Granular Thin Films Research Articles

Charging of highly resistive granular metal films

We have used the scanning Kelvin probe microscopy technique to monitor the charging process of highly resistive granular thin films. The sample is connected to two leads and is separated by an insulator layer from a gate electrode. When a gate voltage is applied, charges enter from the leads and rearrange across the sample. We find very slow processes with characteristic charging times exponentially distributed over a wide range of values, resulting in a logarithmic relaxation to equilibrium. After the gate voltage has been switched off, the system again relaxes logarithmically slowly to the new equilibrium. The results cannot be explained with diffusion models, but most of them can be understood with a hopping percolation model, in which the localization length is shorter than the typical site separation. The technique is very promising for the study of slow phenomena in highly resistive systems and will be able to estimate the conductance of these systems when direct macroscopic measurement techniques are not sensitive enough.

Cobalt content- and magnetic field-dependent transmission behaviors of red laser light for Co[sbnd]Al2O3 granular thin films

The transmission of red laser light with wavelength λ = 632.8 nm of Cox-(Al2O3)1-x magnetic granular thin films was investigated under the effect of external magnetic field H (0–500 Oe). Results showed the behaviors of variation in the T transmittance ratio as a function of H exhibit three trends depending on the Co content x: (1) T declines to a saturation value called the floor level of the transmission Tflo when x < 0.49; (2) T tends to reach a maximum and then decreases to a saturation floor level when x = 0.49; and (3) T increases to a saturation value called the ceiling level of the transmission Tcei, when x > 0.49. Tflo increases in accordance with x when x < 0.49, whereas Tcei decreases rapidly when x approaches 1. These phenomena were considered to be related to the magnon–plasmon interactions from excitation by photons for the spins polarized in the Co particles.

English

The reflection of visible light (wavelength from 350 to 800 nm) on magnetic granular thin films Cox-Ag1-x with x from 0 to 35 at % was studied. Dependence of reflection (R) is strong on magnetic and percentage of Co in the thin films, R strongly increase in wavelengths 560 and 660 nm. In this paper, some arguments relating to the spin-plasmon phenomenon were used to explain the results. Key words: Granular magnetic thin films, visible light, reflection, spin-plasmon.

Optical properties of Au–TiO2 and Au–SiO2 granular metal thin films studied by Spectroscopic Ellipsometry

We report on the optical properties in the dielectric regime of gold nanostructured granular thin films fabricated through sputter deposition with a composite target at room temperature and over a wide photon energy range (0.62–4.13eV) by means of Spectroscopic Ellipsometry. The thickness and the films effective optical constants are successfully determined using an approach based on multiple Gaussian oscillators. In the quasi-static regime, i.e., 2R≪λ, and in the dipole approximation, examining the real and imaginary parts, ε1, ε2, of the dielectric function, it is shown that the dc optical conductivity is almost negligible (σ=ωε0ε2≪10−5Ωcm−1) and only the capacitive contribution holds for the electron-phonon relaxation in localized surface plasmon of the gold particles. Furthermore, we find that the resonant frequencies ωp becomes red-shifted when the particles are electromagnetically coupled to each other or when the surrounding medium dielectric constant, εm, increases, thus exhibiting a wide spectral tuning range of 1.95–2.24eV.

Micromagnetic Studies of Finite Temperature M–H Loops for FePt-C Media**Supported by the National Natural Science Foundation of China under Grant Nos 51171086 and 51371101.

We have recently developed a new micromagnetic method at finite temperature, where the Hybrid Monte Carlo method is employed to realize the Boltzmann distribution with respect to the magnetic free energy. Hence, the hysteresis loops and domain structures at arbitrary temperature below the Curie point Tc can be simulated. The Hamilton equations are used to find the magnetization distributions instead of the Landau–Lifshitz (LL) equations. In our previous work, we applied this method on a simple uniaxial anisotropy nano-particle and compared it with the micromagnetic method using LL equations. In this work, we use this new method to simulate an L10 FePt-C granular thin film at finite temperatures. The polycrystalline Voronoi microstructure is included in the model, and the effects of the misorientation of FePt grains are also simulated.

