Search

The use of high‑energy x‑ray diffraction techniques with the latest generation synchrotron sources has created new approaches to study quantitatively the structure of noncrystalline materials. Recently, this technique has been combined with neutron diffraction at pulsed source to provide more detailed and reliable structural information not previously available. This article reviews and summarises recent results obtained from the high energy x‑ray diffraction on several oxide glasses, SiO2, B2O3 and PbSiO3, using bending magnet beamlines at SPring‑8. In particular, it addresses the structural models of the oxide glasses obtained by the reverse Monte Carlo (RMC) modelling technique using both the high energy x‑ray and neutron diffraction data.

We investigated an effect of sputtering gas species (Ar, Kr, and Xe) for deposition of a W insertion layer in the CoFeB/W/CoFeB free layer on magnetic properties of the free layer and tunnel magnetoresistance (TMR) ratio of magnetic tunnel junctions (MTJs) stacks using the free layer annealed at 400 °C for 1 h. As the W insertion layer thickness $t_{W}$ increased, we found the degradation of perpendicular anisotropy and larger reduction of saturation magnetic moment per unit area $m_{S}$ in the free layer using Ar compared to those using Kr and Xe. We also found a smaller TMR ratio for the MTJ stack using Ar compared to those using Kr and Xe. Energy-dispersive X-ray spectrometry line analysis revealed more significant interdiffusion between W and CoFeB layers in the free layer using Ar than those using Kr and Xe, that could result in the smaller $m_{S}$ and perpendicular anisotropy in the free layer and smaller TMR ratio for the MTJ stack using Ar than those using Kr and Xe. We also investigated concentration of Ar, Kr, and Xe in W layers deposited using Ar, Kr, and Xe, respectively, by high-resolution Rutherford backscattering spectrometry, revealing that 0.2 at% Ar was detected in the W layer using Ar, while Kr and Xe were not detected in W layers using Kr and Xe. Such a difference in concentration of inert gas atoms in the W layer could be one possible reason for the difference about degree of interdiffusion between W and CoFeB layers.

The interplay of magnetic and semiconducting properties has been in the focus since more than a half of the century. In this introductory article we briefly review the key properties and functionalities of various magnetic semiconductor families, including europium chalcogenides, chromium spinels, dilute magnetic semiconductors, dilute ferromagnetic semiconductors and insulators, mentioning also sources of non-uniformities in the magnetization distribution, accounting for an apparent high Curie temperature ferromagnetism in many systems. Our survey is carried out from today's perspective of ferromagnetic and antiferromagnetic spintronics as well as of the emerging fields of magnetic topological materials and atomically thin 2D layers., 7 pages, 87 references

We investigated annealing temperature $T_{a}$ dependence of tunnel magnetoresistance (TMR) ratio and magnetic properties for perpendicular-anisotropy (CoFe)100– X B X /MgO magnetic tunnel junctions (MTJs) with single (CoFe)100– X B X /MgO interface (s-MTJ) and double CoFeB–MgO interface (d-MTJ) structures with various boron compositions X. High TMR ratio over 100% was observed in the s-MTJ with $X= 35$ at.% after annealing at 360 °C–400 °C, whereas the s-MTJ with $X = 30$ at.% showed the degradation of TMR ratio with the increase of $T_{a}$ above 360 °C, resulting from the decrease of perpendicular anisotropy. The d-MTJ with $X = 25$ at.% maintained high TMR ratio up to $T_{a} = 400~^{\circ }\text{C}$ owing to its higher perpendicular anisotropy compared with the s-MTJ. The difference of perpendicular anisotropy between the s-MTJ and the d-MTJ can be attributed to higher interfacial anisotropy together with lower saturation magnetization of the d-MTJs. The lower saturation magnetization is attributable to two MgO layers that suppress boron diffusion from CoFeB layers, which was verified by cross-sectional line analysis using electron energy-loss spectroscopy.

