J. Gal

Neutron diffraction study of the ferrimagnetic structures of RFe5Al7 compounds with R=Tb, Dy, Ho, Er, Tm

Abstract Long range order of ternary RFe 5 Al 7 (R = Tb, Dy, Ho, Er, Tm) intermetallics of ThMn 12 -type structure is analysed by neutron powder diffraction. Ferromagnetic order is found for both the heavy rare earth and the iron sublattices. Rare earth and iron moments are oriented antiparallel to each other, resulting in ferrimagnetic structures. The ordering temperatures are about 235 K for the Tb, Dy and Tm compounds and about 60 K for the Ho and Er compounds. Magnitudes and orientations of ordered magnetic rare earth and iron moments are determined; a moment modulation is observed in TbFe 5 Al 7 . The formation of the ferrimagnetic structures is preceded by short range or frozen magnetic states. The temperature behaviour of the magnetization is discussed.

Neutron diffraction on YFe5Al7 as reference of the f-magnetism of isostructural rare earth-iron-aluminium compounds

Abstract YFe5Al7 crystallizes isostructurally with the rare earth(R)-iron-aluminium compounds RFe5Al7 in the tetragonal ThMn12-type structure in space group I4/mmm. Lattice parameters are refined from X-ray diffraction lines and atomic parameters and site occupancies from room and low temperature neutron diffraction intensities. Ordered cation distributions are found with Y on 2a, Fe on 8f and Al on 8i sites. 8j sites are occupied by both Fe and Al according to stoichiometry. A broad diffuse neutron peak indicates short-range structural order originating from iron clusters on 8j positions. Below 100 K, YFe5Al7 exhibits an incommensurate magnetic structure of small Fe moments (〈μ〉 = 0.5 μB) with a propagation vector close to [ 1 7 , 1 7 , 0]. Contrary to the findings in the other rare earth compounds, no long-range ordered ferromagnetic iron sublattice comes off in YFe5Al7.

Neutron diffraction studies of ferrimagnetic RFe6A16 (R = Tb, Ho, Er) intermetallics

Abstract ThMn 12 -type compounds TbFe 6 A1 6 , HoFe 6 A1 6 and ErFe 6 A1 6 order uniformly around 340(10)K by forming ferromagnetic rare-earth and iron sublattices, which are coupled antiparallel to each other within the tetragonal basis plane. Individual rare earth and iron moments are analysed between 300 and 4 K. The ordering process is restricted over a wide temperature range, which is explained by the existence of frozen spins.

Atomic distribution in RFexAl12−x compounds with R = Tb, Dy, Ho, Er, Tm and Fe concentrations x ≈ 5 studied by neutron diffraction

Abstract The ternary intermetallic rare earth—iron—aluminium compounds crystallize isostructurally in the tetragonal ThMn 12− type structure, space group I 4/ mmm . Neutron diffraction reveals atomic distributions, with R on 2a sites and nearly 100% occupancy of 8f sites by Fe and of 8i sites by Al. 8j sites are occupied by both Fe and Al according to stoichiometry. There are indications of a clustering of iron atoms on 8j positions. Refined structural parameters and interatomic distances are discussed with respect to the preferential site occupations of Fe and Al. Unit cell contractions due to lanthanides and different Fe/Al mixing ratios are observed.

High pressure studies of actinide intermetallics

Magnetic properties of NpOs2,NpAl2. NpCo2Si2, NpAs, and NpSn3 are investigated by high pressure Mössbauer spectroscopy using the 60 keV transition in237Np. A wide spectrum between itinerant (band-like) electron behaviour and highly localized 5f electron magnetism is observed. Delocalization of 5f electrons may be determined by direct 5f-5f overlap between neighbouring Np ions or may be mediated by the valence electrons of the alloying partner.

High-resolution67Zn-Mössbauer study of oxide spinels

We report on first experiments to investigate the electronic structure in the normal spinels ZnAl2O4 and ZnFe2O4 and in the inverse spinels Zn2SnO4 and Zn2TiO4 using the high-resolution67Zn-Mössbauer spectroscopy. The electric field gradient for67Zn at the B (octahedral) site in ZnAl2O4 is negative, whereas the A (tetrahedral) site remains essentially cubic, however, with a more positive center shift. ZnFe2O4 orders antiferromagnetically at ≈10K. Due to superexchange a magnetic field is observed at67Zn. In the inverse spinels short range order leads to only few (Zn,Sn) and (Zn,Ti) configurations at the octahedral sites. The s-electron densities at the67Zn sites are distinct and cover a surprisingly broad range for Zn2TiO4. This strongly suggests that d-electrons of Ti play an essential role in the chemical bond of this compound.