Origin of the large positive magnetoresistance of Ge1−xMnx granular thin films

GeMn granular thin films are a unique and promising material for spintronics applications due to large positive magnetoresistance (MR). Previous studies on GeMn have suggested that the large MR is related to nanospinodal decomposition of GeMn into Mn-rich ferromagnetic nanoparticles and Mn-poor paramagnetic matrix. However, its microscopic origin of the MR has not been clarified yet. Here, using X-ray magnetic circular dichroism (XMCD), which is extremely sensitive to the local magnetic state of each atom, we investigate the magnetic properties of the nanoparticles and the matrix in GeMn separately. We find that the MR ratio is proportional to the product of the magnetizations originating from the nanoparticles and the matrix. This result indicates that spin-polarized holes in the nanoparticles penetrate into the matrix and that these holes undergo spin-disorder magnetic scattering by the paramagnetic Mn atoms in the matrix, which induces the large MR.

PdSb添加L1&lt;sub&gt;0&lt;/sub&gt;-FePt系薄膜の結晶配向性および磁気特性

FePt thin films with L10 ordered structure are candidates for high-density magnetic recording media because of their high uniaxial magnetic anisotropy energy. The (Fe0.395Pt0.395Cu0.15P0.02Pd0.04)100–xSbx thin films with various x were deposited directly on quartz glass substrates by a sputtering method. FePt ordered alloy thin films with (001) preferential orientation were achieved in Sb content of x ≥ 8.0 at.% with post-annealing at 600°C for 1 hour under reduced pressure in a H2 atmosphere. However, the coercivity of these thin films decreased dramatically due to growing of the crystal grains. Therefore, in this study, we prepared the PdSb-FePt-(C4F8)n granular thin films to improve the coercivity. The Fe0.44Pt0.44Pd0.04Sb0.08-(C4F8)n granular thin film deposited at C4F8 partial pressure of 0.1 mTorr then annealed at 700°C for 1 hour under reduced pressure in a H2 atmosphere exhibited a large coercivity of 30 kOe.

Non-ohmic behavior of metal-insulator granular thin films in low-field regime (eΔV ≪ kBT)

Non-ohmic behavior is not expected in metal–insulator granular systems in a low-field regime. There is no model to explain this behavior, even though it has been reported in several metal-insulator granular thin films (Fe-Al2O3, Co-Al2O3, and Ti-SiO2). In this paper, we show additional experimental results of Fe-SiO2 granular films and propose an explanation for the electrical properties of all above mentioned systems, based on Mott variable range hopping. The experimental results show that the localization length increases and the electrical resistance decreases with the increase of electrical potential or current. The non-ohmic behavior of the resistance and the increase of the localization length with increasing current are explained by the activation of new pathways for electrons in granular thin films that contain variable grain sizes and/or have different distances between grains.

Resistivity minimum in granular composites and thin metallic films

We analyze the temperature dependence of conductivity in thick granular ferromagnetic compounds NiSiO2 and in thin weakly coupled films of Fe, Ni and Py in vicinity of metal-insulator transition. Development of resistivity minimum followed by a logarithmic variation of conductivity at lower temperatures is attributed to granular structure of compounds and thin films fabricated by conventional deposition techniques. Resistivity minimum is identified as a transition between temperature dependent intra-granular metallic conductance and thermally activated inter-granular tunneling.

Effects of non-magnetic phase and deposition temperature on magnetic properties of FePt–MgO granular thin films on single-crystal MgO substrate

(FePt)100−x(MgO)x granular thin films were fabricated by using co-sputtering on single crystal MgO substrate at different deposition temperatures followed by a high-temperature annealing. The adding MgO was found to have the following effects: 1) high fraction of MgO phase (70vol%) degraded the L10 phase transformation; 2) FePt grains embedded in MgO matrix were well separated, leading to minimized exchange interaction between the FePt grains; 3) 55vol% fraction of MgO was optimal for the L10-FePt phase formation with highest coercivity, due to the formation of granular structure, which was also verified by comparing the structure and properties of FePt/MgO bilayer films. Deposition temperature has also effect on the magnetic properties of FePt–MgO films. Room temperature deposition did not result in the L10 phase even after annealing at 800°C. However, the magnetic properties of the films deposited at 200°C were better than those deposited at 300°C. The low coercivity resulted from elevated deposition temperature was attributed to the thermal expansion and competitively growth among FePt and MgO grains. The results revealed that adding appropriate amount of MgO (55vol%) can be achieved by finding a proper deposition temperature (200°C), well-defined granular L10-FePt films with fine grain distribution, high coercivity and small domain size.

Universality of the electrical transport in granular metals.