Diffusion and redistribution of constituent elements on the CoFeB surface after natural oxidation and annealing were investigated by means of X-ray photoelectron spectroscopy with argon beam sputtering. Boron and iron atoms were found to redistribute into the $\sim 3$ nm-thick oxidized surface by the natural oxidation even at room temperature. After annealing up to 400 °C in vacuum, boron atoms drastically diffused into the oxidized surface to reduce the oxidized iron/cobalt. The migration of boron atoms was observed within the depth of $\sim 6$ nm. In contrast, iron atoms diffused little. The diffusion and redistribution behaviors during the redox reactions, such as the natural oxidation and annealing, are speculated to be related to the values of the standard free energy of formation of oxides. In other words, the reduction nature of elements on the CoFeB surface is speculated to be the driving force in the diffusion and redistribution of iron and boron on the surface during the redox reaction.

Spin-transfer-torque magnetoresistive random access memories using CoFeB-MgO based magnetic tunnel junctions with perpendicular easy axis (p-MTJs) are attracting much attention owing to their high potential in offering electronics with both low-power-consumption and high-performance [1–3].

Junction size (D) dependence of thermal stability (Δ) factor and intrinsic critical current (IC0) were investigated for MgO/CoFeB/Ta/CoFeB/MgO recording structure in magnetic tunnel junctions (MTJs) having a CoFeB reference layer and a synthetic ferrimagnetic (SyF) reference layer. Δ of the recording structure shows almost constant value down to 40 nm, whereas IC0 shows a linear dependence on the recording layer area, as similarly observed in recording structure with single-interface. Average absolute intrinsic critical current density is 3.5 MA/cm2, which is comparable to previously reported value for recording structure with single-interface. A MgO/CoFeB(1.4)/Ta(0.4)/CoFeB(1.0)/MgO double-interface recording structure in MTJ with SyF reference layer shows Δ of 59 at D = 29 nm.

We studied the magnetic and magnetoresistance characteristics of pseudospin-valve magnetic tunnel junctions (MTJs) based on CoFe/Pd multilayer electrodes with perpendicular magnetic anisotropy and an MgO barrier. The MTJs at annealing temperature (T a) of 473 K showed a tunnel-magnetoresistance (TMR) ratio of 1.5%. An fcc (111)-oriented texture of the bottom and top Co90Fe10/Pd multilayer electrodes, together with an imperfectly crystallized MgO, were revealed by cross-sectional TEM images. The TMR properties of perpendicular MTJs with a Co20Fe60B20 or Co50Fe50 layer inserted between the CoFe/Pd multilayer electrodes and the MgO barrier were also studied. The TMR ratio with Co20Fe60B20 insertion was 1.7% at T a= 473 K and monotonically decreased at T a over 523 K. The TMR ratio with Co50Fe50 insertion increased up to 3% at T a= 573 K and then decreased to 0.4% at T a= 598 K. The influence of the Pd layer on CoFeB was studied by using the simplified structures of Pd/CoFeB/MgO/CoFeB/Pd and Ta/CoFeB/MgO/CoFeB/Ta with inplane anisotropy. A former structure with Pd resulted in reduced TMR ratio which decreases with increasing T a, whereas MTJs with a Ta-based structure showed a monotonic increase of a TMR ratio. The low TMR ratio observed in Pd-containing structures appears to result from crystallization of CoFeB in an unfavorable crystal orientation.

