Structure and magnetic properties of co-sputtered Co–C thin films

We studied the structure and magnetic properties of co-sputtered Co_1−xC_x thin films using a transmission electron microscope (TEM) and a SQUID magnetometer. These properties were found to depend critically on deposition temperature, T_S, and composition, x. Generally, phase separation into metallic Co and graphite-like carbon phases proceeds with increasing T_S and decreasing x. Plan view and cross-sectional TEM images of the films prepared showed that Co grains about 10–20 nm in diameter and 30–50 nm in height are three-dimensionally separated by graphite-like carbon layers 1–2 nm thick. Optimum magnetic properties with saturation magnetization of 380 emu/cc and coercivity of 400 Oe were obtained for a film with x=0.5 and T_S=350°C.     

Various properties of sputter-deposited Ta–Ru thin films

This paper discusses several structural, electrical and oxidation characteristics of co-sputtered Ta–Ru alloy films on oxidized Si-substrates. From X-ray examination, the Ta₁Ru₁ phase has formed and dominates in the compositions exceeding 54 at.% Ru content. The resistivity of the Ta–Ru thin films can reach a maximum of ∼320 μΩ cm in the composition range between 35 and 54 at.% Ru. After thermal treatment in air (600°C, 1 h), Ru-rich samples show a less increase in resistivity than Ta-rich ones. The observed preferential oxidation of Ta in the Ta–Ru samples can be further interpreted by thermodynamic calculations. The Ta-rich surface oxide is believed to be responsible for the passivating ability of the Ru atom toward oxidation at high temperatures. This results in the Ru of the metallic state though the oxidation of Ta occurs.     

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si（001） substrate

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films.In this paper,Co films with different thickness of Co Si2buffer layers were grown on Si(001)substrates.In order to investigate morphology,structure,and magnetic properties of films,scanning tunneling microscope(STM),low energy electron diffraction(LEED),high resolution transmission electron microscopy(HRTEM),and surface magneto-optical Kerr effect(SMOKE)were used.The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of Co Si2buffer layers.Few Co Si2monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality.Furthermore,the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process,which is the typical phenomenon in cubic(001)films.     

The high frequency magnetic properties of self assembled Fe–Co–Si–N nanogranular thin films

The effect of a variation in Si and N concentration on the microstructure, crystal structure and high-frequency magnetic properties of Fe–Co–Si–N nanogranular thin films was investigated. The films, prepared by rf magnetron sputtering, consisted of nanosized grains of FeCo as well as a Si and N rich intergranular amorphous phase. The Si concentration had a significant effect on the crystal structure of the FeCo phase. The resistivities of the Fe–Co–Si–N films were significantly enhanced by an increase in Si concentration. The resonant frequency of the Fe–Co–Si–N films could be tuned from 0.45 GHz to 2.1 GHz by controlling Si concentration. The N concentration greatly influenced magnetic properties and the variation in resonant frequency is in agreement with Kittel’s equation.     

Role of electrolyte pH on structural and magnetic properties of Co–Fe films

Co–Fe films were electrodeposited on polycrystalline Titanium substrates from the electrolytes with different pH levels. X-ray diffraction (XRD) was used to study the crystal structure of the films. The XRD patterns showed that the films grown at the pH levels of 3.70 and 3.30 have a mixed phase consisting of face-centred cubic (fcc) and body-centred cubic, while those grown at pH=2.90 have only fcc structure. It was observed that the film composition, by energy dispersive x-ray spectroscopy, contain around 88 at% Co and 12 at% Fe for all films investigated in this study. Morphological observations indicated that all films have grainy structure with the slight change of grain size depending on the electrolyte pH. Magnetoresistance measurements, made at room temperature, showed that all films exhibited anisotropic magnetoresistance, which is affected by the electrolyte pH. From the magnetic measurements made by vibrating sample magnetometer, the saturation magnetization increases as the electrolyte pH decreases. Furthermore, all films have in-plane easy-axis direction of magnetization.     

Microstructures and soft magnetic properties of as-deposited Fe–Sm–O thin films

The influences of O₂ partial pressure on saturation magnetization, coercivity and effective permeability of the as-deposited Fe–Sm–O thin films, which were fabricated by RF magnetron reactive sputtering method, were investigated. The nanocrystalline Fe_83.4Sm_3.4O_13.2 thin film fabricated at O₂ partial pressure of 5% exhibited the best magnetic softness with a saturation magnetization of 1.43 MA/m, coercivity of 65.2 A/m and effective permeability of about 2600 in the frequency range from 0.5 to 100 MHz. The electrical resistivity of Fe_83.4Sm_3.4O_13.2 was 130 μΩ cm. The microstructures and electrical resistivity were investigated in this work.     

