Magnetic properties of CeMn2−xCoxGe2

The structure and magnetic properties of CeMn_2−xCo_xGe₂ (0.0≤x≤1.0) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Co for Mn leads to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Co for Mn shows a depression of ferromagnetic ordering.     

Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

The structure and magnetic properties of Nd_1−xY_xMn₂Ge₂ (0.0≤x≤0.6) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering.     

Structural and magnetic properties of Sm2Fe17−xCrxC2 nanocrystalline carbides with 0≤x≤2

The crystallographic and the Curie temperature of the Sm₂Fe_17−xCr_xC₂ (x=0.5, 1, 1.5 and 2) carbides have been extensively studied. X-ray diffraction studies have shown that all these alloys are approximately single phases corresponding to the Th₂Zn₁₇ type rhombohedral structure with a small amount of -Fe. The amount of this residual -Fe phase decreases with increasing the Cr atomic content. It decreases from 1 wt% for x=0.5 to 0.4 wt.% for x=2. The lattice parameter c increases as a function of the Cr atomic content x from x=0 to x=1.5 and then decreases. This is due to the Cr atoms which prefer to substitute the Fe atoms in the 6c sites located along the c-axis. The lattice parameter a and the unit-cell volume decrease in all substitution ranges. The insertion of the C atoms leads essentially to an increase of the distances between the 9d and 18h sites and the 9d–18f sites. The Curie temperature reaches a maximum value of 583 K for x=1.5 and then decreases to 551 K for x=2. The enhancement of the T_c for lower Cr contents is due to a lowering of the hybridization of the iron atoms with their neighbors, the magnetovolume effect and the reduction of antiferromagnetic interactions. However, the decrease in T_c for higher Cr content is due to the reduction in the number of Fe–Fe pairs due to the magnetic dilution effect. For given interatomic distances, the exchange coupling of the Cr–Cr atoms is not of antiferromagnetic type and the exchange integral of the Cr–Cr pair is higher than that of the Fe–Fe pair.     

Magnetic properties of Pr1−xGdxMn2Ge2 compounds

The structure and magnetic properties of the Pr_1−xGd_xMn₂Ge₂ (0.0≤x≤1.0) compounds have been investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The lattice constants and the unit cell volume obey Vegard’s law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for GdMn₂Ge₂ as a function of Gd concentration x. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The results are summarized in the x–T magnetic phase diagram.     

Effect of Mn concentration on the spin glass transition temperature of Fe0.70−xMnxAl0.30 alloys

Magnetization measurements on zero field cooled (ZFC) and field cooled (FC) samples as a function of temperature where made on Fe_0.70−xMn_xAl_0.30 alloys with 0≤x≤0.10. Our data show the existence of two regimes of the spin glass temperature T_f when increasing the concentration x of the Mn atoms. We found that T_f decreases slowly with increasing x up to x=0.06 and then shows a rapid linear decrease above this concentration. The existence of two regimes of T_f behavior with x may be attributed to the disappearance of ferromagnetic clusters which exist for low values of x (x<0.06). Compared to the Mn-free alloy, the relative changes in T_f are bigger and exceed 40% at x=0.10. The second regime (x>0.06) is associated with more influence of x on T_f.     

HfFe6Ge6-type YbMn6Ge6−xGax compounds (0.07≤x<0.72) with large magnetocrystalline anisotropy

The HfFe₆Ge₆-type YbMn₆Ge_6−xGa_x solid solution (0.07≤x≤0.72) has been studied by X-ray diffraction, microprobe analysis and powder magnetization measurements. All the compounds order antiferromagnetically between T_N=481 K for x=0.07 and T_N=349 K for x=0.72 and display more or less pronounced spontaneous magnetization at lower temperature. The corresponding Curie points increase from 40 K for x=0.07 to 319 K for x=0.72. The maximum magnetization values of the Ga-rich compounds (M≈5 μ_B/f.u. at 6 K) is compatible with a ferrimagnetic order of the Mn and Yb sublattices whereas the smaller values measured in the Ga-poor compounds suggest the stabilization of non-colinear magnetic structures. All the studied compounds are characterized by rather large coercive fields at low temperature (4.0≤H_c≤8.2 kOe).     

