Microstructure and magnetic properties of electrodeposited Co/Cu multilayer nanowire arrays

Highly uniform Co/Cu multilayer nanowire arrays had been electrodeposited into the nanochannels of porous anodic aluminum oxide template. X-ray diffraction pattern showed that Co and Cu grow in their HCP and FCC structures, respectively. Each nanowire had the same length with 20 μm and the diameter with 50 nm. The thickness of Co was 50 nm and Cu layer was about 5 nm. Magnetic measurements of the nanowire arrays showed that the magnetic coercivity for the applied field parallel to the nanowires is larger than that perpendicular to the anowires. The magnetic coercivity of Co multilayer nanowire arrays is smaller than that of the Co/Cu nanowire arrays and the crystal direction of Co layers were not obviously affected by Cu layer. The Co/Cu nanowire arrays exhibited excellent Giant Magneto Resistive ratio of about 75%.     

Oscillatory behavior of the magnetic properties of Nd–Fe–B films with Mo and Mo–Cu additions

A series of Ta/NdFeB/Ta thin films with Mo and Mo–Cu additions embedded by alloying and by stratification have been prepared by r.f. sputtering. The influence of additions, their embedding mode, and annealing temperature on the structural and magnetic behavior of Ta/NdFeB/Ta thin films is presented. The use of additions of Mo and Mo–Cu leads to refined grain structure and improvement in the hard magnetic characteristics of Ta/NdFeB/Ta thin films. The Ta/[NdFeBMo(540 nm)/Ta films and Ta/[NdFeB(180 nm)/MoCu(dnm)] × n/Ta multilayer films present enhanced coercivities and M_r/M_s ratios in comparison with the Ta/NdFeB(540 nm)/Ta films. The stratification of Ta/NdFeB/Ta thin films with Mo–Cu interlayers leads to an oscillatory behavior of hard magnetic characteristics of the Ta/[NdFeB(180 nm)/MoCu(dnm)] × n/Ta multilayer films, when the thickness, d, of Mo–Cu interlayers varies by increments of 1 nm. When the thickness of Mo–Cu interlayers varies by increments of 2 nm the oscillatory behavior of the magnetic characteristics is not revealed. For a thickness of the Mo–Cu interlayer of 3 nm in the Ta/[NdFeB(180 nm)/MoCu(3 nm)] × 3/Ta thin films annealed at 650 °C, the c-axis of part of the hard magnetic Nd₂Fe₁₄B grains is oriented out-of-plane.     

Sb-modified growth of stacked Ge/Si(100) quantum dots

We report on the Sb induced modifications of the morphology of self assembled Ge/Si(100) quantum dot stacks in a Si matrix grown by a molecular beam epitaxy. It is shown that the size of the quantum dots in the stack and the Si spacer layer uniformity inside the stack are regulated by the amount of deposited Sb. We consider the thin Sb layer at the Ge/Si growth interface as a factor limiting the surface migration of Si and Ge ad-atoms. The surface diffusion coefficients of Si ad-atom on uncovered pyramid shaped Ge island and on a Ge island covered by a single monolayer of Sb are estimated to be 2.4 μm²s⁻¹ and 2.3 × 10⁻⁴ μm²s⁻¹ at a temperature of 600 °C, correspondingly. Based on this remarkable reduction of surface diffusion the morphology of the surface can be preserved when the growth is continued after the single monolayer of Sb is at the surface.     

Structural and tunneling properties of magnetic tunnel junctions with Al-O and MgO barrier

Two types of magnetic tunnel junctions (MTJs) with the configuration: substrate Si(1 0 0)/SiO₂ 47 nm/buffer/IrMn 12 nm/CoFe 2.5 nm/Al-O 1.5 nm/NiFe 3 nm/Ta 5 nm and Si(1 0 0)/SiO₂ 47 nm/buffer/IrMn 10 nm/CoFeB 3 nm/MgO 2 nm/CoFeB 4 nm/Ta 5 nm were prepared by the sputtering technique with two different buffers: A-Cu 25 nm and B-Ta 5 nm/Cu 25 nm. The B buffer caused a high texture of MTJs whereas in the case of the A buffer junctions texture was weak. Crystallites in the textured layers grew in a columnar like shape that induced interfacial roughness. High textured buffer B caused high interfacial roughness that reduced the resistance-area (RA) product due to a barrier thickness fluctuation. RA also changed substantially depending on the type of a barrier. The highest RA product ∼15 MΩ μm² was achieved for a low textured junction with Al-O barrier whereas in the high textured MgO sample RA product was ∼100 kΩ μm². Tunnel magnetoresistance (TMR) measured at room temperature was about 45% for the samples with Al-O barrier, whereas for the samples with MgO barrier TMR was about three times higher and achieved 140%.     