Conventional semiconductor devices use electric fields to control conductivity, a scalar quantity, for information processing. In magnetic materials, the direction of magnetization, a vector quantity, is of fundamental importance. In magnetic data storage, magnetization is manipulated with a current-generated magnetic field (Oersted-Ampère field), and spin current is being studied for use in non-volatile magnetic memories. To make control of magnetization fully compatible with semiconductor devices, it is highly desirable to control magnetization using electric fields. Conventionally, this is achieved by means of magnetostriction produced by mechanically generated strain through the use of piezoelectricity. Multiferroics have been widely studied in an alternative approach where ferroelectricity is combined with ferromagnetism. Magnetic-field control of electric polarization has been reported in these multiferroics using the magnetoelectric effect, but the inverse effect-direct electrical control of magnetization-has not so far been observed. Here we show that the manipulation of magnetization can be achieved solely by electric fields in a ferromagnetic semiconductor, (Ga,Mn)As. The magnetic anisotropy, which determines the magnetization direction, depends on the charge carrier (hole) concentration in (Ga,Mn)As. By applying an electric field using a metal-insulator-semiconductor structure, the hole concentration and, thereby, the magnetic anisotropy can be controlled, allowing manipulation of the magnetization direction.

Ferromagnetism in transition-metal-doped III-V and II-VI compound semiconductors and their heterostructures allow exploration of unprecedented possibilities, in which spin degrees of freedom and more common charge degrees of freedom are combined. These include quantum heterostructures that incorporate ferromagnetism, electric field as well as light control of ferromagnetism, and magnetization reversal by electrical means. Possibilities of achieving high ferromagnetic-transition temperature are also discussed

We report on molecular-beam epitaxy (MBE) of Mn-doped GaAs films grown at relatively high temperatures and their properties. The samples were grown at 400, 450, 530, and 560 °C and their growth fronts were monitored by in situ reflection high-energy electron diffraction (RHEED) during the entire growth. From the RHEED pattern, we found that the maximum doping composition of Mn is subpercent at these temperatures. The RHEED pattern and the surface morphology of the samples grown at 400 and 450 °C were influenced strongly by As overpressure and Mn doping.

We have developed a perpendicular-anisotropy magnetic tunnel junction (p-MTJ) stack with CoFeB free layer and Co/Pt multilayer based synthetic ferrimagnetic (SyF) pinned layer that withstand annealing at a temperature up to 420°C (that compatible with CMOS BEOL process) by controlling boron diffusion. We demonstrated the 10 nmφ p-MTJ with double CoFeB/MgO interface tolerable against 400°C annealing which is a requisite building block for realization of high density spin transfer torque magnetic random access memory (STT-MRAM) in reduced dimensions.

This Review discusses recent advances towards electric-field control of magnetism in ferromagnetic semiconductors and metals, and in multiferroics. The electrical manipulation of magnetism and magnetic properties has been achieved across a number of different material systems. For example, applying an electric field to a ferromagnetic material through an insulator alters its charge-carrier population. In the case of thin films of ferromagnetic semiconductors, this change in carrier density in turn affects the magnetic exchange interaction and magnetic anisotropy; in ferromagnetic metals, it instead changes the Fermi level position at the interface that governs the magnetic anisotropy of the metal. In multiferroics, an applied electric field couples with the magnetization through electrical polarization. This Review summarizes the experimental progress made in the electrical manipulation of magnetization in such materials, discusses our current understanding of the mechanisms, and finally presents the future prospects of the field.

