Mechanism of magnetic ordering in Dy<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-Ni bilayer films

A mechanism for the magnetic ordering of dysprosium in Dy_1?x Ni _x -Ni bilayer films is proposed. This ordering was discovered earlier by the authors when studying magnetic circular dichroism. For x exceeding a threshold value (～0.05), the contribution from the Dy_1?x Ni _x layer in a bilayer film to the magnetic circular dichroism over the temperature range 80–300 K is approximately equal in magnitude to the magnetic circular dichroism observed in a single-layer Dy film at temperatures below the ferromagnetic phase transition temperature of Dy (～100 K). Since magnetic circular dichroism is an effect linear in magnetization, the observed effect is associated with magnetic ordering of the Dy_1?x Ni _x layer in bilayer films due to the simultaneous influence of two factors: the incorporation of Ni into the Dy layer and the influence of the continuous Ni sublayer. The ferromagnetic ordering of a dysprosium layer doped with nickel (under conditions of an atomic contact with a continuous nickel layer) was confirmed by the field dependences of the polar and longitudinal Kerr effects. It was shown that both layers in the bilayer structure are magnetized in the same direction and characterized by an anisotropy of the easy-plane type. The magnetic ordering is assumed to be due to the change in the density of states of the Dy_1?x Ni _x alloy caused by hybridization with the narrow peaks near the Fermi level characteristic of nickel.     

Hyperfine interactions in Sc<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript>x</Subscript>Fe<Subscript>2</Subscript> cubic Laves alloys

Effects of hybridization of 3d bands of iron with 3d bands of scandium and 4d bands of yttrium in Sc_1?xY_xFe₂ cubic Laves alloys (0≤_x≤1) are studied by the nuclear magnetic resonance method. The concentration dependences of the lattice parameters a, saturation magnetization σ, and hyperfine fields at the ⁵⁷Fe, ⁴⁵Sc, and ⁸⁹Y nuclei—as well as the ²⁷Al impurity nuclei, whose atoms substitute iron atoms in the lattices of these alloys—are measured. The “local” and “induced” contributions to hyperfine fields at the ⁵⁷Fe nuclei are separated and the magnetic moments at iron atoms are estimated. It is found that the hybridization effect leads to the formation of magnetic moments at Sc and Y atoms (whose direction is opposite to the direction of the magnetic moment at iron atoms) and is responsible for the ferrimagnetic structure in Sc_1?xY_xFe₂ alloys.     

Formation of nickel magnetic nanoparticles and modification of nickel phthalocyanine matrix by sodium doping

Data for the vapor-phase doping (300°C) of nickel phthalocyanine (NiPc) by sodium taken in different concentrations (x), as well as structural analysis data for Na _{x = 0.2}NiPc, Na _{x = 1}NiPc, and Na _{x = 3}NiPc samples, have been reported. The structure of the samples and their atomic configuration versus the doping level have been studied by transmission electron microscopy, Raman scattering, X-ray diffraction, X-ray absorption spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The structural parameters of Ni–N, Ni–C, and Ni–Ni bonds have been determined, and it has been found that, at a low level of doping by sodium, local structural distortions are observed in some molecules of the NiPc matrix near nickel atoms. The fraction of these molecules grows as the doping level rises from x = 0.2 to x = 1.0. It has been shown that doping changes the oscillation mode of light atoms, which indicates a rise in the electron concentration on five- and six-membered rings. At a high level of sodium doping (x = 3.0), nickel nanoparticles with a mean size of 20 nm and molecule decomposition products have been observed in the NiPc matrix. It has been found that the fraction of nickel atoms in the Na _{x = 3}NiPc nanoparticles as estimated from EXAFS data is sufficient for the room-temperature magnetic properties of the samples to persist for a long time.     

Spin-reorientation transitions and domain structure in TbFe<Subscript>11−<Emphasis Type="Italic">x</Emphasis></Subscript> Co<Subscript>x</Subscript>Ti single crystals

The magnetic structure of single-crystal TbFe_11?xCo_xTi compounds has been studied over a broad temperature range and in strong magnetic fields (up to 14 T). Measurements of magnetization and magnetostriction and a study of the domain structure revealed that spin-reorientation transitions (SRTs) in TbFe_11?xCo_xTi single crystals depend substantially on the cobalt concentration. It was established that the SRT temperatures and threshold magnetic fields are governed by the interplay between the magnetic anisotropies of the 3d and terbium sublattices. It is shown that, in these compounds, the low-temperature phase with planar anisotropy is separated in temperature from the high-temperature phase with uniaxial anisotropy by an intermediate metastable phase containing domains of the uniaxial or planar phase.     

