MgO Single Crystal Substrates Research Articles

New Type of Microwave High-$T_{c}$ Superconductor Microstrip Resonator and Its Application Prospects

A new type of planar high-Tc superconductor (HTS) resonator, proposed by the authors, is described. Its feature is excitement with whispering gallery modes (WGM) unlike the known microstrip resonators excited with the lower modes. The resonant structure of two-planar disks and two microstrip lines was simulated and made on the basis of a DyBa 2 Cu 3 O 7-δ superconductor epitaxial film of 600-nm thickness using photolithography technique. The film was deposited on MgO single crystal substrate of thickness h = 0.5 mm. Microwave response in a form of Q-factor and resonant frequency depending on temperature was measured in Kand Ka-bands. The obtained results show higher values of Q-factor of WGM microstrip HTS resonators in comparison with the known HTS microstrip resonators with lower modes. In the authors' opinion, these resonators open prospects for development of new planar microwave devices and techniques. The response of HTS disk resonators obtained earlier and about six months later at approximately the same conditions showed the lowering Q-factor, which may be explained by the degradation of HTS film properties and requires further study.

Picosecond nonlinear optical properties of SrTiO3 composite films doped with gold and nickel nanoparticles.

Nanoparticles composite thin films formed by nanometer-sized gold and nickel particles embedded in SrTiO3 matrices were fabricated on MgO single-crystal substrates by co-depositing the metal and ceramic targets using the pulsed laser deposition technique. The linear optical absorption properties were measured from 350 to 800nm, and the absorption peak due to the surface plasmon resonance of Au metal particles was observed around 557nm. The ultra-fast third-order nonlinear optical properties of the films were determined by a single-beam z-scan method at a wavelength of 532nm with laser duration of 55ps. The nonlinear refractive index n2 and the nonlinear absorption coefficient β were determined, respectively, and the figure of merit χ(3)/α (with χ(3) being the third-order nonlinear susceptibility and α the linear optical absorption coefficient) was discussed. Whether gold or nickel, metal particles have little effect on the figure of merit, but significantly affect the ratio of the real part to the imaginary part of χ(3) [Reχ(3)/Imχ(3)]. The obtained Reχ(3)/Imχ(3) of Au/SrTiO3 is 1.43, which is more than 3 times as large as that of Ni/SrTiO3.

Fabrication and characterization of high-quality all-NbN Josephson tunnel junctions for superconductive quantum interference devices

We have developed high-quality NbN/AlN/NbN Josephson tunnel junctions for direct current-superconducting quantum interference devices (DC-SQUIDs). The junctions were fabricated with epitaxial NbN/AlN/NbN trilayers on single crystal MgO (100) substrates and showed excellent tunneling properties with a quality factor Rsg/Rn above 20, a large gap voltage about 5.7mV, and a large IcRn product above 3.8mV. The critical current density Jc of the NbN/AlN/NbN junctions is controlled by the AlN barrier thickness dAlN in the junctions. The performance of DC-SQUIDs comprising NbN junctions was measured at 4.2K, and the white noise in a flux-locked-loop mode was 5µΦ0/Hz1/2, thus demonstrating the feasibility of all-NbN junctions for SQUID development.

Tuning the magnetic properties of Fe50−xMnxPt50 thin films

The magnetic and structural properties of highly ordered (S∼0.82) epitaxial Fe50−xMnxPt50 thin films were investigated. L10 Fe50−xMnxPt50 (x=0, 6, 9, 12 and 15) thin films with a constant thickness of 45nm were prepared by co-sputtering Fe50Pt50 and Mn50Pt50 on to MgO (100) single crystal substrate. We find a significant increase in the coercivity for FeMnPt thin films. We have shown that this increase coincides with a tetragonal distortion, while a recent first principles study of Mn doped FePt showed the sub lattice ordering of ferromagnetically aligned Mn atoms would lead to increase in magnetic anisotropy in the FeMnPt ternary alloy system with fixed Pt concentration. At x=12 the coercivity has increased by 46.4% relative to that of Fe50Pt50 (x=0). We attribute the increase in coercivity to the tetragonal distortion as the experimentally determined c/a ratio is larger than the expected c/a ratio for ferromagnetically ordered Mn atoms in the sublattice at the concentration x=12. High temperature deposition and high temperature annealing was applied to achieve large coercivity in Mn doped FePt as these process lead to the observed tetragonal distortion.

Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces

The interaction of radiation with materials controls the performance, reliability, and safety of many structures in nuclear power systems. Revolutionary improvements in radiation damage resistance may be attainable if methods can be found to manipulate interface properties to give optimal interface stability and point defect recombination capability. To understand how variations in interface properties such as misfit dislocation density and local chemistry affect radiation‐induced defect absorption and recombination, a model system of metallic CrxV1−x(0 ≤x≤ 1) epitaxial films deposited on MgO(001) single crystal substrates has been explored. By controlling film composition, the lattice mismatch between the film and MgO is adjusted to vary the misfit dislocation density at the metal/oxide interface. The stability of these interfaces under various irradiation conditions is studied experimentally and theoretically. The results indicate that, unlike at metal/metal interfaces, the misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry and the location of the misfit dislocation extra half‐plane (in the metal or the oxide) drive radiation‐induced defect behavior. Together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation‐tolerant nanomaterials for next‐generation nuclear power plants.

Substrate Induced Strain Field in FeRh Epilayers Grown on Single Crystal MgO (001) Substrates

Equi-atomic FeRh is highly unusual in that it undergoes a first order meta-magnetic phase transition from an antiferromagnet to a ferromagnet above room temperature (Tr ≈ 370 K). This behavior opens new possibilities for creating multifunctional magnetic and spintronic devices which can utilise both thermal and applied field energy to change state and functionalise composites. A key requirement in realising multifunctional devices is the need to understand and control the properties of FeRh in the extreme thin film limit (tFeRh < 10 nm) where interfaces are crucial. Here we determine the properties of FeRh films in the thickness range 2.5–10 nm grown directly on MgO substrates. Our magnetometry and structural measurements show that a perpendicular strain field exists in these thin films which results in an increase in the phase transition temperature as thickness is reduced. Modelling using a spin dynamics approach supports the experimental observations demonstrating the critical role of the atomic layers close to the MgO interface.

The effect of annealing temperature on the magnetic anisotropy in Co ultrathin film on MgO(001) substrate

Co epitaxial thin films with 2.5nm thickness were prepared on single-crystal MgO(001) substrates and annealed at different temperatures. The contribution of each interface of the MgO/Co/Cu trilayer to the in-plane magnetic anisotropy (IMA) was studied by changing interfacial coupling through annealing. The structure was measured by low energy electron diffraction (LEED), and the magnetic properties were measured using the anisotropic magnetoresistance (AMR) measurements and the longitudinal Magneto-optical Kerr effect magnetometer (MOKE). We found that the magnetic anisotropy of the as-deposited one shows superposition of a two-fold symmetry with a weak four-fold contribution caused by the stress of the interface between Co/Cu, which is along the easy axis [-110]. After annealing at 200°C, the symmetry of magnetic anisotropy was changed from uniaxial magnetic anisotropy (UMA) into four-fold symmetry due to the significant increasing of four-fold magnetocrystalline anisotropy. When the films were annealed above 300°C, the damage of the MgO/Co interface additionally decreased the IMA to isotropy. Meanwhile, the coercivity raised from 45Oe (without annealing) to 1200Oe (annealed at 400°C) along the easy axis direction. Our experimental results prove that the Co/Cu interface and the MgO/Co interface play an essential role in manipulating the four-fold and the UMA in the system.

