Ultrathin Cr Research Articles

Growth and surface magnetism of ultrathin Cr(001) films

We investigate the growth of ultrathin Cr films on a Au(001) surface and observe that the growth of 1.5 nm thick Cr layers at 290 K, followed by post-annealing at 520 K, results in high-quality epitaxial Cr(001) films with atomically flat large terraces and distinct surface states. Subsequently, these optimized growth conditions are successfully applied to the growth of 1 nm and 3 nm thick Cr films. Magnetic imaging of 1 and 1.5 nm thick Cr(001) films prepared under the optimized growth conditions is performed using spin-polarized scanning tunneling microscopy. Distinct magnetic contrasts featuring a topological antiferromagnetic (TAF) order are observed in both films; however, spin frustration originating from the density of screw dislocations for both films shows a significant difference. The 1.0 nm thick Cr film, which exhibits a clear TAF order with the suppression of a large spin-frustrated area, is suitable for application to spin-electronic devices.

Growth of Magnetron-Sputtered Ultrathin Chromium Films: In Situ Monitoring and Ex Situ Film Properties

We report a systematic nanoscale investigation on the ultrathin Cr film growth process and properties. Polycrystalline metallic films were manufactured by magnetron sputtering on fused silica substrates. The film growth was observed in situ by broad-band optical monitoring (BBM) and plasma-emission spectroscopy (OES) methods. The ex situ characterization of the Cr films with thicknesses varying from 2.6 nm up to 57 nm were performed by both non-destructive and destructive techniques. Recently, we reported on a novel set of data for optical and electrical properties of sputtered chromium films. The optical and electrical properties of the films are known to be governed by their structure and microstructure, which were analyzed in detail in the present research. Moreover, the optical properties of the films were studied here in a significantly wider optical range and obtained using both in situ and ex situ measurements. Reliable in situ nanoscale characterization of metal films was shown to ensure an unfailing approach in obtaining ultrathin layers with desirable thickness and stable and well-determined optical constants and electrical conductivity. This is of high importance for various industries and novel upcoming applications.

An exchange bias observed in Tb/Cr/FeCo trilayers with ultrathin Cr layer at low temperature

Positive exchange bias field (He) is observed in Tb/Cr (tCr)/FeCo trilayers at 5 K without cooling field, and negative He for Tb/FeCo bilayer. The negative He of Tb/FeCo implies the FM coupling at the interface due to Co and Fe dominate in the magnetization of the ferrimagnetic interlayer alloy of FeCo and Tb. With the inserting of Cr layer, this situation is broken, and the positive He implies the antiferromagnetic interlayer coupling. A peak of He = 6.0 mT for trilayers with tCr = 1.5 nm is corresponding to the minimum value of coercivity as a function of tCr at 5 K, which is used to study the effect of the cooling field (Hfc) on He as a function of temperature. It is found that Hfc of 100 mT triggers He from positive to negative at T ≤ 15 K. The magnetoresistance results also confirm the coexistence of multiple MR mechanisms in these trilayers.

Glass poling as a substrate surface pre-treatment for in situ metal nanoparticle formation by reduction of metal salt

Metal nanoparticles are used in optical coatings and sensors due to their absorption in optical part of spectrum and its sensitivity to the environment induced by localized surface plasmon resonance. Glass is the most common substrate used for optical coatings. However, its surface does not have optimal properties for coating with metal nanoparticles grown in situ by reduction of metal salt. Glass surface optimization methods may involve environmentally hostile chemicals or processes that have time limited or atmosphere sensitive effects. In this study it is demonstrated and discussed effectiveness, mechanisms and advantages of glass poling as pre-treatment method for improving glass surface properties for maximization of coatings plasmonic performance. Pre-treatment of glass surfaces by poling is highly efficient for the purpose. Glass poling quenches ion exchange between metal ions from the solution and alkali ions from glass, favouring nanoparticles formation. Surface prepared in such way is not affected by ageing in normal atmosphere and is effective even after coating with ultrathin dielectric or Cr layers.

Mid-Infrared Response from Cr/n-Si Schottky Junction with an Ultra-Thin Cr Metal.

