Magnetic properties of Gd1−xThxFe2 and Gd1−xCexFe2

Saturation magnetization, magnetization vs temperature, Curie temperatures, and lattice parameters are presented for the ternary alloys Gd_1?xTh_xFe₂ and Gd_1?xCe_xFe₂. Quadrivalent Th and Ce were introduced into the lattice in an effort to induce ferromagnetic GdFe coupling. Experiment showed that the antiferromagnetic GdFe coupling in GdFe₂ is preserved in the ternaries. The Fe moment and Curie temperature decrease as the Gd content of the sample is decreased. This is ascribed to electron transfer from Th or Ce to the Fe d shell. Failure to achieve ferromagnetic coupling is ascribed to electron capture by iron, which prevents a rise in electron concentration as Gd is replaced by Ce or Th.     

Magnetic and structural properties of Y6−xErxFe23 alloys and their hydrides

The magnetic characteristics and structural features of the Y_6?xEr_xFe₂₃ alloys and their hydrides are reported. The parent alloys all formed in a face-centered cubic structure. A pronounced minimum in the lattice parameter was observed for Er₄Y₂Fe₂₃. A similar minimum was also observed in the 1 atm hydrogen capacity of the alloys. The hydrides of the Y-rich compounds were found to retain the cubic structure of the parent compounds, whereas the hydrides of Er₅YFe₂₃ and Er₆Fe₂₃ adopt tetragonally distorted structures. Both the intermetallics and their hydrides were observed to be ferrimagnetic, exchange-dominated systems. In all cases, absorption of hydrogen resulted in an increase in both the saturation magnetization and the Curie temperature.     

Structural and magnetic properties of RMn2−xFexD6 compounds (R=Y, Er; x≤0.2) synthesized under high deuterium pressure

RMn_2−xFe_xD₆ compounds were obtained by applying a deuterium pressure of several kbar to RMn_2−xFe_x compounds for x≤0.2 and R=Y, Er. These compounds are isostructural to RMn₂D₆ compounds and crystallize in a K₂PtCl₆ type structure with a random substitution of R and half the Mn atoms in the same 8c site whereas the other Mn atoms are located on the 4a site and surrounded by six D atoms (24e site). According to neutron powder diffraction analysis the Fe atoms are preferentially substituted on the 4a site. YMn_2−xFe_xD₆ compounds are paramagnetic and their molar susceptibility follows a modified Curie-Weiss law. ErMn_2−xFe_xD₆ compounds display a ferromagnetic behavior at 2 K, but their saturation magnetization (M_S∼4.0 μ_B/f.u.) is half that of their parent compounds (M_S∼8.0 μ_B/f.u.). The neutron diffraction patterns of ErMn_1.8Fe_0.2D₆ display below 13 K both ferromagnetic and antiferromagnetic short range order, which can be related to a disordered distribution of Er moments. The paramagnetic temperatures of ErMn_2−xFe_xD₆ compounds are negative and decrease versus the Fe content whereas they are positive and increase for their parent compounds.     

Magnetic interactions in ternary cobalt oxides with cubic perovskite structure

Magnetic properties were measured on the cubic perovskite systems SrCoO_3?δ, (La_1?xSr_x)CoO₃ (0.5 ≦ x ≦ 1.0), and Sr(Co_1?xMn_x)O₃ (0 ≦ x ≦ 1.0). It is found that S²⁺ and La³⁺ ions strongly affect the spin state of the Co²⁺ ion and that the Mn⁴⁺ ion located at the octahedral site affects the spin state of Co⁴⁺ ion. The magnetic properties (T_c, T_θ, and σ) are explained by the magnetic interaction Co³⁺OCo³⁺, Co³⁺OCo⁴⁺, Co⁴⁺OCo⁴⁺, Mn⁴⁺OMn⁴⁺, and Mn⁴⁺OCo⁴⁺ in these systems.     

