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.     

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.     

Structure and physical properties of titanium diselenide intercalated with nickel

The structure, electrical resistivity, thermopower, and magnetic susceptibility of titanium diselenide intercalated with nickel (N_xTiSe₂) are studied systematically in the nickel concentration range x=0–0.5. In accordance with a model proposed earlier, strong hybridization of the Ni3d/Ti3d states is observed, giving rise to suppression of the magnetic moment because of delocalization of the nickel d electrons. It is shown that the strain caused by the Ni3d/Ti3d hybridization does not change the local coordination of a titanium atom.     

Isotropic positive magnetoresistance in Co-Al<Subscript>2</Subscript>O<Subscript>n</Subscript> nanocomposites

The magnetotransport properties of Co_x(Al₂O_n)_{100 ? x} nanocomposites were studied in a wide concentration range (34 ≤ x ≤ 74 at %). Negative tunnel magnetoresistance reaching 6.5% in a field of 10 kOe was established. In addition to the negative magnetoresistance, the Co_x(Al₂O_n)_{100 ? x} composites were found to exhibit positive magnetoresistance reaching 1.5% in fields of 10 kOe over the concentration range corresponding to the percolation threshold (54 ≤ x ≤ 67 at %). The positive magnetoresistance is assumed to be due to the simultaneous existence in the composite structure of clusters and individual nanoparticles characterized by different values of the magnetic anisotropy and due to the dipole-dipole interaction between the clusters and nearest neighbor particles.     

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.     

Determination of the region of existence of ferromagnetic nanostructures in the paraphase of La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ba<Subscript>x</Subscript>MnO<Subscript>3</Subscript> by the EPR method

In the paraphase of a number of La_1?x Ba_xMnO₃ single crystals with 0.1 ≤ x ≤ 0.2 below 340 K, signals of the ferromagnetic resonance are observed, which indicates the presence of magnetically ordered nanoscopic objects (ferrons). The region of the existence of ferrons on the Ba density-temperature phase diagram has an approximate triangular shape, which is characteristic of the Griffiths phase. Investigations of the angular and frequency dependences of the position of the ferromagnetic resonance line indicate that the nanostructures have a spherical shape. The parameters of their magnetic anisotropy are found to be H _a1 = 2500 Oe and H _a2 = ?700 Oe.     

Spin-Wave Resonance in Chemically Deposited Fe-Ni Films: Measuring the Spin-Wave Stiffness and Surface Anisotropy Constant

Single-layer Fe _x Ni_{1 - x} thin magnetic films have been investigated by the spin-wave resonance technique in the entire concentration range. The surface anisotropy and exchange stiffness constants for the films with a Ni content from 30 to 80 at % have been measured from the experimental standing spin wave spectra. The surface exchange spin wave penetration depth δ _C = 20–30 nm has been determined from the dependences of the surface anisotropy and exchange coupling constants on the Fe₂₀Ni₈₀ film thickness in the range of 250–400 nm.     

Magnetic Structures and Magnetic Phase Transitions in Rare-Earth <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Si<Subscript>2</Subscript> Intermetallic Compounds (<Emphasis Type="Italic">R</Emphasis> = Sm,Tb)

The magnetic structures that form in La_1–xR_xMn₂Si₂ (R = Sm, Tb) layered compounds with various concentrations x have been determined by magnetic neutron diffraction and magnetic measurements, and the magnetic phase diagrams have been built. It is shown that the formation of the magnetic structures is dependent not only on exchange interactions, but also on the type of the magnetic anisotropy of a rare-earth atom. It is found that, in La_1–xTb_xMn₂Si₂ compounds with 0.2 < x < 0.5, the competition of the Tb–Mn and Mn–Mn interlayer exchange interactions and the existence of a strong uniaxial magnetic anisotropy in the Mn and Tb sublattices leads to the frustrated magnetic state and prevents the formation of the long-range magnetic order in the Tb sublattice.     

Multiferroic properties and structural features of <Emphasis Type="Italic">M</Emphasis>-type Al-substituted barium hexaferrites

Precise studies of the crystal and magnetic structures of M-type substituted barium hexaferrites BaFe_12–x Al _x O₁₉ (0.1 ≤ x ≤ 1.2) have been performed by powder neutron diffraction in the temperature range 300–730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in M-type substituted barium hexaferrites BaFe_12–x Al _x O₁₉ (x ≤ 1.2) with a collinear ferromagnetic structure.     

Magnetic and magnetoresistive properties of Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Se selenides

Gd _x Mn_1–x Se (0 ≤ х ≤ 0.15) solid solutions are synthesized on the basis of manganese monoselenide. Their magnetic and electrical properties are studied in the temperature range of 80–900 K in magnetic fields up to 10 kOe. An FCC lattice with the Fm3m space group and antiferromagnetic ordering of the magnetic moments of manganese ions is found. A monotonic reduction in the Néel temperature and an increase in the effective magnetic moment along with the gadolinium concentration are observed. Anomalies in the temperature dependence of electrical resistivity and a shift in the temperatures of anomalies in a magnetic field are found.     

Influence of the technique of preparation of (Co)<Subscript>x</Subscript>(LiNbO<Subscript>3</Subscript>)<Subscript>100 − <Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposites on their magnetic properties

The field and concentration dependences of the ferromagnetic resonance spectra and static magnetic parameters of granular Co_x(LiNbO₃)_{100 ? x} nanocomposites (26 ≤ x ≤ 81 at %) with a matrix of amorphous ferroelectric LiNbO₃ have been investigated. It is shown that oxidative processes affect the magnetic parameters of this material.     

