Magnetic studies on ZnTe:Cr film grown on glass substrate by thermal evaporation method

ZnTe and ZnTe:Cr films were prepared on glass substrate by using thermal evaporation method. X-ray diffraction analysis revealed the presence of ZnCrTe phase. X-ray photoelectron spectroscopy was used to estimate the composition of as-prepared films. The valence state of Cr in ZnTe:Cr film is determined to be +2 by using electron spin resonance spectroscopy. Magnetic moment data as a function of magnetic field was recorded by using superconducting quantum interference device magnetometry at 300 K. The result showed a clear hysteresis loop with coercive field of 48 Oe. Magnetic domains were observed by using magnetic force microscopy and the average value of domain size was 3.7 nm.     

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.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Ge(1 1 1) films

Influences of oxygen exposure on the magnetic properties of Co/Ge(1 1 1) ultrathin films have been investigated by surface magneto-optic Kerr effect technique. As the oxygen exposure increases on Co/Ge(1 1 1) films, their magnetic properties could be modified. As an example for 15 ML Co/Ge(1 1 1) films, the coercivity increases from 730 to 920 Oe and the remanence Kerr intensity is reduced for 500 Langmuir (L) of oxygen exposure. Corresponding compositions analyzed by Auger electron spectroscopy measurement shows that the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. Oxygen distributes on the topmost layers of the film. The adsorbed oxygen influences the electronic density of states of Co and results in the changes of the magnetic properties. Besides, the appearance of O/Co/Ge interface could modify the stress anisotropy, and as a result the coercivity of ultrathin Co/Ge(1 1 1) film is enhanced.     

Thickness-dependent magnetization reversal in CoZrNb amorphous films

Structure and magnetization of CoZrNb amorphous films prepared by DC magnetron sputtering have been studied as a function of film thickness (t), from 35 to 840 nm. Using comprehensive characterization, we show that the CoZrNb amorphous films possess a single phase and no nanocrystalline can be detected. The magnetic measurements indicate that the magnetization reversal of CoZrNb films is strongly dependent on t. That is, the coercivity is abruptly reduced to be lower than 4 Oe with t increasing from 35 to 105 nm, and then gradually decreases to ∼0.2 Oe as t increases. This coercivity transition versus t is accompanied by the strong magnetization reversal when t is larger than 105 nm. The results reveal that CoZrNb amorphous films with comparatively large film thickness (>100 nm) are suitable for sensors and anti-faked materials.     

Planar Hall effect and magnetoresistance in Ni81Fe19 and Co square shaped thin films

Ni₈₁Fe₁₉ and Co thin films have been fabricated and their transport properties have been investigated for potential applications in ultra sensitive magnetic field sensors. The Ni₈₁Fe₁₉ films exhibit an anisotropic magnetoresistance (AMR) of 2.5% with a coercivity 2.5 Oe and the Co films exhibit an AMR of 0.7% with coercivity 11 Oe. Large planar Hall effect magnetoresistance values at room temperature are reported for both cases. An unbalanced Wheatstone bridge model is proposed to describe quantitatively the observed experimental Planar Hall Effect data.     

Effect of Co layer thickness on structural and magnetic properties of C/Co/C films

Granular C/Co/C films have been prepared by magnetron sputtering from C and Co onto glass substrates at room temperature and subsequent in situ annealing. It has been found that the structure and magnetic properties of the C/Co/C films depend strongly on the Co layer thickness. Vibrating sample magnetometer measurements indicate that the in-plane coercivities reach maximum in 20 nm Co thickness of both as-deposited and annealed films. The squareness ratio of annealed films was more than 0.8. X-ray diffraction shows that majority Co nanograins are formed as the hexagonal-close-packed (HCP) structure in 20 nm Co thickness with annealing at 400 °C. Scanning probe microscope was used to scan surface morphology and magnetic domain structures. The values of the surface roughness were lower than 0.6 nm in all annealed samples. The average magnetic cluster size was estimated to be about 10 nm in annealed 20 nm Co thickness films.     

Soft magnetic properties of FeCo films with Co underlayer

Bilayered Fe₆₅Co₃₅ (=FeCo)/Co films were prepared by facing targets sputtering with 4πM_s∼24 kg. The soft magnetic properties of FeCo films were induced by a Co underlayer. H_c decreased rapidly when the Co underlayer was 2 nm or more. The films showed well-defined in-plane uniaxial anisotropy with the typical values of H_ce=10 Oe and H_ch=3 Oe, respectively. High frequency characteristics of the films show the films can work at 0.8 GHz with real permeability as high as 250.     

