Inverse Heusler alloy Mn2CoAl thin films, known as a spin-gapless semiconductor (SGS), grown by three different methods—ultra-high vacuum magnetron sputtering, Ar-ion beam assisted sputtering, and molecular beam epitaxy—are investigated by comparing their electric transport properties, microstructures and atomic-level structures. Of the samples, the Mn2CoAl thin film grown by molecular beam epitaxy consists of Mn- and Co-rich phases, the structures of which are determined to be the L21B-type and disordered L21-type, respectively, according to anomalous X-ray diffraction analysis. None of them forms the XA-type structure expected for SGS Heusler alloy, although they all exhibit SGS characteristics. To validate the SGS characteristics, it is necessary to extract not only the magnetic and electric transport properties but also information about microstructures and atomic-scale structures of the films including defects such as atomic swap.

This work demonstrates that graphene/diamond (carbon sp2-sp3) heterojunctions can be used as multibit optoelectronic memory, where light information is stored as multilevel resistance in a nonvolatile manner. The carbon heterojunctions exhibit a large memory switching ratio of ∼104 and a retention time of >100 min, which allows for multilevel and nonvolatile data storage of optical information. The carbon heterojunctions also exhibit an apparent response to optical pulses, and the output current (conductivity of the junctions) increased linearly in response to the total number of optical pulses. Simple optical arithmetic operations such as accumulation, subtraction, and counting can be performed by using the multiple resistance states of the heterojunctions. The carbon heterojunctions have light sensing, memory, and arithmetic functions in a single device, and they are expected to pave the way for the production of innovative optical computing devices with multifunctional integration of sensing, memory, and calculation functions.

Abstract

We demonstrate ambipolar transport and modulation of electronic properties of Mn2CoAl (MCA), one of the most promising candidates for spin gapless semiconductors (SGSs), by using ionic liquid gating in electronic double-layer transistors. The carrier concentration and mobility of MCA films were systematically changed with carrier polarity inversion from the p- to the n-type by the gating technique. The ambipolar transport is one of the most significant properties of SGSs and strongly promotes the gapless features of MCA. The present results pave the way for the use of MCA as a spin source of both spin-polarized electrons and holes. Published by AIP Publishing.

We performed X-ray magnetic circular dichroism (XMCD) photoemission electron microscopy imaging to reveal the magnetic domain structure of anti-perovskite nitride Co3FeN exhibiting a negative spin polarization. In square and disc patterns, we systematically and quantitatively determined the statistics of the stable states as a function of geometry. By direct imaging during the application of a magnetic field, we revealed the magnetic reversal process in a spatially resolved manner. We compared the hysteresis on the continuous area and the square patterns from the magnetic field-dependent XMCD ratio, which can be explained as resulting from the effect of the shape anisotropy, present in nanostructured thin films. Published by AIP Publishing.

Graphene/diamond (carbon sp(2)-sp(3)) heterojunctions are demonstrated as photo-controllable memristors with photoswitchable multiple resistance states and nonvolatile memory functions. The ratio of conductivity change between the higher and lower resistance states of the junctions was similar to 10(3). The junctions exhibit light wavelength selectivity, and the resistance states can be switched only by blue or violet light irradiation. The mechanism for the change in photoconductivity is considered to be caused by oxidation-reduction of the graphene and/or graphene-diamond (sp(2)-sp(3)) interfaces through the movement of oxygen ions by bias with photo-irradiation because they have wavelength selectivity and require air exposure for several days to exhibit memristive behavior. These results indicate that graphene-diamond, carbon sp(2)-sp(3) heterojunctions can be used as photo-controllable devices with both photomemory and photoswitching functions. Published by AIP Publishing.

We present a systematic study of the structural and electrical properties of electron-doped infinite-layer Sr0.875La0.125CuO2 (SLCO) thin films grown on BaySr1-yTiO3 (BSTO; y = 0.2-0.7) buffer layers. The BSTO (y = 0.2-0.6) buffer layers were found to allow the coherent growth of SLCO thin films. The coherently tensile-strained SLCO thin films grown on BSTO (y = 0.6) buffer layers at a low deposition rate of 0.2nm/min showed a low room-temperature resistivity (118 mu Omega cm) and a high T-c (36 K). All the other SLCO thin films exhibited a reduced T-c of around 20 K. The present results are discussed in terms of correlation among epitaxial strain, c(SLCO)/a(SLCO) ratio, and T-c. (C) 2016 The Japan Society of Applied Physics

The epitaxial thin films of electron-doped infinite-layer cuprate superconductor Sr1-xLaxCuO2 (SLCO; x = 0.1) were grown by dc magnetron sputtering, and their structural and electrical properties were systematically investigated by changing mismatch to substrate and SLCO film thickness, with a reduction annealing period under vacuum to remove the excess apex oxygen. Thin films of BaySr1-yTiO3 (BSTO) with y = 0.55, 0.6, and 0.7, prepared on (001) (La0.18Sr0.82)(Al0.59Ta0.41)O-3 substrates, were used as the epitaxial buffer layers to induce different levels of tensile strain in c-axis-oriented SLCO, while (001) LaAlO3 substrates were used for the growth of a-axis-oriented SLCO. The highest Tc value of c-axis oriented SLCO was obtained on the BSTO (y = 0.6) layers and the mismatch dependence of strain relaxation thickness was clarified. The strain in a-axis SLCO films was analyzed and controlled by changing the thickness of SLCO thin films. As a result, strong thickness dependence of superconducting properties was observed, and T-c(zero) was obtained in the a-axis SLCO for the first time. Based on these results, the strain effects of the SLCO film properties are discussed.

High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes were investigated. Cu Schottky diodes showed clear rectification up to similar to 700 degrees C. The current-voltage characteristics of the diodes at 400 degrees C were almost unchanged after keeping them for 30 h, implying they have high stability at similar to 400 degrees C. The diodes showed specific on-resistance and breakdown voltage of 83.4m Omega cm(2) and 713 V at 400 degrees C, respectively, which are comparable to reported highest values for diamond Schottky diodes and close to the theoretical limit for 6H-SiC at several hundred degrees C. These results indicate that Cu/diamond Schottky diodes are promising for high-temperature power applications. (C) 2015 Elsevier B.V. All rights reserved.

Epitaxial bilayers of antiferromagnetic Mn3GaN/ferromagnetic Co3FeN with an antiperovskite structure were grown by reactive magnetron sputtering, and their structural, magnetic, and electrical properties were investigated. Exchange coupling with an exchange field H-ex of 0.4 kOe at 4K was observed for Mn3GaN (20 nm)/Co3FeN (5 nm) bilayers. Negative anisotropic magnetoresistance (AMR) effect in Co3FeN was observed and utilized to detect magnetization reversal in exchange-coupled Mn3GaN/Co3FeN bilayers. The AMR results showed evidence for current-induced spin transfer torque in antiferromagnetic Mn3GaN. (C) 2015 AIP Publishing LLC.