Features of Electronic Structure of Intermetallic Compounds CeNi<Subscript>4</Subscript>M (M = Fe,Co, Ni,Cu)

The evolution of the electronic structure of CeNi₄M (M = Fe, Co, Ni, Cu) intermetallics depending on the type of nickel substitutional impurity is explored. We have calculated band structures of these compounds and considered options of substituting one atom in nickel 3d sublattice in both types of crystallographic positions: 2c and 3g. The analysis of total energy self-consistent calculations has shown that positions of 2c type are more energetically advantageous for single iron and cobalt impurities, whereas a position of 3g type is better for a copper impurity. The Cu substitutional impurity does not change either the nonmagnetic state of ions or the total density at the Fermi level states. Fe and Co impurities, on the contrary, due to their considerable magnetic moments, induce magnetization of 3d states of nickel and cause significant changes in the electronic state density at the Fermi level.     

Electronic Structure and Exchange Interactions in RNi<Subscript>4</Subscript>Co (R = Eu,Yb) Compounds

The electronic structure and the exchange interactions in EuNi₄Co and YbNi₄Co compounds have been calculated in terms of a theoretical approach with the inclusion of electronic correlations (LSDA + U method); the variants of substitution of cobalt ion for nickel in the 3d lattice in both types of crystallographic positions 2c and 3g are considered. The total energies obtained in self-consistent calculations show that individual cobalt impurities are more preferably arranged in position of the 3g type. A Co ion in RNi₄Co (R = Eu, Yb) is characterized by a significant magnetic moment, which leads to significant increase in the exchange interaction of Co and Ni ions in the 3d metal sublattice.     

Galvanomagnetic properties of Heusler alloy Co<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Al (<Emphasis Type="Italic">Y</Emphasis> = Ti,V, Cr,Mn, Fe,and Ni)

The Hall effect and the magnetoresistance of ferromagnetic Heusler alloys Co₂ YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at T = 4.2 K in magnetic fields H ≤ 100 kOe. Normal R ₀ and anomalous R _S Hall coefficients are shown to be maximal in magnitudes in the middle of the 3d period of the periodic table of elements. Coefficient R ₀ changes the negative sign to positive sign in going from weak (Y = Ti, V) to strong (Y = Cr, Mn, Fe, and Ni) ferromagnetic alloys. Constant R _S is positive and proportional to ρ^2.9 in all the alloys. The magnetoresistance of the alloys is not higher than several percent and its magnitude is changed fairly significantly in the dependence on the number of valence electrons z; the magnetoresistance signs vary arbitrarily.     

Effect of nickel on the magnetic state of dysprosium in Dy<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript>x</Subscript>-Ni bilayer films

This paper reports on the results of investigations into the temperature and spectral dependences of the magnetic circular dichroism in Dy_1?xNi_x-Ni bilayer films prepared through thermal sputter deposition of components under ultrahigh vacuum. The distribution of the components over the layer thickness is examined by Auger spectroscopy. The nickel content x in Dy_1?xNi_x layers varies from 0.005 to 0.06. It is shown that, in the temperature range 80–300 K, the contribution made to the magnetic circular dichroism by a Dy_1?xNi_x layer in a bilayer film with a nickel content higher than the threshold value is approximately equal to the magnetic circular dichroism observed in an isolated Dy_1?xNi_x film at temperatures below the temperature of the phase transition to a ferromagnetic state (～100 K). This phenomenon is explained by magnetic ordering in the Dy_1?xNi_x layer of the bilayer film due to the combined effect of two factors, namely, the incorporation of nickel into a dysprosium layer and the presence of a continuous nickel sublayer in the film.     

FMR and EPR in Ni@C nanocomposites: Size and concentration effects

One-domain Ni@C nanoparticles encapsulated in carbon coating have been investigated depending on the size and concentration of Ni in carbon. The nanoparticles of nickel were prepared with the average diameters changing in a broad range of 4–45 nm, and the concentration of Ni in C varies in 2–12 wt%. To prepare the Ni@C nanocomposites the solid solutions of nickel phthalocyanine–metal-free phthalocyanine (NiPc) _x (H₂Pc)_1–x , 0 ≤ x ≤ 1 were synthesized and the solidphase pyrolysis of these compounds was performed. In the case of ultradispersive Ni nanoparticles (the interval of quantum dots is 1–10 nm), a considerable shift of the resonance field and broadening of resonance absorption field were revealed in the spectra of FMR at room temperature. The data were interpreted taking into account the essential contribution of the surface magnetic anisotropy, the magnetic field of which far exceeds the magnetic field of volume anisotropy.     

