Mechanisms controlling magnetic properties of pseudobinary compounds TbNi5−xMx (M=Cu or Al)

Magnetic properties and heat capacity of the pseudobinary TbNi_5−xCu_x and TbNi_5−xAl_x alloys have been studied for x⩽2.5 and x⩽1.5, respectively. The substitution of Cu or Al for Ni decreases sharply the spontaneous magnetic moment, increases the magnetic anisotropy in the “easy” basal plane, and leads to strong magnetic domain wall pinning. The Curie temperature T_C in TbNi_5−xCu_x depends on x non-monotonously and has a maximum value at x=1, while T_C in TbNi_5−xAl_x does not depend on composition up to x=1 and sharply decreases at x>1. These results are explained by the effects of random local crystal fields, band magnetism and heterogeneous polarization of the mixed 3d band.     

Structure and magnetic properties of RFe13−xSix (R = Pr,Nd or Gd,x = 2.5–5)

RFe_13−xSi_x (R = Pr, Nd and Gd) alloys with x = 2.5–5 have been synthesized and characterized in a magnetic field up to 17 kOe and in a temperature range 10–1173 K to ascertain whether they might be useful as high temperature, high-energy permanent magnet materials. It was found that a body-centered tetragonal (BCT) Ce₂Ni₁₇Si₉-type structure forms in PrFe_13−xSi_x alloys when x ⩾ 4. The Curie temperatures T_c of this BCT phase are in a range 50–90 K, higher than that of the corresponding PrCo_13−xSi_x BCT phase (∼ 20 K). The PrFe_13−xSi_x alloys with x⩾ 4 show spin reorientation at cryogenic temperatures (15–47 K) and exhibit significant coercivity in loose powder samples below their spin-reorientation temperature. Using the information about the magnetization of LaFe_13−xSi_x BCT alloys, one can estimate the moment of Pr ion in PrFe_13∼-xSi_x alloys. It is found to be very close to that in PrCo_13−xSi_x BCT alloys, around 2 μ_2/atom. For RNd or Gd, the RFe_13−xSi_x alloys occur only as mixtures of RFe₂Si₂, R(Fe, Si)₁₁ and FeSi. The Ce₂Ni₁₇Si₉-type structure cannot be formed in these alloys even when x = 5. The low T_c for the PrFe_13−xSix alloys precludes their use as a permanent magnet material, except perhaps at low temperature.     

Crystal structure and magnetic properties of pseudoternary TbRh2−xMxSi2(M = Ru,Ir) compounds

The structural and magnetic characteristics of the pseudoternary TbRh_2−xM_xSi₂(M = Ru, Ir) compounds were studied. The compounds crystallize in the tetragonal ThCr₂Si₂-type structure. The magnetic data were collected in the temperature range 70–300 K. Their magnetic susceptibilities satisfy the Curie-Weiss law in the temperatures higher than 130 K. The magnetic moment of the rare earth atom is larger than of the free Tb³⁺ ion. A modified RKKY theory with included interaction between the conduction electrons was applied to explain the variation of properties of the compounds.     

Original magnetic behaviour observed in RNi5−xCux alloys (R = Pr,Gd or Y)

The Curie temperatures, spontaneous magnetizations and initial susceptibilities were investigated in the temperature interval from 1.5 to 40 K for the pseudobinary alloys RNi_5−xCu_x, where R = Pr, Gd and Y, x ≤ 2.6. It was discovered that the solid solutions PrNi_5−xCu_x between the PrNi₅ and PrCu₅ Van Vleck paramagnets are ferromagnets for intermediate Cu concentrations. It was shown that the ferromagnetic ordering in PrNi_5−xCu_x alloys can be explained by concentrational enhancement of 3d band susceptibility which in its turn leads to the enhancement of f-f exchange interactions. The maxima on the concentration dependences of both Curie temperatures for the RNi_5−xCu_x ferromagnets with R = Pr, Gd and susceptibility for the YNi_5−xCu_x Pauli paramagnets are manifestations of 3d band magnetism.     

Magnetic and crystallographic properties of substituted Pr2Fe14−xMxB compounds (M = Si,Ga, CrandCu)

Structural and magnetic properties of Pr₂Fe_14−xM_xB systems (M = Si, Ga, CrandCu) are investigated. These compounds crystallize in a tetragonal system of P4₂/mnm-type for a Si content up to x = 2, Cr content up to x = 3 and for Ga and Cu concentration up to x = 1. The Curie temperature increases when Fe is substituted by Si, Ga and Cu, decreases when Fe is replaced by Cr. The saturation magnetization decreases with increasing content of an M element. The rate of the decrease is larger than that expected by a simple dilution model. The influence of M atoms on the anisotropy field is discussed.     