In-plane magnetic anisotropy of a (Ga,Mn)As film has been investigated by measuring anisotropic magnetoresistance as a function of the direction of in-plane magnetic fields. Two Hall bars with channel direction [-110] or [100] were fabricated to detect the in-plane magnetization reversal process from both the longitudinal and the transverse components of anisotropic magnetoresistance. The results show the coexistence of cubic 〈100〉 easy axes and uniaxial [100] hard axis. The data are fitted well by a coherent magnetization model. Temperature dependence of the in-plane anisotropic fields, determined both by the magnetotransport measurements and by ferromagnetic resonance, is presented. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have fabricated a three terminal device integrating a (Ga,Mn)As Esaki diode and a light emitting diode (LED) to investigate the bias-voltage dependence of the injection of spin polarized electrons. The electroluminescence polarization (PEL) from the LED was measured under Faraday configuration as a function of bias voltages applied independently to the Esaki diode and to the LED. The polarization shows strong dependence on the bias applied to the Esaki diode when the LED bias is fixed. The maximum PEL of 32.4% was observed when the valence electrons near the Fermi energy of (Ga,Mn)As are injected into the LED. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Introduction of a high concentration of manganese in III?V semiconductors, such as InAs and GaAs, results in carrier-induced ferromagnetism, which allows us to integrate ferromagnetism in nonmagnetic heterostructures and which modifies their magnetic properties through electric-field control of carrier concentration. The properties of ferromagnetism can in many cases be semi-quantitatively understood by the p?d Zener model, which is qualitatively different from conventional ferromagnetic metals. These ferromagnetic III?V semiconductors also offer the unique opportunity of examining spin-dependent phenomena observed so far only in metallic systems. Here, we review our experimental study on electrical manipulation of magnetization in these ferromagnetic III?V semiconductors. We first describe the results of electrically assisted magnetization reversal in ferromagnetic semiconductor (In, Mn)As field-effect transistor structures. The coercivity as well as ferromagnetic transition temperature can be controlled through the modification of carrier concentration by applied electric fields in a gated structure. We then present electrical magnetization reversal by spin-transfer torque exerted by spin-polarized currents at low threshold current density (~105?A?cm?2) in (Ga, Mn)As-based magnetic tunnel junctions.

The surface morphologies of ZnO thin films grown on Al 2 O 3 (1120) (a-plane sapphire) substrate by plasma-assisted molecular beam epitaxy (PAMBE) were systematically investigated by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) as a function of Zn beam flux intensity and substrate temperature. The ZnO films are uniquely (0001) oriented with no trace of secondary orientation. A phase diagram for ZnO growth was established, which described the ZnO growth mode transition at fixed O 2 flow rate of 0.3 sccm. For the growth at 750 °C with Zn beam flux of 3.6 x 10 -7 Torr, the RHEED showed a (3 x 3) pattern and a smooth surface represented by atomically flat terraces and half unit cell high steps (∼0.26 nm, a charge neutral unit of ZnO) was observed corresponding to a monolayer or bilayer thickness of Zn-O along the c-axis of (000 1) ZnO.

A new scheme of magnetization reversal, an electrically assisted magnetization reversal, is realized in a carrier-induced ferromagnetic semiconductor (In, Mn)As structure. The demonstration has been done with field-effect transistors (FETs) having a thin (In, Mn)As channel by the application of a gate electric field to control the hole concentration p.

At a lattice-matched ZnO/ScAIMgO(4) heterointerface, Hall and field-effect mobilities of grain-boundary-free ZnO channels have been simultaneously characterized under a gate electric field (E-G) applied through a ScAIMGO(4) dielectric gate. The field-effect mobility increased linearly with increasing EG (see Figure), clearly in contrast to the supralinear (exponential) dependence that has been previously reported for polycrystalline channels.

Magnetic information storage relies on external magnetic fields to encode logical bits through magnetization reversal. But because the magnetic fields needed to operate ultradense storage devices are too high to generate, magnetization reversal by electrical currents is attracting much interest as a promising alternative encoding method. Indeed, spin-polarized currents can reverse the magnetization direction of nanometre-sized metallic structures through torque; however, the high current densities of 10(7)-10(8) A cm(-2) that are at present required exceed the threshold values tolerated by the metal interconnects of integrated circuits. Encoding magnetic information in metallic systems has also been achieved by manipulating the domain walls at the boundary between regions with different magnetization directions, but the approach again requires high current densities of about 10(7) A cm(-2). Here we demonstrate that, in a ferromagnetic semiconductor structure, magnetization reversal through domain-wall switching can be induced in the absence of a magnetic field using current pulses with densities below 10(5) A cm(-2). The slow switching speed and low ferromagnetic transition temperature of our current system are impractical. But provided these problems can be addressed, magnetic reversal through electric pulses with reduced current densities could provide a route to magnetic information storage applications.