Microstructure and microwave permeability of FeCo thin films with Co underlayer

Microstructure, static magnetic properties and microwave permeability of sputtered FeCo films were examined. Fe₆₀Co₄₀ films (100 nm in thickness) deposited on glass substrates exhibited in-plane isotropy and a large coercivity of 161.1 Oe. When same thickness films were deposited on 2.5 nm Co underlayer, well-defined in-plane anisotropy was formed with an anisotropy field of 65 Oe. The sample had a static initial permeability of about 285, maximum imaginary permeability of 1255 and ferromagnetic resonance frequency of 2.71 GHz. Cross-sectional TEM image revealed that the Co underlayer had induced a columnar grain structure with grain diameter of 10 nm in the FeCo films. In comparison, FeCo films without Co underlayer showed larger grains of 70 nm in diameter with fewer distinct vertical grain boundaries. In addition, the Co underlayer changed the preferred orientation of the FeCo from (1 0 0) to (1 1 0). The improvement in soft magnetic properties and microwave behavior originates from the modification of the film microstructure, which can be well understood by the random anisotropy theory.     

Observation of ferromagnetism at room temperature for Cr ions implanted ZnO thin films

Single crystalline ZnO films were grown on c-plane GaN/sapphire (0 0 0 1) substrates by molecular beam epitaxy. Cr⁺ ions were implanted into the ZnO films with three different doses, i.e., 1 × 10¹⁴, 5 × 10¹⁵, and 3 × 10¹⁶ cm⁻². The implantation energy was 150 keV. Thermal treatment was carried out at 800 °C for 30 s in a rapid thermal annealing oven in flowing nitrogen. X-ray diffraction (XRD), atomic force microscopy, Raman measurements, transmission electron microscopy and superconducting quantum interference device were used to characterize the ZnO films. The results showed that thermal annealing relaxed the stress in the Cr⁺ ions implanted samples and the implantation-induced damage was partly recovered by means of the proper annealing treatment. Transmission electron microscopy measurements indicated that the first five monolayers of ZnO rotated an angle off the [0 0 0 1]-axis of the GaN in the interfacial layer. The magnetic-field dependence of magnetization of annealed ZnO:Cr showed ferromagnetic behavior at room temperature.     

Property changes of electroplated Cu/Co alloys and multilayers by organic additives

We investigated the change of magnetic properties of the electroplated Cu/Co alloys and multilayers caused by organic additives and high temperature annealing. When plated with a pure Cu/Co electrolyte, the alloy contained ∼25% of Cu and ∼75% of Co. The alloy was made of hcp-Co, fcc-Co and Cu(1 1 1) and was super-paramagnetic at room temperature. As we add a few organic additives in the plating electrolyte, the hcp-Co of the films disappeared. The organic additives contained in the electrolytes changed paramagnetic Cu/Co multilayers to ferromagnets. High-temperature thermal annealing increased coercivity due to the growth of the Co grains.     

Tunable resonance frequency of FeNi films by oblique sputtering

FeNi thin films were fabricated by radio frequency magnetron sputtering on Si(1 1 1). Dynamic properties at remanence of the films were systematically investigated in a wide frequency range from 100 MHz to 5 GHz. The results show that both thickness of FeNi films and oblique angle have important effects on the magnetic properties of the films, the magnetic resonant frequency of the films can also be adjusted by the two factors. The in-plane uniaxial magnetic anisotropy field can be adjusted from 82 Oe to 220 Oe by increasing the oblique angle. As a consequence, the magnetic resonant frequency of the films increased from 2.7 GHz to 4.2 GHz.     

Influence of ternary addition of transition elements (Cr,Si and Mn) on the microstructure and magnetic properties of nano-structured CuCo alloy

The current state of studies presents the effect of ternary addition of transition elements such as Mn, Cr and Si (10 wt%) on the mechanically driven non-equilibrium solubility of 40 wt% Co containing Cu–Co alloy. X-ray powder diffraction analysis indicates that addition of Mn has been found to be the most effective in enhancing the solubility and formation of a complete solid solution between Co and Cu in a short duration (30 h) of ball milling. The microstructure of the ball milled CuCoMn alloy was found to be stable after the isothermal annealing up to a temperature of 450 °C for 1 h. The magnetic properties such as magnetic saturation, coercivity and remanence of ball milled CuCo alloy in the presence of Mn significantly altered after annealing in the temperature range 350–650 °C for 1 h. The best combination of magnetic properties of CuCoMn alloy has been found after annealing at 550 °C for 1 h.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Influence of Si-N interlayer on the microstructure and magnetic properties of γ′-Fe4N films