Crystal structure of β-CaNi5D0.77 by rietveld analysis of neutron powder-diffraction data

β-CaNi₅D_0.77 is orthorhombic (a = 8.6033(13) b = 5.0810(9) c = 7.8557(15)Å, Pmcm, Z = 4, D_x = 6.48 g cm^?3, V = 343.4ų, FW = 1339.8 amu. Rietveld analysis of a neutron powder-diffraction pattern shows that D atoms occupy the centers of [Ni₄Ca₂] square dipyramids (“octahedra”) linked through the Ca atoms at opposite apices and forming chains. About 80% of the deuterium atoms occupy fairly regular sites (DNi = 1.53, 1.80, 1.86, 1.86 Å; DCa = 2.42, 2.45 Å). The remaining 20% occupy very distorted sites (DNi = 1.21, 1.38, 2.07, 2.07 Å; DCa = 2.54, 2.60 Å). The average DD distance in a chain is 4.3 Å.     

Molecular reorientation of liquid benzene. Anisotropic Raman scattering of the CH and CD stretching modes in the deuterated molecules

The anisotropic Raman spectra of the CH and CD stretching modes in seven deuterated benzenes of D_6h, D_3h, D_2h and C_2h symmetry are reported. The reorientational linewidths are interpreted within the model of anisotropic rotational diffusion. The data are consistent with NMR relaxation studies. The study covers the temperature range between T/T_c = 0.49 and T/T_c = 0.97.     

Structural,morphological and magnetic properties of nano-crystalline zinc substituted cobalt ferrite system

Nano-crystalline zinc-substituted cobalt ferrite powders, Co_1−xZn_xFe₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1), have been synthesized by the combustion route. The structural, morphological and magnetic properties of the products were determined and characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometer (VSM). X-ray analysis showed that the samples were cubic spinel. The increase in zinc concentration resulted in an increase in the lattice constant, unit cell volume, X-ray density, ionic radii, the distance between the magnetic ions and bond lengths on tetrahedral sites and octahedral sites of cubic spinel structure. Opposite behavior was observed for the average crystallite size of the as synthesized solids. The variation of saturation magnetization (M_s) value of the samples was studied. The maximum saturation magnetization value of the Co_o.25Zn_0.75Fe₂O₄ sample reached 76.87 emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high-quality nano-crystalline magnetic ferrites for practical applications.     

Magnetoresistance in the ferromagnetic metallic perovskite SrFe1−xCoxO3

The investigation of the magnetic and transport properties of the oxygen deficient perovskites SrFe_1−xCo_xO_3−δ shows that these compounds exhibit both ferromagnetism and metallicity in a wide compositional range (0≤x≤0.70). Negative magnetoresistance is evidenced for the first time in these oxides, in contrast to SrCoO_3−δ. These properties are explained by superexchange interactions between cobalt and iron according to the scheme Fe³⁺OCo⁴⁺↔Fe⁴⁺OCo³⁺. This model is strongly supported by ⁵⁷Fe Mössbauer measurements which show the existence of two sites at room temperature, high spin localized Fe⁴⁺ and delocalized Fe^3+α sites, whereas magnetic disordering suggesting spin fluctuations is observed at 5 K as soon as cobalt is introduced into the SrFeO₃ structure.     

Magnetic studies of nickel ferrite doped with rare earth ions

The magnetic properties nickel ferrites having general chemical formula Ni _x Fe_{(3 ? x)}O₄ have been studied. The X-ray diffraction measurements confirmed the formation of a cubic spinel structure. The magnetic properties were studied by vibrating sample magnetometer (VSM). It exhibited lower coercivity and higher saturation magnetization due to high crystallinity. The saturation magnetization (M _s) and coercivity (H _c) for Nd and Gd are different ions since the magnetic moment of Gd³⁺ is greater than Nd³⁺.     

