Discontinuous Multilayers Research Articles

Effect of insulator matrix on electrical transport properties of iron-insulator discontinuous multilayers

This work presents the results of the study of structural and electrical transport properties of the iron-insulator discontinuous multilayers (DMIMs). SiO2, HfO2, and MgO were chosen as the materials for the insulating layers. The discontinuous multilayers were prepared by the sequential magnetron sputtering on ceramic substrates. Obtained systems were annealed at different temperatures in an Ar + N2(2 %) environment using an annealing furnace with a continuous gas flow. Crystal structure analysis showed that the effect of annealing on the crystal structure of the samples varies and depends on the annealing temperature and the type of insulator. It has been demonstrated that the formation of iron oxide during annealing can be reduced by using HfO2 as the insulating layer material. The resistivity and temperature coefficient of resistance variation with temperature and the effective thicknesses of the ferromagnetic layers were analyzed.

Size and heat treatment effects in the electrical conduction of the [Fe/SiO]n discontinuous multilayers

The present work shows the comprehensive investigations of crystal structural and electrical properties of [Fe/SiO]n discontinuous multilayers with various effective thickness of ferromagnetic (dFe) and insulator (dSiO) layers. Results of the study performed by the transmission electron microscopy have demonstrated that the formation of discontinuous magnetic layers occurs at the decrease of dFe from 10 nm to 4 nm. Namely, the discontinuous multilayers at dF = 4 nm are granular nanostructures that consist of an array of Fe particles separated by the SiO insulator channels. It was shown that depending on the heat treatment conditions and an effective thickness of ferromagnetic and insulator layers, three modes of conductivity (metallic, transitional and insulation) were realized. The temperature coefficient of resistance takes a positive, negative, or close to zero value depending on the conductivity mode. The correlation between the crystal structure and electrical properties were discussed.

Peculiarities of Magnetoresistance of [Fe/SiO]n Discontinuous Multilayers

The influence of the magnetic and insulator layer thickness on the crystal structure and magnetoresistive properties of [Fe/SiO]n discontinuous multilayers has been studied. The results indicate that the decreasing of Fe layer thickness within the range from 10 to 3 nm leads to the transition of microstructure of the samples from nanocrystalline to amorphous. For all investigated systems in an as-deposited state, the isotropic nature of the field dependences of magnetoresistance is observed. For samples with layer thickness dFe = 4–6 nm and dSiO = 5 nm, the transition to isotropic MR occurs after annealing to 400 K. However, for samples with Fe layer thickness of 7–10 nm, such transition is not observed regardless of the effective thickness of insulator layer and annealing temperature.

A Bilayer 2D-WS2/Organic-Based Heterojunction for High-Performance Photodetectors.

Two-dimensional (2D) tungsten disulfide (WS2) has inspired great efforts in optoelectronics, such as in solar cells, light-emitting diodes, and photodetectors. However, chemical vapor deposition (CVD) grown 2D WS2 domains with the coexistence of a discontinuous single layer and multilayers are still not suitable for the fabrication of photodetectors on a large scale. An emerging field in the integration of organic materials with 2D materials offers the advantages of molecular diversity and flexibility to provide an exciting aspect on high-performance device applications. Herein, we fabricated a photodetector based on a 2D-WS2/organic semiconductor materials (mixture of the (Poly-(N,N′-bis-4-butylphenyl-N,N′-bisphenyl) benzidine and Phenyl-C61-butyric acid methyl ester (Poly-TPD/PCBM)) heterojunction. The application of Poly-TPD/PCBM organic blend film enhanced light absorption, electrically connected the isolated WS2 domains, and promoted the separation of electron-hole pairs. The generated exciton could sufficiently diffuse to the interface of the WS2 and the organic blend layers for efficient charge separation, where Poly-TPD was favorable for hole carrier transport and PCBM for electron transport to their respective electrodes. We show that the photodetector exhibited high responsivity, detectivity, and an on/off ratio of 0.1 A/W, 1.1 × 1011 Jones, and 100, respectively. In addition, the photodetector showed a broad spectral response from 500 nm to 750 nm, with a peak external quantum efficiency (EQE) of 8%. Our work offers a facile solution-coating process combined with a CVD technique to prepare an inorganic/organic heterojunction photodetector with high performance on silicon substrate.