Magnetic phase diagram of the Bi<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

The magnetic and elastic properties of the Bi_1-xCa_xMnO₃ manganites are studied. The phase transformations revealed are ferromagnet-spin glass (x≥0.15) and spin glass-charge-ordered antiferromagnet (x≥0.25). The ferromagnetic state is characterized by ordering of the Mn³⁺d _x ²-y orbitals. It is suggested that thespin glass state originates from local static Jahn-Teller distortions. The antiferromagnetic charge-ordered and the spin-glass disordered phases coexist in samples with 0.25<x<0.32, which may be due to the charge order-disorder phase transformation being martensitic in character. The magnetic phase diagram is constructed.     

Ferrimagnetic nanoparticles for self-controlled magnetic hyperthermia

Based on the Heisenberg model including single-site uniaxial anisotropy and using aGreen’s function technique we studied the influence of size and composition effects on theCurie temperature T _C , saturationmagnetization M _S and coercivityH _C of spherical nanoparticles with astructural formulaM e _1?x Zn _x Fe₂O₄,Me = Ni, Cu, Co, Mn. It is shown that for x = 0.4–0.5and d = 10–20 nm these nanoparticles have aT _C  = 315 K and are suitable for aself-controlled magnetic hyperthermia.     

Crystal structure and magnetic susceptibility of (CuInSe<Subscript>2</Subscript>)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(2MnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The crystal structure of samples in the (CuInSe₂)_{1 ? x} (2MnSe) _x system at room temperature and their magnetic susceptibility in the temperature range 77–1000 K are investigated. It is established that compositions with concentrations 0≤ x ≤ 0.2 form solid solutions with a tetragonal structure, space group I \(\bar 4\)2d (122). The specific magnetic susceptibility χ of samples with 0.1 ≤ x ≤ 0.4 at 77 K lies in the range 9 × 10^?4?1.6 × 10^t-3cm³/g. The temperature dependence of the inverse magnetic susceptibility of the sample with x = 0.4 suggests the presence of a component with antiferromagnetic ordering and a reliably measured Néel temperature that is characteristic of MnSe. The dependences χ = f(T) of the compositions with x = 0.1, 0.2, 0.3, and 0.4 indicate the occurrence of magnetic phase transitions with a change in the spin state.     

Lattice instability induced by 3<Emphasis Type="Italic">d</Emphasis> impurities in II–VI compound semiconductors

Nickel-impurity-induced transverse displacements of ions in a Zn_1?xNi_xSe lattice (x = 0.0025) were detected. This type of displacement correlates with macroscopic distortions of a crystal associated with transverse ultrasonic waves that are propagated along the 〈110〉 direction. The shear instability is assumed to be due to the hybridization of the sp³ bonds with the 3d states of the impurity centers.     

Hyperfine nuclear interaction at copper lattice sites of high-temperature superconductors studied by <Superscript>61</Superscript>Cu(<Superscript>61</Superscript>Ni) Mössbauer emission spectroscopy

Mössbauer emission spectroscopy on the ⁶¹Cu(⁶¹Ni) isotope has been used to determine the quadrupole coupling constant C(Ni) and magnetic induction B(Ni) for the ⁶¹Ni²⁺ probe at copper sites in Cu₂O, CuO, La_{2 ?x}Ba_xCuO₄, Nd_2?xCe_xCuO₄, RBa₂Cu₃O₆, and RBa₂Cu₃O₇ (R=Y, Nd, Gd, Yb). The compounds containing divalent copper were found to exhibit linear C(Ni) vs. C(Cu) and B(Ni) vs. B(Cu) relations [C(Cu) and B(Cu) are the quadrupole coupling constant and magnetic induction for the ⁶³Cu probe, respectively, found by NMR], which is interpreted as an argument for the copper being in divalent state. The deviation of the data points corresponding to the Cu(1) sites in RBa₂Cu₃O₆ and RBa₂Cu₃O₇ from the C(Ni) vs. C(Cu) straight line may be due either to the copper valence being other than 2+ (in the RBa₂Cu₃O₆ compounds) or to the principal axes of the total and valence electric field gradient being differently oriented (in the RBa₂Cu₃O₇ compounds).     

On the nature of differences in the Ni charge states in barium and strontium titanates

XAFS studies of nickel-doped solid solution Ba_1–xSr_xTiO₃ show that the Ni charge state changes from 4 in SrTiO₃ to ~2.5 in BaTiO₃ as x is varied. First-principles electronic structure calculations show that nickel creates an impurity band in the forbidden gap of BaTiO₃ and SrTiO₃. Calculations of the formation energy of the oxygen vacancies explain the difference between the Ni charge states in these compounds by the different formation energies of these vacancies.     