Magnetic phase diagrams and helical magnetic phases inMxNi1−xBr2 (M =Fe,Mn): A neutron diffraction and magneto-optical study

BelowT_N = 44K, NiBr₂ orders as an easy-plane antiferromagnet, but atT_IC ～ 23K it undergoes a transition to a helimagnetic phase whose propagation vector, τ, increases smoothly to a limiting value [0.027,0.027,0] at 4.2 K. The magnetic order in NiBr₂ andM_xNi_1?xBr₂ (M =Fe, Mn) is investigated by single-crystal neutron diffraction. In Fe_xNi_1?xBr₂ τ changes from the [110] to [100] direction at 3.7(17)% Fe. Its magnitude, andT_IC, remain constant up to 9.9(14)% Fe, but above this concentration a collinear, easy-axis structure is stable at all temperatures belowT_N, which decreases towards the value of 11 K in FeBr₂. Doping 2.9% Mn²⁺ into NiBr₂ decreasesT_IC slightly, in the manner previously observed for diamagnetic dopants. Refinement of powder X-ray diffraction profiles show the lattice distortions in the doped materials to be small, while the structures of some Fe_xNi_1?xBr₂ samples are confirmed by refinement of the powder neutron diffraction profiles. The temperature and field dependence of hot and cold exciton-magnon combination bands in the optical absorption spectra of Fe²⁺- and Mn²⁺-doped crystals give additional information about magnetic order. The magnetic behaviour is interpreted in terms of perturbations of the exchange constantsJ_1,2,3, andJ′ and the anisotropy constantD.     

Exchange interactions in R2Fe17−xGax (R=Y, Sm, Gd, Tb, Ho and Tm) compounds

We calculated the molecular field coefficients, n_FeFe and n_RFe (R=Sm, Gd, Tb, Ho and Tm), for R₂Fe_17−xGa_x and the values of n_FeFe and n_SmFe for R₂Fe_17−xT_x (T=Al and Si) using the experimental values of the Curie temperature. The values of n_FeFe increase in spite of the decrease of μ_Fe for 0?x?5. The values of n_SmFe have large values when the magnetic anisotropy is axial. For 6?x?8, the values of n_FeFe, n_HoFe and n_TmFe increase largely, which is related to the change of the easy magnetization direction. For Y₂Fe_17−xT_x (T=Ga and Al), the values of n_FeFe have a maximum value with increasing those of μ_Fe. With increasing V⁻¹, the values of n_FeFe have a maximum value around the same value of V⁻¹ for Y₂Fe_17−xT_x (T=Ga and Al). For Y₂Fe_17−xSi_x, the values of n_FeFe increase with increasing V⁻¹.     

A neutron powder diffraction study of the κ phase in the FeWC system

The crystal structure of the κ-carbide in the FeWC system has been refined from neutron powder diffraction data using the Rietveld profile analysis method. κ-(FeWC) is isostructural with κ-(CoWC); space group $\text{P6}{}_3\text{mmc}$; unit cell dimensions a = 7.7982(2)Å, c = 7.8298(4) Å. The structure refinement indicates $\text{Fe}\text{W}$ substitution at two of the tungsten sites, and 46% vacancies at one of the carbon sites. The composition corresponds to the formula Fe_3+xW_10?xC_4?y, with x = 0.57(3) and y = 0.46(1).     

Crystal structure of Mg0.65Sc0.35Dx deuterides studied by X-ray and neutron powder diffraction

The structural properties of the Mg_0.65Sc_0.35D_x deuterides have been investigated by X-ray and neutron powder diffraction at different deuterium content (0?x?2.2 D/f.u.). The metallic phase adopts a pseudo-CsCl structure (Pm-3m space group (SG); a=3.5921(2) Å) that transforms upon hydrogenation into a face centered cubic (FCC) phase involving an elongation of the c-axis, a shrinkage of the a-axis and a re-ordering of the metallic atoms. The fully hydrided compound (2.2 D/f.u.) adopts a cubic structure (Fm-3m SG; a=4.8087(7) Å) and deuterium is located in fully occupied tetrahedral sites and partially filled (24%) octahedral sites. Upon desorption, a two-phase domain appears with coexistence of a hydrogen-rich (1.55 D/f.u.) and a hydrogen-poor (0.85 D/f.u.) phase (Fm-3m SG; a=4.7598(3) and 4.6936(3) Å, respectively). All deuterium atoms are located in the tetrahedral sites with different occupancy factors: 77% for the H-rich phase and 43% for the H-poor phase. The appearance of a plateau in the pressure-composition-isotherm curve measured at 573 K confirms this two-phase behavior. The structural properties of the Mg_0.65Sc_0.35D_x system are discussed and compared with other body centered cubic (BCC) alloys adopting the same structure.     