Electron-doped infinite-layer (IL) Sr1-xLaxCuO2thin films were grown on bare and lattice-matched Ba0.55Sr0.45TiO3-buffered LaAlO3(100) substrates by magnetron sputtering, and their structures and electrical properties were investigated systematically for various deposition and reduction conditions. The a- and c-axis-oriented films were obtained on LaAlO3and LaAlO3//Ba0.55Sr0.45TiO3substrates, respectively. For the caxis-oriented films, short-time (&lt
6 min) reduction annealing in vacuum followed by deposition was effective for the removal of excess apical oxygen that was incorporated during deposition. The removal of excess apical oxygen was evidenced by the shortened c-axis and the appearance of superconductivity. However, subsequent annealing led to the elongation of the c-axis and loss of superconductivity. The a-axis-oriented films exhibited a similar behavior, although the time scale of such films for reduction annealing was significantly larger than that of the c-axis films. The implications of these results are discussed in terms of the oxygen sublattice in the IL compound.

Thin films of Co3FeN with an antiperovskite structure were epitaxially grown on (La0.18Sr0.82)(Al0.59Ta0.41)O-3 (001) substrates by reactive magnetron sputtering. The influence of the N-2 volume concentration in the sputtering gas mixture on the structure and properties of Co3FeN thin films was systematically investigated. The optimized Co3FeN thin films exhibited a saturation magnetization, M-s, of 1350 emu/cc, which is comparable with the theoretical value. A negative anisotropic magnetoresistance (AMR) effect with an AMR ratio of up to -0.88% was observed at 4.2 K.

The high-temperature electrical characteristics and stability of Cu/diamond Schottky diodes were examined and compared with those of Schottky diodes using Ag and Ni electrodes. The Cu/diamond Schottky diodes exhibited clear rectification up to 700 °C, indicating that high-temperature operation is possible using these diodes. This is thought to be due to their large Schottky barrier height of ∼1.6 eV. The high-temperature stability of the Cu/diamond Schottky diodes was better than that for diodes using Ag or Ni, probably because of less interfacial reaction or interdiffusion between the Cu and diamond. © 2014 The Japan Society of Applied Physics.

Epitaxial MgAl2O4 thin films were deposited on a lattice-matched Heusler alloy, Fe2CrSi, by reactive magnetron sputtering of an MgAl2 target in an Ar+O-2 atmosphere. Epitaxial Fe2CrSi/MgAl2O4 junctions were obtained by inserting an ultrathin MgAl2 interlayer, which worked as a protective layer for oxidization at the surface of the Fe2CrSi. The growth of MgAl2O4 was found to be very sensitive to the MgAl2 thickness and the oxygen partial pressure during the deposition of MgAl2O4. Both epitaxial growth and characteristics of the efficient tunneling barrier were obtained in an Fe2CrSi/MgAl2O4 (3 nm)/CoFe tunneling device for MgAl2O4 thin films grown by reactive sputtering. The present epitaxial MgAl2O4 barrier deposited by reactive sputtering is expected to realize high performance spintronic devices.

Polycrystalline bulk and epitaxial thin film samples of double-perovskite SrLaVRuO6 were prepared, and their magnetic and electrical properties were investigated. Structural characterization on the bulk samples has revealed that a double-perovskite phase can be formed in coexistence with the secondary phase of nonmagnetic semiconducting Sr3V2O8. Magnetic and electrical investigations have shown that the double-perovskite SrLaVRuO6 compound is an antiferromagnetic metal. Moreover, it has been found that epitaxy using a lattice-matched substrate leads to suppression of the Sr3V2O8 secondary phase and promoting growth of phase-pure double-perovskite SrLaVRuO6. © 2013 The Korean Physical Society.

Structural and magnetic properties were investigated for Heusler-type alloy Ru2MnGe thin films. Ru2MnGe films on MgO substrate were subjected to an in-plane compressive strain, and exhibited enhanced antiferromagnetic (AFM) transition temperature (T N) up to 353 K, which exceeds by 37 K from the cubic bulk material (T N = 316 K). We also observed the exchange coupling between Ru2MnGe and Heusler-type ferromagnetic (FM) half-metal Fe2CrSi thin films. The present AFM Heusler alloy with relatively high T N is useful to fabricate high-quality all Heusler-type half-metal AFM/FM junctions and is a promising material for the emerging field of AFM spintronics. © 2013 The Korean Physical Society.

Magnetoresistance (MR) devices using degenerated oxide semiconductors, Nb-doped SrTiO3, (Nb-STO) as intermediate layers between two ferromagnets were fabricated and their magnetic and transport properties were evaluated. Magnetic junctions using the trilayer films showed MR of ∼5% at 4.2 K, and the sign and the magnitude of the MR were changed depending on the thickness of Nb-STO layers. The origin of the inversion of the MR is not clear, however we consider spin injection from ferromagnetic electrodes to Nb-STO through Schottky tunnel barriers plays an important role. © 2013 The Korean Physical Society.

Multiferroic heterostructures of the ferromagnetic, half-metallic Heusler Fe2CrSi (FCS) and the ferroelectric perovskite Ba0.7Sr0.3TiO3 (BSTO) have been formed by magnetron sputtering, and their magnetic and ferroelectric properties have been investigated. FCS/BSTO bilayer structures were epitaxially grown on LaAlO3 substrates with epitaxial relationships of FCS (001)[110]//BSTO (001)[100]. Multiferroic properties with a remanent polarization of 10.6 μC/cm2 and a saturation magnetization of 417 emu/cc were observed for the FCS/BSTO heterostructures at room temperature. These results suggest that the Heusler/perovskite epitaxial heterostructure is a promising candidate for fabricating multiferroic devices. © 2013 The Korean Physical Society.

Thin films and heterostructures of Mn3GaN with an inverse perovskite structure were grown epitaxially on SrTiO3 (001) and (La0.18Sr0.82)(Al0.59Ta0.41)O-3 (001) (LSAT) substrates by ion beam sputtering, and their structural and electrical properties have been investigated. Mn3GaN epitaxial thin films showed metallic behavior of temperature-dependent resistivity with a small maximum at 290-340 K. The maximum resistivity could be attributed to the magnetic transition from antiferromagnetism to paramagnetism. It has been found that epitaxial heterostructures formed by ferroelectric Ba0.7Sr0.3TiO3 and Mn3GaN layers exhibit a large magnetocapacitance effect of more than 2000% in an applied magnetic filed of 1.5 T.

