Magnetic, magnetocaloric and neutron diffraction studies on TbNi5−xMx (M = Co and Fe) compounds

The effect of substitution of Co and Fe for Ni in TbNi₅ on the structural, magnetic and magneto-thermal properties has been investigated. Considerable enhancement of Curie temperature is observed with Fe substitution, whereas the increase is nominal in the case of Co. Neutron diffraction measurements reveal the redistribution of moments and site preference of substitutional ions in Ni 2c and 3g sites. In TbNi₄Fe, both Ni and Fe as well as Tb are found to carry moment while in the case of TbNi₄Co, mainly Tb carries the moment. Magnetocaloric behavior has been investigated from the magnetization and the heat capacity measurements. The magnetic and magnetocaloric properties are found to be strongly correlated in these compounds.     

Magnetic and neutron diffraction studies on potassium uranate KUO3

The magnetic properties of KUO₃ with the cubic perovskite structure have been studied. It shows an antiferromagnetic-type of transition at ca. 17 K in the magnetic susceptibility vs. temperature curve. Magnetic hysteresis is observed at 4.5 K, but not observed at 20 K. Neutron diffraction measurements have been performed on the powdered KUO₃ below and above the transition temperature. The neutron diffraction pattern measured at 10 K shows no appreciable difference from that measured at 50 K. A small ordered magnetic moment of U⁵⁺ ion will make it very difficult to observe magnetic diffractions, even if a magnetic ordering occurs. If the magnetic spin alignment occurs in a small domain, no magnetic diffraction will be found in the neutron experiments. On the other hand, the magnetic phenomena of KUO₃ have been analyzed on the basis of an octahedral crystal field model.     

Magnetic ordering in ternary RMn2Ge2 compounds (R = Tb, Ho, Er, Tm, Lu) from neutron diffraction study

The compounds RMn₂Ge₂ (R = Tb, Ho, Er, Tm, Lu) have been investigated by neutron diffraction. TbMn₂Ge₂ is a collinear ferrimagnet with the Mn and Tb moment aligned along the c axis (μ_TB = 8.81(59) μ_B: μ_Mn = 2.21(44) μ_B). HoMn₂Ge₂ exhibits incommensurale ordering below 2.1 K characterized by two wavevectors at 1.3 K: q₁ = (0.1543(4), 0.1543(4), 0) and q₂ = (0.210(1), 0.007(1), 0). The Mn sublattice remains antiferromagnetic down to 1.3 K (μ_Mn = 2.38(6) μ_B). The Er moments order ferromagnetically below 5.5 K in ErMn₂Ge₂ (μ_Mn = 6.81(31) μ_B). The moments are perpendicular to the c axis. The Mn sublattice remains antiferromagnetic down to 1.8 K (μ_Mn = 2.34(18) μ_B). The magnetic structure of TmMn₂Ge₂ is characterized by the propagation vector (0.0.1/2). the Tm moments lying in the basal plane. The ordering of the Tm moments yields a canting of the Mn moments (τ = 21(3)°); μ_Tm = 6.63(18) μ_B; μ_Mn = 2.28(27) μ_B). The antiferromagnetic structure of LuMn₂Ge₂ has been determined (μ_Mn = 2.32(14) μ_B). The evolution of the magnetic properties of the heavy rare earth compounds RMn₂Ge₂ is discussed.     

Nonstoichiometry and P–T–x diagrams of binary systems

A P–T–x three-dimensional space diagram forms the complete representation between pressure (P), temperature (T) and composition (x) of coexisting phases—solid (S), liquid (L) and vapor (V). It gives the number of nonstoichiometric compounds that may be formed by the components and the stability limits of phases which are in equilibrium. Definitions of stoichiometry and nonstoichiometry are given. Some features of P–T–x diagrams with nonstoichiometric compounds are considered: maximum (T_m^max) and congruent (T_m^c) melting points, difference between the compositions of solid (x^S), liquid (x^L) and vapor (x^V): x^L≠x^S≠x^V at maximum melting point T=T_m^max, the width and position of the homogeneity range, and the nonstoichiometry caused by defects are also discussed. A new technique for the investigation of P–T–x diagrams is presented.     

Magnetic and electronic properties of RCoxGe2 (R = Pr, Nd) compounds

Magnetization, magnetic susceptibility, neutron diffraction and X-ray photoemission spectroscopy measurements were performed on polycrystalline samples: PrCo_0.85Ge₂ and NdCo_0.82Ge₂, crystallizing in an orthorhombic structure of the CeNiSi₂-type. The magnetometric data indicate that both compounds are antiferromagnetic at low temperatures, PrCo_0.85Ge₂ below 5 K and NdCo_0.82Ge₂ below 2.1 K. The neutron diffraction data indicate an antiferromagnetic structure in PrCo_0.85Ge₂ at 1.5 K and give no evidence of any magnetic ordering in NdCo_0.82Ge₂. The X-ray photoemission data indicate that the valence bands are formed predominantly by R 4f5d6s and Co 3d bands. The spin-orbit splitting values determined from the Pr and Nd, 3d_5/2 and 3d_3/2 XPS spectra are equal to 20.9 eV for the Pr compound and 23.0 eV for the Nd compound. The analysis of these spectra performed on the basis of the Gunnarsson–Schönhammer model revealed a small hybridization between 4f-electrons of the rare earths with the conduction band which implies rather good stability of the f shell in these compounds.     

