Walter Steiner

Magnetization processes caused by iron substitution in cubic laves phases

Abstract The magnetic properties of cubic Laves phases RE(Fe, M) 2 , M = Al, Co, Rh are discussed. Pauli-paramagnetism is obtained for all three YM 2 compounds. The appearance of a percolation limit for the onset of long-range ferromagnetic order which depends strongly on the matrix susceptibility is characteristic for all investigated series Y (Fe, M) 2 . Mictomagnetic behaviour can be deduced from magnetic, Mossbauer- and neutron depolarization measurements below the percolation limit. A Co moment of approximately 1 μ B is induced in RECo 2 compounds, RE = Gd, Dy, Ho. On the contrary Rh has no magnetic moment in these compounds. Within the RE(Fe, Co) 2 series a Co moment is induced by both the RE and Fe atoms, whereas within RE(Fe, Rh) 2 the Rh atoms will only be influenced by the substituted Fe. In all compounds the spin of the heavy RE atoms couples antiparallel to the spin of the transition metal atoms. Local environment effects are important for all Fe containing series whereas the properties of RECo 2 compounds evidence itinerant electron metamagnetism.

Structural, magnetic, electronic and transport properties of NdCu2

NdCu2 crystallizes in the orthorhombic CeCu2 structure. Using temperature dependent X-ray diffraction the variations of the lattice parameters with temperature have been measured and compared with those of YCu2. A magnetic ordering temperature at TN=6.5+or-0.1 K and a spin-reorientation temperature (TR=4.1+or-0.1 K) have been observed in the susceptibility, specific heat and resistivity. Further field-induced magnetic transitions around 1, 3 and 15 T (depending on the measuring temperature) were found in the magnetization and magnetoresistance. A preliminary magnetic phase diagram of NdCu2 is presented. The crystal field excitation temperatures determined by inelastic neutron scattering experiments are E1/kB=34 K, E2/kB=58 K, E3/kB=84 K and E4/kB=164 K. For the determination of the nine crystal field parameters Vlm the superposition model has been applied. The deduced set of parameters was used to calculate the crystal field influence on the magnetization, specific heat, thermal expansion and electrical resistivity. From an analysis of the X-ray powder diffraction data the lattice constants a=4.3844 AA, b=7.0350 AA and c=7.4186 AA and the atomic positional parameters (yCu=0.0506, zCu=0.1659, zNd=0.5383) inside the unit cell were obtained.

Influence of the Tl-content on the properties of Bi-doped Tl-1223 superconductors

Abstract Bi-doped Tl-1223 superconductors of the composition Tl x Bi 0.4 Sr 1.6 Ca 2 Cu 3 O y ( x − 0.59–0.92) were prepared from oxidic Bi-Ba-Sr-Ca-Cu precursor material. Upon calcination of the spray-dried metal nitrates, the respective amounts of Tl 2 O 3 were added to the oxidic powders by milling. The sintered and annealed samples consisted predominantly of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. T c(0) -values ranged between 108 and 116 K with transition widths as small as 1 K. T c -onsets from magnetic measurements varied between 110 and 116 K. The transition widths in the magnetization curves were 2–4 K. Transport critical current densities up to 2.1 kA cm −2 at 77 K were measured. Texturing could be improved by grinding, repressing and resintering of the superconducting material increasing the transport critical current density in bulk material to 4.2 kA cm −2 at 77 K.

Magnetism of DyMn2 and HoMn2 - 57Fe and 119Sn Mössbauer studies

Abstract Mossbauer spectra were measured for two Laves phase compounds DyMn 2 and HoMn 2 in which manganese was substituted to 0.5% with 57 Fe and to 0.2% with 119 Sn. At 4.2 K the 57 Fe and 119 Sn spectra of the Dy compound were unambiguously fitted each with two Zeeman patterns (with relative contributions to the spectra 3:1) corresponding to two different Mn sites — magnetic and nonmagnetic. Transferred hyperfine fields at 119 Sn were found to be proportional to the magnetic moment of Dy and its ferromagnetic component, corroborating the magnetic structure found in neutron diffraction (ND) experiment. The same procedure was carried on with the spectra measured for the Ho compound, but the above mentioned proportionality was not found.

