Thickness Of MgO Layer Research Articles

Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films

Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin–orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 °C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtain the as-deposited PMA. Interestingly, contrary to the previous understanding, post-annealing does not significantly influence the well-established as-deposited PMA but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co– and Fe–O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in PMA degradation at high temperatures. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.

Spin Reflection-Induced Field-Free Magnetization Switching in Perpendicularly Magnetized MgO/Pt/Co Heterostructures.

Field-free magnetization switching is critical towards practical, integrated spin-orbit torque (SOT)-driven magnetic random-access memory with perpendicular magnetic anisotropy. Our work proposes a technique to modulate the spin reflection and spin density of states within a heavy-metal Pt through interfacing with a dielectric MgO layer. We demonstrate tunability of the effective out-of-plane spin torque acting on the ferromagnetic Co layer, enabling current-induced SOT magnetization switching without the assistance of an external magnetic field. The influence of the MgO layer thickness on effective SOT efficiency shows saturation at 4 nm, while up to 80% of field-free magnetization switching ratio is achieved with the MgO between 5 and 8 nm. We analyze and attribute the complex interaction to spin reflection at the dielectric/heavy metal interface and spin scattering within the dielectric medium due to interfacial electric fields. Further, through substituting the dielectric with Ti or Pt, we confirm that the MgO layer is indeed responsible for the observed field-free magnetization switching mechanism.

Photoelectron spectroscopy of tetraphenylporphyrin layers on MgO films on Ag(100)

The molecular orbitals of magnesium tetraphenyl porphyrin (MgTPP) deposited on epitaxial MgO films on Ag(100) are studied by ultraviolet photoelectron spectroscopy and two-photon photoemission. The energies of the electronic states are pinned to the vacuum level and the band gap is 3.88±0.07 eV largely independent of the MgO layer thickness.

A Physically Transient Self-Rectifying and Analogue Switching Memristor Synapse

A physical transient memristor synapse with self-rectifying and analogue switching behaviors based on Mo/MgO/AZO (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 2 wt %, ZnO 98 wt %)/W is presented. By modulating thickness of MgO layer, the device with 5 nm MgO insulator layer shows stable resistive switching memory with a rectification ratio up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and precise tuning synaptic functions. Meanwhile, the formation of Schottky barrier and oxygen vacancy conductive filament is proposed to further explain self-rectifying and analogue switching behaviors. Additionally, the electrical characteristics of the Mo/MgO/AZO/W devices degraded after being immersed in deionized water (DI) for 3 min, which demonstrated the physically transient features successfully. The physical transient self-rectifying memristors have great potential for applications in secure neuromorphic computing systems, and bio-integrated electronics.

The Band-Gap Studies of Short-Period CdO/MgO Superlattices

Trends in the behavior of band gaps in short-period superlattices (SLs) composed of CdO and MgO layers were analyzed experimentally and theoretically for several thicknesses of CdO sublayers. The optical properties of the SLs were investigated by means of transmittance measurements at room temperature in the wavelength range 200–700 nm. The direct band gap of {CdO/MgO} SLs were tuned from 2.6 to 6 eV by varying the thickness of CdO from 1 to 12 monolayers while maintaining the same MgO layer thickness of 4 monolayers. Obtained values of direct and indirect band gaps are higher than those theoretically calculated by an ab initio method, but follow the same trend. X-ray measurements confirmed the presence of a rock salt structure in the SLs. Two oriented structures (111 and 100) grown on c- and r-oriented sapphire substrates were obtained. The measured lattice parameters increase with CdO layer thickness, and the experimental data are in agreement with the calculated results. This new kind of SL structure may be suitable for use in visible, UV and deep UV optoelectronics, especially because the energy gap can be precisely controlled over a wide range by modulating the sublayer thickness in the superlattices.

Potential of low-resistivity Cu2Mg for highly scaled interconnects and its challenges

Materials with low resistivity in small dimensions are urgently desired to replace Cu for highly scaled interconnection in advanced integrated circuits. This study reports the possibility of Cu2Mg intermetallic compound as a Cu alternative by showing adequate adhesion on SiO2, a low resistivity of 25.5 μΩcm at 5 nm film thickness and good trench-filling capability in trench width of 38 or 23 nm by sputtering reflow. However, annealing at 400 °C for 30 min led to the dielectric current leakage associated with the formation of a thick MgO layer. Furthermore, Mg composition inside the trenches was less than the stoichiometric Cu2Mg composition. Theoretical calculation of surface diffusion process revealed that the adatom hopping of Mg atoms was slower than that of Cu atoms, which resulted in the Mg-poor composition inside the trenches.

