Critical Thickness Tc Research Articles

Quasi‐Two‐Dimensional Ferromagnetic SrRuO3 Grown by Pulsed Laser Deposition with Layer‐by‐Layer Growth Fashion

AbstractOne of the keys to the construction of metal oxide heterostructures is the short characteristic length scale, which requires controlled growth and interface engineering on an atomic level. At present, the growth mode of SrRuO3 (SRO) thin films grown on TiO2‐terminated (001) SrTiO3 (STO) substrates usually transitions from 2D layer‐by‐layer to step‐flow at the first few growth periods, which is not conducive to the construction of superlattices or multilayer films. In this paper, persistent layer‐by‐layer growth of SRO thin films is demonstrated by regulating the growth conditions. As a result, the thickness can be precisely controlled down to a single unit cell (u.c.) and achieve precise regulation of superlattices. Interestingly, the physical properties are well comparable to that of traditional step‐flow mode and the critical thickness (tc) for the dead layer is only 3 u.c. Through constructing SRO/STO superlattices, the SRO layers become more conductive and a quasi‐2D ferromagnetic monolayer film is achieved. The results pave a path toward SRO heterostructure synthesis and properties control at an atomic scale.

Strain engineering of the magnetic anisotropy and magnetic moment in NdFeO3 epitaxial thin films

Strain engineering is a powerful mean for tuning the various functionalities of ABO3 perovskite oxide thin films. Rare-earth orthoferrite RFeO3 materials such as NdFeO3 (NFO) are of prime interest because of their intriguing magnetic properties as well as their technological potential applications especially as thin films. Here, using a large set of complementary and advanced techniques, we show that NFO epitaxial thin films, successfully grown by pulsed laser deposition on (001)-SrTiO3, show a strong magnetic anisotropy below a critical thickness tc of 54 nm, associated with the occurrence of structural modifications related to symmetry and domain pattern changes. By varying the tensile misfit strain through the decrease of film thickness below tc, the amplitudes of in and out-of-plane magnetization can be continuously tuned while their ratio stays constant. Furthermore, different low-temperature magnetic behaviors are evidenced for strained and relaxed films, suggesting that the strain-induced structural state impacts the magnetic phase stability.

Relaxation Mechanisms and Strain-Controlled Oxygen Vacancies in Epitaxial SrMnO3 Films.

SrMnO3 has a rich epitaxial strain-dependent ferroic phase diagram, in which a variety of magnetic orderings, even ferroelectricity, and thus multiferroicity, are accessible by gradually modifying the strain. Different relaxation processes, though, including the presence of strain-induced oxygen vacancies, can severely curtail the possibility of stabilizing these ferroic phases. Here, we report on a thorough investigation of the strain relaxation mechanisms in SrMnO3 films grown on several substrates imposing varying degrees of strain from slightly compressive (−0.39%) to largely tensile ≈+3.8%. First, we determine the strain dependency of the critical thickness (tc) below which pseudomorphic growth is obtained. Second, the mechanisms of stress relaxation are elucidated, revealing that misfit dislocations and stacking faults accommodate the strain above tc. Yet, even for films thicker than tc, the atomic monolayers below tc are proved to remain fully coherent. Therefore, multiferroicity may also emerge even in films that appear to be partially relaxed. Last, we demonstrate that fully coherent films with the same thickness present a lower oxygen content for increasing tensile mismatch with the substrate. This behavior proves the coupling between the formation of oxygen vacancies and epitaxial strain, in agreement with first-principles calculations, enabling the strain control of the Mn3+/Mn4+ ratio, which strongly affects the magnetic and electrical properties. However, the presence of oxygen vacancies/Mn3+ cations reduces the effective epitaxial strain in the SrMnO3 films and, thus, the accessibility to the strain-induced multiferroic phase.

Superplastic dual-phase nanostructure Mg alloy: A molecular dynamics study

The introduction of amorphous phase and amorphous-crystalline interfaces is a new approach for enhancing mechanical performance of the Mg-based composite materials. In this work, we use molecular dynamics simulation method to explore the effect of amorphous phase size on the mechanical behavior of dual-phase nanostructure Mg alloy under tensile loading. The results show that two different deformation mechanisms of the dual-phase nanostructure Mg alloy occur depending on crystalline phase size (d) and amorphous thickness (t). There is a critical amorphous thickness (tc) for each sample to achieve nearly perfect plasticity, regardless of d. When t < tc, the plasticity of dual-phase nanostructure Mg alloy is provided by amorphous and crystalline phase. However, the plasticity is provided only by amorphous phase, the crystalline phase hardly participates in plastic deformation when t > tc. The results also indicate that reducing d and increasing t is consistent for improving the plastic effect of the dual-phase nanostructure Mg alloy. The optimal matching relationship between d and t is given. Moreover, some qualitative and quantitative analysis about the plastic deformation behavior of dual-phase nanostructure Mg alloy are also presented.