The Debye temperature of the Fe-Cr sigma-phase alloys

57Fe Mossbauer spectra were recorded for microcrystalline and nanocrystalline samples of the σ-phase of Fe100−xCrx alloys with x between 44.6 and 49.6 in the temperature range between 4.3 and 300 K. From the temperature dependence of the average centre shift derived from the spectra measured between 80 K and 300 K, Debye temperatures, θD, were determined. θD-values for the two types of sample show the same linear increase with x, namely ~15 K/at.%, which is faster than a similar trend observed for the α-phase Fe–Cr alloys.

Low-temperature magnetization reversal processes in permanent magnets based on R2T14B

Abstract The temperature dependence of the saturation magnetization, remanence and coercivity in NdFeB alloys partially substituted by Tb, Dy and Co was investigated in the temperature range 4.2–230 K. In addition to a ‘dip’ in the demagnetization curve, both discontinuous changes in the magnetic moment and the influence of the magnetic history of the samples were found in the demagnetization process at 4.2 K.

The effect of Cu/Zn substitution in Nd1.85Ce0.15CuO4 on superconductivity studied by magnetic, transport and specific heat measurements and Raman spectroscopy

Abstract We report on the structural chemistry, DC and AC magnetic, magnetoresistance, specific heat and Raman measurements of Cu/Zn substituted Nd 1.85 Ce 0.15 CuO 4 . Single phase solid solutions of Nd 1.85 Ce 0.15 Cu 1− x Zn x O 4− δ were observed for x ≤0.05 with the Nd 2 CuO 4 -type and with a random distribution of (Nd, Ce) and (Cu, Zn) on their respective lattice sites. Cu/Zn substitution suppresses superconductivity with an initial rate of −4.5 K/mol% Zn. With increasing x , the critical fields and critical current densities are strongly reduced whereas the number of effective pinning centers increases. The normal state electronic specific heat coefficient is estimated to be 11 mJ/(mol K 2 ). Anomalies of the resistivity and magnetoresistance are discussed in terms of a two-dimensional superconducting network in the perlocation regime. Raman spectroscopy indicates that with increasing Zn content the oxygen at the 4d site slightly displaced from the original position in the x − y plane.

Magnetic interactions in pseudobinary rare earth-3d intermetallics

Magnetic measurements on pseudobinary intermetallic compounds of the series R(Mn x Fe 1-x ) 2 (R = Y, Gd, Er) R 6 (Mn x Fe 1-x ) 23 (R = Y, Gd, Tb, Dy, Ho, Er, Tm) R 2 (Fe x Co 1-x ) 17 (R = Y, Gd, Dy) R 2 (Fe x Ni 1-x ) 17 (R = Y, Gd) are reported. Magnetization and susceptibility measurements have been performed at temperatures from 4 K to 1500 K. The magnetic interactions between Mn and Fe are discussed in terms of a localized-moment model. In the case of the R 2 (Fe x M 1-x ) 17 (M = Co, Ni) compounds, a band model seems to be more appropriate. Mossbauer Fe57spectra obtained on the R 2 (Fe,Co) 17 series can be explained by a superposition of four different six-line patterns corresponding to the four crystallographically nonequivalent 3d sublattices.

The Debye temperature of a nanocrystalline σ-Fe55.4Cr44.6 alloy

The Debye temperature ?D = 411 ? 12?K was determined for a ?-Fe55.4Cr44.6 alloy, with average grain size of 27?nm, from M?ssbauer spectra recorded in the temperature range of 4?300?K. The result fits well with those found previously for samples of different Cr content, x, with mean grain sizes in the micrometre range. ?D was found to increase linearly with x at the rate of 14.6?K/at.%.

High field Mössbauer investigations of paramagnetic Y(Fe, Al)2

Abstract 57Fe Mossbauer measurements in external fields up to 13.5 T in the temperature range 4–300 K are reported for Y(FexAl1−x)2. A negative sign was found for the quadrupole splitting. In the paramagnetic state different induced hyperfine fields for different Fe environments were observed.