Magnetic properties of NpM4Al8−x (M=Cr, Fe, Cu) intermetallics: Mössbauer effect and neutron diffraction studies

Mössbauer effect and neutron diffraction studies on the tetragonal NpM4Al8−x (M=Cr, Fe, Cu) of the 14/mmm space group are reported. In NpFe4Al8−x, both the Np and Fe sites order magnetically at close temperatures: the Np order ferromagnetically at 115(15) K (μ(Np)) ∼ 0.6 μB and the Fe moments order antiferromagnetically at 130(10) K (μ(Fe) = 1.05(15) μB) with a ++−− sequence. In NpCr4Al8−x, the Np order magnetically around 50K. From the susceptibility data we conclude possible antiferromagnetic order of both Np and Cr sites. NpCu4Al8−x does not order magnetically down to at least 2 K. The magnetic hyperfine splitting observed below 45 K is explained by slow paramagnetic relaxation. The Np isomer shifts and also its magnetic behavior point to a trivalent ion (5I4 Hunds rule ground state). The observed relaxation and magnetic phenomena are discussed in terms of crystalline electric field effects. In contrast to the isostructural rare-earth RM4Al8 compounds with practically independent order of R and M magnetic sublattices, we show that Np and M sublattices in NpM4Al8 are strongly coupled. This is caused by hybridization of both Np and M atoms with Al.

Electronic structure of Zn in oxide spinels

Using67Zn Mössbauer absorption and emission spectroscopy, we have investigated the electronic structure at the A and B sites in the normal spinels (Zn)[Al2]O4, (Zn)[Fe2]O4 and (Zn)[Ga2]O4. Within each system, the center shift Sc at the A site is more positive. In all systems investigated, the electric field gradientVu at the B site is negative. The values for SC andVU scale with oxygen nearest-neighbour distance to Zn. In the Fe spinel, a transferred magnetic hyperfine field is observed at the Zn site below the antiferromagnetic ordering temperatureTN=10 K. For a more detailed discussion of the chemical bond, we have performed ab initio Hartree-Fock cluster calculations for the Al and Fe spinels. Our experimental and theoretical results show that all hyperfine parameters are essentially determined by covalency effects. Our data on the Ga spinel raise the question of a partially inverse structure.

Dynamic short-range order observed in the (Zn)[Fe2]O4 spinel by neutron diffraction, μSR, and Mössbauer experiments

Using neutron diffraction (ND), muon-spin rotation/relaxation (μSR), and57Fe-Mössbauer spectroscopy (MS) we have investigated magnetic properties of the normal spinel (Zn)[Fe2]O4. In compounds which are slowly cooled from 1200°C to room temperature inversion is below detection limits. AtTN = 10.5 K the spinel exhibits long-range antiferromagnetic order (LRO). The transition as seen in thermal-scan spectra by MS is very sharp. However, ND andμSR experiments show that already at temperatures of ∼ 10TN a short-range antiferromagnetic ordering (SRO) develops which extends through ∼70% of the sample volume just aboveTN. BelowTN SRO and LRO coexist. At 4.2 K still ∼25% of the sample is short-range ordered. The regions over which the SRO extends have a size of ∼ 3 nm. Their fluctuation rates are in the GHz range. Modern ab initio cluster calculations successfully describe the magnetic hyperfine fields as well as the electric field gradient (EFG) tensor at the Fe sites. Covalency of the Fe-O and Zn-O bonds is important. The physical origin of the regions exhibiting SRO, however, remains unresolved at this point.

Coexistence of long range order and spin glass similar behaviour in HoFe4Al8

Abstract We report on powder neutron diffraction and susceptibility studies of the magnetic behaviour of the HoFe 4 Al 8 intermetallic compound. Coexistence of long range magnetic order of the Fe sublattice, together with a spin glass state in the Ho sublattice is established at low temperatures.