped field was observed in samples after the activation process at 20 K in a high applied magnetic field. This is interpreted in the framework of a model of magnetic stress induced damage.

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.

Rare earth transition metal borocarbides (nitrides): superconducting, magnetic and normal state properties

Preface. Introduction and Overview. Electronic Structure and Properties and Phonon Spectra. Magnetic Properties and CEF Effects. Interplay between Superconductivity and Magnetism. Vortex Lattice. Thin Films and Crystal Structure. Nature of the Superconducting State in Borocarbides. Summary and Outlook. Subject Index. Author Index. List of Participants.

Ordering of nanocrystalline Fe-Pt alloys studied by in situ neutron powder diffraction

The ordering in nanocrystalline Fe100−xPtx (x=40, 45, 50, and 55) alloys prepared by mechanical ball milling was studied by neutron powder diffraction during in situ heat treatment and differential scanning calorimetry. Independent of milling time and stoichiometry the onset of ordering is at around 300°C. For alloys with a multilayer-type microstructure an enhanced kinetics of ordering is observed, characterized by an activation energy of 1.03±0.08eV. The transformation to the L10 phase proceeds via a metastable state, which is a mixture of highly ordered L10 and L12 phases. For samples with homogeneous A1 phase, the A1→L10 transformation is characterized by a lower driving force and higher activation energy. For nonequiatomic compositions, the A1→L10 transformation is considerably decelerated.

Superconducting permanent magnets and their application in magnetic levitation

Abstract Superconducting permanent magnets form a completely new class of permanent magnets. Of course, they must be cooled to 77 K or below. At very low temperatures (24 K) their magnetization can be a factor of 10 higher than that of the best conventional magnets, providing magnetic forces and energies which are up to two orders of magnitude higher. These new supermagnets became only recently available by the extreme improvement of the quality of melt-textured massive YBa2Cu3Ox samples. Besides having a high magnetization, these superconducting permanent magnets can freeze in any given magnetic field configuration allowing completely new applications like superconducting transport systems or superconducting magnetic bearings.

Description of texture for permanent magnets

A least mean square procedure was used to fit five types of trial texture functions, each of them containing one fitting parameter, to demagnetization curves of Zn-bonded Sm/sub 2/Fe/sub 17/N/sub x/ samples with varying degree of easy-axis alignment as well as Zn-bonded Sm/sub 2/Fe/sub 17/C/sub x/. The demagnetization curves were measured in the first quadrant of polarization-field plane, parallel and perpendicularly to the texture axis. The temperature dependence (above room temperature) of the intrinsic magnetic properties K/sub 1/K/sub 2/J/sub s/ was determined for both materials. The sensitivity of the calculated anisotropy constants to the degree of easy-axis alignment turns out to be the best criterion to find out the correct texture function which can be described by a Gaussian density in the case of our Zn-bonded Sm/sub 2/Fe/sub 17/N/sub x/. >

Intergrain interactions in nanocomposite Fe–Pt powders

The structure and magnetic properties of nanocomposite Fe100−xPtx (x=40, 50, and 60) powders prepared by mechanical alloying followed by annealing are investigated. Various combinations of phases, away from thermodynamic equilibrium, were obtained using this technique. Remanence curves and magnetic force microscopy data were used to provide insight into the nature of intergrain interactions in such powders. The analysis points to strong intergrain coupling in the Fe100−xPtx powders. An additional small magnetostatic contribution can be observed for the Pt-rich powders.