The universality of the ac electrical transport in granular metals has been scarcely studied and the actual mechanisms involved in the scaling laws are not well understood. Previous works have reported on the scaling of capacitance and dielectric loss at different temperatures in Co-ZrO2 granular metals. However, the characteristic frequency used to scale the conductivity spectra has not been discussed, yet. This report provides unambiguous evidence of the universal relaxation behavior of Pd-ZrO2 granular thin films over wide frequency (11 Hz–2 MHz) and temperature ranges (40–180 K) by means of Impedance Spectroscopy. The frequency dependence of the imaginary parts of both the impedance Z″ and electrical modulus M″ exhibit respective peaks at frequencies ωmax that follow a thermal activation law, ωmax ∝ exp(T1/2). Moreover, the real part of electrical conductivity σ′ follows the Jonscher’s universal power law, while the onset of the conductivity dispersion also corresponds to ωmax. Interestingly enough, ωmax can be used as the scaling parameter for Z″, M″ and σ′, such that the corresponding spectra collapse onto single master curves. All in all, these facts show that the Time-Temperature Superposition Principle holds for the ac conductance of granular metals, in which both electron tunneling and capacitive paths among particles compete, exhibiting a well-characterized universal behavior.

Manifestation of unusual size effects in granular thin films prepared by pulsed laser deposition

We demonstrate manifestation of some rather unusual size effects in granular thin films prepared by a pulsed laser deposition technique. We observed that the temperature dependence of resistivity ρ(T) notably depends on the relation between the grain size Rg and the film thickness d. Namely, more granular LaNiO3 thin films (with small values of Rg) grown on LaAlO3 substrate are found to follow a universal ρ(T)∝T3/2 law for all the measured temperatures. While less granular thin films (with larger values of Rg), exhibit a more complicated behavior accompanied by a clear-cut crossover (around Tcr=200K), from ρ(T)∝T3/2 (for 20K<T<200K) to ρ(T)∝T (for 200K<T<300K). The obtained results are attributed to manifestation of the finite temperature size effects (when an average grain size Rg becomes comparable with the thermal de Broglie wavelength Λ) leading to the crossover temperature Tcr∝(d/Rg)2.

Nanoconstrictions for investigating spin transfer torque effect in granular Ag–Co films

AbstractWe used lithographically fabricated nanoconstrictions made of Ag–Co granular thin films to achieve current densities above 10 A/cm during magnetotransport measurements. Systematically measured variations of the resistance with current under high applied magnetic fields did not reveal any dramatically large magnetoresistance effect, in contrast to the previous observations using mechanical point contact based techniques. Our experimental results on resistance switching by current injection in the Ag–Co nanoconstrictions reveal spin torque effects. The switching amplitudes are considerably smaller when compared to the results obtained for pillar shaped trilayers due to the large number of Co grains in the nanoconstrictions. We demonstrate that the use of point contact patterns with nanometer dimensions, where effects resulting from current induced mechanical distortions at high magnetic fields are negligible, has obvious advantages over the mechanical point contact technique for studying spin injection effects.

The dependence of electromagnetic noise suppress characteristics on magnetic and high-frequency properties of FeCoSiO thin films

A series of soft magnetic granular FeCoSiO films is fabricated using radio frequency magnetron sputtering under pure argon ambient conditions. The magnetic, high-frequency, and microwave noise suppression characteristics of these thin films are studied in detail. By using microwave probe station, integrated-type electromagnetic noise suppressors based on a coplanar waveguide transmission line and incorporated with a SiO2 dielectric and a granular FeCoSiO magnetic thin film with excellent high frequency and soft magnetic properties are investigated. The relationship between transmission properties of electromagnetic noise suppressors and magnetic characteristics of FeCoSiO magnetic thin films were studied. The experimental values of the loss peak frequencies agree well with the calculated ferromagnetic resonant frequency, which considers the demagnetizing field. The results indicate that the main loss mechanism of the integrated devices is the ferromagnetic resonance, but not the L–C resonance. The FeCoSiO magnetic films are considered to be promising as ideal candidate materials for electromagnetic noise suppressor.

Microstructural, electronic and magnetic characterization of Fe-based nanoparticles embedded in Al matrix

Nominal granular iron oxide-aluminum thin films have been prepared by simultaneous deposition of iron oxide nanoparticles, grown by the gas-phase aggregation technique, and an aluminium matrix, grown by conventional magnetron sputtering. Composition, structure and magnetic behavior have been analyzed by different techniques including TEM and AFM microscopies, EDX, RBS, X-ray absorption and Mössbauer spectroscopies and SQUID magnetometry. Both, structure and magnetic behavior, are found to be highly dependent on the preparation conditions. In particular, our work shows that for low matrix/nanoparticle ratios the aluminum is able to partially displace the iron oxide and form a core–shell iron metal-iron oxide structure. For higher ratios, on the other hand, the oxygen atoms become very diluted and their role negligible. In this case a core–shell structure consisting of an iron metal core and an iron–aluminum alloy shell is formed. Magnetization measurements indicate that in the first case the core and the shell are magnetically coupled while in the second case the two phases are magnetically uncoupled, the Fe–Al alloy presenting strong coercivity.

