Magnetic properties of field-annealed FeCo thin films

Fe₅₀Co₅₀ thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 °C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films.     

CoPt–Al 2 O 3 Nanocomposite Films: Synthesis,Structure, and Magnetic Properties

The structure and magnetic properties of CoPt–Al2O3 nanocomposite films synthesized by the annealing of Al/(Co3O4 + Pt) bilayers on a MgO(001) substrate at 650°C in vacuum are investigated. The synthesized composite films contain ferromagnetic CoPt grains with an average size of 25–45 nm enclosed in a nonconducting Al2O3 matrix. The saturation magnetization (Ms ~ 330 G) and coercivity (Hc ≈ 6 kOe) of the films are measured in the film plane and perpendicular to it. The obtained films are characterized by a spatial rotational magnetic anisotropy, which makes it possible to arbitrarily set the easy magnetization axis in the film plane or perpendicular to it using a magnetic field stronger than the coercivity (H > Hc).     

Enhanced imaging of magnetic structures in micropatterned arrays of Co dots and antidots

A specific technique of numerical treatment of atomic force microscopy (AFM) and magnetic force microscopy (MFM) signal has been developed to enhance the quality of raw images, in order both to improve their contrast and to gain better insight on the sample topography and on the local arrangement of the magnetisation vector. Basically, the technique consists in computing the optimum conformal transformation that allows one to superimpose two AFM images of the same area, acquired performing subsequent scans whose fast scan axis were mutually perpendicular, and applying the inverse transform to the second image. After MFM image superposition, the two datasets were either summed or subtracted, in order to improve the magnetic contrast. Computations have been done in a Matlab^® workspace with the help of Image Processing Toolbox 4.2. Improved MFM images obtained on both dots and antidots thin evaporated Co arrays in the demagnetised state (after performing alternate field demagnetisation parallel and perpendicular to the array plane) have been interpreted. Samples consisting of large-size patterns (1×1 mm) of circular dots/antidots with square/hexagonal lattices and minimum diameters of 1 μm were prepared by optical lithography. The magnetic film thickness was chosen depending on resist thickness, and varied between 25 and 150 nm, with a fixed ratio 1:4 between metal/resist film thickness. MFM was exploited to obtain images of either intra-dot or inter-antidot magnetic structures.     

Thickness dependent phase transformation of magnetron-sputtered Ni–Mn–Sn ferromagnetic shape memory alloy thin films

In this study, the influence of film thickness on the first-order martensite–austenite phase transformation of Ni–Mn–Sn ferromagnetic shape memory alloy thin films has been systematically investigated. Different thicknesses of the Ni–Mn–Sn films (from ~100 to 2,500 nm) were deposited by DC magnetron sputtering on Si (100) substrates at 550 °C. X-ray analysis reveals that all the films exhibit austenitic phase with the L2₁ cubic crystal structure at room temperature. The grain size and crystallization extent increase with the increase in film thickness, but the films with thickness above ~1,400 nm show structural deterioration due to the formation of MnSn₂ and Ni₃Sn₄ precipitates. The improvement in the crystallinity of the film with thickness is attributed to the decrease in film–substrate interfacial strain resulting in preferred oriented growth of the films. Temperature-dependent magnetization measurements as well as electrical measurements demonstrate the complete absence of phase transformation for the film of thickness of ~120 nm. For thickness greater than 400 nm, film exhibits the structural transformation, and it occurs at higher temperature with better hysteresis as film thickness is increased up to ~1,400 nm, after which degradation of phase transformation phenomenon is observed. This degradation is attributed to the disorders present in the films at higher thicknesses. Film with thickness ~1,400 nm possesses the highest magnetization with the smallest thermal hysteresis among all the films and therefore best suited for the actuators based on first-order structural phase transformation. Nanoindentation measurements reveal that the higher values of hardness and elastic modulus of about 5.5 and 215.0 GPa obtained in film of 1,014 nm thickness can considerably improve the ductility of ferromagnetic shape memory alloys (FSMA) and their applicability for MEMS applications. The exchange bias phenomenon is also found to be present in the films of thickness 1014, 1412, and 2022 nm exhibiting prominent martensitic transformation. Film of thickness 2,022 nm exhibits maximum exchange bias of ~50 Oe and higher exchange bias blocking temperature of 70 K as compared to other films.     