Crystallographic site of Mn in the icosahedral cluster of LaCo13−xMnx compounds

Studies on the structure and the crystallographic site of Mn in LaCo_13−xMn_x compounds were carried out by using X-ray diffraction and X-ray absorption fine structure (XAFS) of the Mn K-edge. These compounds with x≤3.0 adopt a NaZn₁₃-type structure consisting of icosahedral clusters. The lattice constant increases with the Mn concentration. The calculated XAFS curves of the center and the corner sites in the icosahedral clusters for the Mn K-edge are obtained by using the program . The fitting result for the corner site agrees much better with the observed XAFS spectrum than that for the center site. Therefore, the Mn site is determined to be the corner Co site in the icosahedral clusters of all the compounds. In comparison with the crystallographic parameters of LaCo₁₃, the icosahedral clusters composed of Mn atoms expand and the crystallographic Mn site is slightly more close to the La atom.     

Crystal structure, magnetic behaviour and valence state of ytterbium in the Yb4Ni10+xGa21−x phase

The ternary phase Yb₄Ni_10+xGa_21−x has been synthesised from the elements by high frequency melting in argon atmosphere. The homogeneity region has been established from X-ray powder data and confirmed by EDX analysis for 0.3≤x≤1. The crystal structure of Yb₄Ni_10+xGa_21−x has been estimated from X-ray single crystal data: space group C2/m (no. 12), Z=2, a=20.6815(9) Å, b=4.0560(4) Å, c=15.3520(7) Å, β=124.800(3)°, R(F)=0.023 for 1701 symmetry independent reflections with F(hkl)>4σ(F). A special feature of the structure is the local disorder within the gallium/nickel network. Neglecting atomic disorder in the region of the Ga9 and Ga11 positions, the Yb₄Ni_10+xGa_21−x structure is an occupation variant of the Ho₄Ni₁₀Ga₂₁ type with nickel atoms partially replacing the Ga atoms in the 2d sites at the centers of distorted icosahedra. From magnetic susceptibility and from L_III-XAS spectra, the valence state of ytterbium is near 3+.     

Evolution of magnetic hardness in the HfFe6Ge6-type RMn6Sn6−xGax and RMn6Sn6−xInx compounds (R=Tb, Dy; 0.2≤x≤1.4)

The HfFe₆Ge₆-type RMn₆Sn_6−xX_x′ solid solutions (R=Tb, Dy, X′=Ga, In; x≤1.4) have been studied by powder magnetization measurements. All the series are characterized by ferrimagnetic ordering and by a decrease in Curie temperatures with the substitution (ΔT_c/Δx≈−39 K for X′=Ga and ΔT_c/Δx≈−75 K for X′=In). The RMn₆Sn_6−xGa_x systems are characterized by a strong decrease in the spin reorientation temperature with substitution (ΔT_t/Δx≈−191 K and −78 K for R=Tb and Dy, respectively) while this transition almost does not change in systems containing indium. The coercive fields drastically decrease with the substitution in the TbMn₆Sn_6−xGa_x system while the substitution of In for Sn has a weaker effect. The coercive fields of the Dy compounds do not vary greatly with the substitution in both series. The behaviour of the TbMn₆Sn_6−xGa_x is compared with the evolutions observed in the TmMn₆Sn_6−xGa_x series. This comparison strongly suggests that the replacement of Sn by Ga changes the sign of the A₀² crystal field parameter.     

Physical properties of Mo6−xRuxTe8 and Mo6Te8−xSx

A series of the Chevrel phases, Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x (x=0, 1, 2), has been prepared and the various physical properties, such as the elastic modulus, Debye temperature, and electrical resistivity, have been evaluated. The relationships between several properties of the compounds have also been studied. Young’s modulus and Debye temperature of Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x increase with increasing x value. The relationship between the Vickers hardness and Young’s modulus shows ceramic characteristics for Mo_6−xRu_xTe₈, while they show glass-like characteristics for Mo₆Te_8−xS_x. The electrical resistivities of Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x increase with increasing x value.     