Enhancement of L1 ordering with the c-axis perpendicular to the substrate in FePt alloy film by using an epitaxial cap-layer

FePt alloy thin films with cap-layers of MgO or C are prepared on MgO(001) single-crystal substrates by using a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. The FePt film thickness is fixed at 10 nm, whereas the cap-layer thickness is varied from 1 to 10 nm. The influences of cap-layer material and cap-layer thickness on the variant structure and the L10 ordering are investigated. Single-crystal FePt(001) films with disordered fcc structure (A1) grow epitaxially on the substrates at 200 °C. Single-crystal MgO(001) cap-layers grow epitaxially on the FePt films, whereas the structure of C cap-layers is amorphous. The phase transformation from A1 to L10 occurs when the films are annealed at 600 °C. The FePt films with MgO cap-layers thicker than 2 nm consist of L10(001) variant with the c-axis perpendicular to the substrate surface, whereas those with C cap-layers involve small volumes of L10(100) and (010) variants with the c-axis lying in the film plane. The in-plane and the out-of-plane lattices are respectively more expanded and contracted in the continuous-lattice MgO/FePt/MgO structure due to accommodations of misfits of FePt film with respect to not only the MgO substrate but also the MgO cap-layer. The lattice deformation promotes phase transformation along the perpendicular direction and L10 ordering. The FePt films consisting of only L10(001) variant show strong perpendicular magnetic anisotropies and low in-plane coercivities. The present study shows that an introduction of epitaxial cap-layer is effective in controlling the c-axis perpendicular to the substrate surface.

Probing the electronic structure of Ni–Mn–In–Si based Heusler alloys thin films using magneto-optical spectra in martensitic and austenitic phases

Thin films of Ni52Mn35−xIn11+xSi2 were fabricated by magnetron sputtering on MgO (001) single crystal substrates. Magnetization as function of temperature for Ni52Mn35In11Si2 exhibits features consistent with a magnetostructural transition (MST) from an austenitic phase to a martensitic phase, similar to the bulk material. We observed that the martensitic transformation is externally sensitive to small changes in chemical composition and stoichiometry. It has been found that thin films of Ni52Mn34−xIn11+xSi2 with x=0 and 1 undergo a temperature-induced MST or remain in a stable austenitic phase, respectively. Comparison of magneto-optical transverse Kerr effect spectra obtained at 0.5–4.0eV in the 35–300K temperature interval reveal insignificant differences between the martensitic and austenite phases. We found that the field and temperature dependencies of the transverse Kerr effect are quite different from the magnetization behavior, which is attributed to magnetic inhomogeneity across the films. To elucidate the effects of magnetostructural phase transitions on the electronic properties, we performed density functional calculations of the magneto-optical Kerr effect.

Synthesis of sub-millimeter tall SWNT forests on a catalyst underlayer of MgO single crystal

ABSTRACTWe have demonstrated the high yield and highly single wall selective synthesis of carbon nanotube forest grown using a MgO single crystal as a catalyst underlayer without any surface treatment. Such efficiency has not been previously reported using this underlayer system. Our investigation revealed that the growth ambient which contained no hydrogen but small amounts of water is essential for such efficiency. Evaluation of the growth kinetics by in-situ height monitoring revealed that the 350 μm tall forest with 65% single wall selectivity possessed a catalyst lifetime of about 5 minutes. Investigation by AFM, and XPS depth profiling revealed that this longer lifetime compared with as-deposited MgO can be attributed to the stability of the catalyst particle array with small size and high number density on MgO single crystal substrate. Shorter growth lifetime, when compared to the alumina underlayer system, resulted from an instability in the catalyst nanoparticle size and composition.