Magnetic anisotropy and magnetic properties of RTSi (R=Gd, Y; T=Mn, Fe) compounds

Magnetic properties of Gd_xY_1−xMnSi and GdMn_xFe_1−xSi compounds with CeFeSi-type structure have been studied by magnetization measurements in the temperature range 77–600 K in static magnetic fields up to 12 kOe. The measurements for some compounds have been carried out on single crystal samples. The easy magnetization direction of all single crystal samples was found to be the c axis. The value of the anisotropy constant K₁ for GdMnSi was estimated to be 2.0·10⁶ erg cm⁻³ at 77 K. The substitution in both rare earth and 3d sublattices leads to a sharp increase in magnetic anisotropy of these compounds. The concentration dependencies of magnetic ordering temperatures, effective magnetic moments and paramagnetic Curie temperatures have been determined. The obtained results can be explained by the modification of the band structure due to the change of interatomic distances and the filling of 3d band.     

Magnetic phase diagram of CrTe1−xSbx (0.0≤x≤1.0)

Measurements of the high field magnetization of CrTe_1−xSb_x (0.0≤x≤1.0) were carried out at 4.2 K in pulsed magnetic fields up to 300 kOe. The temperature dependence of the magnetization of CrTe_1−xSb_x was measured in the temperature range from 4.2 K to 800 K. The magnetic phase diagram of CrTe_1−xSb_x (0.0≤x≤1.0) was determined, which is similar to the typical one for the mixed crystals of the layered antiferromagnetic and ferromagnetic compounds proposed by de Gennes.     

Magnetic properties of R2Zn17 compounds (R = rare earth)

Magnetization measurements have been performed on polycrystalline R₂Zn₁₇ compounds (R Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb) in the temperature range 1.5–300 K. In Yb₂Zn₁₇, the rare earth exhibits a valence of 2+ and the compound is not magnetic. All the other compounds order antiferromagnetically at low temperature. The main magnetic characteristics have been determined.     

Magnetic properties of R2CoGa3 (R = rare earth) compounds

R₂CoGa₃ compounds (R = Gd, Th, Dy, Ho, Er and Y) crystallize in the hexagonal P6,lmcm space group. In these compounds the Co atoms have practically no magnetic moment. The heavy rare earth compounds order ferromagnetically with relatively low Curie temperatures ( 3 K (Er) T_c 50 K (Gd)). The compounds with Tb and Dy, and to a smaller extent the Ho compound, present irreversibilities in their magnetization processes, and there are maxima in the zero-field-cooled susceptibility curves.     

Ti(C,N) cermets — Metallurgy and properties

An overview of the metallurgical reactions during the vacuum sintering process of powder mixtures for the manufacture of cermets is presented. The relatively complex phase reactions in the multi-component system Ti/Mo/W/Ta/Nb/C,N-Co/Ni are discussed. The liquid binder phase reacts with titanium carbonitride by preferentially dissolving titanium carbide leaving titanium nitride undissolved. The compositions and the amounts of the gas species set free during the sintering process were monitored and led —together with differential thermal analysis — to a better understanding of the mechanisms that govern the sintering behaviour.

The properties and the microstructure of cermets depend on the nature and the alloy status of the prematerials. The composition of the prematerials with respect to the carbon-nitrogen ratio, the stoichiometry of the hard phase and the amount and composition of the binder phase have a decisive influence on the properties and the cutting performances of the final products. Optimization of the properties with respect to the desired performance is possible. Examples of the cermet cutting performance in various applications are discussed.     

Magnetic,electronic and transport properties of RAg2Ge2 (R=Pr,Nd) compounds

Magnetization, magnetic susceptibility, electrical resistivity, thermoelectric power, neutron diffraction and X-Ray photoemission spectroscopy measurements were performed on polycrystalline samples of PrAg₂Ge₂ and NdAg₂Ge₂, both crystallizing with a tetragonal structure of the ThCr₂Si₂-type. The data indicate that both compounds order magnetically at low temperatures, PrAg₂Ge₂ at 12 K and NdAg₂Ge₂ at 2 K. Neutron diffraction data for the latter indicate antiferromagnetic collinear structure described by the propagation vector k=(½, 0, ½) at 1.5 K. Both compounds exhibit metallic-like electrical behavior. The X-Ray photoemission data indicate that the valence bands are formed mainly by the Ag 4d bands. The spin-orbit splitting values determined from the XPS spectra of Pr and Nd, 3d_5/2 and 3d_3/2, equal 20.5 eV for the Pr compound and 22.7 eV for the Nd compound. The analysis of these spectra performed on the basis of the Gunnarsson-Schönhammer model indicates a weak hybridization of the 4f electrons with the conduction band.     