The effects of magnetic anisotropy on domain structures in half-metallic Heusler alloy Fe2CrSi thin film elements were investigated using high resolution x-ray magnetic circular dichroism photoemission electron microscopy. The transition of the dominating contribution from the magnetocrystalline anisotropy to the shape anisotropy is observed in square-shaped elements when reducing the size below 2.0-2.5 mu m. In particular, we identify in disk-shaped Heusler elements the vortex state as the ground state. The shape-anisotropy dominated, well-defined magnetization configuration shows the potential of the Fe2CrSi Heusler alloy for applications in vortex-core- or domain-wall-devices, where the high spin polarization is desirable. (C) 2013 AIP Publishing LLC.

We demonstrate half-metallic Heusler Co2MnSi films epitaxially grown on diamond semiconductors using the ion-beam assisted sputtering method. Lower temperature growth below similar to 400 degrees C is key for obtaining abrupt Co2MnSi/diamond interfaces. The Co2MnSi films on diamond showed a negative anisotropic magnetoresistance of similar to 0.2% at 10 K, suggesting the half-metallic nature of the Co2MnSi films. Schottky junctions formed using the Co2MnSi/diamond heterostructures at 400 degrees C showed clear rectification properties with a rectification ratio of similar to 10(3). The Schottky barrier heights of the Co2MnSi/diamond interfaces were estimated to be similar to 0.8 eV. These results indicate that Co2MnSi is a promising spin source for spin injection into diamond. (C) 2013 AIP Publishing LLC.

A-axis oriented, electron-doped, Sr1-xLaxCuO2 thin films were grown on LaAlO3 and YAlO3 substrates by magnetron sputtering. X-ray diffraction analysis revealed that in an as-grown state, the a-axis oriented films grew with the long-c phase, which is related to oxygen vacancies in the CuO2 planes, unlike the c-axis oriented films. Another feature of our a-axis oriented films is that the films contain 90 degrees rotated domains from in-plane X-ray diffraction and planer transmission electron microscopy image. We discuss features of a-axis oriented films in terms of oxygen vacancies in the CuO2 planes.

The structure and magnetic properties of Heusler ferromagnet/ antiferromagnet (FM/AFM) bilayers were investigated. Structural characterization revealed that Fe2CrSi/Ru2MnGe bilayers were epitaxially grown with an L21 ordered structure when the Ru2MnGe growth temperature was 573 K. The magnetization curve indicates that exchange bias occurs in the Fe2CrSi/Ru2MnGe bilayers at 77 K. The appearance of an exchange-anisotropy field Hex depends on the growth of Ru2MnGe layer, indicating that the exchange bias originates from the epitaxial Fe2CrSi/Ru2MnGe interface. The exchange anisotropy energy Jk of the Fe2CrSi/Ru2MnGe and Co2MnGe/Ru2MnGe bilayer systems appears to follow a Heisenberg-like exchange coupling model. This suggests that the Cr and Mn interface atoms in the FM layers play significant roles in exchange coupling in the Fe2CrSi/Ru2MnGe and Co2MnGe/Ru 2MnGe bilayer systems, respectively. © 2013 American Institute of Physics.

The high-temperature characteristics of diamond Schottky diodes fabricated using Ag or Ni on in-situ boron-doped diamond were examined. Up to 600 C, Ag Schottky diodes exhibited a high rectification ratio of the order of 10 4. Even at ∼ 750 C, their rectification ratio was about 10, indicating that diamond field effect transistors with Ag Schottky diodes can operate at this temperature. In contrast, Ni Schottky diodes did not show clear rectification above 600 C. An analysis of the I-V curves indicated that the Ag Schottky diodes have a higher rectification ratio than the Ni Schottky diodes at high temperatures due to their higher barrier heights (φB = ∼ 2.0 and ∼ 0.7 eV for Ag and Ni, respectively). © 2013 Elsevier B.V.

Structural and electrical transport properties were investigated for Heusler-type alloy Ru-2 MnGe thin films. Ru-2 MnGe films onMgO substrate were subjected to an in-plane compressive strain due to their small lattice mismatch (-0.7%), and exhibited enhanced antiferromagnetic transition temperature (T-N) up to 353 K, which is much higher than that of the bulk material (T-N = 316 K). In contrast, the films on MgAl2O4 were almost in a relaxed-strain state, and showed T-N close to the bulk value (304 K). It was also found that the T-N of Ru-2 MnGe thin films on MgO exhibited oscillating behavior depending on c/a ratio. We argued that the next-nearest neighbor magnetic interactions (J(2)) of Mn-Mn atoms has oscillated depending on the degree of strain in Ru-2 MnGe, which contribute to the oscillating behavior of T-N against c/a.

Schottky junctions, formed using various ferromagnetic metals and alloys, were fabricated on hydrogen-terminated (H-terminated) diamond, and their interfacial characteristics were evaluated. A clear work function dependence of the Schottky barrier height (phi(B)) was obtained for these junctions, indicating that the Schottky barrier height between H-terminated diamond and ferromagnetic metals can be controlled by selecting metals with appropriate work functions. phi(B) for Ni and NiFe, which have higher work functions above 5 eV, is lower than phi(B) for other ferromagnetic metals. These results indicate that ferromagnetic metals with higher work function are promising for spin injection into diamond semiconductors. (C) 2012 Elsevier B.V. All rights reserved.

Half-Heusler La-Pt-Bi thin films have been deposited on YAlO3(001) substrate by 3-source magnetron co-sputtering. Control of the Bi content was the critical factor to obtain single phase, c-axis-oriented thin films. Generation of secondary phases was effectively prevented by precise control of the deposition rate for separate targets as well as adjustment of the deposition temperature. The realization of single-phase LaPtBi thin films will provide new potential applications to topological insulating devices based on Heusler alloys. (C) 2012 American Institute of Physics. [doi:10.1063/1.3675986]

Epitaxial MgAl2O4 thin films were grown on Heusler alloy Fe2CrSi by reactive magnetron sputtering of a MgAl2 target in an O-2+Ar atmosphere. To grow MgAl2O4 on Fe2CrSi, we inserted a protective layer of MgAl2 between Fe2CrSi and MgAl2O4 to prevent Fe2CrSi from being oxidized. Growth of MgAl2O4 was found to be very sensitive to the MgAl2 thickness and P-O2 during deposition of MgAl2O4. A strong XRD peak of MgAl2O4 (004) was observed with an ultrathin (0.2 nm) MgAl2 layer. The saturation magnetic moment of Fe2CrSi was measured to be 370 emu/cm(3) (1.84 mu B/f.u.) at room temperature and it is expected to have a high spin polarization. The Fe2CrSi/MgAl2O4 heterostructure is promising for use in future spintronic devices. (C) 2012 The Japan Society of Applied Physics

Epitaxial thin films of SrLaVMoO6 with an ordered double perovskite structure have been grown on (001) and (111) SrTiO3 substrates by magnetron sputtering. The optimized (111) film exhibited a clear (111) diffraction peak, which is a superlattice reflection of double perovskite unite cell, indicating clear B-site ordering. Temperature dependences of resistivity ρ show metallic behavior and transition point at 140-150 K, of which behavior is reminiscent of the electrical properties of materials showing long-range magnetic or antiferromagnetic order. XPS results of the Mo 3d core level spectra are discussed in terms of the B-site ordering and oxygen nonstoichiometry. © 2012 Materials Research Society.

