xSix Compounds Research Articles

Magnetic properties of Ho2Co17−xSix compounds

We have studied the magnetic properties of Ho2Co17−xSix compounds (x≤3) by means of magnetic measurements and X-ray diffraction on magnetically aligned powders. The Curie temperature and the spontaneous moment decrease strongly with increasing Si concentration. We have constructed the magnetic phase diagram for the Ho2Co17−xSix system. The easy magnetization direction is parallel to the c-axis in an extended temperature region below the Curie temperature. A spin–reorientation transition takes place at lower temperatures, leading to an easy magnetization direction perpendicular to the c-axis. In the Ho2Co17 compound, the spin–reorientation transition occurs at 1005 K.

Structural and magnetic properties of R2(Fe,Si)17 compounds with R=Tb and Er

The structural and magnetic properties of R2Fe17−xSix compounds have been investigated by means of X-ray diffraction and magnetization measurements. Substitution of Si for Fe leads to a clear reduction of the unit-cell volume. With increasing Si content, a phase transition from the Th2Ni17- to the Th2Zn17-type of structure has been observed for R=Tb, while all compounds crystallize in the Th2Ni17-type of structure for R=Er. A clear decrease of the saturation moment and an increase of the Curie temperature have been found upon Si substitution. The exchange–interaction constant JTbFe between the Tb and the Fe moments has been obtained from a mean-field analysis of the Curie temperature Tc. The X-ray diffraction patterns of magnetically-aligned powder samples show that the easy magnetization direction is in the plane for all compounds investigated.

Magnetic and structural properties of PrCo3−xSix compounds

We have systematically investigated the magnetic properties of the rare-earth compounds PrCo8Si and PrCo3−xSix, the crystal structures of which are hexagonal CeNi3-type and rhombohedral Be3Nb-type, respectively. Sharp cusps were observed in the real and imaginary parts of the ac susceptibility at temperatures between 282 (x=0.08) and 209 K (x=0.23). Also observed were bifurcations of the zero-field-cooled and field-cooled magnetization curves at the same temperatures, showing that the magnetization was irreversible below these temperatures. The effect of Si substitution for Co in this system is to decrease the average ferromagnetic exchange interaction, thus decreasing the Curie temperature, without introducing significant random magnetism effects due to exchange fluctuations or random magnetic anisotropy.

Neutron diffraction study of the TbFeSi2-type NdMnCu0.5Ge1.5−xSix (0≤x≤1.5) solid solution

Antiferromagnetic TbFeSi2-type NdMnCu0.5Ge1.5−xSix compounds have been investigated by bulk magnetometric measurements and neutron diffraction. The Néel temperatures decrease from 370 K (x = 0) to 200 K (x = 1.5). The (010) Mn planes are antiferromagnetic in the germanium rich compounds (x≤0.3), and become ferromagnetic in the silicon rich compounds (x > 1.2). In the intermediate range (0.4<x<1.2), the compounds display more complicated magnetic structures with “mixed” Mn planes and/or helimagnetic arrangements. Whatever the concentration x, the Nd sublattice orders around Tt K. The magnetic behaviour of the Mn sublattice is compared to that observed in the RMnCu0.5Ge1.5 and closely related RMnSi, RMn2Ge2, and RMnSi2 compounds. The various magnetic arrangements observed in NdMnCu0.5Ge1.5−xSix and RMnSi2 compounds are connected with modifications in the chemical bond in each series.