Probing the morphology of epitaxial Fe/MgO discontinuous multilayers by magnetometric technique

Abstract Discontinuous metal-insulator multilayers (DMIMs) are a special type of nanostructures with a layered arrangement of metallic particles sandwiched between continuous insulating layers. DMIMs exhibit moderate tunneling magnetoresistance (TMR) ratio but enhanced low-field sensitivity, which makes them promising candidates for magnetic field sensors. Recently we have grown epitaxial Fe/MgO DMIMs on MgO (0 0 1) single crystal substrates at different deposition conditions. Here, based on the analysis of magnetic isotherms in a broad temperature range, the effect of deposition temperature (TS) on microstructure of DMIMs is being studied and compared with the results of Transmission Electron Microscopy. It is shown that metallic layers consist of flat nanoparticles whose average size decreases, and their crystallinity improves with the increase of TS.

Fabrication and optical properties of nanostructured plasmonic Al2O3/Au–Al2O3/Al2O3 metamaterials

Discontinuous multilayer (DML) thin films, which consist of nano-granular metals (NGMs) embedded in a dielectric matrix, have attracted significant interest as engineered plasmonic metamaterials. In this study, a systematic layer-by-layer deposition of three-dimensional sub-wavelength periodic plasmonic DML structures via the radio frequency sputtering of a composite target has been reported. The overall optical response of the DML films composed of Au–Al2O3 NGM homogenous layers, which are periodically sandwiched between two amorphous Al2O3 layers, are studied using reflection spectroscopic ellipsometry and transmission spectroscopy techniques. By applying the analytical optical approaches based on multiple Gaussian oscillators, ambient DML sub-wavelength structures have been successfully modeled. As a result, the effects of the size and shape of the Au nanoparticles as well as of the surrounding and interfacial media on their localized surface plasmon resonance (LSPR) are elucidated, and the related films thickness and effective optical constants are determined. Interestingly, during the examination of resonance frequencies and dielectric functions, the obtained DML structures exhibit unusual characteristics that are different from those of their NGM constituents due to the electromagnetic interactions of the NGM layers with the LSPR, which represent metamaterial features.

Ferroic superglasses: Polar nanoregions in relaxor ferroelectric PMN versus CoFe superspins in a discontinuous multilayer

Superdipolar glass properties of polar nanoregions (PNRs) in relaxor ferroelectric $\mathrm{PbM}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}{\mathrm{O}}_{3}$ (PMN) are compared to those of ferromagnetic nanoparticles (FNPs) in the superspin glass ${[\mathrm{C}{\mathrm{o}}_{80}\mathrm{F}{\mathrm{e}}_{20}(0.9\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}(3\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{10}$. Both the dynamic critical properties at $Tg{T}_{g}$ and nonergodicity phenomena at $Tl{T}_{g}$ are comparable, but this does not apply to their polydispersivity at $T\ensuremath{\le}{T}_{g}$. Due to the quenched random size and position distribution of the FNP, the superspin glass exhibits standard Cole-Cole broadening of the spectrum of relaxation frequencies at all temperatures. Contrastingly, the relaxation spectrum of the PNRs in PMN is subject to a crossover from Lacroix-B\'en\'e to Cole-Davidson statistics on cooling toward ${T}_{g}$. Surprisingly, it becomes replaced by relaxation and creeplike domain-wall dynamics below ${T}_{g}$ as a consequence of volume percolation of the PNR at ${T}_{g}$ and formation of a ferroelectric microdomain state under the simultaneous control by quenched random electric fields and the softening ferroelectric ${F}_{1u}$ lattice mode.

