A. Szytuła

Magnetic properties of ternary RMn2Si2 and RMn2Ge2 compounds

Abstract Magnetic properties of RMn2Si2 and RMn2Ge2 compounds, where R is a rare earth metal, have been investigated by magnetometric measurements. RMn2Ge2 (where R is a light rare earth) and LaMn2Si2 are ferromagnets. Remaining compounds have antiferromagnetic properties. DyMn2Si2 and ErMn2Si2 show ferromagnetic properties at low temperatures. It was confirmed that the value of Curie (or Neel) temperature for the Mn sublattice decreases with increasing c constant.

Crystals and magnetic structure of RMn2Si2 (R = Pr, Nd, Y) and YMn2Ge2

Abstract Crystallographic and magnetic properties of PrMn2Si2, NdMn2Si2, YMn2Si2 and YMn2Ge2 intermetallics were studied by X-ray, neutron diffraction and magnetometric measurements. The crystal structure of all four compounds was confirmed to be body-centered tetragonal (space group I4/mmm). All were found to be antiferromagnetic with Neel points at 368, 380, 460 and 395 K respectively. Neutron diffraction results indicate that their magnetic structure consists of ferromagnetic layers composed of Mn ions piled up along the c-axis. Each layer is antiferromagnetically coupled to adjacent layer. The magnetic space group is Ip4/m′m′m′. No magnetic ordering of the R sublattice was observed at 1.8 K in the case of R = Pr and Nd.

Magnetic properties of RERu2Si2 (RE=Pr, Nd, Gd, Tb, Dy, Er) interm etallics

Abstract Neutron diffraction and magnetometric measurements on polycrystalline samples of (Pr, Nd, Gd, Tb, Dy, Er)Ru 2 Si 2 compounds were performed in the temperature range between 1.8 and 293 K. All compounds have the tetragonal, ThCr 2 Si 2 -type crystal structure. In (Pr, Nd)Ru 2 Si 2 ferromagnetic ordering within the Pr and Nd sublattices is observed at low temperatures. The magnetic moment is parallel to the c -axis. For (Tb, Dy, Er)Ru 2 Si 2 the magnetic spin alignment is of a linear transverse wave mode. This static moment wave is propagating along the b -axis with τ=[0,τ,0] and is polarized in the c -axis for TbRu 2 Si 2 and DyRu 2 Si 2 and in the b -axis for ErRu 2 Si 2 . The observed magnetic ordering schemes are discussed in terms of isotropic RKKY exchange interactions.

Crystal and magnetic structure of CoxNi1-xMnGe system

Abstract The crystal and magnetic structures of the Co x Ni 1 − x MnGe compounds have been studied by neutron diffraction experiments. Magnetic phase diagram has been constructed. The compounds with the low Co concentration have helicoidal magnetic structure. The compounds with large Co concentration are ferromagnets. The magnetic ordering of the compounds with 0.3⩽ x ⩽0.5 changes with growing temperature: • - at low temperatures it is helicoidal antiferromagnetic, • - in the middle range non-collinear or ferromagnetic spiral, • - near T c collinear ferromagnetic structure is observed.

Magnetic properties of SmMn2Ge2 compounds

The magnetic characteristics of SmMn2Ge2 compounds have been studied by measurements of ac susceptibility, magnetization, electrical resistivity and X-ray diffraction; the p-T diagram has also been determined. With decreasing temperature the phase transitions occur in the following sequence, ferromagnetism is observed in the temperature region of 153 K < T < 341 K, antiferromagnetism becomes stable for 106.5 K < T < 153 K and re-entrant ferromagnetism appears below 106.5 K. The observed magnetic phase transitions correspond with the change of the lattice parameters. In the antiferromagnetic phase the magnetic field H ≈6 kOe induces the transition into the ferromagnetic phase. The pressure dependence of the phase transition temperatures was also observed.

Neutron diffraction study of RECo2Si2 intermetallics

Abstract A neutron diffraction study of polycrystalline RECo2Si2 intermetallics (RE = Pr, Nd, Tb, Ho, Er) carried out at liquid helium temperature shows the presence of a collinear antiferromagnetic ordering of +−+− type. Magnetic moment is localized on RE ions only and amounts to the RE3+ free ion value. In ErCo2Si2 the magnetic moment is normal to the tetragonal unique axis, whereas in the remaining compounds the magnetic moment is aligned along it. Neel points were determined from the temperature dependence of magnetic peak heights.

Stability and excitation of potassium promoter in iron catalysts – the role of KFeO2 and KAlO2 phases

Well‐characterized catalyst model compounds of KAlO2 and KFeO2 are investigated by thermal desorption of potassium from the material. The desorbing fluxes of ions, atoms and highly excited states (field ionizable Rydberg states) were studied with surface and field ionization detectors in a vacuum apparatus. From the Arrhenius plots the activation energies for desorption of K and K+ were determined. The chemical state of potassium at the surfaces is concluded to be: ionic on KAlO2 (with the K desorption barrier of 1.76 eV) and covalent on KFeO2 (barrier of 2.73 eV). These results agree with the data obtained earlier for industrial catalysts for ammonia and styrene production. They are interpreted in terms of the Schottky cycle, which is completed for KAlO2 and fails for KFeO2. This failure indicates a non‐equilibrium desorption process. K Rydberg states are only found to desorb from KFeO2, in agreement with the suggestion that such states in some way are responsible for the catalytic activity.

Magnetic properties of the CoxNi1−xMnGe system

Abstract Magnetic properties of the Co x Ni 1−x MnGe system have been studied by magnetometric measurements. It was shown that the solid solutions exist for 0 ⩽ χ ⩽ 1, with the crystal structure of NiTiSi type. The samples with the low Co concentrations (0 ⩽ χ ⩽ 0.2) have antiferromagnetic properties. At higher concentrations (0.3 ⩽ χ ⩽ 0.5) the samples exhibit a more complex behaviour, at low temperatures having antiferromagnetic properties. At growing temperatures the change of the magnetic ordering is observed. The samples with large Co concentration (0.6 ⩽ χ ⩽ 1.0) are ferromagnets.

Electronic structure of RTX (R=Pr, Nd; T=Cu, Ag, Au; X=Ge, Sn) compounds

Abstract Electronic structure of ternary RTX (R=Pr, Nd; T=Cu, Ag, Au; X=Ge, Sn) compounds was studied by X-ray photoemission spectroscopy. Core-levels and valence bands were investigated. The XPS valence bands are compared with the ones calculated using the spin-polarized tight binding linear muffin-tin orbital (TB LMTO) method. The obtained results indicate that the valence bands are mainly determined by Cu3d, Ag4d and Au5d bands. The spin–orbit splitting values Δ E LS determined from the XPS spectra of Pr and Nd 3d 5/2 and 3d 3/2 are equal to 20.5 eV for Pr-compounds and 22.5 eV for Nd-compounds. The analysis of these spectra on the basis of the Gunnarsson–Schonhammer model gives a hybridization of f-orbitals with the conduction band.

Magnetic properties of the Sm1−xYxMn2Ge2 system

Abstract The magnetic phase diagram ( x , T , p ) of the Sm 1− x Y x Mn 2 Ge 2 system has been studied by susceptibility and magnetization measurements. With increase in yttrium concentration the temperature region of ferromagnetic ordering decreases on the ( x , T ) phase diagram. A similar effect is observed when pressure increases.