Crystallographic and magnetic properties of NdCo11−xSix compounds

The effects of substitution of Si for Co on the crystal structural stability and magnetic properties of NdCo11−xSix compounds have been studied. X-ray diffraction and magnetic measurement show that the single phase region can be expressed as NdCo9.8–9.2Si1.2–1.8 other than NdCo9Si2. X-ray-diffraction study indicates that the compounds adopt the tetragonal BaCd11-type struc- ture. The magnetic properties of a series of NdCo11−xSix solid solutions have been studied by magnetic measurement. Our investigations show that the Curie temperature of NdCo11−xSix increases drastically from 528 K (x=1.8) to 833 K (x=1.2) as the Si content decreases. The Co magnetic moment reaches a maximum between NdCo9.8Si1.2 and NdCo9.6Si1.4, and decreases with the further increase in x. The substitution of Si for Co in NdCo11−xSix leads to a contraction of lattice constant a by 0.2%, c by 0.2%, and a consequent reduction of the unit-cell volume by about 0.6%. The Rietveld refinement results show that the silicon atoms preferentially occupy the 8(d) sites. The occupation of 8(d) sites by proper amount of Si will shift the Fermi level EF upward and to a valley with a lower local density of states in NdCo11−xSix compounds, which is favorable for the stability of the BaCd11-type structure.

Crystal structure and magnetic properties of LaCo13−xSix compounds

Tetragonal LaCo13−xSix compounds have been obtained by annealing cubic LaCo13−xSix compounds. The space group for the tetragonal structure is I4/mcm. Atomic ordering occurs in the tetragonal LaCo13−xSix compounds, i.e., Si atoms show a strong preference for occupying a specific crystallographic position. The tetragonal LaCo13−xSix compounds are ferromagnetic with Curie temperatures higher than 900 K. The magnetic moment per Co atom in the tetragonal LaCo13−xSix compounds decreases with Si content, which can be explained by the rigid-band model. The tetragonal LaCo13−xSix compounds are expected to be promising candidates for permanent magnets purpose.

Magnetic hyperfine fields in Y2Fe17?x Si x compounds

Y2Fe17−xSix compounds withx=0, 0.5, 1.0, 1.5, 2.0, 2.5 and3.0 were investigated by magnetic measurement andNMR. It is found that with increasing Si content the Curie temperatureTC increases while the average Fe magnetic moment μFe decreases. NMR study indicates that Si preferentially substitute the Fe atoms at 4f sites, which is responsible for the increase ofTC.

Structure and Atomic Position in Gd2Fe17−xSix (x = 0, 1, 2) Compounds

Structure, atomic positions, and Curie temperature of Gd2Fe17−xSix (x = 0, 1, 2) compounds are reported. The refinement of the structure is carried out for rhombohedral Gd2Fe16Si (Th2Zn17 type) by Rietveld's profile fitting. The results show that the silicon atoms preferentially substitute iron atoms on site 18h and cause the dumbbell of iron atoms to extend along its axis, the atomic parameters of gadolinium atoms on site 6c and iron atoms on site 18f are also changed. The increase of the number of silicon atoms (x = 2) changes the lattice structure from rhombohedral to hexagonal CaCu5 type. Struktur, Atompositionen und Curie-Temperatur von Gd2Fe17−xSix (x = 0, 1, 2) werden mitgeteilt. Fur rhomboedrisches Gd2Fe16Si (vom Typ Th2Zn17) wird die Struktur mit Rietvelds Profilanpassung verfeinert. Es zeigt sich, das Si-Atome bevorzugt auf 18h Fe-Platzen sitzen und die Fe-Atom-Hantel langs ihrer Achse verlangern, die Parameter der 6c Gd-Atome und der 18f Fe-Atome werden ebenfalls verandert. Mit zunehmende Zahl von Si-Atomen (x = 2) geht die rhomboedrische Struktur in die hexagonale CaCu5-Strukur uber.

Hyperfine interaction experienced by181Ta nuclei at the Hf site in HfFe2−xSix compounds through a decoupling experiment

The time-differential perturbed angular correlation (TDPAC) experiments involving the 133–482 keV γ-γ cascade in181Ta have been performed in the presence of an external magnetic field along the quantization axis to measure the hyperfine magnetic field experienced by181Ta nuclei at the Hf site in the pseudobinary compounds HfFe2−xSix with x=0.1 and x=0.3. The hyperfine magnetic fields measured at 298 K are Hhf=133.1±12.0 kG in the cubic (C15) Laves phase compound HfFe1.9Si0.1 and Hhf=76.8±7.0 kG in the hexagonal (C14) Laves phase compound HfFe1.7Si0.3. The measured hyperfine fields are discussed within the framework of the Campbell-Blandin model.