Magnetic properties of ultrathin discontinuous Co/Pt multilayers: Comparison with short-range ordered and isotropicCoPt3films

Magnetic properties of thin Co/Pt multilayers have been investigated in order to study the dependence of magnetization $M$, uniaxial anisotropy ${K}_{u}$, and Curie temperature ${T}_{\mathrm{C}}$ on the multilayer thickness, composition, and structure. A comparison between epitaxial submonolayer multilayers and epitaxial fcc ${\mathrm{CoPt}}_{3}$ alloy films with large perpendicular magnetic anisotropy (PMA) attributed to growth-induced Co clustering reveals significant differences in the temperature dependence of magnetization $M(T)$, despite the presence of thin planar Co platelets in both cases. Even the thinnest discontinuous multilayered structure shows a Langevin-like $M(T)$, while the alloy films with PMA show a broadened and enhanced $M(T)$ indicating a distribution of environments, including monolayer Co platelets separated by only 1--2 layers of Pt. These differences have been reproduced in Monte Carlo simulations, and are shown to be due to different distributions of Co-Co and Co-Pt nearest neighbors. The relatively uniform Co-Co coordination of even a discontinuous rough multilayer produces a Langevin-like $M(T)$, whereas the broader distribution associated with platelets in the PMA films results in a nearly linear $T$ dependence of $M$.

Pressure dependence of the magneto-transport properties in Fe/MgO granular systems

The effect of hydrostatic pressure at room temperature on the resistance and magnetoresistance (MR) of a discontinuous metal-insulator multilayer (DMIM) of nominal composition [Fe(t Fe = 0.7 nm)/MgO(t MgO = 3 nm)]15 has been studied. The resistivity of the DMIM, ρ, decreases linearly with pressure indicating an increase in conduction via tunneling effect. The value of coefficient (1/ρ 0)dρ/dP = −3.9 × 10−2 kbar−1 is higher than reported values in other granular films implying that the electronic state of the DMIM is close to the iron percolation threshold. At the maximum applied magnetic field (3 kOe) the MR ratio increases from 0.6% at ambient pressure to 1.1% at 7 kbar. This result can be explained by a reduction of the tunnel barrier width induced by the hydrostatic pressure.

Magnetic Properties of Epitaxial Discontinuous Fe/MgO Multilayers

We report magnetic, dynamic and transport properties of discontinuous metal-insulator multilayers Fe/MgO grown on amorphous Corning glass and single-crystalline MgO (001) substrates. The films of structure Substrate/MgO (3 nm)/[Fe (0.6 nm)/MgO (3.0 nm)] x 10 were prepared in ultra-high vacuum conditions using Pulsed Laser Deposition. It was shown that conditions of epitaxial growth are favorable for MgO substrates. As a result a substantial increase of tunneling magnetoresistance caused by spin-filtering effect was observed and reasonably theoretically explained. The value of TMR - 9.2% at room temperature in 18 kOe magnetic field is three times higher comparing to that for the samples grown on Corning glass substrates.

Effect of spacer layer thickness on optical properties of [SnO 2/Mn] n and [SnO 2/Co] n discontinuous multilayers

Multilayers (ML) of [SnO 2/Mn] n and [SnO 2/Co] n were fabricated by rf sputtering. The effect of spacer layer (SnO 2) thickness, x on the optical properties of these ML was studied. The transmittance initially decreases as x increases from 12 to 24 Å and then decreases as x increases to 36 Å. Similar trend is followed by band gap. For [SnO 2/Mn] ML, it decreases from 4.47 to 3.16 eV then increases to 4.28 eV and for [SnO 2/Co] ML, it decreases from 4.06 to 3.75 eV then increases to 4.27 eV as x changes from 12 to 36 Å. This variation is attributed to the quantum interference effects in the SnO 2 layers. The ML with x = 24 Å is found to have low defect concentration compared to the ML with x = 12 and 36 Å. The optical properties of the ML are more sensitive to the SnO 2 layer thickness than that of Mn or Co. The variation of optical properties of the present ML is an artifact of the solubility limit of Mn or Co in the interface layers.

Soft magnetic multilayer films with wide frequency band losses and high resistivity

AbstractA series of CoFeB/MgO multilayers was fabricated by means of DC/RF magnetron sputtering. It is observed that the electromagnetic properties of the multilayers of CoFeB/MgO are closely related with the thickness of CoFeB sublayers, and the soft magnetic properties of discontinuous multilayers are affected by the cycle number of CoFeB/MgO bilayers. The optimized film of [Co64Fe24B12(0.7 nm)/MgO(0.4 nm)]50 exhibits a magnetic loss µ″ larger than 100 and µ′ larger than 80 between 0.9 and 2.1 GHz. The electrical resistivity ρ of this film is as high as 3.75 × 103 µΩ cm. The results indicate that this discontinuous CoFeB/MgO multilayer film has potential to serve as a noise suppressor or EMC materials in the GHz frequency range.

