Structural studies of evaporated CoxCr1−x/Si (1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si (1 0 0) and glass substrates. Chemical composition and interface properties have been studied by modelling Rutherford backscattering spectra (RBS) using SIMNRA programme. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Simulation of the energy spectra shows an interdiffusion profile in the thickest films, but no diffusion is seen in thinner ones. Microscopic characterizations of the films are done with X-ray diffraction (XRD) measurements. All the samples are polycrystalline, with an hcp structure and show a 〈0 0 0 1〉 preferred orientation. Atomic force microscopies (AFM) reveal very smooth film surfaces.     

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.     

Brillouin light scattering study of the magnetic hysteresis loop in Fe-Ni/Si(100) film with induced magnetic anisotropy

The magnetic hysteresis of Fe₅₇Ni₄₃/Si(100) with magnetic anisotropy induced by an external field has been studied by Brillouin light scattering (BLS). The results are compared with those of the magneto-optic-Kerr-effect (MOKE) measurement and the vibrating sample magnetometer (VSM). The BLS results show that the sample film has strong in-plane anisotropy. The angle between the magnetization and a 4.6 G applied magnetic field H reaches a maximum value of 45° when H lies along the hard axis. The coercivity and magnetic anisotropy field for the film obtained by the BLS are compared with the values obtained by the VSM and MOKE measurement.     

Impact of electric currents on the insulator-metal phase transition in epitaxial thin films of La1−xAxMnO3 (A = Sr, Ca, and Ba)

The influence of dc currents with a high current density on the transport properties of epitaxial La_1−xA_xMnO₃ (A = Sr, Ca, and Ba) thin films were studied. An application of a large current could lead to a remarkable reduction in the insulator-metal phase transition peak, demonstrating a significant electroresistance effect. After removing such currents the samples could completely return to its pristine state. Our experiments reveal that such an electroresistance should be a common feature for the perovskite manganites, rather than the results caused by the self-heating or self magnetic field. It may be ascribed to the two aspects: one is the strong interaction between carrier spins and localized spins in Mn ions, the other is the percolative mechanism of phase separation.     

Two-jump magnetic switching in ultrathin Co(0 0 1) films probed by giant magnetoresistance measurements

Ultrathin epitaxial FCC-Co films, which form part of a spin-valve structure, were found to undergo one- or two-jump magnetic switching, in GMR and MOKE measurements depending upon the field orientation. The transitions are mediated by the propagation of 180° or 90° domain walls. The Co two-jump spin switching in the spin-valve structure has contributed to the formation of three stable GMR states: parallel, antiparallel and a new intermediate state.     

Effect of Si-spacer layer thickness on magnetic and magnetoresistive properties of Co/Si/Co/GaAs(0 0 1)

The magnetization process of Co/Si/Co/GaAs was studied as a function of Si-spacer layer thickness. Coercivity of Co/Si decreased with increasing Si-spacer layer thickness.The Hysteresis loop changed from two phases to a single phase with decreasing temperature and Si-spacer layer thickness. Magnetoresistance (MR) ratio in current-perpendicular-to-plane (CPP) configuration increased with decreasing Si-spacer layer thickness.     

Mössbauer spectroscopy and magnetic properties in thin films of FexNi100−x electroplated on silicon (1 0 0)

Fe_xNi_100−x thin films were produced by galvanostatic electrodeposition on Si (1 0 0), nominal thickness 2800 nm, and x ranging 7-20. The crystalline structure of the sample was determined by X-ray diffraction (XRD). The magnetic properties were investigated by vibration sample magnetometry (VSM) and room temperature ⁵⁷Fe Mössbauer spectroscopy. Conversion Electron Mössbauer spectroscopy (CEMS) in both film surfaces for the thick self-supported films showed that the magnetic moment direction is in the plane and conventional transmission (MS) that the directions are out of the plane films. The results were interpreted assuming a three-layer model where the external layer has in-plane magnetization and the internal one, out of plane magnetization.     

Dependence of magnetic anisotropy of the La0.67Ca0.33MnO3 films on substrate and film thickness

Strain in the La_0.67Ca_0.33MnO₃ films has been tuned by varying substrate and film thickness, and its effects on magnetic anisotropy are studied based on the measurements of isothermal magnetization. Measuring the strain in the films by the out-of-plane lattice parameter (c), we found a strong dependence of the magnetic anisotropy constant (Ku) on strain. Ku decreases linearly from ∼−1.1×10⁶ erg/cm³ for c=0.763 nm to 1.2×10⁶ erg/cm³ for c=0.776 nm, corresponding to a change from tensile strain to compressive strain. Positive Ku signifies a uniaxial anisotropy with the easy axis perpendicular to the film plane, while negative Ku demonstrates an anisotropy of the easy plane character. Smaller or larger c leads a decrease or increase in Ku, which indicates the presence of other effects in addition to those associated with strain. Three distinctive processes for the magnetization are observed along the hard magnetic axis of the films on (001)SrTiO₃, suggesting a possibility of strain relaxation even in ultra-thin films.     