Magnetic phase diagrams of the TbRh2−xPdxPdxSi2 and TbRu2−xPdxSi2 systems

The a.c. susceptibility and high field magnetization on TbRh_2−xPd_xPd_xSi₂ and TbRu_2−xPd_xSi₂ compounds were investigated up to 140 kOe. The (T,x) magnetic phase diagrams were determined. For both systems, an increase in the Pd content causes a decrease in the Néel temperature and changes the magnetization curves.     

The role of valence electron concentration in the cohesive properties of YBxN1−x, YCxN1−x and YNxO1−x compounds

The mechanical properties are not yet understood at basic levels. Previous works shows that the greatest hardness for rock-salt structures (such as TiC_xN_1−x) is attained for a valence electron concentration (VEC) of 4.2 electrons per atom. The present work is aimed to explore this concept for yttrium-based compounds. By means of first principles calculations we did a systematical investigation where nitrogen in YN (VEC = 4) was supplanted by either of B, C or O to reduce or increase its VEC, forming YB_xN_1−x, YC_xN_1−x and YN_1−xO_x ternary compounds. We have calculated the cohesive energy (E_O), cell volume (V_O), bulk modulus (B_O) and density of states (DoS) as a function of VEC. The Fermi level (E_f,) is shifted toward the valence band by substituting B or C in YN, and toward the conduction band by means of O. It is concluded that the optimal position for E_f (maximum B_O) is linked to the saturation of electronic states with e_g-symmetry. At this point the excess of electrons provided by O starts filling antibonding states with t_2g-symmetry. That is, B_O increases monotonically as a function of VEC until VEC  4.1, after that point B_O decrease.     

Magnetic properties and electronic structures of GdCo3−xSix compounds

Magnetic measurements and band structure calculations were performed on GdCo_3−xSi_x system with x ≤ 0.3. The experimentally determined magnetizations are in rather good agreement with those obtained from band structure calculations. The composition dependence of cobalt moments, at various lattice sites, are analyzed in correlation with the effects of Co 3d–Si 3p bands hybridization.     

Synthesis and properties of Ba3NaRu2O9−δ and Ba3(Na, R)Ru2O9−δ (R=rare earth) crystals

Crystals of Ba₃NaRu₂O_9−δ (δ≈0.5) and Ba₃(Na, R)Ru₂O_9−δ (R=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) were grown by an electrochemical method, and their crystallographic, magnetic, and electric properties were studied. All crystals have a hexagonal structure of space group P6₃mmc. Ba₃NaRu₂O_9−δ and Ba₃(Na, R)Ru₂O_9−δ (except Ce) have a negative asymptotic Curie temperature suggesting the existence of an antiferromagnetic order; however, they are paramagnetic at temperatures above 1.7 K. Ba₃NaRu₂O_9−δ has an effective magnetic moment P_eff of 0.91 μ_B, while P_eff of Ba₃(Na, R)Ru₂O_9−δ (except Ce) reflects the large free-ion moment of the rare earth ions. Ba₃(Na, Ce)Ru₂O_9−δ shows peculiar magnetic behavior that differs from the magnetism of other Ba₃(Na, R)Ru₂O_9−δ crystals. The resistivity of all crystals exhibits an activation-type temperature dependence with an activation energy in the range of 0.10.2 eV.     

Structural and electrochemical properties of TixZr7−xNi10

Simple ternary alloys with formula Ti_xZr_7−xNi₁₀ (x between 0 and 2.5) were studied as a potential replacement for Laves phase alloys used in the negative electrodes of nickel metal hydride batteries. The samples were prepared by arc-melting and were not annealed. The samples retained a high degree of disorder, which contributed positively to activation and other electrochemical properties. Before hydrogenation, the alloys have a Zr₇Ni₁₀ orthorhombic structure mixed with some C15 and ZrO₂ secondary phases. The amount of C15 secondary phase is important to the bulk diffusion of hydrogen and the surface electrochemical kinetics. That is, the diffusion coefficient and the exchange current both increase in the presence of C15 secondary phase. The proportion of C15 secondary phase is controllable by stoichiometry design. For instance, a slightly higher Zr content reduces the C15 content. Further, as the titanium substitution level increases: (1) the lattice constants decrease; (2) the PCT plateau pressure increases; (3) activation becomes easier; and (4) the high rate dischargeability improves.     