Structure, growth and magnetic property of hard magnetic CoPtP nanowires synthesized by electrochemical deposition

This paper reports the electrochemical synthesis and characterization of one dimensional hard magnetic CoPtP nanowires. Three electrode potentiostatic electrochemical technique was used to deposit nanowires into a nanoporous track-etched polycarbonate membrane with a nominal pore diameter 50 nm and thickness around 6-9 μm. The room temperature electrolyte used for the deposition of nanowires consists of 60 g/lt CoSO₄7H₂O, 4.1 g/lt H₂PtCl₆, 4.5 g/lt NaHPO₂ and 25 g/lt B(OH)₃. The structural morphology was observed by scanning electron microscope and transmission electron microscope. The magnetic property of the nanowires was measured by vibrating sample magnetometer before removing the template. The coercive fields were measured to be 143 kA m^− 1 and 103 kA m^− 1 for parallel (H_║) and perpendicular to the nanowire axis, respectively. The higher coercivity value for H_║ indicating nanowires' easy magnetization direction lies along the nanowires' axis. The average composition of the CoPtP nanowires was determined by electron dispersive spectroscopy and the crystallinity was measured by X-ray diffractometer.     

Effects of MgO and MgO/Pd seed-layers on perpendicular magnetic anisotropy of CoPd thin films

We studied the effects of MgO and MgO/Pd seed-layers on perpendicular magnetic anisotropy in co-sputtered CoPd films. CoPd films with the MgO seed-layer showed perpendicular magnetic properties that were superior to those with another after annealing. The loop squareness was unity, indicating strong perpendicular magnetic anisotropy, when the MgO seed-layer was thicker than 2 nm. We observed that the out-of-plane CoPd (111) texture was strongly developed, as well as the in-plane tensile stress in the CoPd films. The magnetoelastic anisotropy coming from a negative magnetostriction λ₁₁₁ under the in-plane tensile stress dominating over other anisotropies is likely responsible for creating such strong perpendicular magnetic anisotropy. In the case of the MgO/Pd seed-layer, the CoPd films showed mixed anisotropy having both in-plane and out-of-plane magnetic anisotropy components after annealing. The appearance of the strong (100) texture of the CoPd films with the MgO/Pd seed-layer is believed to have caused the decrease in the perpendicular magnetic anisotropy that originated from the magnetoelastic anisotropy due to the additional contribution from the positive magnetostriction λ₁₀₀ but less contribution from the negative magnetostriction λ₁₁₁ when the CoPd films are under in-plane tensile stress.     

Soft magnetic properties of hybrid ferromagnetic films with CoFe, NiFe, and NiFeCuMo layers

Two-layered ferromagnetic alloy films (NiFe and CoFe) with intermediate NiFeCuMo soft magnetic layers of different thicknesses were investigated to understand the relationship between coercivity and magnetization process by taking into account the strength of hard-axis saturation field. The thickness dependence of H_EC (easy-axis coercivity), H_HS (hard-axis saturation field), and χ (susceptibility) of the NiFeCuMo thin films in glass/Ta(5 nm)/[CoFe or NiFe(5 nm-t/2)]/NiFeCuMo(t = 0, 4, 6, 8, 10 nm)/[CoFe or NiFe(5 nm-t/2)]/Ta(5 nm) films prepared using the ion beam deposition method was determined. The magnetic properties (H_EC, H_HS, and χ) of the ferromagnetic CoFe, NiFe three-layers with an intermediate NiFeCuMo super-soft magnetic layer were strongly dependent on the thickness of the NiFeCuMo layer.     

A facile route to sonochemical synthesis of magnetic iron oxide (Fe3O4) nanoparticles

A facile sonochemical approach was applied for the large scale synthesis of iron oxide magnetic nanoparticles (NPs) using inexpensive and non-toxic metal salts as reactants. The as-prepared magnetic iron oxide NPs has been characterized by XRD, TEM, EDS, and VSM. X-ray diffraction (XRD) and EDS analysis revealed that Fe₃O₄ NPs have been successfully synthesized in a single reaction by this simple method. Transmission electron microscopy (TEM) data demonstrated that the particles were narrow range in size distribution with 11 nm average particle size. Moreover, TEM measurements also show that the synthesized nanoparticles are almost spherical in shape. The magnetization curve from vibrating sample magnetometer (VSM) measurement shows that as-synthesized NPs were nearly superparamagnetic in magnetic properties with very low coercivity, and magnetization values were 80 emu/g, which is very near to the bulk value of iron oxide. The estimated value of mass susceptibility of as-synthesized nanoparticles is Xg = 5.71 × 10^− 4 m³/kg.     