Dynamics of Photo-Generated Carriers across the Interface between CsPbBr3 Nanocrystals and Au-Ag Nanostructured Film, and Its Control via Ultrathin MgO Interface Layer.

The dynamics and control of charge transfer between optoelectronically interesting and size-tunable halide perovskite quantum dots and other juxtaposed functional electronic materials are important issues for the emergent device interest involving such a family of materials in heterostructure configurations. Herein, we have grown bimetallic Au–Ag thin films on glass by pulsed laser deposition at room temperature, which bear nanoparticulate character, and the corresponding optical absorption spectra reveal the expected surface plasmon resonance signature(s). Subsequently, spin-coated CsPbBr3 nanoparticle films onto the bimetallic Au–Ag films exhibit surface-enhanced Raman scattering as well as strong photoluminescence quenching, the latter reflecting highly efficient transfer of photo-generated carriers across the CsPbBr3/Au–Ag interface. Surprisingly, when an ultrathin MgO (insulating) layer of optimum thickness is introduced between the CsPbBr3 and Au–Ag films, the charge transfer is further facilitated with the average lifetime of carriers becoming even shorter. By changing the thickness of the thin MgO layer, the carrier lifetime can in fact be tuned; with the charge transfer getting fully blocked for thick enough MgO layers, as expected. Our study thus throws light on the charge-carrier dynamics in halide perovskites, which is of importance to emergent optoelectronic applications.

Bio-corrosion behaviors of hyaluronic acid and cerium multi-layer films on degradable implant

Initial corrosion resistance and bioactivity of biodegradable magnesium are major success factors for implant applications. Electrodeposition containing Ce prevent material damage and local corrosion resulting from implantation, and Ce penetrates hydrogels and consequently maximizes surface self-healing. Ce(NO3) was used to form an electrodeposition layer on magnesium, and hyaluronic acid(HA) was coated through hydrothermal treatment, which created a composite layer consisting of a polymer compound with Ce and a thick and stable MgO layer. The morphology, chemical structure, and scratch tests conducted on various surface treatments showed hydrothermally treated Ce to be the most effective self-healing and corrosion-resistance. In the electrodeposition of Ca/Ce, relatively reactive Ce quickly adhered to the surface, and according to the hydrothermal process, Ca and Ce ions diffused inward as water and HA penetrated the irregular layers. The cell viability of osteoblasts showed no toxicity with Ce, and the osteoblasts were showed the highest differentiation especially in treated with the HA hydrothermal group. Implantation into rat tibias resulted in stable bone marrow and osteoblast growth in the hydrothermal treatment group containing Ce. Consequently, Ce(OH)3/CeO2 penetrating the natural polymer HA had a self-healing ability and resistance to initial corrosion, local damage, and the biodegradation of the magnesium implant.

Switching Performance Comparison With Low Switching Energy Due to Initial Temperature Increment in CoFeB/MgO-Based Single and Double Barriers

Spin-transfer torque magnetic random-access memory (STT-MRAM) based on a single-barrier magnetic tunnel junction (SBMTJ) and a double-barrier magnetic tunnel junction (DBMTJ) has evolved along with a low switching current and low energy consumption to obtain a high areal density and a fast switching speed. The increment of initial temperature, ${T}_{{\text {in}}}$ , in STT-MRAM can achieve low switching energy, ${E}_{{\text {SW}}}$ . However, this leads to an unavoidable decrease in $\Delta $ and switching efficiency. In this paper, SBMTJ and DBMTJ were analyzed in terms of the switching efficiency factor with ${E}_{{\text {SW}}}$ reduction by increasing the ${T}_{{\text {in}}}$ . The switching temperature, ${T}_{{\text {SW}}}$ , was investigated using a finite-element method simulation. The results show that the DBMTJ(A) and SBMTJ with the same MgO layer thickness of 0.9 nm provide a higher switching current, ${I}_{{\text {C}}}$ , than the DBMTJ(B) with a MgO layer thickness of 1.3 nm. The ${T}_{{\text {SW}}}$ in a DBMTJ(A) is higher than that in an SBMTJ, while ${T}_{{\text {SW}}}$ for a DBMTJ(B) is the smallest because of its low ${I}_{{\text {C}}}$ . The DBMTJ(A) and DBMTJ(B) can be applied in a higher temperature range than the SBMTJ at a $\Delta $ of 40. In addition, the STT efficiency factor, $\Delta /{I}_{{\text {C0}}}$ , for a DBMTJ is better than the factor for an SBMTJ. Although the temperature increment would cause an undesirable reduction in $\Delta $ , it can reach a low ${E}_{{\text {SW}}}$ for the requirement of a fast write access time. Therefore, the control device for increasing the ${T}_{{\text {in}}}$ in the MTJs is attractive and should be promoted in the advancement of memory technology.