Magnetic properties of strontium hexaferrite films prepared by pulsed laser deposition

The magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films fabricated by pulsed laser deposition on the Si(100) substrate with Pt(111) underlayer have been studied as a function of film thickness (50–700 nm). X-ray diffraction patterns confirm that the films have c-axis perpendicular orientation. The coercivities in perpendicular direction are higher than those for in-plane direction which indicates the films have perpendicular magnetic anisotropy. The coercivity was found to decrease with increasing of thickness, due to the increasing of the grain size and relaxation in lattice strain. The 200 nm thick film exhibits hexagonal shape grains of 150 nm and optimum magnetic properties of M_s=298 emu/cm³ and H_c=2540 Oe.     

Microstructure and magnetic properties of Co(Cr)/Ti(Cr)/ZnO films prepared by laser ablation on Si substrates

High-quality Co(Cr)(50–66 nm)/Ti(Cr)(25–30 nm)/ZnO(165–225 nm) films were grown on Si substrates by the pulsed-laser deposition technique at 450–500°C. The microstructure of the films and their magnetic properties were investigated by X-ray diffraction, transmission electron microscopy and a vibrating sample magnetometer. It was found that the Co(Cr)/Ti(Cr)/ZnO thin films had a highly preferential c-axis direction normal to the surface of the substrate, and the ZnO sublayer had columnar structures in the growth direction. The interface of the multi-layer films is smooth without interface reaction. The as-produced films had in-plane easy magnetization. It indicates that the enhancements of the magnetic properties of the films are mainly ascribed to the grain structure and multi-domain state of the Co-alloy layer, which can be controlled by the ZnO sublayer.     

Magnetic domain structure in small diameter magnetic nanowire arrays

Fe_0.3Co_0.7 alloy nanowire arrays were prepared by ac electrodepositing Fe²⁺ and Co²⁺ into a porous anodic aluminum oxide (PAO) template with diameter about 50 nm. The surface of the samples were polished by 100 nm diamond particle then chemical polishing to give a very smooth surface (below ±10 nm/μm²). The morphology properties were characterized by SEM and AFM. The bulk magnetic properties and domain structure of nanowire arrays were investigated by VSM and MFM respectively. We found that such alloy arrays showed strong perpendicular magnetic anisotropy with easy axis parallel to nanowire arrays. Each nanowire was in single domain structure with several opposite single domains surrounding it. Additionally, we investigated the domain structure with a variable external magnetic field applied parallel to the nanowire arrays. The MFM results showed a good agreement with our magnetic hysteresis loop.     

Photoinduced self-limited low-temperature growth of ultra-thin silicon-oxide films with water vapor

The growth of ultra-thin (<2 nm) silicon-oxide films was investigated on Si(100):H, Si(111):H, and a-Si:H surfaces in a pure water atmosphere (0.1–10 Pa) at low temperatures of 30–250 °C. Oxidation was induced photochemically by pulsed F₂-laser radiation at 157 nm. The thickness and composition of the growing oxide films were monitored in real time by spectroscopic ellipsometry in the photon energy range of 1.15–4.75 eV. The mechanism of laser-induced silicon oxidation in a H₂O atmosphere is shown to differ fundamentally from the classical Deal–Grove mechanism of thermal oxidation at 900–1200 °C, as well as from the photoinduced low-temperature oxidation in an O₂ atmosphere. In particular, the film thickness essentially does not depend on temperature below 250 °C. A kinetic model is developed for low-temperature silicon oxidation in a H₂O atmosphere. According to this model, the growth is limited at small thicknesses by the oxidation reaction and at larger thicknesses by reactions of the diffusing oxidizing species in the oxide layer. Very good agreement is established between this kinetic model and the ellipsometric measurements and the temperature and pressure dependence of the water oxidation process. PACS 82.65.+r; 07.60.Fs; 81.65.Mq; 82.50.Hp     