Synthesis,microstructure, the complex nature of the magnetic state of lanthanum manganite weakly doped by bismuth

Structural, magnetic, and resonance studies of Bi _x La_1–x MnO₃ are performed. Nanopowders of weakly doped compounds (x = 0.0–0.1) with particle sizes of ≤40 nm are prepared using the sol–gel technique. Nanopowder sintering under the same conditions leads to a sharp increase in the mean granule size with increasing Bi concentration. The 55Mn NMR signal of manganites prepared via solid synthesis corresponds to rapid exchange Mn⁴⁺ ? Mn³⁺. Measurements of the magnetic susceptibility reveal an inhomogeneity in the ferromagnetic state of the Bi _x La_1–x MnO₃ system.     

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.     

Phase transitions and spin excitations in new multiferroics with modulated magnetic structure

Coexistence of an antiferromagnetic (modulated) structure and electric polarization has been revealed in single crystals of Eu_{1 ? x} Y_xMnO₃ (0.2 ≤ x ≤ 0.5) and Gd_{1 ? x} Y_xMnO₃ (0 ≤ x ≤ 0.2) manganites. Hence, these compounds can be considered as a new family of multiferroics. Various phase transitions, both spontaneous and induced by magnetic fields up to 250 kOe, accompanied by anomalies in magnetization, magnetostriction, permittivity, and electric polarization, have been found, and phase T-x diagrams have been constructed. In the submillimeter range (8–40 cm^?1), new spin excitations—electromagnons—have been revealed; they are excited by an electric field. It is established that suppression of the modulated structure by a magnetic field leads to the disappearance of electromagnons; this process is accompanied by significant changes in the permittivity in a wide frequency range.     

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.     

Change in the Magnetocapacity in the Paramagnetic Region in a Cation-Substituted Manganese Selenide

The capacity and the dielectric loss tangent of a Gd _x Mn_1–xSe (x ≤ 0.2) solid solution have been measured in the frequency range 1–300 kHz without a magnetic field and in a magnetic field of 8 kOe in the temperature range 100–450 K, and the magnetic moment of the solid solution has been measured in a field of 8.6 kOe. The magnetocapacity effect and the change in the magnetocapacity sign have been observed in room temperature in the paramagnetic region. A correlation of the changes in the dielectric permittivity and the magnetic susceptibility with temperature has been revealed. The magnetocapacity is described using the model with orbital electron ordering and the Maxwell–Wagner model.     

Anomalous Hall effect and ferromagnetism in the new diluted magnetic semiconductor Sb<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript>x</Subscript>Te<Subscript>3</Subscript>

The magnetic and galvanomagnetic properties of single crystals of the new diluted magnetic semiconductor p-Sb_2?xCr_xTe₃ (0 ≤ x ≤ 0.02) have been studied in the temperature range 1.7–300 K. A ferromagnetic phase with the Curie temperature T_c ≈ 5.8 K and the maximum Cr content x = 0.0215 has been revealed. The easy magnetization axis is parallel to the C₃ crystallographic axis. In the presence of strong magnetic fields, the Shubnikov-de Haas effect has been observed. Analysis of this effect shows that doping with chrome reduces the concentration of holes. Negative magnetoresistance and the anomalous Hall effect are observed at liquid helium temperature.     

Effect of the Composition and the Crystal Structure on the Electrophysical Properties of the Ni<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>W<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> System at Low Temperatures

The problem of establishing the correlation between, on the one hand, the chemical and phase compositions of Ni_1–xW_x alloys (0 ≤ x ≤ 0.5) and, on the other hand, the character of the temperature dependences of the electrical resistivity, is considered. Based on the experimental ρ(T) curves, the concentration dependences of are reconstructed in the wide temperature range (50 K ≤ T ≤ 273 K). The ρ(x) curves have features related to a change in the crystal structures of the alloys (concentration fcc–bcc phase transition), their magnetic structures and percolation processes occurring in the two-phase fcc + bcc medium.     

(Pr–Ca) Manganites That Display Multiferroic Properties: High-Temperature Magnetic,Resistive, and Dielectric Measurements

Complex magnetic, resistive, and dielectric studies of Pr_1–xCa_xMnO₃ (х = 0.15–0.30) manganites reveal multiferroic properties at T?T_C in these solid solutions. States with local magnetization in the form of ferromagnetic clusters (nucleation temperature T* ≈ 700 K) and high dielectric constants coexist in the temperature window T_C ≤ T ≤ T*. There is a correlation between the temperature dependences of specific resistance and specific magnetization.     

Synthesis and investigation of the structure of nanocomposites based on nickel nanoparticles dispersed in a phthalocyanine matrix

A method based on doping of pure nickel phthalocyanine (NiPc) with a polycrystalline potassium powder at relatively low temperatures (300°C) has been proposed for the synthesis of a magnetic nanocomposite containing nickel nanoparticles stabilized in the NiPc matrix. The structural analysis of the synthesized nanoparticles and changes in the NiPc initial matrix has been performed using X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy. It has been found that, at the doping level used in this study, the synthesized samples of the K_xNiPc nanocomposites contain from 9 to 18% Ni in the form of metallic magnetic nanoparticles with an average size of more than 40 nm. It has been shown that the formation of nanoparticles is accompanied by a relative misorientation of persistent NiPc molecules with the unchanged structure of each of these molecules. The stabilization of nickel nanoparticles by the phthalocyanine matrix leads to the fact that the synthesized nanocomposites acquire time-conserving magnetic properties.