Spin generation in completely MBE-grown Co2FeSi/MgO/GaAs lateral spin valves

We demonstrate first measurements of successful spin generation in crystalline ${\mathrm{Co}}_{2}\mathrm{FeSi}$/MgO/GaAs hybrid structures grown by molecular-beam epitaxy (MBE) with different MgO interlayer thicknesses. Using nonlocal spin valve and nonlocal Hanle measurement configurations, we determine spin lifetimes of $\ensuremath{\tau}\ensuremath{\approx}100$ ns and spin diffusion lengths of $\ensuremath{\lambda}\ensuremath{\approx}5.6\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ for different MgO layer thicknesses, proving the high quality of the GaAs transport channel. For an optimized MgO layer thickness, the bias dependence of the spin valve signals indicates the verification of the half-metallic gap (upper edge) of ${\mathrm{Co}}_{2}\mathrm{FeSi}$ in accordance with first-principles calculations. In addition to that, spin generation efficiencies of up to $18%$ reveal the high potential of MgO interlayers at the ${\mathrm{Co}}_{2}\mathrm{FeSi}$/GaAs interface for further device applications.

Epitaxial growth of Co2FeSi/MgO/GaAs(0 0 1) heterostructures using molecular beam epitaxy

We demonstrate the growth of crystalline Co2FeSi/MgO/GaAs(001) hybrid structures of different MgO interlayer thicknessess by molecular beam epitaxy. The MgO layer thickness was verified by X-ray reflectivity measurements and high-resolution transmission electron microscopy images. Secondary ion mass spectroscopy confirmed that the MgO layer works successfully as a diffusion barrier. Co and Fe defect densities in the GaAs layer could be reduced by three orders of magnitude using a 1.3 nm thick MgO film. High-resolution transmission electron microscopy images together with X-ray diffraction phi scans also revealed an in-plane rotation of the Co2FeSi layer by 45° for samples with MgO interlayers with respect to the GaAs substrate. Investigations of the magnetic properties revealed that the MgO interlayers do not affect the value of the magnetic moment of the Co2FeSi layer. Further, an in-plane rotation of the easy axis of the Co2FeSi layer by 90° for all samples with MgO interlayers was observed and will be discussed.

Magnetic Anisotropy in [Pt/Co]&lt;sub&gt;4&lt;/sub&gt;/MgO/[Co/Pt]&lt;sub&gt;2&lt;/sub&gt; Multilayers

A serial of [Pt/Co]4/MgO/[Co/Pt]2multilayers were prepared by magnetron sputtering in this study and Antiferromagnetic (AF) interlayer coupling was observed with increasing MgO spacer thickness. Perpendicular magnetic anisotropy of soft layer in [Pt/Co]4/MgO/[Co/Pt]2multilayers were measured by Extraordinary Hall Effect (EHE). The direction of the magnetic field was parallel to the direction of the current. The results revealed that perpendicular magnetic anisotropy constant Kuoscillates with the thickness of MgO layer which is between 1.3 to 3.3 nm. The period of this oscillation is about 0.43 nm. This is very close to the monolayer (100) MgO. With increasing MgO spacer, the periodic variation of interface between [Pt/Co]nand MgO layer may be one of the reasons for this oscillation behavior of Ku.