Infrared detection technology has been widely applied in many areas. Unlike internal photoemission and the photoelectric mechanism, which are limited by the interface barrier height and material bandgap, the research of the hot carrier effect from nanometer thickness of metal could surpass the capability of silicon-based Schottky devices to detect mid-infrared and even far-infrared. In this work, we investigate the effects of physical characteristics of Cr nanometal surfaces and metal/silicon interfaces on hot carrier optical detection. Based on the results of scanning electron microscopy, atomic force microscopy, and X-ray diffraction analysis, the hot carrier effect and the variation of optical response intensity are found to depend highly on the physical properties of metal surfaces, such as surface coverage, metal thickness, and internal stress. Since the contact layer formed by Cr and Si is the main role of infrared light detection in the experiment, the higher the metal coverage, the higher the optical response. Additionally, a thicker metal surface makes the hot carriers take a longer time to convert into current signals after generation, leading to signal degradation due to the short lifetime of the hot carriers. Furthermore, the film with the best hot carrier effect induced in the Cr/Si structure is able to detect an infrared signal up to 4.2 μm. Additionally, it has a 229 times improvement in the signal-to-noise ratio (SNR) for a single band compared with ones with less favorable conditions.

Ultrabroadband and ultrathin absorber based on an encapsulated T-shaped metasurface.

Ultrabroadband absorbers are vital for applications such as solar energy harvesting and integrated optoelectronic devices. Herein, we design, fabricate and characterize a novel ultrabroadband and ultrathin absorber based on the encapsulated T-shaped metasurface (ETM). The ETM consists of a 20 nm Cr film and a Cr substrate sandwiched by the T-shaped polymethyl methacrylate (PMMA) arrays. The Cr film provides a robust absorptive surface with improved impedance matching, and ultrabroadband absorption can be achieved via the excitation of the localized surface plasmon (LSP) of this ultrathin film. The average absorption of simulated and experimental results of the ETM in the visible range of 400-800 nm for the TM (TE) polarization are 96.4% (96.3%) and 90.6% (89.4%), respectively. Three-dimensional (3D) power dissipation density distributions of the proposed structure have been investigated, which indicates that the synergistic absorption effect of different parts of the T-shaped ultrathin Cr film contributes to the major absorption enhancement. The absorption of the ETM is very robust to the changes of geometrical parameters and the symmetry of the structure, and it can be maintained almost the same even if T-shaped profiles are changed to L-shaped profiles. Moreover, the absorption performance of the ETM exhibits polarization-insensitive and wide-angle features, which has advantages for many potential applications.

Accurate Design of Solar Selective Absorber Based on Measured Optical Constants of Nano-thin Cr Film

Solar selective absorbers have significant applications in various photothermal conversion systems. In this work, a global optimization method based on genetic algorithm was developed by directly optimizing the solar photothermal conversion efficiency of a nano-chromium (Cr) film-based solar selective absorber aiming to work at the specified working temperature and solar concentration. In consideration of the semi-transparent metal absorption layer employed in multilayered solar selective absorbers, the optical constants of ultrathin Cr film were measured by spectroscopic ellipsometer and introduced into the optimization process. The ultrathin Cr film-based solar selective absorber was successfully designed and fabricated by the magnetron sputtering method for the working temperature at 600 K and a solar concentration of 1 Sun. The measured reflectance spectra of the sample show a good agreement with the numerical simulations based on measured optical constants of ultrathin Cr film. In comparison, the simulated results by using the optical constants of bulk Cr film or literature data exhibit a large discrepancy with the experimental results. It demonstrates the significance of considering the actual optical constants for the semi-transparent metal absorption layer in the design of nano-metal film-based solar selective absorber.

A Highly Efficient Visible Absorber Coating on a Curved Substrate

In this study, we propose and fabricate a perfect absorber on a planar substrate using alternate silicon dioxide and ultrathin metallic lossy chromium (Cr) films. Furthermore, we transfer the absorber to a curved substrate via an optimization design of symmetric structures. The absorber exhibits a highly efficient absorption and large incident-angular tolerance characteristics in the whole visible region. We investigate each layer contribution to the absorption theoretically, and find that ultrathin (~5 nm) lossy Cr films play a dominant absorptive role. Using the effective interface method, we calculate the phase difference on the lossy Cr front surface. It is close to the destructive interference condition, from which we clarify why the proposed structures can produce a highly efficient absorption.

Graphene as an Ideal Buffer Layer for the Growth of High-Quality Ultrathin Cr2O3 Layers on Ni(111).