In this study, we have fabricated Heusler alloy Fe2CrSi/Fe2VSi superlattice on MgAl2O4 substrate and have investigated interlayer exchange coupling among Fe2CrSi layers. Biquadratic coupling is dominated below the Neel temperature of Fe2VSi. By comparing magnetic coupling energy at 77 K and room temperature, it is suggested that the inter-layer exchange coupling is dominated by the proximity effect, i.e. the antiferromagnetic alignment of magnetic moments in Fe2VSi spacer layer.

The magnetic and electronic properties of double perovskite Sr 2-xLaxVMoO6 (x = 0:9-1:2) polycrystalline bulk samples have been investigated. Magnetization and electrical resistivity measurements indicate that Sr2-xLaxVMoO6 is an antiferromagnetic metal with an antiferromagnetic transition temperature in the range 123-146 K. X-ray photoemission spectroscopy measurements reveal that Mo has a tetravalent state (Mo4+) and V has a trivalent state (V 3+) in SrLaVMoO6. These results indicate that V 3+ (3d2) ions are antiferromagnetically coupled with Mo4+ (4d2) ions. Electron-doping effects of Sr 2+xLaxVMoO6 by La substitution differed from those of half-metallic Sr2-xLaxFeMoO6. This can be qualitatively explained by considering the different electron-doped orbitals and ordering ratios of B-site ions between Sr2-xLa xVMoO6 and Sr2-xLaxFeMoO6 (Mo/V or Mo/Fe). © 2011 The Japan Society of Applied Physics.

The effects of biaxial strain on the electrical and magnetic properties of an antiferromagnetic Heusler compound Fe2VSi were systematically investigated. A series of epitaxial Fe2VSi thin films on MgAl2O4 and MgO substrates were fabricated with different tensile strains by varying the substrate-lattice mismatch and the film thickness. The strain was characterized by the ratio of the out-of-plane lattice parameter c to the in-plane lattice parameter a; this ratio c/a varied from 0.987 to 0.998 at room temperature. The tensile epitaxial strain was found to increase the antiferromagnetic Neel temperature T-N to 193 K, which is 70 K higher than that for the unstrained bulk material. A clear dependence of T-N on c/a was observed, which is consistent with theoretical predictions in which the band Jahn-Teller effect plays a significant role. (C) 2011 American Institute of Physics. [doi:10.1063/1.3555089]

SrLaVMoO6 thin films have been grown on various substrates by magnetron sputtering in Ar+H-2 mixture gas. High-quality c-axis oriented SrLaVMoO6 films have been obtained in the growth temperature of 630 degrees C and Ar + 5% H-2 mixture gas. The SrLaVMoO6 films showed low resistive metallic behavior, which is comparable to bulk SrLaVMoO6. X-ray photoemission spectroscopy measurements indicate that the dominant valence state for Mo and V in the SrLaVMoO6 films is tetravalent (Mo4+) and trivalent state (V3+), respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3556898]

Diamond field effect transistors (FETs) with Pt or Al Schottky gate electrodes were fabricated on high-quality boron (B)-implanted layers formed by combining ion-implantation and high-pressure and high-temperature annealing. The high temperature characteristics of these FETs were examined. Pt-gate B-implanted diamond FETs showed maximum drain current (I-DS) of 0.16 mA/mm at gate voltage of -2 V and drain voltage of -20 V at 25 degrees C. The I-DS increased as temperature increased because of the activation of boron, and it showed maximum value of 3.9 mA/mm at similar to 200 degrees C. Al-gate FETs showed similar temperature dependence of I-DS, though the operation temperature and I-DS were higher than those of Pt-gate FETs. High-temperature operation of the B-implanted diamond FETs was possible above 550 degrees C without severe drain bulk leakage, though the maximum I-DS gradually decreased as temperature increased because of drain bulk leakage above 300 degrees C. (C) 2010 The Japan Society of Applied Physics

A dielectric barrier separating hydrogen induced p-type channel and Al gate metal contact of diamond FET has been investigated. The separation barrier is necessary to prevent tunneling current between the H-induced channel and the gate contact. In this investigation, CV measurements, fitting of forward IV characteristics, TEM and SIMS profiles have been used to obtain a more detailed picture of this barrier layer. While the composition of this layer is not clear, TEM and SIMS measurements indicate that this layer may be connected to a diamond phase or aluminium oxide. Using material properties of these materials, thickness of the separation layer extracted from the CV measurements was between 5-10 nm and the channel sheet change density was above 1 x 10(13) cm(-2). This thickness is in good agreement with the TEM observations. Frequency dependent CV measurements showed almost no frequency dependence, and no UV light dependence has been observed. Temperature dependent CV measurements showed a decrease of the dielectric constant at 100 degrees C. Fitting of the forward tunnelling current indicated a thickness of the barrier layer of about 5 nm with a barrier height of 2.4 eV. (C) 2009 Elsevier B.V. All rights reserved.

We performed beryllium (Be) doping to diamond by inserting a solid Be-rod into the plasma ball during microwave plasma CVD growth. The Be concentration can be controlled in the range from similar to 10(16) to similar to 10(18) cm(-3) by changing the Be-rod insertion depth in the plasma ball and the microwave power. In cathodoluminescence (CL) spectra of Be-doped CVD films, we observed a peak at 4.760 eV and its phonon replica as well as free-exciton (FE)related emissions. Peak energies of these emissions are close to those of Be-related emissions from Be-implanted diamond films. (C) 2008 Elsevier B.V. All rights reserved.

We tried to dope various ions (B, Al, Ga, Mg, and Be) into diamond films by combining ion-implantation and high-pressure and high-temperature annealing. In cathodoluminescence spectra of Be-implanted films, previously unreported emissions appeared at 4.843, 4.687, 4.533 eV. These emissions were only observed from Be-implanted films, and they were not observed from B, Al, Ga, and Mg-implanted ones. The 4.843-eV line is assigned to zero phonon line, and the 4.687- and 4.533-eV lines are its phonon replicas because the energy difference between each peak is close to the optical phonon energy of diamond (similar to 0.15 eV). The temperature dependence of the 4.843-eV line is similar to that of bound excitons. (C) 2008 Elsevier B.V. All rights reserved.

On the basis of the RF characteristics of p-type diamond field-effect transistors (FETs) with hydrogen surface termination, we establish an equivalent circuit (EQC) model. From comparisons of three cases we reveal that to represent the device performance in the EQC, the source, gate, and drain resistance should be considered but that the gate-source and gate-drain resistance can be ignored. The features of diamond FETs are (1) a plateau of the gate capacitance in a certain gate voltage range. (2) maximum f(T) and f(MAX) cut-off frequencies near the threshold gate voltage, and (3) a high f(MAX)/f(T) ratio similar to 3.8. We discuss these features in terms of the energy barrier between the gate metal and the two-dimensional hole channel and drift region below the gate.