Etude structurale et magnétique de pérovskites de formule Ba3Fe2−xHoxUO9

The compounds crystallize in cubic perovskite-type system. The space group is Fm3m and the cell is entirely ordered. 4a positions (Wyckoff's notation) contain iron and uranium, 4b positions, barium and iron, 8c positions, barium and any holmium, which therefore fills coordinance-12 crystallographic sites. For iron-rich compounds (0 < x ? 0.8), interactions between Fe³⁺ ions from two magnetic sublattices 4a and 4b are antiferromagnetic. The interactions between the third magnetic sublattice, which corresponds to Ho³⁺ ions filling 8c positions, and the resultant of the first two, are slightly ferromagnetic. Such data have been obtained with all the other compounds for which the formula is Ba₃Fe_2?xLn_xUO₉, where Ln is Yb, Tm, Er, Dy, Tb, and Gd.     

Phase relations of ternary compounds in the BaFeS system

The phase relations of the ternary phases at 900 and 700°C were determined by heating mixtures in the regions BaSFeSFeS₂ and FeFeSBaS and identifying the products by powder X-ray diffraction. The stable ternary phases at 900°C are BaFe₂S₃, Ba₂FeS₃, Ba₆Fe₈S₁₅, a solid solution BaFeS₂Ba₇Fe₆S₁₄, Ba₂FeS₄, the infinitely adaptive series $\text{Ba}{}_3\text{Fe}{}_{\mathrm{1+x}}\text{S}{}_5\text{,}\text{1}\text{3}\text{≤ x ≤}\text{2}\text{5}$, and Ba₉Fe₁₆S₃₂ which belongs to the infinitely adaptive series Ba_1+xFe₂S₄. At 700°C the stable ternary phases are: BaFe₂S₃, Ba₂FeS₃, BaFeS₂Ba₇Fe₆S₁₄ (solid solution), Ba₆Fe₈S₁₅, Ba₂FeS₄, Ba₅Fe₄S₁₁, and two infinitely adaptive series: Ba₃Fe_1+xS₅ and Ba_1+xFe₂S₄. A stable ternary phase at 700 and 900°C with probable composition Ba₂Fe₄S₅ was found in the FeFeSBaS section.     

Thermodynamic properties and cation distribution of the ZnFe2O4Fe3O4 spinel solid solutions at 900°C

The thermodynamic properties of the Fe₃O₄ZnFe₂O₄ spinel solid solution were determined at 900°C by the use of the solid electrolyte galvanic cell Fe₂O₃ + Fe₃O₄|O^2?|Fe₂O₃ + Zn_xFe_3?xO₄The activity values obtained exhibit slight negative deviation from the ideal solution model. An analysis of the free energy of mixing of the spinel solid solution provided information on the distribution of cations between the tetrahedral and octahedral sites of the spinel lattice. This is the basis for the estimation of the free energy of formation of pure zinc ferrite from oxides. ΔG⁰_ZnFe2O4 = ?2740 ? 1.6 T cal mole^?1     

Solid-state ionics: The electrical conductivity in the Ag1−xCuxI-substituted ammonium iodide double salts

The temperature dependence of the electrical conductivity in the Ag_1?xCu_xIQI (Q = NH₄, (CH₃)₄N, and C₅H₅NCH₃) systems were measured at temperatures ranging from ?20 to 130°C under nitrogen gas flow. Temperature T_d, at which a high ionic conductivity compound decomposes to a poor conductivity compound, was determined as a function of x. Temperature T_d was raised with the increase of x in the Ag_1?xCu_xINH₄I and Ag_1?xCu_xI(CH₃)₄NI systems, but did not depend on the x value in the Ag_1?xCu_xIC₅H₅NCH₃I system; that is, the disproportionation reaction in the Ag_1?xCu_xINH₄I and (CH₃)₄NI systems was different from that in the Ag_1?xCu_xIC₅H₅NCH₃I system.     

