Papers by Keyword: Magnetism

Abstract: This study reports on the effect of Gd concentrations on the properties of Gd-doped ZnO films. The films were prepared using co-sputtering method at room temperature. Characterization tools such as X-ray diffraction (XRD), atomic force microscopy (AFM), and vibrating sample magnetometer (VSM) were used to analyze the properties of the prepared films. XRD results observed that all the films are well crystalline and designated to the hexagonal wurtzite structure of ZnO with no secondary phases, which confirmed the successful of doping the Gd into ZnO. Topography analysis from AFM discovered the increase of Gd concentrations of Gd-doped ZnO films leads to the increase in grain size and rougher surface of the films. The magnetization of the films effectively depends on the Gd concentrations, which the diamagnetic behavior changed to ferromagnetic behavior upon Gd doping. A film with higher Gd doping concentration is more effective than lower Gd doping in terms of saturation magnetization (M_s), coercivity (H_c) and remanent magnetization (M_r). These findings revealed that optimizing the Gd concentration is very crucial in enhancing the magnetic properties of Gd-doped ZnO films.

Abstract: We utilize the Metropolis algorithm to obtain statistical averages of the domain wall length in a FePt granular structure after remagnetization is performed by an ultrashort polarized laser impulse. We propose and check the cluster-size-based order parameter, which along with magnetization, shows the domain wall length, and as a consequence, the average cluster size in the system. We treat the inverse Faraday effect as an external directed magnetic field and show impulse time- and laser power- dependent estimates within the Heisenberg model.

Abstract: A theoretical study was made of the dependence of the blocking temperature of the core/shell system of nanoparticles on the intensity of their magnetostatic interaction. It is shown that with an increase in the concentration of nanoparticles (intensity of the magnetostatic interaction), the blocking temperature increases. Moreover, the of large nanoparticles changes more significantly.

Abstract: Herein, the spinel Co_1-xZn_xFe₂O₄ (x = 0.0, 0.2, 0.4 and 0.6) powder samples have been prepared by the solid-state reaction method. We have carried out the measurements of crystal structure, element analysis, material characterization, magnetic property and Curie temperature using the X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer, and the first-principles calculations within the framework of the density functional theory (DFT). The EDS measurement indicates that the Co_1-xZn_xFe₂O₄ powder samples have been successfully synthesized and exhibited the cubic spinel structures. Both the lattice constant and crystallite size increase with the Zn concentration due to the larger ionic radius of Zn²⁺ ion than the Co²⁺ ion. The concentration ratio of the Co²⁺ and Co³⁺ ions can be predicted by the distribution of cations between the A and B sites by the XPS measurement. For the magnetic properties, the residual magnetization, coercivity and Curie temperature decrease monotonically as the Zn concentration increases, while the saturation magnetization initially increases and then decreases at the room temperature. For the Co_0.8Zn_0.2Fe₂O₄ sample, the magnetic saturation reaches the maximum value of 62.98 Am²kg^-1, due to a large amount of the Co³⁺ ions. The adequate replacement of Zn ion for the Co site can improve the magnetic properties of spinel Co_1-xZn_xFe₂O₄ powders, and effectively regulates the Curie temperature.

Abstract: The electronic structure and magnetic properties of C atoms in Co, Ni-substituted graphene single-layers were studied by first-principles calculation method based on density functional theory. The study found that the pure graphene single-layer is an insulator, does not have magnetism, and we found that the doping of Co and Ni atoms alone does not make the system magnetic. Both Co and Ni atoms are capable of generating impurity levels in the graphene single-layer system. The impurity level of Co atom doping is 0.75 eV below the Fermi level, and the impurity level of Ni atom doping is 0.4 eV above the Fermi level. Studies on the coupling doping of Co and Ni atoms show that two different distance Co atoms or Ni atoms in the graphene single-layer are not always ferromagnetically coupled, and a stable magnetic ground state cannot be obtained. It can produce different magnetic ground states by controlling different doping distances, thus we provide one new way to control the spin properties.

Abstract: Possible processes of the carbon isotopes fraction under hydrodynamic caviation at carbon-containing liquids are considered. We have made corresponding experiments with benzene, toluene and ethanol to provide carbon nanoforms with different crystal structures. Specific bonding forces for metals and n-diamond are modeled using developed numerical method. The task is to identify the key parameters affecting the pairs force potentials of carbon atoms and consider the application of the results to microelectronics and natural processes.