Cross-sectional transmission microscopy images of hydrogen-terminated diamond field-effect transistor reveal interfacial layers between the metal and hydrogen-terminated diamond layer. Especially interesting is that an interfacial layer between Al and H-terminated diamond is clearly seen. This layer corresponds to the energy barrier, which we confirmed from an RF analysis of a diamond FET. During growth, an amorphous-like subsurface layer with vacancies has already formed on H-terminated diamond, and during subsequent metal evaporation, metal diffuses through vacancies into the subsurface layer, and eventually the interfacial layer forms. (C) 2008 Elsevier B.V. All rights reserved.

We show that high-pressure and high-temperature (HPHT) annealing of ion-implanted diamond is efficient as a doping technique. The HPHT annealing condition is located in the thermodynamically stable region for diamond. The HPHT annealing is highly effective for the recovery of damage induced by ion implantation. In the entire annealing temperature range, the HPHT annealing is more efficient than conventional thermal annealing methods such as vacuum annealing. At 1400 degrees C, we obtained the highest boron doping efficiency of 7.1%, which is ten times higher than that by vacuum annealing. (C) 2007 Elsevier B.V. All rights reserved.

Diamond layers were grown top a thickness of 25 to 35 mu m either on type-Ib synthetic or type-IIa natural diamond substrates by using high or low microwave-power densities. To evaluate defects and impurities depth profiles, the samples were angled by polishing and characterized by cathodoluminescence. The first important finding is that boron, nitrogen and structural defects seem to be concentrated in a 7-10-mu m-thick region near the substrate/layer interface, as evidenced by strong defect or impurity-related luminescences and a weak free-exciton peak. The diamond quality is thus much improved in the bulk or near the surface, especially when high microwave-power densities are used, due to the inhibition of unepitaxial crystals formation. The second important finding is that using type-IIa natural substrates, which contain extremely low levels of impurities, results in a strong decrease in the amount of boron and the intensity of the nitrogen-related peak both in the bulk and at the interface of the CVD layer. These results suggest that most impurities in our CVD layers could originate from the substrate itself (C) 2008 Elsevier B.V. All rights reserved.

Three types of growth defects commonly found epitaxial diamond films grown by chemical vapour deposition (CVD), namely unepitaxial crystals (UCs), hillocks with flat top (FHs) and pyramidal hillocks (PHs), were etched using hydrogen/oxygcn plasma to discuss their origin. UCs formed at random locations on the grown layer without any apparent relation with the substrate. Their nucleation might be due to contaminants and their development controlled by the growth conditions in the plasma. In contrast, dislocations formed from impurities segregated at the interface between the substrate and the CVD layer, were found to be the origin of the FHs and the PHs. A simple crystal model that involves micro-faceting or twinning at an intrinsic stacking fault originating from the dislocation core is proposed to explain the formation and the evolution of the growth defects. (C) 2007 Elsevier B.V. All rights reserved.

By combining ion-implantation and high-pressure and high-temperature (HPHT) annealing, we have obtained boron implanted diamond layers with a high a hole concentration and mobility and succeeded in fabricating diamond FETs using the B-implanted layers. For a B dose of 3 x 10(15) cm(-2), as heet hole concentration and mobility of 1.6 x 10(13) cm(-2) and 41 cm(2)/Vs at 300 K were obtained. Diamond FETs fabricated on the B-implanted layer showed maximum drain current of 0.15 mA/mm at gate voltage of -2 V and maximum break-down voltage of 530 V, corresponding to similar to 1.1 MV/cm. The breakdown voltage value is comparable to those of Schottky diodes using B-doped CVD diamond films. These results confirm the high quality of the B-implanted and HPHT-annealed layers. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Diamond semiconductor possesses exceptional physical properties, such as high thermal conductivity, high breakdown field, and high mobility, and is therefore expected to offer the highest performance, among semiconductors as high-frequency, high-power transistors. At present the most critical issue in achieving diamond transistors is the lack of an n-type or p-type dopant with low activation energy. This paper reviews approaches towards electronic-device application of diamond done mainly by NTT Basic Research Laboratories. First it describes our diamond field-effect transistors with 0.1-mu m gate length, which exhibit high cut-off frequencies for the current and power gains, f(T) and f(MAX), of 45 GHz and of 120 GHz, and output power density of 2.1 W/mm at 1 GHz. Next it shows how doping efficiency in ion implantation can be improved by using high-pressure high-temperature annealing. Finally, it describes Our concept of diamond/nitride heterostructure and presents results that confirm its feasibility. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigated the interface microstructure of sandwich-type MgB2/Al-A1O(x)/MgB2 Josephson tunnel junctions by cross-sectional transmission electron microscopy (TEM) in order to clarify the non-idealities in the junction characteristics. The results indicate that there are poor-crystal line MgB2 layers and/or amorphous Mg-B composite layers of a few nanometers between the AlOx barrier and upper MgB2 layer. The poor-crystalline upper Mg-B layers seem to behave as normal metal or deteriorated superconducting layers, which may be the principal reason for all non-idealities of our MgB2/Al-AIO(x)/MgB2 junctions.

The present status of diamond-based transistors for high-frequency and high-power applications is reviewed. We have achieved the drain current density of 550 mA/mm, cut-off frequencies for current gain (f(T)) and power gain (f(MAX)) of 45 GHz and 120 GHz, respectively, and output-power density of 2.1 W/mm at 1 GHz in class-A operation of a field-effect transistor (FET) with hydrogen (H)-surface-terminated diamond. We have found that gate capacitance can be separated into depletion-layer capacitance and insulator capacitance. Concerning a stability of H-surface termination, no apparent decrease in the current for an FET without a gate contact was observed, but gate bias stress results in a slight decrease in the drain current and simultaneously an increase in the gate leakage current. (c) 2007 Elsevier B.V. All rights reserved.

The authors show that high-pressure and high-temperature (HPHT) annealing is very effective for the activation of ion-implanted dopants in diamond. The HPHT annealing condition is located in the thermodynamically stable region for diamond in the phase diagram and is, therefore, much more efficient for the recovery of implantation-induced damage and for the activation of ion-implanted dopants than thermal annealing in vacuum. The B-implanted film after HPHT annealing showed a high mobility of 632 cm(2)/V s with a sheet hole concentration of 4.8x10(10) cm(-2) at 300 K and the doping efficiency of similar to 7%. The mobility is the highest so far for ion-implanted diamond. In the entire annealing temperature range, the HPHT annealing is more efficient than the thermal annealing in vacuum. (c) 2007 American Institute of Physics.