Enhancement of superconducting properties of MgB2 thin films by using oxygen annealing atmosphere

In this work, we show that significant enhancement of superconducting properties of MgB2 thin films prepared by vacuum evaporation and ex-situ annealing can be obtained when the annealing is performed in O2 atmosphere instead of the usually employed inert atmosphere (Ar in the current study). Rapid annealing in O2 significantly improves the critical transition temperature (Tc), critical current density (jc), and upper critical magnetic field (Hc2) compared to the control sample prepared in the same way, but annealed in Ar. Structural and composition analyses of the films demonstrate that the annealing in O2 atmosphere produces a sufficiently thick MgO layer on the surface of MgB2 thin films, which acts as a protective barrier preventing out-diffusion of Mg from the films during the high-temperature annealing. This consequently leads to better stoichiometry (B/Mg ratio), increased MgB2 grain size and enhanced intergrain connectivity. The higher residual resistivity of the films annealed in O2 atmosphere also indicates higher concentration of intragrain impurities, which causes a further increase of the jc and Hc.

Sequential magnetic switching in Fe/MgO(001) superlattices

Polarized neutron reflectometry is used to determine the sequence of magnetic switching in interlayer exchange coupled Fe/MgO(001) superlattices in an applied magnetic field. For 19.6 A thick MgO layers we obtain a 90 periodic magnetic alignment between adjacent Fe layers at remanence. In an increasing applied field the top layer switches first followed by its second-nearest neighbor. For 16.4 A MgO layers, a 180 periodic alignment is obtained at remanence and with increasing applied field the layer switching starts from the two outermost layers and proceeds inwards. This sequential tuneable switching opens up the possibility of designing three-dimensional magnetic structures with a predefined discrete switching sequence.

Negative tunneling magnetoresistance of Fe/MgO/NiO/Fe magnetic tunnel junction: Role of spin mixing and interface state

Motivated by a recent tunneling magnetoresistance (TMR) measurement in which the negative TMR is observed in MgO/NiO-based magnetic tunnel junctions (MTJs), we have performed systematic calculations of transmission, current, and TMR of Fe/MgO/NiO/Fe MTJ with different thicknesses of NiO and MgO layers based on noncollinear density functional theory and non-equilibrium Green's function theory. The calculations show that, as the thickness of NiO and MgO layers is small, the negative TMR can be obtained which is attributed to the spin mixing effect and interface state. However, in the thick MTJ, the spin-flipping scattering becomes weaker, and thus, the MTJs recover positive TMR. Based on our theoretical results, we believe that the interface state at Fe/NiO interface and the spin mixing effect induced by noncollinear interfacial magnetization will play important role in determining transmission and current of Fe/MgO/NiO/Fe MTJ. The results reported here will be important in understanding the electron tunneling in MTJ with the barrier made by transition metal oxide.

MgO thickness-induced spin reorientation transition in Co0.9Fe0.1/MgO/Co0.9Fe0.1 structure

The magnetic anisotropy (MA) of Mo/Au/Co0.9Fe0.1/Au/MgO(0.7–3nm)/Au/Co0.9Fe0.1/Au heterostructure has been investigated at room temperature as a function of MgO layer thickness (tMgO). Our studies show that while the MA of the top layer does not change its character upon variation of tMgO, the uniaxial out-of-plane MA of the bottom one undergoes a spin reorientation transition at tMgO of about 0.8nm, switching to the regime where the coexistence of in- and out-of-plane magnetization alignments is observed. The magnitudes of the magnetic anisotropy constants have been determined from ferromagnetic resonance and dc-magnetometry measurements. The origin of MA evolution has been attributed to a presence of an interlayer exchange coupling (IEC) between Co0.9Fe0.1 layers through the thin MgO film.

Interface matching and intermixing of thin MgO barriers and ferromagnetic layers deposited on GaAs (001)

MgO tunneling barriers are extensively studied as a spin filtering and diffusion barrier for deposition of ferromagnetic layers on GaAs (001) surfaces. The relatively large lattice mismatch of the MgO and GaAs substrate and probable formation of interface states at the metal layer side require a careful barrier design. We present a study of deposition, microstructure, and strain relaxation in MgO barriers deposited on GaAs. The dependence of morphology of MgO layers on their thickness was characterized using a combination of X-Ray Reflectivity and High Resolution Transmission Electron Microscopy. The stress at the MgO/GaAs interface was observed to be released through the formation of misfit dislocations and partially by the formation of mis-oriented domains. The deposited MgO and Fe layers were found to be highly textured irrespective of the thickness of MgO layers. Energy dispersive X-ray microanalysis was used to observe intermixing at the interface of Fe and MgO layers.