Low loss and highly tunable (Ba,Sr)(Ti,Mn)O3/(Ba,Sr)TiO3 bilayered films for electrically tunable microwave device applications

Ba0.05Sr0.95TiO3 (BST), 5 mol% Mn-doped BST (Mn-BST) and bilayered Mn-BST/BST films with a thickness of about 100 nm were fabricated by using pulsed laser deposition technique on MgO single crystal substrates, and their microwave dielectric properties were comparatively investigated. It was confirmed that all the BST-based films were crystallized in pure single-oriented perovskite phase. There appeared nanosized pores in the BST film; after addition of Mn, however, the film surface morphology became denser while exhibiting an increased roughness, and ultimately, a smooth and dense morphology was achieved by the design of bilayered Mn-BST/BST film. Coherently, the microwave dielectric properties of BST films were enhanced by Mn doping and more significantly enhanced in the bilayered Mn-BST/BST film. As a result, for the bilayered film, a very low loss tangent of 0.017–0.007 and a high dielectric tunability of 70.9% under E = 36 V/μm were simultaneously obtained at 79 K and L-band frequency (1–2 GHz), thereby resulting in an extremely high commutation quality factor of 14492. The excellent microwave dielectric properties obtained in the Mn-BST/BST bilayered film originate not only from the Mn-doping effect, but are also ascribed to the pre-deposition of BST layer, which serves as a good template for the growth of Mn-BST film.

Magnetic properties of &amp;lt;italic&amp;gt;L&amp;lt;/italic&amp;gt;1&amp;lt;sub&amp;gt;0&amp;lt;/sub&amp;gt;-FePt(001)/&amp;lt;italic&amp;gt;A&amp;lt;/italic&amp;gt;1-FePt bilayer exchange springs

Soft A 1-FePt (30 nm) films were magnetron-sputtered on MgO(001) single-crystal substrates at an elevated temperature of 400°C, and subsequently annealed at diffrent temperatures in range of T a=[400°C, 700°C] for 6 h in order to turn them into hard magnets with different degrees of A 1→ L 10 transformation. Then second layers of A 1-FePt with different thicknesses were covered at 100°C to obtain L 10-FePt/ A 1-FePt bilayer magnetic exchange springs with (001) textures. At T a≤600°C, the coverage of hard layer was ~100%, though the grain size increased with T a. The surface roughness reduced from 2.45 to 1.02 nm if the film was annealed at T a=500°C. The surface of hard layer flattened by annealing should benefit to improve the strength of interlayer exchange interaction. At T a=500°C and 600°C, A 1-FePt and L 10-FePt coexisted in the hard layer with an ordering degree of 0.61 and 0.84, respectively. The direction of magnetic easy axis began to switch from in plane to out of plane at T a=500°C, indicating a vertical magnetocrystalline anisotropy was generated. The out-of-plane magnetization curves showed rectangle-like shapes at 500°C≤ T a≤600°C, in spite of the soft phase left in hard layer, and a moderate coercive force of ~5 kOe was obtained at T a=500°C. This indicates the exchange interaction between coexistent two phases in hard layer was strong enough to pin the soft magnetic moments with the hard ones. At T a=700°C, the coercive force exceeded 20 kOe, which is too huge to be affected by exchange springs. By using the film of T a=500°C as hard layer, L 10/ A 1-FePt bilayer film with 20 nm thick soft layer kept rigid behaviors with almost the same coercive force of single hard layer, and respective reversal steps for soft layer and hard layer appeared in the magnetization curve if the thickness of soft layer was 30 nm, with a reduced coercive force of ~4 kOe. As double shoulders should appear in the hysteresis loop if the thickness of soft layer exceeds twice of domain wall thickness of hard layer, i.e., the so-called interlayer exchange length, this indicates that the hard/soft interlayer exchange length was in the range of 20 nm l ex L 10/ A 1-FePt exchange spring. The reason is due to that the coexistence of A 1-FePt and L 10-FePt in hard layers lowered the efficient uniaxial magnetocrystalline anisotropy. This lengthened distance of interlayer exchange could be maintained at T a=600°C. This is benefit to design the properties of magnetic springs for different applications.

