R. Groessinger

Magnetoelectric Properties of CoFe

The CoFe₂O₄-BaTiO₃ core-shell structure composite has been successfully synthesized by wet chemical method. X-ray characterization showed that the composite consisted of two single phases: CoFe₂O₄ and BaTiO₃. The saturation magnetization of the CoFe₂O₄ component in composite was found to be similar to those of the bulk sample. It was observed that the longitudinal and transverse magnetoelectric coefficients of the core-shell structure are 18 times higher than those of the mixed structure

_2

This paper reports the magnetoelectric properties of CoFe2O4-BaTiO3 core-shell structure composites. Crystalline structures of the composites were investigated by X-ray diffractometer (XRD). Magnetic properties were studied using a pulse field magnetometer. Magnetostriction was measured using strain gauge method. AC susceptibility and ME coefficient were investigated using lock-in technique in which AC fields with frequencies from 1 Hz to 4 kHz and amplitudes from 0.1 Oe to 20 Oe superimposed onto a DC magnetic field up to 6500 Oe were employed. All measurements were carried out at room temperature and ambient pressure.

O

On different soft magnetic materials (such as pure Fe, Ni, Fe3% Si, Fe6.5% Si) the frequency dependence (from 0.5 to 50 Hz) of the hysteresis loop was measured on ring shaped samples. The samples were plastically deformed at different temperatures using the so-called high pressure torsion (HPT) method. From these data the frequency dependence of the coercivity for all samples can be deduced. A simple model based on eddy currents in the material is developed in order to describe the change of the coercivity with frequency. The most important parameter here is the relative permeability, which can be estimated from this model. The relative permeability was additionally determined from a plot of dB/dt versus the external field, which was in this case a triangular shaped H(t). The model parameters are compared and discussed. The internal stress due to the HPT treatment was estimated directly from the coercivity.

_4

The nonlinear dependence of magnetization on history and direction of the applied magnetic field in Ba- and Sr-ferrites is described by statistical domain behavior using phenomenological parameters. These parameters are related to spontaneous magnetization, magnetocrystalline anisotropy, magnetostriction, and microstructure geometry, using dimensionless microscopic constants.

-BaTiO

Abstract Elastic powder neutron diffraction experiments on the ThMn12-type compound HoFe10Mo2 in the paramagnetic state proved that Mo occupies the 8i sites only, while the 8j and 8f sites are fully occupied by Fe. Below TC in the entire temperature range a magnetic refinement yielded a simple ferrimagnetic structure with a magnetization oriented along the c-axis. The results of the magnetic refinement are compared with data previously obtained from bulk magnetization measurements.

_3

Hysteresis loops and magnetostriction were measured on ring-shaped Fe85Ga15 samples. The samples were annealed at T = 980 °C for 100 h and cooled from annealing temperature in two different ways: one was slowly cooled in an oven and the other one was quenched in cold water. X-ray diffraction results showed a strong texture in these samples, which is dependent on the cooling process. The anisotropy energy and magnetostriction of the samples were calculated based on these determined textures. These results agree qualitatively with magnetization and magnetostriction experiments and support the thermal history influence on the magnetostriction values.

Core-Shell Structure Composite

We have proposed to build a 100 T/10 ms, 70 T/100 ms, 60 T/1 s pulsed field user facility with a 50 MJ capacitor bank at the Forschungszentrum Rossendorf near Dresden. This would provide the appealing possibility to have access to Zeeman energies in the energy range of the infrared free-electron-lasers (5 μm to 150 μm; 2 ps; cw; > 10 W) now under construction at the radiation source ELBE (superconducting electron linear accelerator; 40 MeV; 1 mA; 2 ps; cw) in Rossendorf. The work is accompanied by computer simulations of the planned coil systems, of the power supply, and by the development of high-strength conductors aiming at a tensile strength of about 1.5 GPa at σ ≈ σ Cu/2 (microcomposite CuAg alloys and Cu-steel macro compounds). With a view of gaining experience in the construction and operation of pulsed magnets, a pilot pulsed field laboratory was established at the Institute of Solid State and Materials Research Dresden (IFW Dresden). The laboratory includes short pulse magnets with peak field up to 60 T in a 24 mm bore and a rise time of about 10 ms (coil from NHMFL, Tallahassee), and a 40 T long pulse magnet with 24 mm bore and rise time of about 80 ms (coil from METIS, Leuven). The repetition rate of 20 min between pulses is limited by the cooling time of the coils. The coils are energized by a 1 MJ, 10 kV capacitor bank with some special features. With this set-up measurements of magnetization and magnetotransport on 4f-electron systems, for example RECu2, have been out in the temperature range of 1.5 to 300 K and at fields up to 52 T using high precision pick-up coils.

Magnetoelectric Properties of CoFe2O4-BaTiO3 Core-Shell Structure Composites

Abstract Magnetoresistance (MR) of amorphous ribbons of Fe86Zr7Cu1B6 have been investigated on the nanocrystallization by direct DC Joule heating. 3-D road map of these samples was illustrated by the crystal sizes, coercivities and magnetizations, started from as-cast to the total annealing. The largest negative MR of 0.5% was found for this system at T = 10 K.