Effect of annealing temperature on the microstructure and optical–electrical properties of Cu–Al–O thin films

We have successfully prepared Cu–Al–O thin films on silicon (100) and quartz substrates by radio frequency (RF) magnetron sputtering method. The as-deposited Cu–Al–O film is amorphous in nature and post-annealing treatment in argon ambience results in crystallization of the films and the formation of CuAlO₂. The annealing temperature plays an important role in the surface morphology, phase constitution and preferred growth orientation of CuAlO₂ phase, thus affecting the properties of the film. The film annealed at 900 °C is mainly composed of CuAlO₂ phase and shows smooth surface morphology with well-defined grain boundaries, thus exhibiting the optimum optical–electrical properties with electrical resistivity being 79.7 Ω·cm at room temperature and optical transmittance being 80% in visible region. The direct optical band gaps of the films are found in the range of 3.3–3.8 eV depending on the annealing temperature.     

Synthesis and magnetic properties of Co–Zn magnetic fluid

Co_1−xZn_xFe₂O₄ (with x varying from 0 to 0.7) nanoparticles to be used for ferrofluid preparation were prepared by chemical co-precipitation method. The fine particles were suitably dispersed in transformer oil using oleic acid as the surfactant. The magnetization (M_s) and the size of the particles were measured at room temperature. The magnetization (M_s) was found to decrease with the increase in zinc substitution. The magnetic particle size (D_m) of the fluid was found to vary from 11.19 to 4.25 nm decreasing with the increase in zinc substitution.     

The optical properties of MgxZnl-xO thin films

Mg_xZn_1-xO thin films have been prepared on silicon substrates by radio frequency magnetron sputtering at 60℃. The thin films have hexagonal wurtzite single-phase structure and a preferred orientation with the c-axis perpendicular to the substrates. The refractive indices of Mg_xZn_1-xO films are studied at room temperature by spectroscopic ellipsometry over the wavelength range of 400--760\,nm at the incident angle of 70℃. Both absorption coefficients and optical band gaps of Mg_xZn_1-xO films are determined by the transmittance spectra. While Mg content is increasing, the absorption edges of Mg_xZn_1-xO films shift to higher energies and band gaps linearly increase from 3.24.eV at x=0 to 3.90\,eV at x=0.30. These results provide important information for the design and modelling of ZnO/ Mg_xZn_1-xO heterostructure optoelectronic devices.     

Effect of preheating temperatures on microstructure and optical properties of Na-doped ZnO thin films by sol–gel process

The chemical composition, crystalline structure, surface morphology and photoluminescence spectra of Na-doped ZnO thin films with different heat treatment process were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and a fluorescence spectrometer. The results show that preferred orientation, residual stress, average crystal size and surface morphology of the thin films are strongly determined by the preheating temperature. The effects of preheating temperature on microstructure and surface morphology have been discussed in detail. The photoluminescence spectra show that there are strong violet & UV emission, blue emission and green emission bands. The violet & UV emission is ascribed to the electron transition from the localized level below the conduction band to the valence band. The blue emission is attributed to the electron transition from the shallow donor level of oxygen vacancies to the valence band, and the electron transition from the shallow donor level of interstitial zinc to the valence band. The green emission is assigned to the electron transition from the level of ionized oxygen vacancies to the valence band.     

Improvement of microstructure and magnetic properties of Nd–Fe–B alloys by Nb and Co additions

In order to establish the role of niobium on the hydrogenation, disproportionation, desorption and recombination (HDDR) behavior of near-stoichiometric alloys, two alloys: NdI3Fe8OB7 and Nd13Fe78Nb1Co1B7 (at%) were investigated before, during and after the HDDR process. The microstructure of the as-cast Nb-free alloy before employing the HDDR process was found to consist of three phases, the matrix Nd₂Fe₁₄B (φ) phase, Nd-rich phase and a significant amount of free iron; whereas, the microstructure of the Nb-containing alloy consisted of only the first two phases.     

Synthesis and characterization of highly textured Pt–Bi thin films

Pt–Bi films were synthesized on glass and thermally oxidized silicon substrates by e-beam evaporation and annealing. The structures were characterized using X-ray diffraction (XRD) and transmission electron microscopy/selected area electron diffraction (TEM/SAED) techniques. Single-phase PtBi was obtained at an annealing temperature of 300°C, whereas a higher annealing temperature of 400°C was required to obtain the highly textured γ-PtBi₂ phase. TEM/SAED analysis showed that the films annealed at 400°C contain a dominant γ-PtBi₂ phase with a small amount of β-PtBi₂ and α-PtBi₂ phases. Both the PtBi and γ-PtBi₂ phases are highly textured in these two kinds of film: the c-axis of the hexagonal PtBi phase is mostly in the film plane, whereas the c-axis of the trigonal γ-PtBi₂ phase is perpendicular to the film plane. The electrical resistivity of the film with the γ-PtBi₂ phase was smaller by one order of magnitude than that of the film with the PtBi phase.     

The influence of the composition of eight-atom Pt–Ir clusters on the magnetic properties

The energetic stability, electronic structure and magnetic properties of Pt_8–nIr_n clusters have been investigated by employing the spin-polarised generalised gradient approximation. The cubic structure is expected to be the effective building block in Ir-rich clusters after optimisation extensively. The average binding energy of all the clusters presents the linear increment trend with iridium atoms, due to the stronger interaction between Ir atoms than Pt atoms. Bader charge analysis shows how tiny charge transfers from iridium to platinum. The atomic moments of Ir are larger than that of Pt, and the Ir-rich clusters show greater moments than the Pt-rich cluster, with the exception of Ir₈ and Ir₇Pt. A unique magnetic property is found in the Pt₄Ir₄ cluster, where two Pt atoms show antiferromagnetic alignment and the other atoms are found to be aligned ferromagnetically.     