Linear and quadratic magneto-optical Kerr effect investigation of Co2Mn1.30Si0.84 epitaxially grown on MgO

Abstract We investigated the magneto-optical properties of a L21 ordered nonstoichiometric Co2Mn1.30Si0.84 film epitaxially grown on a MgO-buffered MgO (001) single-crystal substrate. Longitudinal magneto-optical Kerr effects (LMOKE) and rotating magneto-optical Kerr effect (ROT-MOKE) measurements suggest that the film exhibits a cubic magnetic anisotropy with the extracted cubic anisotropy constant of K C = 6.7 × 10 4 e r g / c m 3 . Orientation-dependent ROT-MOKE suggest that the quadratic magneto-optical Kerr effects (QMOKE) components also show fourfold symmetry with a modest amplitude of 3 mdeg, which is in accordance with complex Kerr angle expression for cubic symmetry systems. Our results suggest that ROT-MOKE is not only an efficient method to determine magnetic anisotropy parameters but also a good method to extract QMOKE components.

Effect of Cu buffer layer on magnetic anisotropy of cobalt thin films deposited on MgO(001) substrate

Cobalt thin films with 5 nm thickness were prepared on single-crystal MgO (001) substrates with different thickness Cu buffer (0 nm, 5 nm, 10 nm, 20 nm). The structure, magnetic properties and transport behaviors were investigated by employing low-energy-electron-diffraction (LEED), magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR). By comparing the magnetic properties of the sample as-deposited (without Cu buffer layer) one with those having the buffer Cu, we found that the magnetic anisotropy was extremely affected by the Cu buffer layer. The magnetic anisotropy of the as-deposited, without buffer layer, sample shows the uniaxial magnetic anisotropy (UMA). We found that the symmetry of the magnetic anisotropy is changed from UMA to four-fold when the thickness of the Cu buffer layer reaches to 20 nm. Meanwhile, the coercivity increased from 49 Oe (without buffer layer) to 300 Oe (with 20 nm Cu buffer), in the easy axis direction, as the thickness of the buffer layer increases. Moreover, the magnitudes of various magnetic anisotropy constants were determined from torque curves on the basis of AMR results. These results support the phenomenon shown in the MOKE.

What determines the interfacial configuration of Nb/Al2O3 and Nb/MgO interface.

Nb films are deposited on single crystal Al2O3 (110) and MgO(111) substrates by e-beam evaporation technique. Structure of Nb films and orientation relationships (ORs) of Nb/Al2O3 and Nb/MgO interface are studied and compared by the combination of experiments and simulations. The experiments show that the Nb films obtain strong (110) texture, and the Nb film on Al2O3(110) substrate shows a higher crystalline quality than that on MgO(111) substrate. First principle calculations show that both the lattice mismatch and the strength of interface bonding play major roles in determining the crystalline perfection of Nb films and ORs between Nb films and single crystal ceramic substrates. The fundamental mechanisms for forming the interfacial configuration in terms of the lattice mismatch and the strength of interface bonding are discussed.

Improvement of Oxidation Behavior in Cr(N,O) By Increasing the Oxygen Content

Chromium oxynitride(Cr(N,O)) thin films have been developed by controlling the oxygen partial pressure during the deposition 1). The Cr(N,O) thin films show superior properties not only the hardness but also the oxidation behavior than those of CrN, which are suitable for machining tool applications. However, the reason of the oxidation improvement was not known because of the presence of grain boundaries. Recently, we have succeeded in preparation of epitaxially grown Cr(N,O) on MgO and other single crystal substrates to reduce the density of grain boundaries2). In this work, oxidation behavior of the epitaxially grown Cr(N,O) thin films was investigated from crosssectional transmission electron microscope (TEM) observations. Thin films of Cr(N,O) were epitaxially grown by a pulsed laser deposition method. In the deposition chamber, oxygen gas with a partial pressure of 2x10-5Pa and N radicals from an RF radical source were introduced. A plate of Cr was rotated in the deposition chamber was irradiated by a Nd:YAG pulsed laser with a wavelength of 355 nm. Thin films were deposited on MgO (100) single crystal substrates kept at 973 K in the deposition chamber. After the deposition, oxidation tests were carried out in a furnace at 673-1173 K in air for one hour. After the oxidation tests, powder X-ray diffraction (XRD), crosssectional TEM and electron energy loss spectroscopy (EELS) observations/analyses were conducted for precise phase, microstructure and composition analysis. From the XRD and TEM analyses, the as-deposited thin films were of epitaxially grown Cr(N,O) on the substrates. After the oxidation tests, from XRD patterns, peaks for Cr2O3were seen from samples heated above 873 K. On the other hand, peaks for Cr(N,O) remained up to 1073 K. From TEM and EELS analyses of the sample heated at 1073K, on the existing Cr(N,O) epitaxially grown thin film, a polycrystalline oxide layer was covering. It was found that the oxide layer works as an oxygen diffusion barrier from the surface. References 1) T. Suzuki, J. Inoue, H. Saito, M. Hirai, H. Suematsu, W. Jiang, and K. Yatsui, Thin Solid Films 515, 2161 (2006). 2) K. Suzuki, T. Endo, A. Sato, T. Suzuki, T. Nakayama, H. Suematsu, and K. Niihara, Mater. Trans. 54, 1957 (2013).