The (γ′-Fe₄N/Si-N)_n (n: number of layers) multilayer films and γ′-Fe₄N single layer film synthesized on Si (1 0 0) substrates by direct current magnetron sputtering were annealed at different temperatures. The structures and magnetic properties of as-deposited films and films annealed at different temperatures were characterized using X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. The results showed that the insertion of Si-N layer had a significant influence on the structures and magnetic properties of γ′-Fe₄N film. Without the addition of Si-N lamination, the iron nitride γ′-Fe₄N tended to transform to α-Fe when annealed at the temperatures over 300 °C. However, the phase transition from γ′-Fe₄N to ?-Fe₃N occurred at annealing temperature of 300 °C for the multilayer films. Furthermore, with increasing annealing temperature up to 400 °C or above, ?-Fe₃N transformed back into γ′-Fe₄N. The magnetic investigations indicated that coercivity of magnetic phase γ′-Fe₄N for as-deposited films decreased from 152 Oe (for single layer) to 57.23 Oe with increasing n up to 30. For the annealed multilayer films, the coercivity values decreased with increasing annealing temperature, except that the film annealed at 300 °C due to the appearance of phase ?-Fe₃N.     

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films

Effect of oxygen exposure on the magnetic properties of ultrathin Co/Si(1 1 1)-7×7 films have been studied. In ultrahigh vacuum environment, Auger electron spectroscopy (AES) analysis shows that no oxygen adsorption occurs on Si(1 1 1)-7×7 surface and Co-Si compound interfaces. As the thickness of Co films increases above 5 monolayers (ML), pure cobalt islands form on the surface and the amount of oxygen on the surface layers increases with increasing the oxygen exposure time. From the results of slight chemical shift and depth profiling measurements, the oxygen is weakly adsorbed on the topmost layer of 15 ML Co/Si(1 1 1) films. The adsorbed oxygen influences the electronic density of states of Co and leads to the changes of the magnetic properties. The appearance of the O/Co interface could modify the stress anisotropy, as a result, the coercivity of ultrathin Co/Si(1 1 1) films are enhanced. As an example for 15 ML Co/Si(1 1 1), the coercivity increases from 140 to 360 Oe with 5000 Langmuir of oxygen exposure.     

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.     

Effects of layering and magnetic annealing on the texture of CoPt films

The effect of magnetic field annealing of magnetron sputtered CoPt alloy films and Co/Pt bilayers on the crystallographic texture of the obtained chemically ordered (L1₀) CoPt films is presented. In CoPt alloy films the main effect of the magnetic field is to suppress (1 1 1) growth in the early stages of L1₀ formation whereas the development of (0 0 1) versus (1 0 0) texture is related to chemical ordering strain. A higher degree of (0 0 1) texture is obtained by magnetically annealing Co/Pt bilayers since the initial (1 1 1) texture in the as-sputtered films is avoided and Co-Pt alloying occurs in the presence of the magnetic field.     

Magnetic properties of co-sputtered Tb-Co2FeAl films with perpendicular anisotropy

We present a systematic study on magnetic properties of co-sputtered Tb-Co₂FeAl (TCFA) films. The TCFA films with suitable Tb content have perpendicular magnetic anisotropy (PMA). The PMA deteriorates with both decreasing film thickness and high temperature annealing. Under a certain thickness, the perpendicular coercivity of the TCFA films with PMA can be reduced down to 60 Oe, which is comparable with normal soft ferromagnets. After annealing at 100 °C, a large remanence squareness of 0.95 is observed in the TCFA film with 33% Tb and a thickness of 30 nm.     

Influences of the interfacial state between Co and a Si substrate on the magnetic properties of Co/Si(1 1 1) films

Morphology and magnetic properties of Co/Si(1 1 1) interfaces have been investigated using scanning tunneling microscope and surface magneto-optic Kerr effect techniques. As deposited at room temperature for Co/Si(1 1 1), defects have been observed with shapes of dark patches and bright islands on the surface with different Co coverage. The defect formation causes a rough interface. For subsequently deposited Co layers, the interfacial state between Co and the Si substrate results in the appearance of both the longitudinal and polar Kerr loops. After annealing treatments, interdiffusion of Co atoms and Si(1 1 1) substrate occurs as revealed by Auger electron spectroscopy. Scanning tunneling microscope images show the formation of Si clusters with average diameter of 10 nm at high temperatures. The disappearance of ferromagnetism of the films occurs due to the structural and compositional changes.