Magnetic susceptibility and surface properties of EuAlO3 nanocrystals

Nanocrystalline single-phase europium aluminate (EuAlO₃) has been synthesized by modified sol-gel method with a new complexing agent - malic acid. The material has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive analysis (EDS). The nanoparticles have an average size of 50 nm, a density of 7.01 g/cm³ at T = 293 K, a specific surface area 15.0 m²/g and form agglomerates. The binding energies of core-level electrons of europium, aluminium, and oxygen in EuAlO₃ have been found by means of X-ray photoelectron spectroscopy (XPS) and compared with the values for the respective elements. The temperature dependence between 300 K and 2 K of the experimental AC magnetic susceptibility of EuAlO₃ nanocrystals has been determined and compared with the theoretical one for Eu³⁺. It has been confirmed that the material is paramagnetic in the entire temperature range.     

A neutron diffraction study of the hexagonal pseudo-ternary compounds ErMn6−xFexSn6 (x = 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0)

The hexagonal ErMn_6−xFe_xSn₆ solid solution (0.2 < x < 4) has been studied by magnetisation measurements and neutron diffraction. The ordering temperature of the T = (Mn,Fe) sublattice almost continuously increases from T = 386 K for x = 0.2 to T = 498 K for x = 4. The T sublattice orders in the successive magnetic structures helimagnetic H1, antiferromagnetic AF2, helimagnetic H2 and antiferromagnetic AF1 with increasing iron content. While structures AF2 and H1 were already observed in ternary Mn compounds and AF1 in ternary iron compounds, the structure H2 is of a new kind characterized by an AF slab around the Er(1a) site. At low temperature, a change of the easy direction of the Er moment from easy plane to easy axis is observed. The iron-rich compounds display a ferromagnetic order of the Er sublattice. A new kind of magnetic structure characterized by a sine-wave modulated arrangement with a propagating vector Q = (0, 0, q_z) is also observed. The evolution of the magnetic properties (enhancement of the AF character of the (Mn,Fe) sublattice and magnetocrystalline anisotropy of erbium) is discussed.     

Separation of macroscopic, elastic mismatch and thermal expansion misfit stresses in metal matrix composite quenched plates from neutron diffraction measurements

Neutron diffraction measurements of the strain profile in a quenched plate of an aluminium-dash;silicon carbide particle-reinforced metal matrix composite are reported. The results have been used to evaluate the efficacy of an analysis technique which allows distinction of the stiffness mismatch and shape misfit stresses between the matrix and reinforcement, as well as between these and any macrostress present. The analysis is presented for measurements made on a metal matrix composite plate which, as a consequence of quenching from elevated temperature, shows large variations in residual stress as a function of position through the plate thickness. The measurements illustrate the additional insight which can be obtained through the separation of the elastic mismatch and thermal misfit stresses. The stress components thus obtained show good agreement with calculated long-range residual and mismatch stresses.     

Magnetic properties of novel compounds R3(Fe,Cr)29Xy (R=Nd, Sm and X=N, C)

The novel ternary rare-earth iron-rich interstitial compounds R₃(Fe,Cr)₂₉X_y (R=Nd, Sm and X=N, C) with the monoclinic Nd₃(Fe,Ti)₂₉ structure have been successfully synthesized. Introduction of the interstitial nitrogen and carbon atoms led to a relative volume expansion ΔV/V of about 6% and an enhancement of Curie temperatures T_c about 268 K for the nitride and about 139 K for the carbide, respectively. The Nd₃Fe_24.5Cr_4.5X_y compounds have a planar anisotropy at room temperature. A first-order magnetization process (FOMP) with critical field B_cr=4.4 T and 3.1 T at room temperature were observed for the Nd-nitride and carbide compounds, respectively. The Sm₃Fe₂₄Cr₅X_y compounds were found to have a large uniaxial anisotropy of about 18 T at 4.2 K and about 11 T at 293 K. A FOMP with B_cr=2.3 T was also observed in the Sm-nitride compounds at 4.2 K. Magnets with coercivity of μ_OjH_c0.8 T at 293 K has been successfully developed from the Sm₃Fe₂₄Cr₅X_y (X---N and C) phases.     

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.     

Magnetic properties and magnetocaloric effect in the aluminide RENiAl2 (RE = Ho and Er) compounds

Two single-phased aluminide RENiAl₂ (RE = Ho and Er) compounds have been synthesized by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) of HoNiAl₂ and ErNiAl₂ have been studied by magnetization measurements. A second-order paramagnetic (PM) to ferromagnetic (FM) phase transition together with a considerable reversible MCE were observed around the Curie temperature T_C ∼7.5 K and 5.0 K for HoNiAl₂ and ErNiAl₂, respectively. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 14.0 J/kg K, 171 J/kg, 213 J/kg for HoNiAl₂ and 21.2 J/kg K, 267 J/kg, 357 J/kg for ErNiAl₂, respectively.     

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.