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

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

Exchange bias effects in Heusler alloy Ni2MnAl/Fe bilayers

Ni2MnAl Heusler alloy thin films were epitaxially grown on MgO(1 0 0) single crystal substrates by ultra-high-vacuum magnetron sputtering technique. X-ray diffraction and transmission electron microscopy observation revealed that the structures of all the Ni2MnAl thin films were B2-ordered regardless of the deposition temperature ranging from room temperature to 600 °C. The temperature dependence of electrical resistivity showed a kink about 280 K, which was consistent with a reported value of the Néel temperature for antiferromagnetic B2-Ni2MnAl. The magnetization curves of Ni2MnAl/Fe bilayer samples showed a shift caused by the interfacial exchange interaction at 10 K. The maximum value of the exchange bias field Hex was 55 Oe corresponding to the exchange coupling energy Jk of 0.03 erg cm−2.

(Invited) Metalation of Porphyrins on Surfaces in Liquid and Vacuum

Adsorption and metalation of porphyrins on metal single-crystal surfaces under ultrahigh vacuum has been studied extensively, but less is known about the adsorption behavior of porphyrins on oxide surfaces. On metal surfaces porphyrin molecules will react with coadsorbed metal atoms or the surface itself, producing the metalated molecules and coadsorbed hydrogen atoms, which subsequently combine and desorb as hydrogen molecules. Examples will be given of 5,10,15,20-tetraphenylporphyrin (2HTPP) on Cu(111). A similar reaction is also possible for oxide surfaces. As 2HTPP is adsorbed on MgO an ion-exchange reaction happens already at room temperature and the two aminic protons of the porphyrin molecule are exchanged with a Mg2+ ion from the oxide surface, producing MgTPP and, locally, Mg(OH)2. Examples will be given of adsorption on both single-crystal MgO(100) substrates and nanostructured powders. Examples will also be given of metalation of 2HTPP adsorbed on a Au(111) surface immersed in an aqueous zinc acetate solution, metal-exchange reactions with Cu2+ and unsuccessful attempts to demetalate the adsorbed metalloporphyrins in acidic solutions.

Dielectric properties of BaTi2O5 thick films prepared on Pt-coated MgO(110) single-crystal substrate by laser chemical vapor deposition

BaTi2O5 (BT2) films were deposited on Pt-coated MgO(110) single-crystal substrates by laser chemical vapor deposition. The single-phase BT2 thick films were deposited at high deposition rates. The BT2 thick films consisted of elongated grains with columnar cross-section. With increasing the deposition temperature (Tdep), the orientation of BT2 thick films changed from (112) to (511), and the grain size increased. The (112)-oriented BT2 thick film deposited at Tdep=956K had dielectric constant (ε′) of 67 and dielectric loss (tanδ) of 0.015, while the (511)-oriented BT2 thick film deposited at Tdep=964K had ε′ of 74 and tanδ of 0.019 at 300K and 1MHz.