Irreversible effects in LaGa1−xMnxO3

Quasi-irreversible increase in the electrical conductivity is observed in single crystals of LaGa_1−xMn_xO₃. The effect lasts for long time at room temperature and can be erased by heating of the crystal above the phase transition temperature. We explain the observed effects in terms of ionization and local lattice distortion processes.     

Preparation of (CdO)1−x(PbO)x and (CdS)1−x(PbS)x thin films by spray pyrolysis technique and their characterization

Mixed thin films of (CdO)_1−x(PbO)_x and (CdS)_1−x(PbS)_x (x=0.25) were prepared on glass substrates by spray pyrolysis technique for various substrate temperatures 300, 320 and 340 °C. Structural and optical properties were studied. XRD studies reveal the formation of mixed films. The substrate temperature of 340 °C seems to be critical for the formation of CdO-PbO mixed films. It is observed that (CdS)_1−x(PbS)_x mixed films were formed at all the three substrate temperatures. The direct band gap value of (CdO)_1−x(PbO)_x and (CdS)_1−x(PbS)_x mixed films is about 2.6 and 2.37 eV, respectively.     

Influence of spin-glass-like phase on magnetic and electrical transport properties of reactive sputtered Al1−xFexN films

Nanocrystalline Al_1−xFe_xN films were fabricated using the reactive sputtering method. A large amount of spin-glass-like moments are in the films. With the decrease of temperature, the films turn from the spin-glass-like behavior to ferromagnetism. At low temperatures, the saturation magnetization increases and the coercivity decreases with the increase of x. The coercivity increases significantly below 50 K due to the pinning effect of the frozen disordered spin-glass-like moments. All of the films are semiconducting. The low-temperature transport mechanism turns from tunneling to hopping as x increases. Magnetoresistance (MR) shows weak saturation trend with the applied field because of the hard alignment of the frozen moments. Meanwhile, MR follows the relation of log |MR|=a+bT−1, and the spin polarization satisfies P(T)=P₀e−βTα, related with the disordered spin-glass-like moments.     

Structure and magnetic properties of CoxPb1−x nanowire arrays

Ordered Co_xPb_1−x nanowire arrays embedded in the porous anodic aluminum oxide (AAO) template have been fabricated by electrodeposition. XRD experiments prove that neither hexagonal-close-packed (hcp) nor face-centered-cubic (fcc) Co peaks are detected when the Co component (x) is below 0.91. The coercivity (H_c) and squareness (M_r/M_s) are found to increase with ferromagnetic Co component and the maximum value is at the position x=1 (pure Co nanowires). Annealing effects cause H_c and M_r/M_s increase, which surpasses the pure Co nanowires in the 0.2<x<0.6 at the annealing temperature of 700 °C. Microstructure change during annealing process is proposed to explain the magnetic properties change of samples.     

Perpendicular magnetic anisotropy of ultrathin FeCo alloy films on Pd(0 0 1) surface: First principles study

Using the full potential linearized augmented plane wave (FLAPW) method, thickness dependent magnetic anisotropy of ultrathin FeCo alloy films in the range of 1 monolayer (ML) to 5 ML coverage on Pd(0 0 1) surface has been explored. We have found that the FeCo alloy films have close to half metallic state and well-known surface enhancement in thin film magnetism is observed in Fe atom, whereas the Co has rather stable magnetic moment. However, the largest magnetic moment in Fe and Co is found at 1 ML thickness. Interestingly, it has been observed that the interface magnetic moments of Fe and Co are almost the same as those of surface elements. The similar trend exists in orbital magnetic moment. This indicates that the strong hybridization between interface FeCo alloy and Pd gives rise to the large magnetic moment. Theoretically calculated magnetic anisotropy shows that the 1 ML FeCo alloy has in-plane magnetization, but the spin reorientation transition (SRT) from in-plane to perpendicular magnetization is observed above 2 ML thickness with huge magnetic anisotropy energy. The maximum magnetic anisotropy energy for perpendicular magnetization is as large as 0.3 meV/atom at 3 ML film thickness with saturation magnetization of . Besides, the calculated X-ray magnetic circular dichroism (XMCD) has been presented.     

Structure and magnetic properties of FexPd1−x thin films

Fe_xPd_1−x films were epitaxially grown on Au(001). The structure changes from face-centered-cubic (fcc) to face-centered-tetragonal (fct) at x∼0.6, then to body-centered-cubic (bcc) at x∼0.85. Ferromagnetism shows up at 300 K when x is 0.06. The cubic magnetocrystalline anisotropy constant K₁ switches from negative to positive as x increases to 0.34.     

Structural and magnetic properties of Cr1+x(Se1−yTey)2 compounds

The structural and magnetic properties of Cr_1+x(Se_1−yTe_y)₂ having a NiAs structure has been studied for (1+x)=1.27, 1.32 and 1.36 and y=0.75 by means of the Korringa-Kohn-Rostoker (KKR) band structure method. The sub-stoichiometry and the disorder on the chalcogenide sub-lattice has been treated by means of the coherent potential approximation (CPA) alloy theory. From total energy calculations a preferential site occupation on the Cr sub-lattice was found together with an antiparallel alignment of the magnetic moments on the two inequivalent Cr layers. The magnetic properties at finite temperature has been studied by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed to determine the critical temperature in good agreement with experiment.     