Probing the effect of nitrogen gas on electrical conduction phenomena of ZnO and Cu-doped ZnO thin films prepared by spray pyrolysis

Zinc oxide (ZnO) and Cu-doped ZnO (CZO) thin films were prepared on borosilicate glass substrates by spray pyrolysis technique. The X-ray diffraction study revealed that Cu doping caused a reduction in crystallite size. AFM study showed an increase in roughness with doping. This is attributed to the aggregation of particles to form clusters. From transmission electron microscopy analysis, the particle size is measured to be in the range 30–65 nm (average particle size 48 nm) for undoped ZnO, whereas it is in the range 24–56 nm (average particle size 40 nm) for CZO film. The electrical resistivity of the thin films was investigated in the presence of air as well as N₂ mixed air at different temperatures in the range 30–270 °C. The change in resistivity properties was explained on the basis of conduction phenomena within the grain along with the grain boundaries as well as Cu- and N₂-induced defect states. The thermal activation energy of ZnO was found to be in the range 0.04–0.7 eV and dependent on Cu doping and N₂ level in air.     

Temperature dependence of a nanoporous Pd film hydrogen sensor based on an AAO template on Si

In this study, hydrogen sensing properties of nanoporous Pd films based on Anodic Aluminium Oxide (AAO) templates grown on a silicon substrate have been investigated at various temperatures (25–100°C) and hydrogen concentrations (100–1000 ppm) to determine the temperature-sensitivity relationship. For this purpose, a hexagonally shaped AAO template of approximately 50 nm in diameter and 700 nm in length with 80 nm interpore distances was fabricated using two-step anodization of an Al film deposited on an n-type (100) oriented oxidized Si substrate. Then, the nanoporous surface of the AAO template was used as a substrate for supporting a nanoporous Pd film of an approximately thickness of 60 nm. The morphologies of the AAO template and Pd film coated on the AAO template were studied mainly by Scanning Electron Microscopy (SEM). Hydrogen sensing properties of the nanoporous Pd film were measured using a resistance transient method. It was found that the sensor response of the nanoporous Pd films on the AAO template was better than the traditional Pd thin film sensors, the sensitivity of the sensor was approximately 1.8% for 1000 ppm H₂, and the detection limit was lower than 100 ppm at room temperature. The highest sensitivity was measured at room temperature.     

FMR and TEM Studies of Co and Ni Nanoparticles Implanted in the SiO<Subscript>2</Subscript> Matrix

Fused silica plates have been implanted with 40 keV Co⁺ or Ni⁺ ions to high doses in the range of (0.25–1.0) × 10¹⁷ ions/cm², and magnetic properties of the implanted samples have been studied with ferromagnetic resonance (FMR) technique supplemented by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The high-dose implantation with 3d-ions results in the formation of cobalt and nickel metal nanoparticles in the irradiated subsurface layer of the SiO₂ matrix. Co and Ni nanocrystals with hexagonal close packing and face-centered cubic structures have a spherical shape and the sizes of 4–5 nm (for cobalt) and 6–14 nm (for nickel) in diameter. Room-temperature FMR signals from ensembles of Co and Ni nanoparticles implanted in the SiO₂ matrix exhibit an out-of-plane uniaxial magnetic anisotropy that is typical for thin magnetic films. The dose and temperature dependences of FMR spectra have been analyzed using the Kittel formalism, and the effective magnetization and g-factor values have been obtained for Co- and Ni-implanted samples. Nonsymmetric FMR line shapes have been fitted by a sum of two symmetrical curves. The dependences of the magnetic parameters of each curve on the implantation dose and temperature are presented.     