Highly Ductile and Ultra-Thick P-Doped FeSiB Amorphous Alloys with Excellent Soft Magnetic Properties.

Herein, we demonstrate the successful synthesis of novel Fe80Si9B(11−x)Px (x = 0, 1, 3, 5, 7) ultra-thick amorphous ribbons by planar flow casting. The influence of P alloying on glass forming ability (GFA), microstructure, thermal stability, soft magnetic properties, and ductility has been systematically investigated. The results reveal that introduction of P into Fe80Si9B11 alloy can remarkably enhance the GFA and increase critical thickness (tc) of the alloy from 45 to 89 um. Furthermore, the annealed FeSiBP amorphous alloys exhibited excellent soft magnetic properties, including high saturation magnetic flux density of 1.54 T, the low coercivity of 1.5 A/m, and low core losses of 0.15 W/kg. In addition, the representative Fe80Si9B8P3 ultra-thick amorphous alloy demonstrate excellent ductility even after annealing at 400 °C for 10 min, which indicates the superior performance of P-doped FeSiB alloys as compared to the commercial Fe78Si9B13 (Metglas 2605 S2) alloy. The combination of high GFA, excellent ductility, and low core losses of newly developed FeSiBP amorphous soft magnetic alloys makes them attractive candidates for magnetic applications in the high-frequency and high-speed electric devices.

Controlling the formation and stability of ultra-thin nickel silicides - An alloying strategy for preventing agglomeration

The electrical contact of the source and drain regions in state-of-the-art CMOS transistors is nowadays facilitated through NiSi, which is often alloyed with Pt in order to avoid morphological agglomeration of the silicide film. However, the solid-state reaction between as-deposited Ni and the Si substrate exhibits a peculiar change for as-deposited Ni films thinner than a critical thickness of tc = 5 nm. Whereas thicker films form polycrystalline NiSi upon annealing above 450 °C, thinner films form epitaxial NiSi2 films that exhibit a high resistance toward agglomeration. For industrial applications, it is therefore of utmost importance to assess the critical thickness with high certainty and find novel methodologies to either increase or decrease its value, depending on the aimed silicide formation. This paper investigates Ni films between 0 and 15 nm initial thickness by use of “thickness gradients,” which provide semi-continuous information on silicide formation and stability as a function of as-deposited layer thickness. The alloying of these Ni layers with 10% Al, Co, Ge, Pd, or Pt renders a significant change in the phase sequence as a function of thickness and dependent on the alloying element. The addition of these ternary impurities therefore changes the critical thickness tc. The results are discussed in the framework of classical nucleation theory.

Thickness-dependent on the static magnetic properties and dynamic anisotropy of FeNi films with stripe domain structures

FeNi stripe domain (SD) films with broad range of film thicknesses were prepared by electrodeposition method. The static magnetic properties and domain structures as well as the fundamental magnetic parameters of perpendicular anisotropy constant Kp, quality factor Q, critical thickness tc, and domain width w for SD films were studied both from experiment and calculation way respectively. The results showed that the Kp, Q, and w of SD films increased when the film thickness increased. In addition, the thickness dependence of the dynamic properties of FeNi SD films was further studied by a vector network analyzer ferromagnetic resonance. The results indicated a decreased dynamic anisotropy when the films thickness or the applied magnetic field increased. This was related to the enhanced SD structures of film when the thickness increased.

Depth resolved lattice-charge coupling in epitaxial BiFeO3 thin film.