Magnetic field penetration into the weak-link system of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> granular superconductor

The differential magnetic susceptibility χ_d(H) of YBa₂Cu₃O_7?x polycrystalline samples is studied experimentally in fields H<150 Oe. The empirical χ_d(H) dependence is determined. The results are explained on the basis of the critical-state model of a Josephson medium with hypervortices.     

Effect of cation substitution on the lattice dynamics and ferroelectric instability of cubic BaTiO<Subscript>3</Subscript> and BaZrO<Subscript>3</Subscript> doped with Bi and La ions

The spectra of lattice vibration frequencies of solid solutions Ba_{1 ? x} Bi_2x/3? _x/3Ti(Zr)O₃ and Ba_{1 ? x} La _x Ti(Zr)_{1 ? x/4}? _x/4O₃ are calculated in terms of a generalized Gordon-Kim model with inclusion of the dipole and quadrupole polarizabilities. Over the entire concentration range, the calculated phonon spectra contain a ferroelectric soft mode. The effect of various interactions on the ferroelectric instability of these solid solutions is studied. It is shown that the character of ferroelectric instability is largely determined by the mechanism of charge compensation.     

The two-phase crystal structure and magnetic properties of the LaCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>x</Subscript>O<Subscript>3−<Emphasis Type="Italic">d</Emphasis></Subscript> system

The preparation conditions and magnetic properties of the La(Co_1?xFe_x)O3?d system of solid solutions were studied. Irrespective of synthesis conditions, the samples with x ≤ 0.15 were rhombohedral (space group \(R\bar 3c\)), and those with x ≥ 0.7, orthorhombic (space group Pnma). In the intermediate region of 0.15 < x < 0.7, the samples could be mixtures of rhombohedral and orthorhombic phases. Structural heterogeneity considerably decreased as the temperature of the synthesis increased or after annealing in a reducing medium. The samples containing more than 40% iron exhibited weak spontaneous magnetization, as is characteristic of weak ferromagnets. The magnetic properties and structure depend on the content of oxygen. A decrease in the concentration of oxygen sharply increases magnetic anisotropy; the coercive force is then close to 10 kOe, which is more than that known for other oxide systems. The magnetic properties are explained in terms of the model according to which Co³⁺ ions are in the low-spin state.     

Transport properties and ferromagnetism of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S sulfides

We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of Co _x Mn_1?x S sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT ₁ = 200–270 K and ΔT ₂ = 530–670 K and at T ₃ ～ T _N. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (T _N), the antiferromagnetic Co _x Mn_1?x S sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t _2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.     

Magnetic ordering in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3−<Emphasis Type="Italic">x</Emphasis>/2</Subscript> anion-deficient manganites

The structural, magnetic, and electrotransport properties of La_1?xSr_xMnO_{3? x/2}(0≤x≤0.30) manganites with perovskite structure are investigated experimentally as a function of oxygen deficiency. In the solid solutions La_1?xSr_xMnO₃, a change in the type of symmetry of the unit cell is observed at x=0.125. Samples with x≤0.125 are characterized by an O′-orthorhombic unit cell, whereas samples with x>0.125 are characterized by a rhombohedral unit cell. The structural properties of the anion-deficient solid solutions La_1?xSr_xMnO_3?x/2 are analogous to those of the stoichiometric system. It is assumed that, as the oxygen content decreases, La_{1? x}Sr_xMnO_3?x/2 anion-deficient solid solutions experience a series of successive magnetic phase transformations in the ground state: from an A-type (x=0) antiferromagnet to a cluster spin-glass-type inhomogeneous magnetic state (0.175>x≤0.30) through a two-phase (antiferromagnetic and ferromagnetic) state (0>x≤0.175). The anion-deficient solid solution with x=0.175 has the maximal value of the ferromagnetic component. As the oxygen deficiency increases, the resistivity of La_{1? x}Sr_xMnO_3?x/2 samples first decreases (up to a value of x=0.175), acquiring an activation character, and then increases (up to a value of x=0.30). In this case, none of the anion-deficient solid solutions exhibits a metal-semiconductor transition in the whole range of concentrations considered. A peak of magnetoresistance at a temperature below the point of magnetic ordering is observed only in the sample with x=0.175. The results of experiments carried out with a series of La_1?xSr_xMnO_3?x/2 anion-deficient solid solutions are summarized in the concentration diagrams of the spontaneous magnetic moment and the critical temperature of magnetic phase transitions. Hypothetical magnetic phase states are pointed out. The experimental results obtained can be interpreted in terms of the phase-separation model and the competition between ferromagnetic and antiferromagnetic indirect superex-change interactions. It is assumed that Mn³⁺-O-Mn³⁺ indirect superexchange interactions in the orbitally disordered phase are positive in the case of octahedral coordination of manganese ions and are negative when the coordination of at least one Mn³⁺ ion is pentahedral.