O. Isnard

Neutron diffraction study of the structural and magnetic properties of the R2Fe17Hx(Dx) ternary compounds (R Ce, Nd and Ho)

Abstract A structural and magnetic characterization of R2Fe17 alloys and their corresponding hydrides has been carried out by means of powder neutron diffraction. The hydrides have been found to retain the host metal symmetry. Hydrogen is accommodated in rare-earth-rich octahedral and tetrahedral sites. The effects of hydrogenation on the magnetic properties are discussed.

A neutron diffraction and Mössbauer effect study of the Nd2Fe17−xSix solid solutions

Abstract The substitution of silicon for iron in Nd 2 Fe 17 strongly raises the Curie temperature but leads to a reduction in the unit cell volume. Refinement of the neutron-diffraction pattern for Nd 2 Fe 12.91 Si 4.09 indicates that silicon preferentially occupies the 18h site in the Nd 2 Fe 17 structure, the site with the most neodymium near neighbors. This occupation is surprising because conventional arguments would suggest that replacement of iron on the 6c site, which has a very short iron to near-neighbor iron bond length, would yield an increase in the Curie temperature.

Structural and magnetic properties of ternary nitrides R2Fe17Nx (R ≡ Nd, Sm)

Abstract A structural and magnetic characterization of the new ternary R2Fe17Nx compounds has been carried out by means of neutron powder diffraction and χac susceptibility measurements. Preliminary high field magnetization measurements are also presented. These new nitrides have been found to retain the 2:17 host metal symmetry. Nitrogen is accommodated in octahedral sites. The effects of nitrogenation on the magnetic properties are discussed. The high field measurements have allowed us to determine the anisotropy field of the Sm2Fe17N2.2 compound in the range from 4.2 to 300 K.

Magnetism in Fe-based intermetallics: relationships between local environments and local magnetic moments

Abstract We report on the relationship between the local magnetic moments and crystal sites, mainly in iron-rich intermetallics. The topology of the atomic environment is analysed in terms of numbers of neighbours, disclination lines and atomic domains. The wide range of compounds studied includes rare earth-iron alloys, such as R6Fe23, RFe3, R(FeM)12, R2Fe17, ternary rare earth-iron-metalloid intermetallics such as R2Fe17Hx, R2Fe17N3, R2Fe17Cx, R2Fe14B, ThFe11Cx, R(FeM)12Nx and R(FeM)12Cx, as well as binary iron nitrides (Fe3N, Fe4N, Fe16N2), carbides (Fe5C2, Fe3C) or borides (Fe2B and Fe3B). The sites having major ligand lines are observed to carry the larger magnetic moments encountered in the structures. In the ternary compounds strong bonds are found between iron sites and a neighbouring metalloid atom, thus leading to a lower iron magnetic moment. Finally, a close relationship between the local moment and the volume of the iron site has been observed: a large atomic domain is found to favour a high moment.

A STRUCTURAL ANALYSIS AND SOME MAGNETIC PROPERTIES OF THE R2FE17HX SERIES

Abstract A structural and magnetic characterization of the compounds R 2 Fe 17 H x through the R series (except for R = Eu, Yb) is presented. The starting alloys crystallize with the rhombohedral ( Th 2 Zn 17 - type R -3 m ) or the hexagonal ( Th 2 Ni 17 - type P 6 3 / mmc ) structure for the light and heavy rare earths, respectively. It is seen that the host alloy symmetry is retained upon hydrogenation. Hydrogen is found to accomodate rare earth-rich octahedral and tetrahedral sites. The maximum hydrogen uptake ranges from 3 to 5 hydrogen atoms per formula unit for the heavier and lighter rare earths, respectively. The highly anisotropic cell expansion upon hydrogenation is analyzed. A filling scheme for hydrogen is proposed. The effect of hydrogenation on intrinsic magnetic parameters such as T C and the total magnetization has been studied. It is correlated to the variation of short Fe-Fe distances within the structure and discussed in terms of variations of the exchange interaction. It is also shown that hydrogenation modifies the thermal variation of the anisotropy constants in some compounds.