For epitaxial films, a critical thickness (tc) can create a phenomenological interface between a strained bottom layer and a relaxed top layer. Here, we present an experimental report of how the tc in BiFeO3 thin films acts as a boundary to determine the crystalline phase, ferroelectricity, and piezoelectricity in 60 nm thick BiFeO3/SrRuO3/SrTiO3 substrate. We found larger Fe cation displacement of the relaxed layer than that of strained layer. In the time-resolved X-ray microdiffraction analyses, the piezoelectric response of the BiFeO3 film was resolved into a strained layer with an extremely low piezoelectric coefficient of 2.4 pm/V and a relaxed layer with a piezoelectric coefficient of 32 pm/V. The difference in the Fe displacements between the strained and relaxed layers is in good agreement with the differences in the piezoelectric coefficient due to the electromechanical coupling.

Conducting interfaces between amorphous oxide layers and SrTiO3(110) and SrTiO3(111)

Interfaces between (110) and (111)SrTiO3 (STO) single crystalline substrates and amorphous oxide layers, LaAlO3 (a-LAO), Y:ZrO2 (a-YSZ), and SrTiO3 (a-STO) become conducting above a critical thickness tc. Here we show that tc for a-LAO does not depend on the substrate orientation, i.e. tc (a-LAO/(110)STO)≈tc(a-LAO/(111)STO) interfaces, whereas it strongly depends on the composition of the amorphous oxide: tc(a-LAO/(110)STO)<tc(a-YSZ/(110)STO)<tc(a-STO/(110)STO). It is concluded that the formation of oxygen vacancies in amorphous-type interfaces is mainly determined by the oxygen affinity of the deposited metal ions, rather than orientation-dependent enthalpy vacancy formation and diffusion. Scanning transmission microscopy characterization of amorphous and crystalline LAO/STO(110) interfaces shows much higher amount of oxygen vacancies in the former, providing experimental evidence of the distinct mechanism of conduction in these interfaces.

Effect of Gold Thickness and Annealing on Optical and Electrical Properties of TiO2/Au/TiO2 Multilayers as Transparent Composite Electrode on Flexible Substrate

Multilayer structures of TiO2/Au/TiO2 have been deposited onto flexible substrates by room temperature sputtering to develop indium-free transparent composite electrodes (TCEs). The effect of Au thicknesses on optical, electrical properties and the mechanism of conduction have been discussed. The electrical conductivity of the TCEs is solely contributed by the middle metal layer. The critical thickness (t c) of the Au mid-layer to form a continuous conducting layer is 10 nm, and the multilayer has been optimized to obtain a sheet resistance of 12.2 Ω/sq and an average optical transmittance of 86% at 590 nm. The Haacke figure of merit (FOM) for t c has one of the highest FOM with 18 × 10−3 Ω−1. The samples were annealed in various environments for 24 h up to 150°C.

Determination of bromine in selected polymer materials by a wavelength-dispersive X-ray fluorescence spectrometric method — Critical thickness problem and solutions

The purpose of this study was to develop an accurate method for the determination of bromine in polymer materials using X-ray fluorescence spectrometry when the thickness of the sample is less than the bromine critical thickness (tc) value. This is particularly important for analyzing compliance with the Restriction of Hazardous Substances Directive. Mathematically and experimentally estimated tc values in polyethylene and cellulose matrixes were up to several millimeters. Four methods were developed to obtain an accurate result. These methods include the addition of an element with a high mass absorption coefficient, the measurement of the total bromine contained in a defined volume of the sample, the exploitation of tube-Rayleigh line intensities and using the Br-Lβ line.

Thickness-dependent magnetic properties of Ce9Fe91films

Ce9Fe91 films with different thickness were fabricated by a rf magnetron sputtering method. The critical thickness tc for spin reorientation transition has been determined to be approximately 90 nm using the stripe domain model and magnetic force microscope. Above tc, the films exhibit Bloch stripe domain structure and a superhigh resonance frequency at 6 GHz is found for the ∥ stripe configuration. However, below tc, the films possess an in-plane uniaxial anisotropy caused by order interface tension between the film and substrate, and the resonance frequency breaks through the Snoek limit.