Perpendicular magnetic anisotropy enhancement and thermal stability study in Co/Pt multilayers with MgO/Pt interface

ABSTRACTThe Co/Pt multilayers with Pt and MgO/Pt underlayer were prepared by magnetron sputtering and the perpendicular magnetic anisotropy(PMA) of the multilayers were researched by the means of Anomalous Hall effect (AHE). The annealing effect study of Pt(1.0)/[Co(0.4)/Pt(0.8)]3(unit: nm) shows that the sample has pretty good thermal stability but the coercivity(HC) has little change. The experimental results show that the PMA and Hc of Co/Pt multilayers was improved greatly by the additional scattering of electrons induce from the amorphous insulator-metal interface of MgO-Pt. When MgO layer thickness(tMgO) is 4nm, the Hall resistance (RHall) of MgO(4)/Pt(1.0)/[Co(0.4)/Pt(0.8)]3 multilayer has a maximum value of 2.11Ω which is increased by 16%. Furthermore the Hc increases to 172Oe which is more than 1.5 times to Pt(1.0)/[Co(0.4)/Pt(0.8)]3 multilaye and the Hc meets the maximum value of 204Oe when tMgO is 2 nm. Annealing effects of MgO(4)/Pt(2)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2) show that it has good thermal stability and the Hc reaches its maximum value of 438Oe at annealing temperature(TA) of 300°C. The multilayer begins to lose the PMA property at higher TA. The Hysteresis loop of MgO(4)/Pt(1.0)/[Co(0.4)/Pt(0.8)]3 multilayer annealed at 300°C was tested and the anisotropy constant(Keff) was obtained as 8.9 × 106 erg/cm3, which is about 4.45 times the Pt(1.0)/ [Co(0.4)/Pt(0.8)]3 multilayer. The PMA of Co/Pt multilayers is greatly enhanced by MgO/Pt interface and annealing treatment.

Investigation of local structural and magnetic properties of discontinuous to continuous layer of Co at Co/MgO interface in MgO/Co/MgO trilayer structure

We investigate influence of oxygen migration in MgO/Co/MgO trilayer structure by varying Co and MgO layer thickness. To resolve Co/MgO interface structural and magnetic properties, we used near edge X-ray absorption spectra (XANES), X-ray absorption fine structure (XAFS) spectroscopy and Magneto-optical Kerr effect (MOKE) on MgO/Co/MgO trilayer structure. O K-edge XANES spectra show the variation in hybridization of Co 3d orbitals with neighboring O 2p orbitals as a function of Co layer thickness. Co L3 -edge XANES and Co K-edge XAFS results revealed the quantity of CoO formation as a function of Co layer thickness from discontinuous layer to continuous layer. Formation of cobalt oxide is mainly due to island formation of Co ≤ 10 nm layer and oxygen migration from MgO layer at interface via redox reaction to form CoO bonds. The oxygen migration is driven by imperfect stoichiometry of MgO at the Co/MgO interface. The formation of CoO layer influences the CoO orbital hybridization modifying in-plane magnetic anisotropy as a function of Co and MgO layer thickness. Higher coercivity (Hc) for lower thickness of Co layer confirms the formation of domain wall pinning. A model has been drawn to describe the interaction of Co layer with MgO layer at Co/MgO interface from discontinues to continuous region of Co thin film of magnetic tunnel junctions (MTJ)s, which may help to design high-performance spintronic devices.

Realization of Al2O3/MgO laminated structure at low temperature for thin film encapsulation in organic light-emitting diodes

A laminated structure of Al2O3 and MgO deposited by atomic layer deposition (ALD) is used to realize a thin film encapsulation technology in organic light-emitting diodes (OLEDs). This film was targeted to achieve an excellent barrier performance. As the thickness of MgO layer increased from 0 nm to 20 nm, its physical properties transformed from the amorphous state into a crystalline state. The optimized cyclic ratio of ALD Al2O3 and MgO exhibited much lower water vapor transmission rate (WVTR) of 4.6 × 10−6 gm−2/day evaluated by Calcium (Ca) corrosion at 60 °C&100% RH, owing to the formation of a terrific laminated structure. Top-emitting OLEDs encapsulated with laminated Al2O3/MgO show longer operating lifetime under rigorous environmental conditions. These improvements were attributed to the embedded MgO film that served as a modified layer to establish a laminated structure to obstruct gas permeation, as well as a scavenger to absorb water molecules, thus alleviating the hydrolysis of bulk Al2O3 material.