Metal-oxide nanostructures play a fundamental role in a large number of technological applications, ranging from chemical sensors to data storage devices. As the size of the devices shrinks down to the nanoscale, it is mandatory to obtain sharp and good quality interfaces. Here, it is shown that a two-dimensional material, namely, graphene, can be exploited as an ideal buffer layer to tailor the properties of the interface between a metallic substrate and an ultrathin oxide. This is proven at the interface between an ultrathin film of the magnetoelectric antiferromagnetic oxide Cr2O3 and a Ni(111) single crystal substrate. The chemical composition of the samples has been studied by means of X-ray photoemission spectroscopy, showing that the insertion of graphene, which remains buried at the interface, is able to prevent the oxidation of the substrate. This protective action leads to an ordered and layer-by-layer growth, as revealed by scanning tunneling microscopy data. The structural analysis performed by low-energy electron diffraction indicates that the oxide layer grown on graphene experiences a significant compressive strain, which strongly influences the surface electronic structure observed by scanning tunneling spectroscopy.

Influence of ultrathin chromium adhesion layer on different metal thicknesses of SPR-based sensor using FDTD

Chromium (Cr) layer with ultrathin thickness plays significant role as adhesion layer to attach the prism with various metallic materials in a Surface Plasmon Resonance (SPR) sensor. In this research, the presence of fixed 0.5nm Cr adhesion layer has been investigated to evaluate its influence to reflectivity, resonance angle (θRes) and full-width at half maximum (FWHM) for different thickness of Gold (Au), Silver (Ag) and Copper (Cu) in the range of 20-60nm using finite-difference-time domain (FDTD) analysis. The results show the intensity of the maximum reflectivity (Rmax) decreased to 3-5% and the minimum reflection intensity (Rmin) changed from 2-14% depending on the thickness of the metallic layer. The lowest Rmin for each metals contributed by the presence of Cr adhesion layer was 6.06% 40nm Au, 11.7% 30nm Ag and 0.26% 40nm Cu. However, the resonance angle is almost same for all metals without influenced by the presence of ultrathin Cr adhesion layer. The optimum thickness of 50nm Au shows the same and highest sensitivity by 80.56°RIU for structure I and II. The FWHM increased when Cr adhesion layer exist and broadens the SPR curve. Therefore, the ultra-thin chromium layer has influenced the performance of the SPR sensor and highly dependent on the thickness of the metal.

Optimization of Mo/Cr bilayer back contacts for thin-film solar cells.

Molybdenum (Mo) is the most commonly used material as back contact in thin-film solar cells. Adhesion of Mo film to soda–lime glass (SLG) substrate is crucial to the performance of solar cells. In this study, an optimized bilayer structure made of a thin layer of Mo on an ultra-thin chromium (Cr) adhesion layer is used as the back contact for a copper zinc tin sulfide (CZTS) thin-film solar cell on a SLG substrate. DC magnetron sputtering is used for deposition of Mo and Cr films. The conductivity of Mo/Cr bilayer films, their microstructure and surface morphology are studied at different deposition powers and working pressures. Good adhesion to the SLG substrate has been achieved by means of an ultra-thin Cr layer under the Mo layer. By optimizing the deposition conditions we achieved low surface roughness, high optical reflectance and low sheet resistivity while we could decrease the back contact thickness to 600 nm. That is two thirds to half of the thickness that is currently being used for bilayer and single layer back contact for thin-film solar cells. We demonstrate the excellent properties of Mo/Cr bilayer as back contact of a CZTS solar cell.

Magnetic properties tailored by inserting ultrathin Cr layer in Gd/Ni80Fe20 film

The magnetic properties of Gd(4 nm)/Cr()/(12 nm) films with different thicknesses of Cr layer prepared by magnetron sputtering were investigated. X-ray diffraction proves the coexistence of hexagonal closed-packed (hcp) (0 0 2) and face-centred cubic (fcc) (1 1 1) peaks in the Gd layer. Magnetic measurements show a modulation akin to RKKY coupling in the saturation magnetization () as a result of the coupling sign between Gd and Py layers oscillating with . The strained hcp phase and, in particular, the fcc phase lead to poor magnetic moments and strongly reduced saturation of Gd, which can also be affected by the possible magnetically dead layers, the intermixing at interfaces and a presumably more gradual transition to adjacent phases. Theoretical fittings of the relationship between the ferromagnetic resonance field () and the microwave frequency yield the effective magnetization (4) and perpendicular anisotropy constant (), which are all enhanced by introducing Gd and Cr layers as a result of the increased interfaces. Interestingly, with the increasing thickness of Cr spacer layer, an obvious decrease in the magnetic damping coefficient () is observed, from which we infer that both the spin–orbit interaction due to the interlayer couplings and the electron density of states (DOS) at the Fermi energy (EF) resulting from the intermixing at the interfaces of Gd/Cr/ film are the major origins. In addition, all magnetic parameters are approaching to those of a Py film with increasing to 3 nm.