Crystal structure and magnetic properties of Ba-ordered manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd)

The structure and magnetic properties of the Ba-ordered state in solid solutions of manganites Ln_0.70Ba_0.30MnO_3?δ (Ln = Pr, Nd) with a cation ratio Ln³⁺/Ba²⁺ ? 1 are studied experimentally. The samples are obtained by two-stage synthesis. The initial stoichiometric Ba-disordered solid solutions Ln_0.70Ba_0.30MnO₃ synthesized in air according to traditional ceramic technology are characterized by the orthorhombic (Imma, Z = 4) perovskite-like unit cell and are ferromagnets with Curie temperatures T _C ≈ 173 and ≈ 143 K for Pr and Nd, respectively. The average size <D> of a crystalline in the initial samples is 5 μm. It is found that annealing of the initial samples in a vacuum of P[O₂] = 10^?4 Pa leads to their separation into three phases: (1) the anion-deficient ordered LnBaMn₂O₅ phase described by a tetragonal (P4/mmm, Z = 2) perovskite-like unit cell, as well as the phases (2) Ln₂O₃ (P $\bar 3$ m1, Z = 1) and (3) MnO (Fm $\bar 3$ m, Z = 2). Reduction leads to the formation of a nanocomposite with an average crystallite size <D> = 100 nm. Anion-deficient Ba-ordered phases of LnBaMn₂O₅ exhibit ferrimagnetic properties with Néel temperatures T _N ≈ 113 and ≈123 K for Pr and Nd, respectively. Annealing of anion-deficient samples in air at a moderate temperature of T = 800°C does not change the average size of the nanocrystallite, but noticeably alters their phase composition. Stoichiometric nanocomposites consist of two perovskite-like phases: (1) the Ba-deficient ordered stoichiometric phase LnBaMn₂O₆, which is described by a tetragonal (P4/mmm, Z = 2) unit cell and has the Curie temperatures T _C ≈ 313 (Pr) and ≈303 K (Nd), and (2) the Ba-disordered superstoichiometric phase Ln_0.90Ba_0.10MnO_3+δ, which is described by an orthorhombic (Imma, Z = 4) unit cell and has Curie temperatures T _C ≈ 138 (Pr) and ≈123 K (Nd). The two magnetic phases of the Ba-ordered nanocomposite are exchange-coupled. For the low-temperature magnetic phase, a temperature hysteresis is observed at ΔT ≈ 22 K in a field of 10 Oe and at ΔT ≈ 5 K in a field of 1 kOe. It is shown that states with different degrees of ordering of cations in the A sublattice can be obtained employing different technological conditions of treatment. The significant changes in the magnetic properties of Ba-ordered nanocomposites are explained on the basis of chemical phase separation taking into account the effect of compression, which is a consequence of the action of chemical (cation ordering) and external (surface tension) pressures.     

Electronic structure and magnetic properties of metastable SmCo_7 compound

1 Introduction The rare-earth transition-metal intermetallic compounds have been widely investi-gated for many years, among them the Sm-Co series compounds with 1:5 and 2:17 crys-tal structures. These compounds have been used as sintered and bonded permanent magnets since the 1960s[1,2]. Interest recently has been focused on the TbCu7-type struc-ture Sm-Co intermetallic compounds with a strong uniaxial magnetocrytalline anisot-ropy and a low temperature coefficient (β = ?0.11%)[3―6] due t…     

Electronic structure and magnetic properties of （Mn,N）-codoped ZnO

The electronic structures and magnetic properties of（Mn, N）-codoped Zn O are investigated by using the firstprinciples calculations. In the ferromagnetic state, as N substitutes for the intermediate O atom of the nearest neighboring Mn ions, about 0.5 electron per Mn^2＋ion transfers to the N^2-ion, which leads to the high-state Mn ions（close to ＋2.5）and trivalent N3-ions. In an antiferromagnetic state, one electron transfers to the N2-ion from the downspin Mn2＋ion,while no electron transfer occurs for the upspin Mn^2＋ion. The（Mn, N）-codoped Zn O system shows ferromagnetism,which is attributed to the hybridization between Mn 3d and N 2p orbitals.     