The authors report on a terahertz (600 GHz) mixing experiment with MgB2 microbolometers in the resistive state. The authors observed that for a 20 nm film a mixer gain bandwidth of 2.3 GHz can be achieved, corresponding to an energy relaxation time of 70 ps. The experimental results were analyzed using a two-temperature model. As a result, the phonon escape time of similar to 20 ps was deduced. At 1.6 THz the MgB2 mixer uncorrected noise temperature was 11 000 K. The obtained results show that MgB2 bolometers are good prospects for the terahertz range as both broadband mixers and fast direct detectors.

The radio-frequency characteristics of p-type diamond field-effect transistors with hydrogen surface termination were numerically analyzed using an equivalent-circuit model. From the gate-source capacitance (C-GS)-voltage (V-GS) results extracted from measured s parameters, the authors found a plateau in C-GS within a certain V-GS range. This means that a two-dimensional hole gas channel forms parallel to the surface and that the channel is separated by a thin energy-barrier layer with an infinite height from the gate metal. At a high negative V-GS, as negative V-GS is increased, C-GS increases steeply. This results from holes penetrating the energy barrier. (c) 2007 American Institute of Physics.

High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by - 2 times and the luminescence bands from 270 to 320 min, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm(2)/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films. (c) 2006 Elsevier B.V. All rights reserved.

We characterized high-quality polycrystalline diamond with large grains and fabricated polycrystalline diamond field effect transistors (FETs). The polycrystalline diamond had (110) preferred orientation, and its typical grain size was similar to 100 mu m. Well-resolved free exciton related emissions were observed at room temperature in cathodoluminescence. The FETs showed extremely high DC and RF performance. The cut-off frequency for current gain (f(T)) and power gain (f(max)) were 45 and 120 GHz, respectively. The maximum drain current (I-DS) was 550 mA/mm. These values are the highest among diamond FETs, including those fabricated from single-crystal diamond. These results suggest that high-quality polycrystalline diamond, whose maximum size is 4 in., is very promising for diamond electronic devices. (c) 2006 Elsevier B.V. All rights reserved.

MgB2 thin films with the T-c above 38 K have been fabricated using molecular beam epitaxy (NIBE) at the growth temperature below 300 degrees C. The T-c of 38.2 K was obtained in 1.3-mu m-thick MgB2 films formed on SiC substrates with AIN buffer layers. The T-c is comparable to those of MgB2 single crystals and the highest among MgB2 films fabricated below 500 degrees C. Increase of the film thickness of MgB2 above 1 mu m was the key point to obtain high-quality films at low growth temperature.

Using high-quality polycrystalline chemical-vapor-deposited diamond films with large grains (similar to 100 mu m), field effect transistors (FETs) with gate lengths of 0.1 mu m were fabricated. From the RF characteristics, the maximum transition frequency f(T) and the maximum frequency of oscillation f(max) were similar to 45 and similar to 120 GHz, respectively. The fT and f(max) values are much higher than the highest values for single-crystalline diamond FETs. The dc characteristics of the FET showed a drain-current density-I-DS of 550 mA/mm at gate-source voltage V-GS of -3.5 V and a maximum transconductance g(m) of 143 mS/mm at drain voltage V-DS of -8 V. These results indicate that the high-quality polycrystalline diamond film, whose maximum size is 4 in at present, is a most promising substrate for diamond electronic devices.

Temperature dependent DC and RF characteristics of p-type diamond metal-semiconductor field-effect transistors (MESFETs) on hydrogen-terminated surfaces are investigated. The device is thermally stable up to 100 degrees C. because it does not deteriorate at all at higher temperatures. Temperature coefficients of transconductance (g(m)), drain conductance (g(ds)), gate-source capacitance (C-gs), gate-drain capacitance (C-gd), cut-off frequency (f(T)), and maximum drain current (I-ds) were obtained from small-signal equivalent circuit analysis. The cut-off frequency (fT) is almost totally independent of temperature. Intrinsic g(m), g(ds), and C-gs decrease with increasing temperature. C-gd is almost totally independent of temperature. The threshold voltage shifts to the negative side with increasing temperature. We propose a band model of an Al-gate contact/H-terminated diamond to explain the temperature dependence of these components.

We report great improvement of RF output power for H-terminated diamond field-effect transistors (FETs). For the FET device with a gate width of 1 mm and a gate length of 0.4 mu m, the maximum output power (P-out) is 1.26 W, the maximum power gain is 23.2 dB, and the power added efficiency (PAE) is 56.3%. The increase in the device temperature when output power is 0.84 W is only similar to 0.6 degrees C. This is due to diamond having the highest thermal conductivity. (c) 2006 Elsevier B.V. All rights reserved.

This paper reports the first Studies on temperature dependent DC and RF characteristics of diamond metal-semi conductor field-effect transistors along with circular-transmission-like-method measurements on hydrogen-terminated diamond surface. In general, the device under study is thermally stable up to 100 degrees C as it does not deteriorate at higher temperatures with the cut-off frequency for current gain maintained at 8 similar to 9 GHz. It is found that the sheet resistance is almost totally independent of temperature, contact resistance is negligible, and channel conductance underneath the gate decreases with increasing temperature. The threshold voltage for the device is found to shift to the negative side with increasing temperature. A small-signal equivalent circuit analysis reveals that both transconductance and gate-source capacitance decrease with increasing temperature,. which results in the almost constant cut-off frequency for current gain. The experimental results call be explained by the fact that with increasing temperature, the band near the Al/H-terminated diamond surface bends upward more weakly, which leads to a decrease of buffer capacitance. At the same time the mobility decreases and the transconductance therefore decreases. (c) 2006 Elsevier B.V. All rights reserved.

Great improvement in the output power density of diamond FETs on a diamond homoepitaxial layer grown using a high-purity source gas is reported. For a device with a gate length of 0.1 mu m and gate width of 100 pro, at 1 GHz, maximum output power density of 2.1 W/mm, maximum power gain of 10.9 dB, and power added efficiency of 31.8% were obtained.

Sandwich-type all-MgB2 Josephson tunnel junctions (MgB2/AlOx/MgB2) have been fabricated with as-grown MgB2 films formed by molecular-beam epitaxy. The junctions exhibit substantial superconducting current (IcRN product similar to 0.8 mV at 4.2 K), a well-defined superconducting gap (Delta = 2.2-2.3 mV), and clear Fraunhofer patterns. The superconducting gap voltage of Delta agrees well with the smaller gap in the multigap scenario. The results demonstrate that MgB2 has great promise for superconducting electronics that can be operated at T similar to 20 K. (c) 2005 American Institute of Physics.