Combined spectroscopic and ab initio investigation of monolayer-range Cr oxides on Fe(001): The effect of ordered vacancy superstructure

We investigated the electronic structure of an ultrathin Cr oxide film prepared by growing about 0.8 monolayers of Cr on the oxygen-terminated $\mathrm{Fe}(001)\text{\ensuremath{-}}p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ surface and characterized by the formation of an ordered array of Cr vacancies producing a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R{27}^{\ensuremath{\circ}}$ superstructure. We combined experimental techniques such as angle- and spin-resolved photoemission spectroscopy, low-energy electron diffraction, and scanning tunneling spectroscopy with ab initio calculations, focusing on (i) the peculiar energy dispersion of $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ states and (ii) the orbital and spin character of $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}3d$ states. We show that the experimental $\mathrm{O}\phantom{\rule{0.16em}{0ex}}2p$ dispersion can be related to the presence of an ordered vacancy lattice. The comparison with the existing literature on the oxidation of bulk Cr(001), where a network of Cr vacancies with a short-range crystallographic order is present, reveals a similar effect on O states. The valence electronic structure of the Cr oxide layer is mostly composed by spin-minority Cr states, consistent with an antiferromagnetic coupling with the Fe substrate.

Influence of Cr layer thickness on the static and dynamic performances of Tb/Cr/Ni80Fe20 structure

Using the vibrating sample magnetometer and ex situ vector network analyzer ferromagnetic resonance, the static and dynamic magnetic properties of Tb (4 nm)/Cr (tCr)/Ni80Fe20 (12 nm) trilayers with different thickness of Cr layer (from 0.2 to 4.0 nm) were investigated comparing with those of Ni80Fe20 film. From the results of in-plane static magnetic hysteresis loops, we analyzed the soft magnetic properties and in-plane anisotropy, along with obtaining the saturation magnetization (Ms) which shows no obvious reducing and a modulation akin to RKKY coupling. By fitting the relationship of resonance field and linewidth versus resonance frequency, the effective magnetization 4πMeff, gyromagnetic ratio γ, and effective damping coefficient (αeff) were extracted. It is worthwhile to mention that the inserting of ultrathin Cr layer (0.2 nm) enhanced the above parameters obviously, which then backed down towards their values in Ni80Fe20 film with increasing the thickness of Cr layer as a result of the weakening magnetic proximity effect and the decreasing spin-pumping effect in trilayers. Finally, they became flattening with the Cr layer thicker than 1 nm where oscillations are also observed in the value of γ and αeff. Interestingly, the oscillations in αeff and γ are consistent with each other, which implies the damping mechanism mainly related to the spin-orbit coupling originating from the indirect interaction between Tb and Ni80Fe20. In addition, the inhomogeneous broadening (ΔH0) were determined to be very small which further ascertain the accurate of the αeff.

Gap and spin texture engineering of Dirac topological states at theCr−Bi2Se3interface

The presence of an exchange field in topological insulators reveals novel spin related phenomena derived from the combination of topology and magnetism. In the present work we show the controlled occurrence of either metallic or gapped topological Dirac states at the interface between ultrathin Cr films and the Bi$_2$Se$_3$ surface. The opening and closing of the gap at the Dirac point is caused by the spin reorientation transitions arising in the Cr films. We find that atom thin layers of Cr adhered to Bi$_2$Se$_3$ surfaces present a magnetic ground state with ferromagnetic planes coupled antiferromagnetically. As the thickness of the Cr film increases stepwise from one to three atomic layers, the direction of the magnetization changes twice from out-of-plane to in-plane and to out-of-plane again. The out of plane magnetization drives the gap opening and the topological surface states acquire a circular meron spin structure. Therefore, the Cr spin reorientation leads to the metal/insulator transition in the Bi$_2$Se$_3$ surface and to the correlated modification of the surface state spin texture. Consequently, the thickness of the Cr film provides an effective and controllable mechanism to modify the metallic or gapped nature, as well as the spin texture of the topological Dirac states.