Effects of Co substitution on structural and magnetic properties of R3(Fe1−xCox)29−yVy (R=Tb,Dy)

Synthesis of two novel series of intermetallic compounds Tb₃(Fe_1−xCo_x)_27.4V_1.6 (x=0,0.1, 0.2, 0.3, 0.4) and Dy₃(Fe_1−xCo_x)_27.8V_1.2 (x=0, 0.1, 0.2, 0.3) with the monoclinic Nd₃(Fe,Ti)₂₉-type structure (3:29) is presented. In the Dy series for x=0.4 a disordered variant of the hexagonal Th₂Ni₁₇-type structure is formed. The cell parameters decrease and the Curie temperature increases with increasing of the Co content. In the case of the Tb₃(Fe_1−xCo_x)_27.4V_1.6 series in the M(T) curve a magnetic transition is observed which is attributed to spin reorientation phenomena. This critical temperature decreases with increasing Co from 473 K for x=0.1 to 393 K for x=0.3, and was not observed in the case of 0.4. XRD patterns of magnetically aligned powder samples reveal the presence of a tilted magnetic structure.     

Impact of Pr on structural,superconducting and magnetic properties of Y1−xPrxBaSrCu3O7

We study the structural, superconducting and magnetic properties of the Y_1−xPr_xBaSrCu₃O₇ system with x=0.0, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.75 and 1.0, from X-ray diffraction, AC and DC magnetic susceptibility measurements. X-ray diffraction results reveal that while Pr substitutes isostructurally until x=0.50 in Y_1−xPr_xBaSrCu₃O₇, for x=0.75 and 1.0 few small intensity un-reacted lines are observed in the spectrum. The orthorhombic distortion decreases with an increase in x. Both x=0.75 and 1.0 samples are tetragonal. The c lattice parameter of the substituted samples increases with x, indicating probable substitution of comparatively bigger ionic radii Pr^3+/4+ ion at Y³⁺ in Y_1−xPr_xBaSrCu₃O₇ system. The AC susceptibility results showed that the transition temperature T_c of the Y_1−xPr_xBaSrCu₃O₇ system decreases monotonically with an increase in x. Both x=0.75 and 1.0 samples do not show a superconducting transition down to 4.2 K. The critical concentration of Pr to completely suppress superconductivity in Y_1−xPr_xBaSrCu₃O₇ is higher (0.75) than that reported (0.55) for Y_1−xPr_xBa₂Cu₃O₇. DC magnetic susceptibility measurements done on PrBaSrCu₃O₇ and PrBa₂Cu₃O₇ samples at 0.5 T revealed that while the Pr ordering temperature seems to be 12 K for the former, the same is 17 K for the latter. These results indicate that the ordering temperature of Pr moments in RE_1−xPr_x : 123-type systems, which decides the Pr^4f hybridisation with the Cu–O conduction band has a direct connection to the suppression of superconductivity. The lower T_N of Pr explains the less destructive effect of the same on the superconductivity of the parent undoped system.     

Structure and properties of the anions MF4−, MCl4− and MBr4− (M = C,Si, Ge)

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX₄^?, with M = C, Si, Ge and X = F, Cl, Br, show that GeF₄^?, SiCl₄^?, GeCl₄^? and SiBr₄^? prefer a C_2v conformation, but CCl₄^? is an elongated C_3v structure. CBr₄^? has T_d symmetry in MP2, but is slightly more stable in elongated C_3v form with DFT and CCSD(T). GeBr₄^? has T_d symmetry. CF₄^? and SiF₄^? are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl₄ and SiCl₄, and close to zero for GeF₄ and SiBr₄. Adiabatic EAs range from 0.47 eV for SiCl₄ to 1.78 eV for GeBr₄. The lowest excited states at T_d symmetry are ²T₂ resonances with energies of 2.1–3.5 eV, resulting from excitation of the a₁ singly occupied molecular orbital to vacant t₂ orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl₄^? is made. From dissociation energies of MX₄, MX₄^?, MX₃ and MX₃^?, appearance energies of X^?, MX₃^?, X₂^? and MX₂^? were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl₄^?, SiCl₄^? and GeCl₄^?.     