Effect of oxygen pressure on microstructure and magnetic properties of strontium hexaferrite (SrFe12O19) film prepared by pulsed laser deposition

The effects of oxygen pressure during deposition on microstructure and magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films grown on Si (100) substrate with Pt (111) underlayer by pulsed laser deposition have been investigated. X-ray diffraction pattern confirms that the films have c-axis perpendicular orientation. The c-axis dispersion (Δθ₅₀) increases and c-axis lattice parameter decreases with increasing oxygen pressure. The films have hexagonal shape grains with diameter of 150-250 nm as determined by atomic force microscopy. The coercivities in perpendicular direction are higher than those in in-plane direction, which shows the films have perpendicular magnetic anisotropy. The saturation magnetization and anisotropy field for the film deposited in oxygen pressure of 0.13 mbar are comparable to those of the bulk strontium hexaferrite. Higher oxygen pressure leads to the films having higher coercivity and squareness. The coercivity in perpendicular and in-plane directions of the film deposited in oxygen pressure of 0.13 mbar are 2520 Oe and 870 Oe, respectively.     

Study of nanoconductive and magnetic properties of nanostructured iron films prepared by sputtering at very low temperatures

A combined study of the surface nanostructure and electrical characteristics of iron thin films prepared on naturally passivated silicon wafers is presented. By means of conductive-scanning force microscopy, the influence of the substrate temperature during film preparation on both surface morphology and conductivity response is investigated. In addition, magnetic properties of these films are reported and correlated with the nanostructural properties. Films prepared at 200 K show granular core–shell magnetic behaviour exhibiting exchange bias. Both conductive and magnetic data indicate that samples prepared at low temperature behave as a percolated network of nanometric metallic iron clusters (with typical sizes of 20 nm) interconnected by oxidized chains (of 10 nm in diameter), showing an excellent system for nanotechnological exchange bias applications.     

Structural and magnetic properties of evaporated Fe thin films on Si(1 1 1), Si(1 0 0) and glass substrates

We present experimental results on the structural and magnetic properties of series of Fe thin films evaporated onto Si(1 1 1), Si(1 0 0) and glass substrates. The Fe thickness, t, ranges from 6 to110 nm. X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used to study the structure and surface morphology of these films. The magnetic properties were investigated by means of the Brillouin light scattering (BLS) and magnetic force microscopy (MFM) techniques. The Fe films grow with (1 1 0) texture; as t increases, this (1 1 0) texture becomes weaker for Fe/Si, while for Fe/glass, the texture changes from (1 1 0) to (2 1 1). Grains are larger in Fe/Si than in Fe/glass. The effective magnetization, 4πM_eff, inferred from BLS was found to be lower than the 4πM_S bulk value. Stress induced anisotropy might be in part responsible for this difference. MFM images reveal stripe domain structure for the 110 nm thick Fe/Si(1 0 0) only.     

Quantification of the lift height for magnetic force microscopy using 3D surface parameters

In this work, the quantitative conditions for the lift height for imaging of the magnetic field using magnetic force microscopy (MFM) were optimized. A thin cobalt film deposited on a monocrystalline silicon (1 0 0) substrate with a thickness of 55 nm and a thin nickel film deposited on a glass with a thickness of 600 nm were used as samples. The topography of the surface was acquired by tapping mode atomic force microscopy (AFM), while MFM imaging was performed in the lift mode for various lift heights. It was determined that the sensitivity of the measurements was about 10% higher for images obtained at a scan angle of 90° compared to a scan angle of 0°. Therefore, the three-dimensional surface texture parameters, i.e., average roughness, skewness, kurtosis and the bearing ratio, were determined in dependence on the lift height for a scan angle of 90°. The results of the analyses of the surface parameters showed that the influence of the substrate and its texture on the magnetic force image could be neglected for lift heights above 40 nm and that the upper lift height limit is 100 nm. It was determined that the optimal values of the lift heights were in the range from 60 to 80 nm, depending on the nature of the sample and on the type of the tip used.     