Sol–gel synthesis of nanocrystalline Zn1−xNixFe2O4 ceramics and its structural,magnetic and dielectric properties

Zn_1?xNi_xFe₂O₄ (0.0 ≤ x ≤ 1.0) nanoparticles are prepared by sol–gel method using urea as a neutralizing agent. The evaluation of XRD patterns and TEM images indicated fine particle nature. The average crystallite size increased from 10 to 24 nm, whereas lattice parameters and density decreased with increasing Ni content (x). Infrared spectra showed characteristic features of spinel structure along with a strong influence of compositional variation. Magnetic measurements reveal a maximum saturation magnetization for Zn_0.5Ni_0.5Fe₂O₄ (x = 0.5); however, reduced value of magnetization is attributed to the canted spin structure and weakening of Fe³⁺(A)–Fe³⁺(B) interactions at the surface of the nanoparticles. Impedance analysis for different electro-active regions are carried out at room temperature with Ni substitution. The existence of different relaxations associated with grain, grain boundaries and electrode effects are discussed with composition. It is suggested that x = 0.5 is an optimal composition in Zn_1?xNi_xFe₂O₄ system with moderate magnetization, colossal resistivity and high value of dielectric constant at low frequency for their possible usage in field sensor applications.     

Crystal structure and magnetic properties of high-oxygen pressure annealed Sr1−xLaxCo0.5Fe0.5O3−δ (0?x?0.5)

Structural and magnetic studies are presented for the perovskite type Sr_1−xLa_xCo_0.5Fe_0.5O_3−δ (0?x?0.5) materials annealed under moderately high-oxygen pressures of ∼200 atm. A detailed analysis of the room temperature neutron time-of-flight diffraction data reveals that the crystal structure of the sample SrCo_0.5Fe_0.5O_2.89(1), previously described as vacancy-disordered cubic, is similar to the formerly reported, oxygen-vacancy ordered Sr₈Fe₈O₂₃ compound, i.e. Sr₈Co₄Fe₄O₂₃ is tetragonal with the I4/mmm symmetry. With an increase of the La content the studied materials become nearly oxygen stoichiometric and a lowering of the crystal symmetry is observed from cubic (x=0.1 and 0.2) to tetragonal I4/mcm (x=0.3 and 0.4), and finally to monoclinic I12/c1 (x=0.5). Low-temperature structural and magnetic measurements show a ferromagnetic ordering with the maximum Curie temperature near 290 K at x=0.2.     

Titanates de cuivre substitués à structure bixbyite: Les composés Cu1−xTi1−xFe2xO3 (0.15 ≤ x ≤ 0.33)

A new bixbyite family, Cu_1?xTi_1?xFe_2xO₃ (0.15 ≤ x ≤ 0.33) has been synthesized and characterized. The unit cell is cubic: a ～ 9.40Å. The X-ray powder diffraction study shows up an isotypism with the (Fe, Mn)₂O₃ compounds. There is a disordered distribution of Cu^II, Ti^IV, and Fe^III over the two cyrstallographic sites: P_I and P_II. P_II is highly distorted (two long MO distances) by the Jahn-Teller effect of Cu^II. The bixbyite structure is described in terms of polyhedra arrangement, as a particular case of the CM₂O₃ family. The cation packing is discussed in relation with the existence of the bixbyite structure for the Cu_1?xTi_1?xFe_2xO₃ compounds. The electrical properties (σ ～ 10^?5(Ω cm)^?1 for x = 0.286 at room temperature) show an electron conduction with probably a hopping mechanism.