High frequency properties of Ni75Fe25-SiO2 granular thin films with very high resistivity

Excellent soft magnetic properties have been achieved in low metal volume fraction(x ∼ 0.52) of (Ni₇₅Fe₂₅)_x(SiO₂)_1−x granular film fabricated by magnetron sputtering. The coercivity of 247 A m^− 1 with very high resistivity ρ over 1.18 × 10⁴ μΩ cm has been obtained. At the frequency lower than 1 GHz, the real part μ′ of the complex permeability keeps about 55 and the imaginary part μ? is less than 2. A combined study of TEM and XRD indicates that the sample consists of Ni₇₅Fe₂₅ particles uniformly embedded in insulating SiO₂ matrix. The excellent soft magnetic properties are ascribed to the magnetic dipole interaction between particles. The investigations of field cooled and zero field cooled curves as well as Henkle Plot prove the existence of this intergranular interaction.     

Ion beam mixing of Co/Fe multilayers: Magnetic and structural properties

[Co(7 nm)/Fe(7 nm)]₆ multilayers were electron-beam evaporated onto Si(100) substrates in ultrahigh vacuum and irradiated at room temperature with 200-keV Xe ions, leading to ion beam mixing within the Co/Fe multilayer, but not with the Si substrate. Irradiation-induced changes in structural and magnetic properties were characterized by means of Rutherford backscattering spectroscopy, X-ray diffraction and in-plane magneto-optical Kerr effect. Irradiation with 1 × 10¹⁶ Xe ions/cm² induced Co/Fe intermixing to a 1 : 1 atomic concentration ratio (RBS) and the formation of the Fe₅₀Co₅₀ permendur phase in the intermixed zone (XRD). For lower ion fluences, the coercivity decreased strongly, but then increased slowly for higher fluences. The angular pattern of the relative remanence showed a perfect uniaxial anisotropy. The magnetic energy density was parametrized with the expression E_s / M_s ∝ (K_u1 / M_s) sin²(φ − φ₀) + (K_u2 / M_s) sin⁴(φ − φ₀), M_s being the saturation magnetization and φ₀ the symmetry angle. The second-order term K_u2 / M_s was found to decrease strongly with increasing Xe fluence.     

Effect of P on crystallization behavior and soft-magnetic properties of Fe83.3Si4Cu0.7B12 − xPx nanocrystalline soft-magnetic alloys

The P content dependences of the crystallization behavior, thermal stability and soft-magnetic properties of high Fe content Fe_83.3Si₄Cu_0.7B_12 − xP_x (x = 0 to 8) nanocrystalline soft-magnetic alloys were investigated. P addition is very effective in widening the optimum annealing temperature range and refining of bcc-Fe grain size in addition to the increasing of nanocrystalline grain density. Uniform nanocrystalline bcc-Fe grains with average size of about 20 nm and number density of 10²³-10²⁴ /m³ were prepared at around x = 6-8 for the annealed Fe_83.3Si₄Cu_0.7B_12 − xP_x alloys. The coercivity H_c markedly decreases with increasing x and exhibits a minimum at around x = 6-8, while the saturation magnetic flux density B_s shows a slight decrease. Fe_83.3Si₄Cu_0.7B₆P₆ nanocrystalline alloy exhibits excellent soft-magnetic properties with a high saturation magnetic flux density B_s of 1.77 T, low coercivity H_c of 4.2 A/m and high effective permeability μ_e of 11,600 at 1 kHz.     

Magnetic properties and microstructures of single-layered Fe100 − xPtx films deposited onto heated substrates

The single-layered Fe_100 − xPt_x films of 30 nm thick with Pt contents (x) of 35-57 at.% are deposited on heated Si (100) substrate at a temperature (Ts) of 620 °C by magnetron co-sputtering. When the Pt content in the Fe-Pt alloy film is 35 at.%, the value of in-plane coercivity (Hc_//) is close to perpendicular coercivity (Hc_⊥) and both values are about 800 kA/m. The FePt films exhibit perpendicular magnetic anisotropy when the Pt content increases to the values of between 45 and 51 at.%. The perpendicular coercivity, saturation magnetization (Ms) and perpendicular squareness (S_⊥) for Fe₅₄Pt₄₆ film are as high as 1113 kA/m, 0.594 Wb/m² and 0.96, respectively. These magnetic properties reveal its significant potential as perpendicular magnetic recording media. Upon further increasing the Pt content to 57 at.%, the coercivity of the Fe-Pt film decreases drastically to below 230 kA/m and tends to be closer to in-plane magnetic anisotropy.     