Effect of C layer on the structures and magnetic properties of (0 0 1)-oriented [C/CoPt]n/Ag films

CoPt/Ag and [C/CoPt]n/Ag thin films have been prepared onto the glass substrates by magnetron sputtering. We investigated the evolution of texture and magnetic properties of CoPt/Ag and [C/CoPt]n/Ag films. The results show that C-doping plays an important role in improving (0 0 1) texture, improving the order parameter S, reducing the intergrain interactions, and making the magnetization reversal mechanism more close to Stoner-Wolfarth rotational mechanism. The growth mechanism of (0 0 1) texture also seems to be related strongly to the films thickness. Our results show that the highly (0 0 1)-oriented films with ordered fct phase have a significant potential for the perpendicular media of extremely high-density recording.     

Optical and magnetic properties of (Ga1−xMnx)N/GaN digital ferromagnetic heterostructures

(Ga_1−xMn_x)N/GaN digital ferromagnetic heterostructures (DFHs) and (Ga_1−xMn_x)N/GaN grown on GaN buffer layers by using molecular beam epitaxy have been investigated. The photoluminescence (PL) spectra showed band-edge exciton transitions. They also showed peaks corresponding to the neutral donor-bound exciton and the exciton transitions between the conduction band and the Mn acceptor, indicative of the Mn atoms acting as substitution. The magnetization curves as functions of the magnetic field at 5 K indicated that the saturation magnetic moment in the (Ga_1−xMn_x)N/GaN DFHs decreased with increasing Mn mole fraction and that the saturation magnetic moment and the coercive field in the (Ga_1−xMn_x)N/GaN DFHs were much larger than those in (Ga_1−xMn_x)N thin films. These results indicate that the (Ga_1−xMn_x)N/GaN DFHs hold promise for potential applications in spintronic devices.     

A quantitative understanding of pressure dependent conductivity of FeSi1−xGex

A new density of states model, referred to as the Gaussian density of states, is proposed for the quantitative understanding of the electrical conductivity behaviour of FeSi Kondo insulating system. The effects of electron correlation and disorder, responsible for the physical properties of this system, are judiciously incorporated in this model. Within the framework of this model, a detailed quantitative analysis of the temperature and pressure dependent electrical conductivity data of FeSi_1−xGe_x (x=0.0, 0.05 and 0.20) reported by Awadhesh Mani et al. [Phys. Rev. B 63 (2001) 115103] has been carried out. From these analyses the complicated pressure dependence of energy gap seen experimentally in these samples could be satisfactorily rationalized.     

Sol-gel synthesis and magnetization study of Mn1−xCuxFe2O4 (x=0, 0.2) nanocrystallites

The magnetic nanoparticles of Mn_1−xCu_xFe₂O₄ (x=0, 0.2) were prepared by using a sol-gel method. It is proved that both the MnFe₂O₄ and Mn_0.8Cu_0.2Fe₂O₄ nanoparticle samples have superparamagnetic feature. Although the particle sizes are the same, substitution of a small fraction Cu for Mn results in the increase of magnetocrystallite anisotropy energy, thus enhances the blocking temperature from 130 K for MnFe₂O₄ to 260 K for Mn_0.8Cu_0.2Fe₂O₄. Mössbauer spectroscopy confirms that the anisotropy constant K of the Mn_0.8Cu_0.2Fe₂O₄ material is distinctly higher than that of the MnFe₂O₄ compound. Increase of the blocking temperature suggests that the approach we employed is effective to tackle the ‘superparamagnetic limit’ problem.     

Structural study of CoxGe100−x alloys produced by mechanical alloying

Two alloys of the Co-Ge system were produced by mechanical alloying starting from the elemental powders in the compositions Co₂₀Ge₈₀ and Co₄₀Ge₆₀. The crystalline structures of the Co_xGe_100−x (x=20, 40) alloys obtained were investigated using the X-ray diffraction (XRD) technique. The measured XRD patterns showed the presence of the peaks corresponding to the crystalline m-CoGe phase and also to the high pressure and temperature phase c-CoGe in the as-milled sample for Co₂₀Ge₈₀, although it was milled at room temperature and pressure. For Co₄₀Ge₆₀, the crystalline Co₃Ge₂ phase was obtained, and structural data for all phases were determined by means of a Rietveld refinement procedure. The thermal stability of the phases was investigated performing a heat treatment of the alloys at 450 °C for 6 h and, after that, new XRD measurements were collected and were also studied using a Rietveld refinement procedure. The m-CoGe and Co₃Ge₂ phases seem to be very stable, but the relative amount of c-CoGe decreases a little, indicating a less stable phase, which can be explained by the fact that it is produced usually under extreme conditions.