Low-temperature molecular beam epitaxy has enabled synthesis of magnetic III–V semiconductors that can be epitaxially integrated into nonmagnetic III–V heterostructures. Carrier-induced ferromagnetism in these magnetic III–Vs has allowed us to explore spin-dependent phenomena previously not accessible in semiconductors. I review preparation and properties of ferromagnetic III–Vs together with isothermal and reversible electric field control of ferromagnetism.

Ferromagnetic (Ga,Mn)As epitaxial layers with zincblende structure have been grown on Si (1 0 0) substrates employing a three-step method using molecular beam epitaxy. Magnetic measurements reveal that the ferromagnetic transition temperature is 48 K for a layer with a lattice constant of 0.567 nm, which corresponds to a nominal Mn composition of 2%. When grown directly on Si without the three-step method, the same set of growth parameters results in a (Ga,Mn)As layer with a ferromagnetic order below 25 K. The difference of magnetic property between these two samples is attributed to the quality of the (Ga,Mn)As layer which originates from the different growth methods.

We have studied the surface potential profile of the doping superlattice by observing the cleaved edge of the doping superlattice sample with the electrostatic force microscope (EFM). The surface potential of the structure is estimated from the two frequency components of the electrostatic force between the EFM probe and the superlattice structure. Lateral resolution of the EFM measurements found to be about 200 A. 2ω component was found to suggest an appreciable contribution in (∂C/∂z) term which might modify ω component result.

We investigate the electric-field effect on the magnetic anisotropy in perpendicularly magnetized Pt/Co/Pd/MgO structures deposited on GaAs and Si substrates as a function of temperature. An electric field is applied to the Pd surface through a 2-nm-thick insulating MgO layer by forming an electric double layer using an ionic liquid. We find that the electric-field dependence of the anisotropy is similar but clearly different between the two samples at low temperatures. The difference is expected to be related to the difference in the internal strain of the two structures.

We investigated properties of perpendicular-anisotropy magnetic tunnel junctions (p-MTJs) with [Co/Pt]-multilayer based synthetic ferrimagnetic reference (SyF) layer at elevated annealing temperature Ta from 350°C to 400°C. Shift field HS defined as center field of minor resistance versus magnetic field curve of the MTJs increased with increase of Ta from 350°C to 400°C. The variation of HS is attributed to the variation of saturation magnetic moment in the SyF reference layer. Cross sectional energy dispersive X-ray spectroscopy analysis revealed that Fe element of CoFeB in the reference layer diffuses to Co/Pt multilayers in the SyF reference layer.

We study current-induced magnetization switching properties of a magnetic tunnel junction with junction diameter of 19 nm and resistance-area product of 6 Ωμm2 in the nanosecond regime with and without in-plane magnetic field. At zero field, for both parallel (P)-to-anti-parallel (AP) and AP-to-P switchings, the probability of switching PSW approaches unity with the increase of pulse voltage duration τP. However, under in-plane magnetic field, PSW for P-to-AP switching starts to saturate at a value lower than unity with increasing τP, while AP-to-P switching remains the same as in the absence of in-plane magnetic field. This in-plane field dependence of PSW can be partially explained by the influence of electric-field modulation of magnetic anisotropy.

High-energy ( E ≥ 30 keV) X-ray diffraction with the latest generation synchrotron sources as well as the introduction of advanced insertion devices: wiggler and undulator, has created new approaches to the quantitative study of the structure of non-crystalline materials because of several improvements: higher resolution in real space due to a wide range of Q, smaller correction terms (especially for absorption correction), reduction of truncation errors, the feasibility of running under extreme environments, including high-temperatures and high-pressures, and of obtaining a direct comparison between X-ray and neutron diffraction data. Recently, this technique has been combined with neutron diffraction with a pulsed source to provide more detailed and reliable structural information not previously available. This article reviews and summarizes recent results obtained from high-energy X-ray diffraction on several oxide glasses: SiO2, B2O3, MgP2O6 and PbSiO3, using bending magnet beamlines at Super Photon ring-8 (GeV) (SPring-8). In particular, it addresses the structural models of oxide glasses obtained by the reverse Monte Carlo (RMC) modelling technique using both the high-energy X-ray and neutron diffraction data.