Pure UV photoluminescence from ZnO thin film by thermal retardation and using an amorphous SiO2 buffer layer

ZnO/SiO₂ thin films were fabricated on Si substrates by E-beam evaporation with thermal retardation. The as-prepared films were annealed for 2 h every 100 °C in the temperature range 400-800 °C under ambient air. The structural and optical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL). The XRD analysis indicated that all ZnO thin films had a highly preferred orientation with the c-axis perpendicular to the substrate. From AFM images (AFM scan size is 1 μm×1 μm), the RMS roughnesses of the films were 3.82, 5.18, 3.65, 3.40 and 13.2 nm, respectively. PL measurements indicated that UV luminescence at only 374 nm was observed for all samples. The optical quality of the ZnO film was increased by thermal retardation and by using an amorphous SiO₂ buffer layer.     

Demonstration of ultra-high-resolution MFM images using Co90Fe10-coated CNT probes

We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic storage media using carbon nanotube probes coated by ferromagnetic Co₉₀Fe₁₀ films (20, 30, 40, and 50 nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic domain image with an 40 nm-Co₉₀Fe₁₀-coated tip (50 nm tip diameter) about a lateral detect density of 1200 k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM imaging reported for this material system and structure. The observed dependence of tip dimension on signal contrast and image resolution was successfully explained by a theoretical analysis indicating that the signal contrast, along with the physical probe-tip dimension, should be taken into account to design magnetic probes tips for high-resolution magnetic force microscopy.     

Electrical resistivity due to electron scattering with magnetic domain walls and magnetic properties of Ni-Fe alloy thin films

The physical properties of magnetic domain walls and electrical conductivity of permalloy thin films under external magnetic fields were studied. Using a magnetic force microscope (MFM), we observed the variation of domain configurations with the change of applied magnetic field for different film thicknesses of 245, 320, and 415 nm. A superconducting quantum interference device (SQUID) was exploited to measure the magnetization loop for the applied magnetic field either parallel or perpendicular to the normal direction of the surface. We also found that the resistivity increases significantly as the electrical current conduction changed from parallel to perpendicular to the domain walls.     

Magnetic force microscopy studies of domain walls in nickel and cobalt films

A magnetic force microscopy is used to examine the domain walls in nickel and cobalt films deposited by argon ion sputtering. Thin nickel films deposited at high substrate temperatures exhibit coexistent Bloch and Neel walls. Films grown at room temperature display alternative Bloch lines with cap switches. These films agglomerate to form grains after annealed at high temperatures. The film composed of larger grains behaves better nucleation implying magnetic domains of closure, while the film composed of smaller grains exhibits more defects implying alternative Bloch lines. We have also observed domain displacements and cap switches, which occur due to precipitation of particles in small grain size films. Stripe domains are observed for film thicknesses larger than 100 nm. They become zigzag cells when an external field of 1.5 T is applied perpendicular to the surface of the films. This experiment indicates that the domain sizes in thin films and the strip widths for thick films both depend on the square-root of the film thickness, which varies from 5 to 45 nm and from 100 to 450 nm, respectively.     

Structural, optical and electrical properties of cubic AlN films deposited by laser molecular beam epitaxy

Cubic AlN films were successfully deposited on TiN buffered Si (100) substrates by a laser molecular beam epitaxy (LMBE) technique, and their crystal structure and optical and electrical properties were studied. The results indicate that cubic AlN films show the NaCl-type structure with a (200) preferred orientation, and the lattice parameter is determined to be 0.4027 nm. The Fourier transform infrared (FTIR) pattern of the cubic AlN film displays sharp absorption peaks at 668 cm⁻¹ and 951 cm⁻¹, corresponding to the transverse and longitudinal optical vibration modes. Ellipsometric measurements evidence a refractive index of 1.66–1.71 and an extinction coefficient of about zero for the cubic AlN film in the visible range. Capacitance–voltage (C–V) traces of the metal–insulator–semiconductor (MIS) device exhibit that the cubic AlN film has a dielectric constant of 8.1, and hysteresis in the C–V traces indicates a significant number of charge traps in the film.