We briefly survey recent developments in the thin film synthesis and junction fabrication of MgB2 toward superconducting electronics. The most serious problem in the thin film synthesis of MgB2 is the high vapor pressure required for phase stability. This problem makes in-situ film growth difficult. However, there has been substantial progress in thin film technology for MgB2 in the past three years. The low-temperature thin-film process in a UHV chamber can produce high-quality MgB2 films with T-c similar to 35 K. Furthermore, technology to produce single-crystal epitaxial MgB2 films has recently been developed by using hybrid physical-chemical vapor deposition. With regard to Josephson junctions, various types of junctions have been fabricated, all of which indicate that MgB2 has potential for superconducting devices that operate at 20-30 K, the temperature reached by current commercial cryocoolers.

We briefly survey recent developments in the thin film synthesis and junction fabrication of MgB2 toward superconducting electronics. The most serious problem in the thin film synthesis of MgB2 is the high vapor pressure required for phase stability. This problem makes insitu film growth difficult. However, there has been substantial progress in thin film technology for MgB2 in the past three years. The low-temperature thin-film process in a UHV chamber can produce high-quality MgB2 films with Tc ∼ 35 K. Furthermore, technology to produce single-crystal epitaxial MgB2 films has recently been developed by using hybrid physicalchemical vapor deposition. With regard to Josephson junctions, various types of junctions have been fabricated, all of which indicate that MgB2 has potential for superconducting devices that operate at 20-30 K, the temperature reached by current commercial crvocoolers. Copyright © 2005 The Institute of Electronics, Information and Communication Engineers.

As-grown superconducting thin films of MgB2 were prepared by molecular beam epitaxy (MBE), and studied by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Only films prepared at temperatures between 150 and 320 degreesC showed superconductivity. The best T-c(onset) of 36 K was obtained with a sharp transition width of similar to1 K although the film crystallinity was poor. The in situ photoelectron spectra obtained on the surfaces of the MBE grown MgB2 films were free from dirt peaks. The XP spectra revealed the binding energy of the Mg 2p levels in MgB2 is close to that of metallic Mg and the binding energy of B 1s is close to that of transition-metal diborides. The valence UP spectra showed a clear Fermi edge although the density of states at E-F is low and the major components of the valence band are located between 5 and 11 eV. (C) 2002 Elsevier Science B.V. All rights reserved.

We grew single-crystalline Sr1-xLaxCuO2 thin films of electron-doped infinite-layer compounds by molecular beam epitaxy. Crucial to our success was the use of KTaO3 substrates. The best film showed T-c(onset) = 41.5 K and T-c(zero) = 39.0 K. The resistivity of the optimum-doped films exhibited metallic temperature dependence with a low resistivity of 320 muOmega cm. at room temperature and 120 muOmega cm. just above T-c. Such excellent electrical properties have not previously been reported for this system with either thin films or bulk polycrystalline specimens. (C) 2002 Elsevier Science B.V. All rights reserved.

Single-crystalline Sr1-xLaxCuO2 thin films of electron-doped infinite-layer compounds were grown by molecular-beam epitaxy. Crucial to our success was the use of KTaO3 substrates. The best film showed T-c(onset)=41.5 K and T-c(zero)=39.0 K, which is close to the highest T-c(onset) of 43 K for the bulk value. The resistivity of the optimum-doped film exhibited metallic temperature dependence with a low resistivity of 320 mu Omega cm at room temperature and 120 mu Omega cm just above T-c. (C) 2001 American Institute of Physics.

Superconducting thin films of magnesium diboride (MgB2) were prepared on various substrates [SrTiO3 (001), sapphire R, sapphire C and Si (111)] by molecular beam epitaxy. The growth temperature was examined in the ambient to 650 degreesC. Only films formed at temperatures between 150 and 320 degreesC showed superconductivity. The best T-C(onset) of 36 K with a sharp transition width of similar to1 K was observed. The T-C of the as-grown superconducting MgB2 thin films is close to the bulk value. (C) 2001 American Institute of Physics.

We have demonstrated the possibility of growing p-type ZnO films by a pulsed laser deposition technique combined with plasma gas source. The p-type ZnO film has been fabricated by passing N2O gas through an electron cyclotron resonance (ECR) or RF plasma source. N-O gas is effective to prevent "O" vacancy from occurring and introduce "N" as an acceptor, at the same time. With Ca and N codoping technique, we have observed a room temperature resistivity of 0.5 Omega cm and a carrier concentration of 5 x 10(19)cm (3) in ZnO film on glass substrate. Two-step growth, with a thin ZnO template layer formed at high temperature, is quite effective to realize a well crystallized growth at low temperature. The observed p-type ZnO films will open the door for practical applications in various oxide electronic devices. (C) 2001 Elsevier Science B.V. All rights reserved.

3d-transition-metal-doped ZnO films (n-type Zn1-xMxO (x=0.05-0.25): M=Co, Mn, Cr, Ni) are formed on sapphire substrates using a pulsed-laser deposition technique, and their magnetic and electric properties are examined. The Co-doped ZnO films showed the maximum solubility limit. Some of the Co-doped ZnO films exhibit ferromagnetic behaviors with the Curie temperature higher than room temperature. The magnetic properties of Co-doped ZnO films depend on the concentration of Co ions and carriers. (C) 2001 American Institute of Physics.

The magnetic properties of LaFeO3-LaCrO3 superlattices have been artificially controlled by adjusting the atomic order of the B site ions via changes of the stacking direction of the [100], [110] and [111] planes and adjustment of the stacking periodicity. When Fe and Cr layers are atomically stacked along the [111] direction, ferromagnetism appears. In the case of superlattices growing along the < 100 > and [110] directions, antiferromagnetic properties appear, and the Neel temperature changes systematically as the stacking periodicity increases. Although the total number of Fe and Cr ions are the same in the (100), (110) and (111) stacking planes, different magnetic properties can be created in artificial superlattices. (C) 2001 American Institute of Physics.

LaFe0.5Mn0.5O3 solid solution films have been formed on SrTiO3 (111) substrates using a pulsed laser deposition technique and their magnetic properties have been examined. The films showed ferromagnetic (or ferromagnetic) behavior with a Curie temperature of 380 K and the saturation magnetization was estimated to be about 1.5 mu (B) per magnetic ion site (B site). The x-ray photoemission spectra indicated that this behavior was due to the partial ordering of magnetic ions (Fe and Mn ions) which is caused by the charge disproportion between Fe and Mn ions under the film formation conditions. (C) 2001 American Institute of Physics.

Thin films of (1 1 1)-oriented Mg1.5FeTi0.5O4 spinel have been successfully prepared by a pulsed laser deposition (PLD) technique on alpha -Al2O3 (0001) substrates. The value of the lattice parameter of the (1 1 1) out-of-plane and the Neel temperature of the films depend on the film thickness. The spin-glass behavior has been observed with a freezing temperature of 100 K, which is 80 K higher than that of the bulk material. This freezing temperature is reduced by increasing the oxygen pressure during film preparation. These results show that the PLD technique is promising not only for obtaining thin films of oxide materials under a wide range of conditions but also for controlling the magnetic properties of films. (C) 2000 Elsevier Science Ltd. All rights reserved.