Effect of density and thickness on H2-gas sensing property of sputtered SnO2 films

Undoped and Pd-doped SnO₂ films were deposited under various conditions for the investigation of the effect of Pd doping, porosity, and thickness on their H₂ gas sensing properties. The temperature of the substrate and the pressure of the discharge gas were varied. All films formed were composed of columns with thicknesses between 20 and 30 nm. The film density decreased as the discharge gas pressure increased and the substrate temperature decreased. It showed values between 4.2×10³ and 7.0×10³ kg/m³ depending on the deposition condition. Low film density and Pd doping resulted in high sensitivity and fast response. The largest sensitivity was observed for a Pd-doped film with a low density of 4.7×10³ kg/m³ and a thickness of 20 nm.     

Effect of bead milling on heat generation ability in AC magnetic field of FeFe2O4 powder

Nano-sized FeFe₂O₄ ferrite powder having a heat generation ability in an AC magnetic field was prepared by bead milling for a thermal coagulation therapy application. A commercial powder sample (non-milled sample) of ca. 2.0 μm in particle size showed a temperature enhancement (ΔT) of 3 °C in an AC magnetic field (powder weight 1.0 g, 370 kHz, 1.77 kA m⁻¹) in ambient air. The heat generation ability in the AC magnetic field improved with the milling time, i.e., due to a decrease in the average crystallite size for all the examined ferrites. The highest heat ability (ΔT = 26 °C) in the AC magnetic field in ambient air was for the fine FeFe₂O₄ powder with a 4.7 nm crystallite size (the samples were milled for 6 h using 0.1 mm? beads). However, the heat generation ability decreased for the excessively milled FeFe₂O₄ samples having average crystallite sizes of less than ca. 4.0 nm. The heat generation of the samples showed some dependence on the hysteresis loss for the B–H magnetic property. The reasons for the high heat generation properties of the milled samples would be ascribed to an increase in the Néel relaxation of the superparamagnetic material. The hysteresis loss in the B–H magnetic curve would be generated as the magnetic moment rotates (Néel relaxation) within the crystal. The heat generation ability (W g⁻¹) can be estimated using a 1.07 × 10⁻⁴fH² frequency (f, kHz) and the magnetic field (H, kA m⁻¹) for the samples milled for 6 h using 0.1 mm? beads. Moreover, an improvement in the heating ability was obtained by calcination of the bead-milled sample at low temperature. The maximum heat generation (ΔT = 59 °C) ability in the AC magnetic field in ambient air was obtained at ca. 5.6 nm for the sample calcined at 500 °C. The heat generation ability (W g⁻¹) for this heat treated sample was 2.54 × 10⁻⁴fH².     

Copper phthalocyanine thin-film field-effect transistor with SiO2/Ta2O5/SiO2 multilayer insulator

Top-contact Copper phthalocyanine (CuPc) thin-film field-effect transistor (TFT) with SiO₂/Ta₂O₅/SiO₂ (STS) multilayer as the dielectric was fabricated and investigated. With the multi-layer dielectric, drive voltage was remarkably reduced. A relatively large on-current of 1.1 × 10⁻⁷ A at a V_GS of −15 V was obtained due to the strong coupling capability provided by the STS multilayer gate insulator. The device shows a moderate performance: saturation mobility of μ_sat = 6.12 × 10⁻⁴ cm²/V s, on-current to off-current ratio of I_on/I_off = 1.1 × 10³, threshold voltage of V_TH = −3.2 V and sub-threshold swing SS = 1.6 V/dec. Atomic force microscope images show that the STS multilayer has a relative smooth surface. Experiment results indicate that STS multilayer is a promising insulator for the low drive voltage CuPc-based TFTs.     