We have studied the influence of As flux on InAs nanostructures on GaAs (211)B surfaces grown by molecular beam epitaxy (MBE) at various growth temperatures ( T S ). It is shown that isotropic quantum dots (QDs) are formed at low T S under high As pressure condition. However, non-isotropic nanostructures, quantum dashes (QDHs), are formed under low As flux conditions. It is also shown that the sizes of nanostructure become larger with increasing T S . The shapes of nanostructure depend on As flux as well as T S .

The influence of Mn flux intensity and growth temperature on surface morphologies of III–V based magnetic semiconductor (Ga,Mn)As layers grown by low temperature (LT) molecular beam epitaxy (MBE) on (001) GaAs is studied by atomic force microscopy (AFM). The results show that homogeneous (Ga,Mn)As grows two-dimensionally (2D), whereas 3D growth takes place when hexagonal second phase appears on the growth front.

Molecular beam epitaxial (MBE) growth of novel hybrid ferromagnetic/non-magnetic semiconductor pn junction light emitting diodes (LEDs) is presented. The ferromagnetic p-type (Ga,Mn)As layers were grown on i-(In,Ga)As/n-GaAs structures to form LED structures. The current–voltage (I–V) characteristics and the electroluminescence (EL) spectra were measured at temperatures from 5 K to room temperature. In comparison to the properties of control samples consisting of all non-magnetic p-GaAs/(In,Ga)As/n-GaAs LEDs, the EL intensity of ferromagnetic/non-magnetic pn junction LEDs exhibited unique temperature dependence.

Molecular-beam epitaxial growth and characterization of GaSb films with high concentration of Mn (several percents) are presented. The result shows that almost all Mn atoms in GaSb forms MnSb clusters at normal growth temperature (∼560°C), and that low growth temperature (∼250°C) suppresses the formation of MnSb and a few tens of percent of Mn are incorporated in the host GaSb.

Ferromagnetism in manganese compound semiconductors not only opens prospects for tailoring magnetic and spin-related phenomena in semiconductors with a precision specific to III-V compounds but also addresses a question about the origin of the magnetic interactions that lead to a Curie temperature ( T C ) as high as 110 K for a manganese concentration of just 5%. Zener's model of ferromagnetism, originally proposed for transition metals in 1950, can explain T C of Ga 1− x Mn x As and that of its II-VI counterpart Zn 1− x Mn x Te and is used to predict materials with T C exceeding room temperature, an important step toward semiconductor electronics that use both charge and spin.

The effect of Mn as a surfactant on the formation of InAs nanostructures was studied. The shape of the nanostructures changed drastically when a layer of Mn was added on the growth front before InAs growth. When 6 ML 2 1 1 (6 monolayers in (2 1 1) plane) of InAs was grown on GaAs (2 1 1)B with an initial Mn coverage of 0.5 ML, quantum wire-like structures (QWRs) were formed at growth temperatures ranging from 450 to 510°C. With the same initial coverage of Mn, two kinds of nanostructures, quantum dashes (QDHs) and quantum dots (QDs) were formed at 400 or 425°C, whereas only QDs resulted at 380°C. Nanostructures on (1 0 0) and (3 1 1)B surfaces were also investigated and compared.