Neutron diffraction studies of the magnetic phase transitions in Ce2Fe17 compound under pressure

The influence of hydrostatic pressure (up to 5 kbar) on the magnetic structure of Ce2Fe17 was investigated using neutron diffraction in the temperature range from 2 to 300 K. The existence of a collinear ferromagnetic phase below 95 K with a magnetic moment of Fe, mFe=2.0 μB, was confirmed at ambient pressure. Magnetic peaks present between 95 and 205 K correspond to an incommensurate antiferromagnetic structure with a wave vector changing its value from τ1=0.026 A−1 at 100 K to τ1=0.034 A−1 at 205 K. A helical model is used to describe the magnetic structure. Application of high pressures leads to significant changes of the magnetic structure. The ferromagnetic phase, suppressed in the studied temperature range by pressures higher than 3 kbar, gets substituted by a new incommensurate antiferromagnetic phase. This phase can be described as a superposition of the helical structure with a second antiferromagnetic coupling with propagation vector τ2≈0.078 A−1 at 40 K under pressures above 3 kbar. The correla...

Neutron‐diffraction study of the insertion scheme of hydrogen in Nd2Fe14B

Results are presented of a neutron powder diffraction experiment performed at room temperature on the Nd2Fe14BHx series. The location of the hydrogen (or deuterium) atoms within the crystal lattice is described. After an analysis of the crystal structure of Nd2Fe14B, Nd2Fe14BH, Nd2Fe14BH2, Nd2Fe14BH3, and Nd2Fe14BH4, a filling scheme of the interstitial sites by hydrogen atoms is proposed. This filling scheme is compared and discussed with that previously obtained on other R2Fe14B alloys in particular with R=Ce, Y, Ho, Er. The use of all these compositions provides a better understanding of the way the Nd2Fe14B structure is modified by insertion of hydrogen. The evolution of the shortest iron‐iron interatomic distances is also analyzed and related to the change in magnetism.

The structural and magnetic properties of Yn+1Co3n+5B2n compounds investigated by neutron diffraction

The crystal and magnetic structures of Yn+1Co3n+5B2n (n = 2, 3 and ∞) have been studied by high-resolution powder neutron diffraction. The results are compared to earlier measurements on YCo5 and YCo4B. A change in the regular stacking of the boron-containing plane along the c axis has been observed in the Y2Co7B3 sample. Very short Co–B distances are observed, indicating that strong bonds are formed between cobalt and boron. The YCo3B2 compound is paramagnetic down to 2 K. The magnetic structures of Y3Co11B4 and Y2Co7B3 confirm the large variety of cobalt magnetic moments obtained in these compounds. The magnetic behaviour of the Co(2c) atoms is not significantly affected by the substitution of boron for cobalt. Cobalt atoms with significantly reduced magnetic moments are found on the 3g and 6i2 sites in both Y3Co11B4 and Y2Co7B3. A relationship between the magnitude of the Co magnetic moment and the presence of boron in the neighbourhood of the cobalt atoms is proposed. The hybridization of the cobalt 3d electronic state with the boron 2p state is found to play a major role in the determination of the magnitude of the Co magnetic moment in the Yn+1Co3n+5B2n compounds.

Crystal and magnetic structure of YCo4−xFexB

YCo4−xFexB compounds have been investigated by means of neutron powder diffraction. Substitution of Co with Fe causes substantial modifications of the crystal structure. Iron occupies preferentially one (2c) of the two cobalt sites (2c and 6i). The Curie temperature of the material increases with Fe content from Tc=380 K for YCo4B, and concurrently a spin reorientation is suppressed. The magnetic phenomenology is explained in terms of the preferential occupation of Fe, which carries in the 2c sites an ordered magnetic moment almost double of that in the 6i sites.

Neutron powder diffraction study of R2Fe17Hx compounds with R = Pr and Nd

Abstract The paper deals with a structural and magnetic study of Pr 2 Fe 17 D 4.9 , Nd 2 Fe 17 and Nd 2 Fe 17 H 4.9 by means of neutron diffraction. The hydrides are found to retain the 2:17 host metal symmetry. Hydrogen is accommodated in both octahedral and tetrahedral sites close to the rare-earth metal. The impact of hydrogen on the magnetic moments of Fe and R atoms is discussed.