Oxygen-induced immediate onset of the antiferromagnetic stacking in thin Cr films on Fe(001)

We investigated the magnetic coupling of ultra-thin Cr films grown at 600 K on a Fe(001)-p(1 × 1)O substrate by means of spin-polarized photoemission spectroscopy. Our findings show that the expected antiferromagnetic stacking of the magnetization in Cr(001) layers occurs right from the first atomic layer at the Cr/Fe interface. This is at variance with all previous observations in similar systems, prepared in oxygen-free conditions, which always reported on a delayed onset of the magnetic oscillations due to the occurrence of significant chemical alloying at the interface, which is substantially absent in our preparation.

Structure of Cr monolayer on Ag(001): A buried two-dimensionalc(2×2)antiferromagnet

The growth, morphology, and magnetic structure of ultrathin Cr films grown on a Ag(001) substrate are studied using low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), and ab initio density functional theory (DFT) calculations. The presence and temperature dependence of $c(2\ifmmode\times\else\texttimes\fi{}2)$ half-order spots in the LEED pattern, for low electron energies, along with the presence of characteristic Cr $3d$ bands in the ARPES spectra, confirm the existence of antiferromagnetic ordering for the Cr monolayer case. Our experiments are also consistent with the presence of a $p(1\ifmmode\times\else\texttimes\fi{}1)$ Ag overlayer on top of the Cr layer, suggesting the existence of a Ag/Cr/Ag(001) sandwich structure at the surface. Our DFT calculations confirm that this is the most favored geometric and magnetic structure of the system. The Cr layer is found to retain a ``two-dimensional'' character with enhanced Cr $3d$ magnetic moments, despite being buried below a Ag monolayer, due to the absence of significant hybridization between Cr $3d$ and Ag $4d$ electronic states. The coverage dependence of the magnetic ordering indicates a maximum ordering above the expected monolayer coverage, possibly due to intermixing between Ag and Cr atoms in the overlayer.

Electron spectroscopy investigation of the oxidation of ultra-thin films of Ni and Cr on Fe(0 0 1)

We investigated the room temperature oxidation of ultra-thin Ni and Cr films grown on Fe(0 0 1). In particular, we characterized the degree of crystallinity and the stoichiometry of the oxide layers and addressed the chemical stability of the interface with the highly reactive Fe substrate by means of low-energy electron diffraction and x-ray and UV photoemission spectroscopy. In the Ni case we detected, upon oxidation, the formation of a Fe3O4 layer covering the Ni oxide, due to the diffusion of Fe cations towards the surface. At high temperature and in ultra-high vacuum conditions, the Ni oxide dissolved and the Fe oxide layer was reduced to FeO. In the Cr case, we observed the formation of a thin Cr2O3 oxide layer, showing a diffraction pattern compatible with a defective γ-Cr2O3 phase. A thicker Cr oxide layer could be produced by oxidizing the sample at 300 °C, at the expense of the incorporation of trace amounts of Fe cations.

Oxidation effects on ultrathin Ni and Cr films grown on Fe(001): A combined scanning tunneling microscopy and Auger electron spectroscopy study

The effects of oxygen exposure on ultrathin films of Ni and Cr deposited on Fe(001) are studied by means of low energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy.In the case of the Ni/Fe(001) oxidation we find that, since the first stages of oxygen exposure, Fe atoms segregate toward the sample surface, where their oxidation takes place. Post-annealing treatments in ultra-high-vacuum conditions promote the Fe oxidation and improve the order of the overlayer, leading to the stabilization of an atomically flat FeO(111)-like film covering the sample.On the other hand, the oxidation of the Cr/Fe(001) follows a different path. At low oxygen exposure only the Cr film is oxidized, while upon increasing the oxygen dose Fe oxidation takes place. The morphology resulting from the oxidation of such a system is rough and successive thermal treatments do not result in surface ordering.

Magnetic properties of monolayer range chromium oxides on Fe(001)

Cr ultrathin films have been prepared on the oxygen-passivated Fe(001)-p(1 × 1)O surface, resulting in monolayer range chromium oxide layers. We discuss the presence of first nearest-neighbor Cr pairs, as seen by scanning tunneling microscopy, in the early stages of interface growth and compare the results with previous experimental observations on the Cr/Fe oxygen-free interface. We then derive the magnetic properties of the Cr oxide ultrathin films by analyzing new x-ray magnetic circular dichroism measurements conducted on samples grown both at room temperature and at 400 °C. The ultrathin Cr oxide film forms in both cases a ferromagnetic layer which is antiferromagnetically coupled to Fe.