Effect of Uniform Decoration of Ag2S Nanoparticles on Physical Properties of Granular TiO2 Thin Films Synthesized by Using Spin Coating Technique

Effect of Uniform Decoration of Ag2S Nanoparticles on Physical Properties of Granular TiO2 Thin Films Synthesized by Using Spin Coating Technique

Structural Characteristics and Magnetic Properties of Al2O3 Matrix-Based Co-Cermet Nanogranular Films

Magnetic micro- and nanogranular materials prepared by different methods have been used widely in studies of magnetooptical response. However, among them there seems to be nothing about magnetic nanogranular thin films prepared by a rf cosputtering technique for both metals and insulators till now. This paper presented and discussed preparation, structural characteristics, and magnetic properties of alumina (Al2O3) matrix-based granular Co-cermet thin films deposited by means of the cosputtering technique for both Co and Al2O3. By varying the ferromagnetic (Co) atomic fraction, x, from 0.04 to 0.63, several dominant features of deposition for these thin films were shown. Structural characteristics by X-ray diffraction confirmed a cermet-type structure for these films. Furthermore, magnetic behaviours presented a transition from paramagnetic- to superparamagnetic- and then to ferromagnetic-like properties, indicating agglomeration and growth following Co components of Co clusters or nanoparticles. These results show a typical granular Co-cermet feature for the Co-Al2O3 thin films prepared, in which Co magnetic nanogranules are dispersed in a ceramic matrix. Such nanomaterials can be applied suitably for our investigations in future on the magnetooptical responses of spinplasmonics.

Electric field control of magnetic properties and magneto-transport in composite multiferroics

We study magnetic state and electron transport properties of composite multiferroic system consisting of a granular ferromagnetic thin film placed above the ferroelectric substrate. Ferroelectricity and magnetism in this case are coupled by the long-range Coulomb interaction. We show that magnetic state and magneto-transport strongly depend on temperature, external electric field and electric polarization of the substrate. Ferromagnetic order exists at finite temperature range around ferroelectric Curie point. Outside the region the film is in the superparamagnetic state. We demonstrate that magnetic phase transition can be driven by an electric field and magneto-resistance effect has two maxima associated with two magnetic phase transitions appearing in the vicinity of the ferroelectric phase transition. We show that positions of these maxima can be shifted by the external electric field and that the magnitude of the magneto-resistance effect depends on the mutual orientation of external electric field and polarization of the substrate.

High frequency characteristics of (Ni75Fe2)x(ZnO)1−x granular thin films with tunable damping coefficient**Project supported by the National Natural Science Foundation of China (Grant Nos. 50901050 and 51101079).

The effect of the volume fraction of ferromagnetic metal (x) in (Ni75Fe25)x(ZnO)1−x nanogranular thin films on microstructural, soft-magnetic, and high-frequency properties was investigated. Good soft-magnetic properties were obtained in a broad x range, with 0.55 < x < 0.82. High resolution transmission electron microscopy (HRTEM) observations reveal that the grain size of the samples is lower than 14 nm, and that it decreases with decreasing x. Of special interest, our investigation of the permeability spectra indicates that these films exhibit an adjustable frequency linewidth of resonance peak, dependant upon changing x. Correspondingly, large and adjustable damping coefficients (αeff) from 0.023 to 0.043 were achieved by decreasing x from 0.82 to 0.55. Combined with the HRTEM results, the variation of αeff with x was analyzed in detail.

Controlling the microwave characteristic of FeCoB-ZnO granular thin films deposited by obliquely sputtering

A series of FeCoB-ZnO soft magnetic granular films deposited at different oblique angles were prepared by magnetron sputtering system. A variable in-plane uniaxial magnetic anisotropy field from 27.6 Oe to 212 Oe and an adjustable ferromagnetic resonance frequency from 1.89 GHz to 5.3 GHz were obtained in the as-deposited films just by increasing the oblique angle from 15° to 56°. Frequency line-width and effective Gilbert damping factor were both insensitive to the different oblique angles (αeff decreased from 0.036 to 0.03 and Δf decreased from 1.49 to 1.27), which almost satisfied the requirement that fFMR could be tuned independently in a certain frequency range. Besides, the change of dynamic magnetic anisotropy field versus oblique angle was illustrated and analyzed quantitatively compared with the static magnetic anisotropy.