Abstract: Double layer coating with variations in layer thickness on radar absorbent material (RAM), and Polyaniline dobed by DBSA as a dielectric material has been successfully synthesized. As well as Barium M-Hexaferrite doped by Zn²⁺ at x=0.9 as magnetic material successfully synthesized. Polyaniline has been synthesized by polymerization method and Barium M-Hexaferrite has been synthesized by solid state method. Characterization results obtained the conductivity of Polyaniline and Barium M-Hexaferrite respectively 4.4 × 10^-1 S/m and 2.09 × 10^-3 S/m, both of which are in range of semiconductor materials conductivity. The presence of Zn²⁺ doping successfully reduced hard magnetic properties of Barium M-Hexaferrite to be soft magnetic, with coercivity field (Hc) equals 0.0181 Tesla. Based on research that has been done, represent that double layer design with variations PANi : PANi + BaM (3:1) has maximum reflection loss value about -29,6003 dB, and 96.69% of energy absorbed. Whereas in variation of PANi: PANi + BaM (3: 1) has minimum reflection loss value about -15.2937 dB, and 82.21% of energy absorbed. In addition, the coating thickness variations also affect the absorption band width, with the most effective absorption peaks in “D” variation with bandwidth equals 1.81 GHz in range of frequency 8 to 9.81 GHz.

Abstract: We report the results of studies on the electronic state of the hole-doped Y-based pyrochlore iridate, (Y_1-x-yCu_xCa_y)₂Ir₂O₇. We carried out the resistivity, Muon Spin Relaxation (μSR), X-ray Photoemission Spectroscopy (XPS) measurements and Density Functional Theory (DFT) calculations on the non-doped (x=y=0) and doped (x=0.05, y=0.15) systems. We found in the non-doped system that the magnetic ordering of Ir spins which was accompanied by the metal-insulator transition (MIT) occurred at around 157 K and disappeared in the doped system in which MIT seems to disappear or smeared out. We suggest from the current study that a quantum critical point which shows a change in the electronic ground state from insulating to metallic to exist between those two systems.

Abstract: A study on the formation and stability of new quaternary compounds with the general chemical formula Gd₃TAl₃Ge₂ (T = Mn, Cu) has been undertaken by experimental investigations (SEM-EDX, DTA and XRD) and density functional theory (DFT) calculations. These compounds crystallize in the hexagonal Y₃NiAl₃Ge₂-type structure (hP9, P–62m, Z = 1) (an ordered, quaternary derivative of the ternary ZrNiAl or of the binary Fe₂P prototypes), with lattice parameters values a = 7.0239(2) Å and c = 4.2580(1) Å for Gd₃MnAl₃Ge₂ and a = 7.0434(1) Å and c = 4.2089(1) Å for Gd₃CuAl₃Ge₂. DTA suggests a peritectic reaction for the formation of these compounds (at 1245°C for Gd₃CuAl₃Ge₂). The existence and stability of these phases has been explained on the basis of DFT calculations, and a comparison of ground state properties of the studied compounds with the earlier known Gd₃CoAl₃Ge₂ phase is outlined. The negative formation energies in all three cases govern the stability of compounds from theory as well, predicting Gd₃MnAl₃Ge₂ as the most stable phase with highest formation energy (–13.01 eV/f.u.). The total DOS are generic in nature and suggest the robust magnetism, with the Gd-f moments of ≈7 μ_B. An antiparallel coupling among Gd-f and T-d states is observed for all compounds, as usually seen in rare earth (R) - transition metal (T) compounds. Preliminary magnetization measurements on Gd₃MnAl₃Ge₂ show two ferromagnetic/ferrimagnetic (FM/FIM) like transitions at T_C1 = 142 K and T_C2 = 97 K, with another anomaly seen at ≈15 K. Isothermal magnetization data show no hysteresis even at 5 K, and the magnetization does not saturate up to 50 kOe, further suggesting a possible FIM behavior.

Abstract: In magnetically ordered metals the magnitude of the local atomic moment become temperature dependent. To deal with this problem on the ab-initio level one need to employ a specific methodology for calculation of the electronic structure that takes into the account the magnetic disorder effects. In addition one needs to setup a special statistical models allowing simultaneously for ab-initio mapping and for the variation of the local spin magnitude. To this end here we discuss and employ methodology that is based on the Disordered Local Moment (DLM) formalism, spin-constraint Local Spin Density Approximation (LSDA) and Lichtenstein theorem for calculation of the inter-site exchange interactions. An extended classical Heisenberg Hamiltonian used for mapping allows for the variation of the lattice site spin magnitude. We consider here three representative canonical transition metals ferromagnets hcp Gd, bcc Fe and fcc Ni with quite a different character of the magnetic moment localization and illustrate the relative importance of the longitudinal spin fluctuations and the magnetic disorder induced electronic structure reconstruction. We use recently introduced linear measure [1] for integration over the longitudinal spin component in the classical configurational spin space.