Effect of pre-fatigue deformation on thickness-dependent tensile behavior of coarse-grained pure aluminum sheets

To explore the size effect on the mechanical properties of the coarse-grained materials and the relevant influence of sub-structures, the coarse-grained pure aluminum sheets with the thickness spanning from 0.2 to 2.0mm are selected as experimental materials, on which the pre-fatigue deformation (pre-cycle is equal to 5% of fatigue life Nf) is exerted to alter internal sub-structures. The results show that the uniform strain ε of the annealed sheets almost linearly decreases with the reduction of thickness, and the ultimate tensile strength σUTS first slowly declines, then quickly drops as the thickness is below 0.5mm, but the yield strength σYS does not exhibit an obvious thickness effect. The pre-fatigue deformation not only significantly improves the plasticity, but also decreases the critical thickness tc (below which the decrease in σUTS suddenly becomes notable) from 0.5mm of the annealed state to 0.3mm, meanwhile, a higher σUTS is maintained at the thickness ranging from 0.3 to 2.0mm. For the annealed sheets, the cracks are initiated at grain boundaries (GBs), and with decreasing thickness, the appearance of a lot of straight slip lines leads to the occurrence of single slip separation failure in thinner sheets. The pre-fatigue deformation not only increases the uniform deformation degree in grain interiors, but also promotes the formation of more ill-developed cells in sub-grains and wall-cells and the development of the sub-grains in thinner sheets, so that all the pre-fatigued sheets exhibit ductile failure and a higher σUTS is obtained at the thickness between 0.3 and 1.0mm.

High quality transparent TiO2/Ag/TiO2 composite electrode films deposited on flexible substrate at room temperature by sputtering

Multilayer structures of TiO2/Ag/TiO2 have been deposited onto flexible substrates by room temperature sputtering to develop indium-free transparent composite electrodes. The effect of Ag thicknesses on optical and electrical properties and the mechanism of conduction have been discussed. The critical thickness (tc) of Ag mid-layer to form a continuous conducting layer is 9.5 nm and the multilayer has been optimized to obtain a sheet resistance of 5.7 Ω/sq and an average optical transmittance of 90% at 590 nm. The Haacke figure of merit (FOM) for tc has one of the highest FOMs with 61.4 × 10−3 Ω−1/sq.

X‐ray imaging and diffraction study of strain relaxation in MBE grown SiGe/Si layers

AbstractMolecular Beam Epitaxy (MBE) grown, 50‐800 nm thick SiGe layers on Si are studied by two X‐ray complementary techniques: imaging (X‐ray topography) and High Resolution X‐Ray Diffraction. The measured relaxation rates are spreding from as low as 0.01% to over 70%. These results are examined through the main models for critical thickness tc calculation, the Matthews and Blakeslee approach concerning misfit dislocations (MD) development from existing dislocations and the People and Bean model for homogeneous MD nucleation.The beginning step of the relaxation is found to fit exactly with the People and Bean model, otherwise larger relaxation is reached after an intermediate weak relaxation stage. This leads to a refined approach of the critical thickness: a critical band [t, t] Comparison between observed t and calculated tc indicates that the lower limit of this band can be predicted by equilibrium models, the upper one being linked with multiplication stages (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thick CoFeB with perpendicular magnetic anisotropy in CoFeB-MgO based magnetic tunnel junction

We have investigated the effect of an ultra-thin Ta insertion in the CoFeB (CoFeB/Ta/CoFeB) free layer (FL) on magnetic and tunneling magnetoresistance (TMR) properties of a CoFeB-MgO system with perpendicular magnetic anisotropy (PMA). It is found that the critical thickness (tc) to sustain PMA is doubled (tc = 2.6 nm) in Ta-inserted CoFeB FL as compared to single CoFeB layer (tc = 1.3 nm). While the effective magnetic anisotropy is found to increase with Ta insertion, the saturation magnetization showed a slight reduction. As the CoFeB thickness increasing, the thermal stability of Ta inserted structure is significantly increased by a factor of 2.5 for total CoFeB thickness less than 2 nm. We have observed a reasonable value of TMR for a much thicker CoFeB FL (thickness = 2-2.6 nm) with Ta insertion, and without significant increment in resistance-area product. Our results reveal that an ultra-thin Ta insertion in CoFeB might pay the way towards developing the high-density memory devices with enhanced thermal stability.

Critical thickness and nanoporosity of TiO2 optical thin films

This work reports on the porosity and refraction index of TiO2 thin films as a function of the film thickness. Samples were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a microwave electron cyclotron resonance (MW-ECR) reactor at room temperature using titanium tetra-isopropoxide (TTIP) as precursor. Experimental parameters such as plasma gas composition (pure oxygen and argon/oxygen mixtures) and pressure (either ECR conditions or “normal” pressure, i.e. 10−4 or 10−3torrs correspondently) were varied. The evolution of the thin film microstructure, porosity and optical properties is critically studied by AFM, SEM, water adsorption isotherms, ellipsometry and UV–Vis transmittance and the existence of a certain critical thickness (tc) demonstrated. The porosity of the films with thicknesses ranging from several tens of nanometers up to half a micrometer is evaluated by QCM-isotherms at room temperature. The dependency of this critical thickness with the plasma conditions is evaluated experimental and theoretically. Thus, the microstructure change at tc is attributed to a transition from a surface diffused dominated growth mechanism for t<tc to another where shadowing is predominant. Dynamic scaling analysis of the two regimes and their Monte Carlo simulation complete the reported study.