Giant magnetoresistance in Cu–Co films electrodeposited on n-Si

High quality Cu–Co alloy films with excellent metallic luster have been electrolytically deposited directly onto n-Si (1 0 0) substrate, thereby eliminating the need of a conducting seed layer, which is otherwise required when the films were grown on insulating substrates (Al₂O₃). The as-deposited Cu–Co films exhibit relatively higher magnetoresistance (MR) in comparison with the as-deposited films on Al₂O₃ under identical conditions. The observed increase in MR could be attributed to the reduced substrate current shunting. The MR further improves to 2.67% (at H=10 kOe) with vacuum annealing (at 425°C for 30 min) of the films on Si. This has been ascribed to the separation of Cu and Co phases resulting in a magnetic granular nanostructure. This value of MR of annealed films on Si is, however, lower in comparison with the value obtained for annealed films deposited on Al₂O₃. Glancing angle X-ray diffraction (GAXRD) has revealed the formation of copper silicide in these samples, which is responsible for the lower value of MR. Thus we have observed good MR with a copper silicide host matrix.     

Effects of La–Zn substitution on microstructure and magnetic properties of strontium ferrite nanofibers

Sr_1−x La _x Zn _x Fe_12−x O₁₉/poly(vinylpyrrolidone) (PVP) (0.0≤x≤0.5) precursor nanofibers were prepared by the sol–gel assisted electrospinning method from starting reagents of metal salts and PVP. Subsequently, the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers with diameters of around 100 nm were obtained by calcination of the precursor at 800 to 1000°C for 2 h. The precursor and resultant Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometer and vibrating sample magnetometer. The grain sizes of Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers are in a nanoscale from 40 to 48 nm corresponding to the calcination temperature from 800 to 1000°C. With La–Zn substitution content increase from 0 to 0.5, the grain size and lattice constants for the Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers obtained at 900°C show a steady reduction trend. With variations of the ferrite particle size arising from the La–Zn substitution, the nanofiber morphology changes from the necklace-like structure linking by single elongated plate-like particles to the structure building of multi-particles on the nanofiber cross-section. The specific saturation magnetization of Sr_1−x La _x Zn _x Fe_12−x O₁₉ nanofibers initially increases with the La–Zn content, reaching a maximum value 72 A m² kg⁻¹ at x=0.2, and then decreases with a further La–Zn content increase up to x=0.5, while the coercivity exhibits a continuous reduction from 413 (x=0) to 219 kA m⁻¹ (x=0.5). The mechanism for the La–Zn substitution and the nanofiber magnetic property are analyzed.     

Electrical and optical properties of a-SexTe100–x thin films

The dc electrical conductivity of as deposited thin films of a-Se_xTe_100?x (x=3, 6, 9 and 12) is measured as a function of temperature range from 298 to 383 K. It is observed that the dc conductivity increases exponentially with the increase in temperature in this glassy system. The value of activation energy calculated from the slope of ln σ_dc vs. 1000/T plot, is found to decrease on incorporation of dopant (Se) content in the Te system. On the basis of pre-exponetial factor (σ₀), it is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges. The optical absorption measurements show an indirect optical band gap in this system and it decreases on increasing Se concentration. The optical constants (extinction coefficient (k) and refractive index (n)) do change significantly with the photon energy and also with the dopant Se concentration. The decrease in optical band gap may be due to the decrease in activation energy in the present system. It is also found that the real and imaginary parts of dielectric constants show a significant change with the photon energy as well as with the dopant concentration. With large absorption coefficients and compositional dependence of optical band gap and optical constants (n and k), these materials may be suitable for optical disk applications.     

Influence of CuO and sintering temperature on the microstructure and magnetic properties of Mg–Cu–Zn ferrites

The synthesis of a series of Mg–Cu–Zn ferrites with the substitution of Cu for Mg has been obtained by solid-state reaction method. Microstuctural and structural analyses were carried out using a scanning electron microscope and X-ray diffraction (XRD), respectively. The lattice parameter is found to increase with increasing copper content. A remarkable densification is observed with the addition of Cu ions in the ferrites. Microstructural analyses indicate that CuO influences the microstructure of the ferrites by the formation of liquid phase during sintering. The grain size significantly increases with increasing copper content. Exaggerated grain growth is observed for the samples of x=0.25–0.35. The initial magnetic permeability (μ′) increases sharply with increasing concentration of Cu ions. This increase in μ′ is explained with the grain growth mechanism and enhanced densification of the ferrites. The resonance frequency of all the samples shifts toward the lower frequency as the permeability increases with Cu content. Sintering temperature T_s also affects the densification, grain growth and initial magnetic permeability of the samples.