G. Grosse

Magnetism of crystalline and nanostructured ZnFe2O4

Abstract We have investigated the magnetic properties of three differently prepared samples of the spinel ZnFe2O4 with μSR, neutron diffraction, Mossbauer effect and magnetization measurements. Below T N =10.5 K , the first sample (annealed) shows long-range antiferromagnetic order in coexistence with short-range magnetic order. The second sample (rapidly quenched) shows only short-range magnetic order. The third sample (nanostructured with an average particle size of about 9xa0nm) exhibits ferromagnetism below T c ≈460 K combined with superparamagnetism. Below the blocking temperature of ∼50 K the nanostructured sample exhibits a dependence on applied magnetic fields similar to that observed in spin glass systems. The roles of cation-site occupation and magnetic couplings in governing the different magnetic behaviors exhibited by the samples are discussed.

High-pressure hydrides of iron and its alloys

Hydrides of iron and iron-based alloys are thermodynamically stable only at hydrogen pressures in the gigapascal range and rapidly lose hydrogen under ambient conditions. At low temperatures, however, these hydrides can be retained in a metastable state at atmospheric pressure after being cooled under high pressure to liquid nitrogen temperature. This review will discuss the current state of studies on phase transformations in the Fe?H and related systems and also on the composition, crystal structure and physical properties of the hydrides, both under high hydrogen pressures and in the quenched metastable state at ambient pressure. The studies at ambient pressure include magnetization measurements, x-ray and neutron diffraction, M?ssbauer spectroscopy and inelastic neutron scattering. In the sections on M?ssbauer and structural investigations of hydrides of Fe?Cr and Ni?Fe alloys new experimental results will be presented.

Neutron spectroscopy of fullerite hydrogenated under high pressure; evidence for interstitial molecular hydrogen

Inelastic neutron scattering spectra of a hydrofullerite quenched after synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and of the same sample after annealing at 300 K for 35 h, which reduced the hydrogen content by molecules per unit, were measured at 85 K. The quenched sample is shown to consist of molecules with and of interstitial molecular hydrogen. The interstitial molecular hydrogen left the sample during annealing at room temperature, whereas the molecules were stable at this temperature. The intramolecular and intermolecular vibrations of and in the fullerite are discussed in view of the measured spectra.

On the isomorphous phase transformation in the solid f.c.c. solutions CoH at high pressures

Abstract Samples of Co containing 0.2 at.% Fe and consisting of a mixture of approximately equal amounts of the h.c.p. (stable) and f.c.c. (metastable) phases were loaded with hydrogen by a 24 h exposure to a hydrogen atmosphere at 325°C and pressures up to 9 GPa. X-ray diffraction at ambient pressure and 100 K has revealed a steep increase in the lattice parameter of the f.c.c. phase in the samples hydrogenated at 4–6 GPa, corresponding to an increase in the hydrogen content from few atomic per cent to an H/Co atomic ratio of approximately 0.95. This behaviour is interpreted as a supercritical anomaly of an isomorphous phase transformation in the f.c.c. solid solutions. The topology of the T-P phase diagram of the Coue5f8H system is discussed in view of these observations.

Atomic ordering in the hcp cobalt hydrides and deuterides

Abstract Cobalt hydrides and deuterides with the hcp metal lattice and H(D)-to-metal atomic ratios 0.18≤ x ≤0.5 were prepared under high pressures of hydrogen or deuterium, respectively, and studied by neutron diffraction in a metastable state at 120 K and ambient pressure. A profile analysis of the spectra showed that in all samples the hydrogen and deuterium atoms occupy octahedral interstitial sites. In the samples with x ≤0.26, the hydrogen and deuterium atoms are randomly distributed over these sites. In the samples with x ≥0.34, they form layered superstructures, occupying every third octahedral base layer at x =0.34 and every second layer at x =0.38 and 0.5.

Neutron spectroscopy of γ manganese hydride

Abstract The vibrational spectrum of fcc γ -MnH 0.41 synthesized under high pressure of gaseous hydrogen was studied by inelastic neutron scattering at 2 K in the range of energy transfers from 25 to 400 meV. The fundamental band of optical hydrogen vibrations consists of a peak at 111 meV with a broad shoulder towards higher energies, which extends up to about 140 meV. At higher energy transfers, the spectrum originates from multiphonon neutron scattering and exhibits approximately harmonic behaviour. The results are compared with the available data for other metal hydrides.

Neutron diffraction investigation of γ manganese hydride

Abstract A profile analysis of the neutron diffraction spectrum of the fcc high pressure hydride λ-MnH 0.41 measured under ambient conditions showed that hydrogen is randomly distributed over the octahedral interstices of the fcc metal lattice and that the hydride is an antiferromagnet with the same collinear spin structure as pure λ-Mn, but with a smaller magnetic moment of about 1.9 Bohr magnetons per Mn atom.

Neutron scattering studies of the structure and lattice dynamics of a solid solution of hydrogen in -manganese

A solid solution of hydrogen in -Mn, -MnHx with xD 0:073, was prepared at 623 K and a hydrogen pressure of 0.85 GPa. A profile analysis of the neutron diffraction patterns measured at 225 K and at 300 K showed that hydrogen in-MnH0:073 randomly occupies the interstitial positions 12e (0, 0, 0.538) of the -Mn host lattice. An inelastic neutron scattering study of -MnH0:073 at 90 K revealed a pronounced peak at 6.4 meV and a rather broad optical hydrogen band with peaks at 73, 105 and 123 meV. The splitting of the optical band into three different modes agrees with the low symmetry, 2, of the 12e hydrogen sites. A tentative interpretation is given for the peak at 6.4 meV.

The onset of magnetism in CeNi1−xTxSn (T=Cu, Pt)

Abstract Magnetic order (including spin freezing) is absent down to ∼15xa0mK in CeNiSn due to the dominance of coherent spin fluctuations caused by the Kondo interaction. Partial replacement of Ni by Cu or Pt induces a highly disordered magnetic state. The critical concentration is much lower for Cu replacement (∼7%) than for Pt replacement (∼30%). Results of μSR and specific heat (for CeNi 1− x Cu x Sn) measurements near the critical concentration are presented. The possibility of a quantum critical point is discussed. A tentative phase diagram for the two parameters relevant for the onset of magnetism, the unit cell volume and the d electron density, is derived.

Giant tunnelling effect of hydrogen and deuterium in α manganese

The crystal structure of a solid solution of 3.5 at.% deuterium in α-Mn was determined by neutron diffraction and the spectrum of optical deuterium vibrations at 5 K was studied by inelastic neutron scattering (INS). The results give conclusive evidence of the tunnelling origin of the strong peaks at 6.4 and 1.6 meV observed earlier at temperatures up to 100 K in the INS spectra of hydrogen and deuterium solutions in α-Mn, respectively.