Electronic structure of Yb, Ag, and Cu in the heavy-fermion system YbCu5−x Agx

The method of shifts of x-ray diffraction lines is used to study the electronic structure (the populations of 4f states in Yb, 5s states in Ag, and 4s states in Cu) in the heavy-fermion system YbCu_5?x Ag_x (0≤x≤1, T=300 K for Cu and Ag and 77, 300, and 1000 K for Yb). In the cubic phase (AuBe₅-type structure), Yb is shown to exist in the state with a noniteger valence whose magnitude is independent of the composition and is equal to $\bar m_{cub} = 2.91 \pm 0.01$ . At x<0.125, in the two-phase region (a mixture of cubic AuBe₅-type and hexagonal CaCu₅-type phases), the magnitude of m decreases with decreasing x. Based on the experimental values of m in the cubic and hexagonal phases in the two-phase region, the valence of Yb in the hexagonal phase was found to be $\bar m_{hex} = 2.71 \pm 0.04$ . With increasing temperature in the range of 77–1000 K, a linear decrease in m for the samples from the cubic-phase field and a linear increase in m for the samples from the two-phase field is observed. At T=1000 K, the valence of Yb in the cubic and hexagonal phases is virtually the same: m _cub=2.83±0.02 and m _mix=2.78±0.02. The cubic phase exhibits a composition-independent increase in the population of 5s states of Ag (in comparison with the metal) $\overline {\Delta n} _{5s} (Ag) = 0.69 \pm 0.07$ electron/atom and a simultaneous linear increase in Δn _4s (Cu) from about 0.1 electron/atom for x=1 to about 0.3 electron/atom for x=0.2. The difference in the behavior of the effects of Δn _s for Cu and Ag is explained by the specific features of the crystal structure of YbCu_5?x Ag_x. It follows from the analysis of microscopic and macroscopic properties that YbCu_5?x Ag_x is a system with an intermediate valence and, correspondingly, the increase in the effective mass of electrons in it is related to the shift of the 4f-electron level to the Fermi level (delocalization). The effect of the increase in the population of s states in the partners of Yb is explained by the fact that, upon the transition into the state with an intermediate valence, the 4f electron of Yb is hybridized with s electrons of neighboring atoms of Ag and Cu rather than with the electrons of Yb itself.     

Electronic structures of Al–Si clusters and the magic number structure Al8Si4

The low-energy structures of Al₈Si_m (m = 1–6) have been determined by using the genetic algorithm combined with density functional theory and the Second-order Moller-Plesset perturbation theory (MP2) models. The results show that the close-packed structures are preferable in energy for Al–Si clusters and in most cases there exist a few isomers with close energies. The valence molecular orbitals, the orbital level structures and the electron localisation function (ELF) consistently demonstrate that the electronic structures of Al–Si clusters can be described by the jellium model. Al₈Si₄ corresponds to a magic number structure with pronounced stability and large energy gap; the 40 valence electrons form closed 1S²1P⁶1D¹⁰2S²1F¹⁴2P⁶ shells. The ELF attractors also suggest weak covalent Si–Si, Si–Al and Al–Al bonding, and doping Si in aluminium clusters promotes the covalent interaction between Al atoms.     

Quantum-chemical modeling of the electronic structure and magnetic properties of Sn1 − x − y M x Sb y O2, M = Cr, Mn, Co, or Ni (x = 0.25; y = 0, 0.25)

The electronic and magnetic structures of the Sn_0.75 M _0.25O₂ and Sn_0.5 M _0.25Sb_0.25O₂ (M = Cr, Mn, Co, Ni) compounds with a structure that is derivative of the rutile structure are modeled using the ab initio spin-polarized tight-binding linear muffin-tin orbital (TB-LMTO) method. The magnetic moments of the transition metal atoms are calculated. The data obtained are used to analyze the influence of the composition of Sn_{1 ? x ? y} M _x Sb _y O₂ phases on their electronic spectra and the magnetic and transport characteristics.     

Structural,M?ssbauer spectroscopy,magnetic properties,and thermal measurements of Y(3-x)Dy_x Fe_5O_(12)

Yttrium iron garnet powder samples((3-x)Dy_x Fe_5O_(12)), where part of yttrium ions are substituted by dysprosium ions with different concentrations are prepared by the solid state reaction method. The properties of the prepared samples are examined by different methods such as x-ray diffraction(XRD), Mssbauer spectroscopy, macroscopic magnetization measurements, and thermal measurements. The XRD measurements show that all the samples reveal the presence of a single garnet phase with a BCC structure. Room temperature Mssbauer spectra indicate that iron ions occupy three magnetic sites, i.e., two octahedral sites and one tetrahedral site. The saturation magnetization and the initial magnetic susceptibility decrease with the increase of Dy~(3+) substitution. The Curie temperature obtained from the thermal measurements seems to be independent of Dy~(3+) substitution.