Magnetic field strength and orientation effects on Co-Fe discontinuous multilayers close to percolation

Magnetization and magnetoresistance in function of the magnitude and orientation of applied magnetic field were studied in Co-Fe discontinuous multilayers close to their structural percolation. The high pulsed magnetic fields up to 33 T were used in the 120--310 K temperature range. Comparison between longitudinal and transverse (with respect to the film plane) field configurations was made in the low-field and high-field regimes in order to clarify the nature of the measured negative magnetoresistance. Coexistence of two distinct magnetic fractions, superparamagnetic (SPM, consisting of small spherical Co-Fe granules) and superferromagnetic (SFM, by bigger Co-Fe clusters), was established in this system. These fractions were shown to have different relevance for the system magnetization and magnetotransport. While the magnetization is almost completely (up to $\ensuremath{\sim}97\mathrm{%}$) defined by the SFM contribution and practically independent of temperature (in this range), the magnetoresistance experiences a crossover from a regime dominated by Langevin correlations (suppressed with temperature) between neighbor SPM and SFM moments at low fields, to that dominated by spin scattering (enhanced with temperature) of charge carriers within SFM clusters at high fields. Also, the demagnetizing effects, sensitive to the field orientation, were found to essentially define the low-field behavior and characteristic crossover field.

Thickness dependence of magnetic and transport properties in organic-CoFe discontinuous multilayers

Spin-dependent transport measurement in 3-hexadecyl pyrrole (3HDP) with a CoFe layer and the current-in-plane geometry is reported. Transport properties indicate the CoFe layers are discontinuous when their thicknesses are smaller than 6 nm. The temperature dependence of the conductance suggests that the transport mechanism is likely small polaron hopping. The observed positive magnetoresistance ratio at low temperature gives evidence of spin-conserving transport.

Temperature dependence of the high-field magnetization in Fe-Ag granular alloys and discontinuous multilayers

Fe-Ag co-deposited granular alloy and discontinuous multilayer samples with 10-36 at% Fe average compositions were prepared by vacuum evaporation onto Si(111) single crystal substrates at room temperature. The magnetic properties of the samples were studied by SQUID and in-field Mössbauer spectroscopy. Superparamagnetic behaviour was observed for all the samples with blocking temperatures (TB) steeply increasing with increasing Fe concentration. The bulk magnetization (M) measured in 5 T shows linear temperature dependence for both types of samples, and the relative decrease with temperature is much larger for a granular alloy than for a multilayer sample of equal TB. The Mössbauer spectroscopy results indicate that the faster decrease of the high field magnetization in granular alloys is explained by the larger amount of small grains that are not aligned by the 5 T field.

X-ray absorption spectroscopy study of granular Fe / Si3N4 systems

[Fe (tFe) / Si3N4 (3 nm)] multilayers were prepared by sequential magnetron sputtering. The Fe layer thickness has been varied for every sample. Magnetic properties show a gradual evolution from a ferromagnetic state for samples with larger metal thickness to a granular behaviour for the samples with the smallest Fe layer thickness. Microstructural features such as average thickness and granularity, as well as the local order around Fe atoms, were studied by x-ray reflectometry and x-ray absorption spectroscopy, respectively. X-ray reflectometry suggests that the formation of discontinuous metal-insulator multilayers is produced at tFe ≤ 1.3 nm. Extended x-ray absorption fine structure (EXAFS) analysis shows a reduction, of the Fe-Fe coordination shell as the metal layer thickness decreases. Moreover, a new phase emerges, and it is visible at the samples with tFe ≤ 1.3 nm. This coordination shell is attributed to the formation of Fe-N bonds likely placed at the interface regions. X-ray absorption near edge spectroscopy (XANES) at the Fe K-edge shows as well the evolution from the metallic spectrum to a combined contribution of 2 phases as the metal layer thickness decreases. XANES calculations performed within the real-space multiple-scattering formalism of two nanometric phases for metal bcc Fe and tetrahedral FeN in zinc-blende structure provide a successful explanation of the XANES spectral evolution. The appearance of a new phase linked to the interface regions obtained by X-ray absorption analysis suggests the granular morphology of samples with tFe ≤ 1.3 nm.