Glass-forming ability and magnetic properties of mechanically solid-state reacted Co100−xTix alloy powders

A high-energy ball milling technique using the mechanical alloying method has been employed for fabrication of glassy Co_100−xTi_x (25≤x≤67) alloy powders at room temperature. The fabricated glassy alloys in the Co-rich (33≥x) side exhibit good soft magnetic properties. The binary glassy alloys for which the glass transition temperatures (T_g) have rather high temperatures (above 800 K), show large supercooled liquid regions before crystallization (ΔT_x larger than 50 K). The reduced glass transition temperature (ratio between T_g and liquidus temperatures, T_l (T_g/T_l)) was found to be larger than 0.56. We have also performed post-annealing experiments on the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction), of which the heat of formation was measured directly. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glass state.     

Magnetic properties of Ce3−xGdxCo11B4 borides

The structure and magnetic properties of Ce_3−xGd_xCo₁₁B₄ borides have been studied by X-ray powder diffraction (XRPD), magnetization and differential scanning calorimetry (DSC) measurements. X-ray analysis reveals that the compounds crystallize in the hexagonal Ce₃Co₁₁B₄-type structure with P6/mmm space group. The substitution of Gd for Ce leads to an increase of the unit-cell parameter a and the unit-cell volume V, while the unit-cell parameter c decreases linearly. Magnetic measurements indicate that all samples are ordered magnetically below the Curie temperature. The Curie temperatures increase as Ce is substituted by Gd. The saturation magnetization at 4 K decreases upon the Gd substitution up to x = 1, and then increases.     

Structural and thermodynamic properties of the pseudo-binary TiCr2−xVx compounds with 0.0≤x≤1.2

The TiCr_2−xV_x compounds with 0.0≤x≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x>0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature (P–C–T) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P–C–T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.     

Existence, structure and valence properties of the skutterudites CeyFe4−xCoxSb12

We report on sample preparation, annealing effects, electron microprobe analysis in the series Ce_yFe_4−xCo_xSb₁₂ which shows that a phase separation occurs for substituted samples (0<x<4) annealed at 650 and 550 °C. Single phase samples are obtained for either Ce_yFe₄Sb₁₂ or Ce_yCo₄Sb₁₂ samples annealed at 650 °C and for all compositions when annealed at 700 °C. The valence state of Ce in homogeneous samples has been studied using X-ray absorption spectroscopy (XAS). Ce ions are trivalent throughout the series and the XAS spectra does not show effect of the crystal field on the 5d-final state.     

Magnetocrystalline anisotropy of R2Fe17Hx (x=0, 3) single crystals

The magnetocrystalline anisotropy of R₂Fe₁₇ (R=Y, Gd, Tb, Ho and Er) and their hydrides are studied by analyzing the magnetization curves of single crystal samples in the temperature range 4.2–300 K in magnetic fields up to 140 kOe. There is no noticeable influence of hydrogenation on the magnetic anisotropy in the Y₂Fe₁₇ and Gd₂Fe₁₇ compounds. An easy-plane to easy-cone transition is detected for a Tb₂Fe₁₇H₃ single crystal after hydrogenation. A significant change of the magnetization process has been observed in the hydrides Ho₂Fe₁₇H₃ and Er₂Fe₁₇H₃. Hydrogenation induces a FOMP-type transition in the Ho₂Fe₁₇H₃ compound and, on the contrary, leads to the disappearance of the FOMP type transition in the Er₂Fe₁₇H₃ compound. The obtained results are discussed on the basis of a model based on the interaction of the quadrupolar moment and the magnetic moment of the 4f electron shell of the rare earth ion with the interstitial hydrogen. It is established that the orientation of the quadrupolar moment of the asymmetric 4f shell with respect to the direction of the resulting magnetic moment of 4f electrons plays an important role.