Effect of Seed Layer on Room Temperature Tunnel Magnetoresistance of MgO Barriers Formed by Radical Oxidation in IrMn-Based Magnetic Tunnel Junctions

NiFe-seeded magnetic tunnel junctions (MTJs) of IrMn/CoFe/MgO/CoFeB were successfully formed by radically oxidizing a thin Mg layer. Room temperature (RT) tunnel magnetoresistance (TMR) of up to 211±10% was obtained and found to be strongly dependent on the thickness of the NiFe seed layer. High resolution transmission microscopy (HRTEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and magneto-optic Kerr effect (MOKE) analyses performed on NiFe/IrMn bilayer systems revealed that the IrMn(111)-fcc texture, grain size, surface roughness (rms), and exchange-biasing field (Hex) were strongly affected by the thickness of the NiFe seed layer. A critical NiFe thickness (tc ≈12 Å) was found: For tNiFe≤tc, the IrMn showed a very poor (111)-fcc texture with reduced grain size, very smooth surface, and reduced Hex. For tNiFe > tc, the IrMn showed a complete opposite behavior: much enhanced (111)-fcc texture with larger grain size, rougher surface, and larger Hex. For MTJ-based IrMn systems, a striking behavior is reported: larger TMRs and lower tunnel junction resistance (RA) products are obtained for tNiFe ≤tc while lower TMRs and larger RAs are obtained for tNiFe > tc.

Effect of Seed Layer on Room Temperature Tunnel Magnetoresistance of MgO Barriers Formed by Radical Oxidation in IrMn-Based Magnetic Tunnel Junctions

NiFe-seeded magnetic tunnel junctions (MTJs) of IrMn/CoFe/MgO/CoFeB were successfully formed by radically oxidizing a thin Mg layer. Room temperature (RT) tunnel magnetoresistance (TMR) of up to 211±10% was obtained and found to be strongly dependent on the thickness of the NiFe seed layer. High resolution transmission microscopy (HRTEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and magneto-optic Kerr effect (MOKE) analyses performed on NiFe/IrMn bilayer systems revealed that the IrMn(111)-fcc texture, grain size, surface roughness (rms), and exchange-biasing field (Hex) were strongly affected by the thickness of the NiFe seed layer. A critical NiFe thickness (tc ≈12 Å) was found: For tNiFe≤tc, the IrMn showed a very poor (111)-fcc texture with reduced grain size, very smooth surface, and reduced Hex. For tNiFe > tc, the IrMn showed a complete opposite behavior: much enhanced (111)-fcc texture with larger grain size, rougher surface, and larger Hex. For MTJ-based IrMn systems, a striking behavior is reported: larger TMRs and lower tunnel junction resistance (RA) products are obtained for tNiFe ≤tc while lower TMRs and larger RAs are obtained for tNiFe > tc.

Oxygen Effect on the Surface Structure and Magnetism of Ultrathin CoNi/Cu(001) Alloy Films

We present a study of the structure and the corresponding magnetic property of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">95</sub> /Cu(001) ultrathin films during exposure to oxygen. For a film below the critical thickness (t <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> ) of spin reorientation transition (SRT) from in-plane to a perpendicular direction, the SRT occurs while introducing oxygen for 25 Langmuir (L), followed by decreasing the perpendicular magnetization for further oxygen exposure. For a film above t <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> , the perpendicular magnetization is enhanced while introducing oxygen for 25 L and decreases during additional oxygen exposure. In both these cases, the interlayer distance ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> ) varies only 1.1% during the SRT or the enhancement of the perpendicular magnetization, whereas the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> increases substantially when perpendicular magnetization disappears. This implies that the oxygen-induced SRT of the CoNi/Cu(001) alloy films is not materially affected by the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> of the surface layer. This might indicate the formation of antiferromagnetic oxide that induces the SRT of CoNi/Cu(001) alloy films.