Electromagnetic and microwave properties of discontinuous CoFeB/MgO soft magnetic multilayer films

A series of discontinuous multilayers of CoFeB-MgO was fabricated by using DC/RF magnetron sputtering, and the electromagnetic and microwave properties were investigated. The results show that the microstructure and electromagnetic properties of the CoFeB-MgO discontinuous multilayer films can be altered by varying the relative content of the magnetic CoFeB phase and the insulating MgO phase. High performance microwave soft magnetic properties along with high resistivity are achieved in the discontinuous ［Co64Fe24B12(0.7nm)/MgO(0.4nm)］40 multilayer film. This film has the saturation magnetization of 1.3 T, hard axis coercivity of 130 A/m, and resistivity of 3.4 mΩ·cm. Moreover, the resonance frequency is about 2.1 GHz for this film. Both the real part μ′ and the imaginary part μ″ of the measured permeability are larger than 240 at 1.59 GHz and are both larger than 100 from 0.9 GHz to 2 GHz. So this film has potential for serving as the microwave absorbers and electromagnetic compatibility materials in the GHz frequency range.

FMR IN NANOSYSTEMS — DISCONTINUOUS MULTILAYERS Fe/SiO2/Fe

We present the study on the interlayer and intergranular exchange and dipolar coupling in [Fe 97 Si 3/ SiO 2]5 discontinuous multilayers by means of ferromagnetic resonance. Due to strong ferromagnetic exchange coupling (J~-3 erg/cm 2) the precessional motions of magnetic moments of granules are coupled and results in an acoustic and optical mode. Moreover there is notable line splitting in optical mode under external field normal to the layer, which is explained by an interlayer dipolar coupling, only possible for discontinuous layer. Some aspects of the damping processes in discontinuous multilayers are discussed as well.

Universal Domain Wall Dynamics in Disordered Ferroic Materials

The dynamics of driven domain walls (DWs) is studied in disordered uniaxial ferroic materials such as periodically poled ferroelectric KTiOPO4; quantum ferroelectric SrTi18O3; relaxor ferroelectric Sr0.61Ba0.39Nb2O6(SBN); ultrathin ferromagnetic multilayers Pt/Co/Pt; and discontinuous magnetic metal-insulator multilayers [Co80Fe20/Al2O3]10(DMIM) with ac susceptibility, [Formula: see text], and domain imaging. Sideways creep and slide motion, well known from dc excitation, is mapped onto characteristic dynamic modes in the ac spectra or, more significantly, in Cole-Cole presentations, [Formula: see text]. Creep and slide are complemented by switching at high enough field amplitudes and by DW segmental relaxation at high enough frequencies. The dynamic relaxation-to-creep transition is well understood within scaling theory, whereas the crossover between the modes creep, slide, and switching is numerically modeled on the basis of the Edwards-Wilkinson equation of DW motion. Clear signatures of all dynamical modes are even found at fractal DWs in dipolarly controlled DMIMs and in polar nanoregions of random-field-controlled SBN above Tc.

Ferromagnetic resonance study of exchange and dipolar interactions in discontinuous multilayers

We studied interlayer and intergranular exchange and dipolar coupling in (Fe97Si3∕SiO2)5 discontinuous multilayer (DM) by means of ferromagnetic resonance (FMR). Due to a strong ferromagnetic exchange coupling (J∼−3ergs∕cm2), the precessional motions of magnetic moments of the granules are coupled, which results in acoustic and optical modes. Moreover, there is a notable FMR line splitting in the optical mode under the external field normal to the DM, which is explained by an interlayer dipolar coupling, only possible for the DMs. The detailed structural analysis and electrical measurements confirmed that the metal nanoparticles were separated by an insulating matrix, and the single-electron transport phenomena (Coulomb blockade and tunneling magnetoresistivity) were observed at low temperatures (77–100K).