Artificial superlattices of SrRuO3 and BaTiO3 were formed on SrTiO3 (110) substrates with various stacking periodicities and their magnetic and electric properties were examined. The Curie temperature and the magnetization of the superlattices decreased as the stacking periodicity decreased. Antiferromagnetic behaviors and the anisotropy of resistivity were observed in a two- by five-layer (2/5) superlattice. Low-dimensional effects appeared strongly in the 2/5 superlattice. (C) 2000 Elsevier Science Ltd. All rights reserved.

Magnetic properties have been artificially controlled in LaFeO3-LaCrO3 superlattices by arranging the atomic order of B site inns with changing the stacking direction and periodicity. When Fe and Cr layers are atomically stacked along the (111) direction, ferromagnetism appears, as shown in our previous study. In the case of the superlattices formed along the (100) direction, on the other hand. an antiferromagneiic property appears anal the Neel temperatures systematically change from 250 K to > 400 K, corresponding to the stacking periodicity. Although the total numbers of Fe and Cr ions are the same in (111) and (100) stacking, quite a different magnetic character can be created in the artificial superlattices.

Artificial superlattices of LaFeO3-LaMnO3 were formed on SrTiO3 (111), (100), and (110) substrates with various stacking periodicity using a pulsed laser deposition technique, and their magnetic properties were controlled by altering the ordering of magnetic ions (Fe or Mn). For superlattices constructed on the (111) plane, all the superlattices showed ferromagnetic (or ferrimagnetic) behaviors and the same Curie temperatures (T-C) at 230 K. The magnetization was reduced as the stacking periodicity of the superlattices decreased. On the other hand, in the case of superlattices formed on (110) or (100) substrates, the increase of the spin frustration effect at the LaFeO3-LaMnO3 interface with decreasing the stacking periodicity caused a reduction of T-C and magnetization. In particular, spin-glass-like behavior was observed in superlattices of less than 3/3 stacking periodicity. [S0163-1829(99)50442-6].

(Bi0.7Ba0.3) (Fe0.7Ti0.3)O-3 films have been constructed on Nb-doped SrTiO3 (100) by the pulsed-laser deposition technique, and their physical properties have been examined. The films exhibit both ferroelectricity and ferromagnetism (weak ferromagnetism) with P-r=2.5 mu C/cm(2) and M-r=0.2 emu/g at room temperature. The film-thickness dependence of their magnetic and electric properties (size effect) is also discussed simultaneously. The dielectric constants of the films decrease with reducing film thickness below 1000 Angstrom. The magnetic Curie temperature of the films, on the other hand, does not change at all down to 250 Angstrom. (C) 1999 American Institute of Physics. [S0003-6951(99)02130-0].

Bismuth-based-layer-structure ferroelectric films have been formed epitaxially on Nb-doped SrTiO3 substrates by pulsed laser deposition (PLD). These films show an atomically smooth surface with a wide terrace of 100 nm similar to 200 nm, as observed by atomic force microscopy (AFM) which is ideal for measuring such properties. Thin films of (Bi-0.7, Ba-0.3) (Fe-0.7, Ti-0.3)O-3 exhibit both ferroelectric and ferromagnetic (weak ferromagnetism) properties at room temperature. In these samples, a size effect is discussed at the same time.

We have constructed ferroelectric and/or ferromagnetic superlattices by a layer-by-layer successive deposition technique with a laser MBE. An ideal hetero-epitaxy can be obtained due to the similar crystal structure of the perovskite type ferroelectric BaTiO(3), SrTiO(3), Bi-based layered compounds and ferro- or antiferromagnetic LaFeO(3), LaCrO(3), (La,Sr)MnO(3). In both superlattices, strain effect plays an important role for deterring their physical properties. In the Bi-based layer structured superlattices, we have controlled dielectric constant and ferroelectric properties. We also control the spin order on LaFeO(3)/LaCrO(3) superlattices formed on SrTiO(3)(111) and (100) substrate. Such a spin structure(ferromagnetic order) cannot be realized in bulk samples. (C) 1998 Elsevier Science S.A. All rights reserved.

Ferromagnetic spin order has been realized in the LaCrO3-LaFeO3 superlattices. Ferromagnetic coupling between Fe3+ and Cr3+ through oxygen has long been expected on the basis of Anderson, Goodenough, and Kanamori rules. Despite many studies of Fe-O-Cr-based compounds, random positioning of Fe3+ and Cr3+ ions has frustrated the observation of ferromagnetic properties. By creating artificial superlattices of Fe3+ and Cr3+ layer along the [111] direction, ferromagnetic ordering has been achieved.

We have constructed artificial superlattices with a combination of magnetic/magnetic and ferroelectric/ferromagnetic materials using a laser ablation technique. An ideal hetero-epitaxy can be obtained due to the similar crystal structure of the perovskite type di/ferroelectric BaTiO3, Pb(Zr,Ti)O-3 (so-called PZT), SrTiO3 and ferro/antiferromagnetic LaFeO3, LaCrO3, (La,Sr)MnO3. First of all, we have controlled ferromagnetic order on LaFeO3/LaCrO3 superlattices formed on SrTiO3(111) substrate. Such a spin structure(ferromagnetic order) can't be got in bulk condition. In the heterostructured ferromagnetic/ferroelectric devices, (La,Sr)MnO3/PZT there are remarkable and interesting phenomena. The electric properties of the ferromagnetic material can be controlled by the piezoelectric effect via distortion of the crystal structure.

The dependency of the magnetic susceptibility on temperature has elucidated that the mineral atacamite Cu2Cl(OH)(3) has the antiferromagnetic property. In order to explain this behavior and the spin arrangement derived from experimental studies, the ab initio UHF MO and UNO CASCI calculations were carried out for the tetranuclear cluster of copper(II), Cu4Cl(OH)(3), which is the constitution in the atacamite crystal structure. The results of the CASCI calculations appreciate the relative stabilities among various spin arrangements for the tetranuclear cluster, being consistent with the experiments.

Cu(HCOO)(OH) exhibits a strong ferromagnetic interaction (theta =+43K). The susceptibility shows a sharp maximum at T-m=21K and a plateau below 12K. This magnetic behavior is characteristic of antiferromagnetic materials. The spin-flop behavior is observed below T-m. To examine the spin-flop behavior of Cu(HCOO)(OH), magnetic phase diagram was drawn up. Magnetic phase diagram of Cu(DCOO)(OD) was also constructed to examine whether magnetic interaction between layers is due to hydrogen bond or not. Between them there is little difference in the phase diagram, but there is a remarkable difference in the hysteresis curves at 2K. Besides, temperature dependence of susceptibility and hysteresis curve at 1.8K of Cu-3(HCOO)(2)(OH)(4) was measured. In consequence it was found that this material also showed spin-flop behavior.