Investigations on the interlayer coupling in Co/Pt multilayers with perpendicular anisotropy via the extraordinary Hall effect

The interlayer coupling in Co/Pt multilayers has been investigated via the measurements of extraordinary Hall effect. In the conventional [Co/Pt]_n multilayer, the coercivity H_C has been observed to increase exponentially for n < 6 and almost linearly for n > 6 with the decrease of temperature. Surprisingly, H_C for n = 1 shows the fastest increase at low temperatures, and becomes the largest one at T < 40 K. As a function of the repetition number n, the almost temperature-independent oscillation of H_C has been observed, being indicative of the Ruderman-Kittel-Kasuya-Yosida type ferromagnetic interlayer coupling in the [Co/Pt]_n multilayer. However, the antiferromagnetic interlayer coupling can be realized via the spin-valve configuration of the Co/Pt multilayer. In the Co/Pt/[Co/Pt]_n multilayer, the antiferromagnetic-to-ferromagnetic transition of the interlayer coupling has been observed at low temperatures. These observed phenomena are strongly related to the temperature-dependent polarization in the Co/Pt multilayers.     

Carrier transport mechanism on ZnO nanorods/p-Si heterojunction diodes with various atmospheres annealing hydrothermal seed-layer

Annealing in various atmospheres (vacuum, N₂, and O₂) was employed for a hydrothermal seed-layer. The influence on ZnO nanorods (NRs) and carrier transport of ZnO NRs/p-Si heterojunction diodes (HJDs) was investigated. In this work, a hydrothermal method was employed to prepare a seed-layer on a Si substrate, and then annealing at 450 °C in various atmospheres was carried out to improve the subsequent growth of ZnO NRs according to the same method. Observations indicated that ZnO NRs with an O₂-annealed seed-layer have a higher nucleation density and absorb fewer OH groups or O₂⁻ ions, and hence they have fewer defect-level centres. This leads to a very large rectification ratio of 1.9 × 10⁵ in the ZnO NRs/p-Si HJDs because oxygen atoms compensate for the oxygen vacancy-related defects. More band-gap states are present at the ZnO/p-Si interface for the vacuum annealing sample, and this enables recombination-tunnelling transport with a rather large ideality factor of 7 at forward voltage less than 0.7 V. In contrast, diffusion-recombination transport was obtained in the N₂- and O₂-annealed samples with ideality factors as low as 2.4 and 2.2, respectively.     

Application of 3D image correlation for full-field transient plate deformation measurements during blast loading

A high-speed stereo-vision system is employed to quantify dynamic material response during buried blast loading. Deformation measurements obtained using 3D image correlation of synchronized, patterned stereo-vision images obtained with an inter-frame time in the range 16 μs ≤ t ≤ 40 μs indicate that (a) buried blast loading initially induces highly localized material response directly under the buried blast location, with severity of the blast event a strong function of depth of explosive burial, (b) for relatively shallow (deep) depth of explosive burial, plate surface velocities and accelerations exceed 220 m s⁻¹ (100 m s⁻¹) and 6 × 10⁶ m s⁻² (1.5 × 10⁶ m s⁻¹) during the first 30 μs (80 μs) after detonation, respectively.     

Formation and magnetic properties of M-Sr ferrite hollow fibers via organic gel-precursor transformation process

The M-Sr ferrite hollow fibers have been successfully prepared by the organic gel-precursor transformation process. The phase formation process of M-Sr ferrite is analyzed by FTIR, XRD and consists of the gel-precursor thermal decomposition and the subsequent ferrite phase formation from strontium oxide and iron oxide. The M-Sr ferrite hollow fibers obtained are characterized with SEM and XRD, and show a high aspect ratio, fine diameters around 4 μm and a ratio of the hollow diameter to the fiber diameter being about 1/2. The optimized M-Sr ferrite hollow fibers are composed of nanograins with a hexagonal plate morphology. Magnetic properties are measured with VSM under a maximum applied of 1194 kA m⁻¹. The M-Sr ferrite hollow fibers formed at 1100 °C for 2 h with the specific saturation magnetization of 53.5 A m² kg⁻¹ possess a shape anisotropy characteristic and the coercivity for the aligned hollow fibers parallel and perpendicular to the applied field is correspondingly 385 and 357 kA m⁻¹.     

Tunable coercivity in perpendicular (CoFe/Tb)n/CoFe multilayers

Magnetic coercivity and anisotropy have been investigated in amorphous Co₅₀Fe₅₀t/Tb t multilayers with perpendicular anisotropy. The thin CoFe layer does not crystallize when sandwiched between Tb layers. The saturation magnetization M_s (~ 10⁶ A/m) and perpendicular magnetic anisotropy constant K_u (~ 10⁵ J/m³) combined with low coercivity μ₀H_c (10-10² mT) have been obtained. These magnetic properties can be tuned. These amorphous multilayers could be designed to suit specific spintronic applications.