Intersubband optical transitions in ZnO-based quantum wells grown by plasma-assisted molecular beam epitaxy are investigated by a photocurrent spectroscopy. ZnO/MgZnO multiple quantum well (MQW) structures containing different ZnO well thicknesses are prepared. Photocurrent peaks are observed when the polarization of incident light is TM mode, following the intersubband selection rule. On reducing the ZnO well thickness, photocurrent peaks shifted to higher energies. Calculation indicates that the photocurrent peaks are due to intersubband transitions from the first to the third subband in ZnO/MgZnO MQWs. Comparison between experiment and calculation allows us to estimate the band offset ratio.

High quality AlGaN/GaN nominally undoped single heterostuctures (SH) with different Al contents have been fabricated on sapphire substrates. Magnetotransport investigations are performed on these samples at low temperature in a magnetic field up to 10 T. The existence of a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is confirmed by the observation of Shubnikov-de Hass oscillations in the magnetic field below 3 T and the integer quantum Hall effect. In particular, the Al0.18Ga0.82N/GaN SH shows a Hall mobility of 10300 cm2/Vs at 1.5 K, which to our knowledge is the highest carrier mobility on sapphire substrates grown by MOCVD. The mobility and carrier sheet density are enhanced by increasing Al composition. Based on the piezoelectric field effect, the Al composition dependence of 2DEG sheet density is calculated, which agrees well with the experimental result. This paper also investigates the effect of doping in the AlGaN layer on the properties of 2DEG. When the AlGaN layer is doped, the situation is changed. In the case of lightly doping in the AlGaN layer, the 2DEG cannot be observed. When the doping level in the AlGaN layer is increased, the 2DEG is demonstrated again together with an enhancement in the electron mobility. These behaviors are attributed to the strain-induced piezoelectric effect and silicon doping-induced screening effect. This result should be highly emphasized in designing GaN-based electrical devices.

InAs and (In,Mn)As nanostructures grown on GaAs(1 0 0), (2 1 1)B and (3 1 1)B substrates have been studied. Quantum dots (QDs) were observed when InAs was grown on GaAs(1 0 0) (or (3 1 1)B). QDs with bimodal size distribution were formed when InAs was deposited on GaAs(2 1 1)B at lower growth temperatures (" 4 ) whereas quantum dashes (QDHs) were observed at higher " 4 . (In,Mn)As QDs grown on GaAs(1 0 0) showed a broad range of dot sizes with irregular shape. (In,Mn)As QDs with bimodal size distribution were observed for the structure grown on GaAs(3 1 1)B. (In,Mn)As QDs grown on GaAs(2 1 1)B showed improved size uniformity compared to those grown on GaAs (1 0 0) and (3 1 1)B. The e!ects of Mn as a surfactant on InAs nanostructures were also studied. ( 1999 Elsevier Science B.V. All rights reserved.

Molten NaCl was analyzed by high-pressure X-ray diffraction experiments using synchrotron radiation up to 5 GPa and 1600 degrees C along the melting curve. The interference function Qi(Q) and the correlation function g(r) were derived from the diffraction data. The first-neighbor distance r 1 is about 2.7 Aa and the second-neighbor distance appears around 1.4r 1 -1.5r 1 . The coordination number, CN, of the nearest neighbor ions increases with pressure from 3.5 at 0.1 MPa to 4.5 at 5 GPa. This is the evidence that the NaCl melt has a B1-like structure with large vacancies over this pressure range and becomes densified by an increase in CN as a result of second neighbor compaction.

We fabricated field effect transistors having independent ohmic contacts to individual two-dimensional-electron gases in a double quantum well structure by etching the backgate (BG) area to 30 μ m-thick, while keeping the major part of the substrate thick for ease of handling (60 μ m). By this approach, it was made possible to form independent ohmic contacts with applied voltage to BG of about −60 V. We investigated the device by measuring the tunneling conductance as a function of temperature, in-plane magnetic field and carrier density. The temperature dependence showed that the broadening of resonant tunneling peak originated from the ionized impurity scattering and the electron–electron scattering. The in-plane magnetotunneling has shown two peaks at particular fields expected from the carrier densities in each well. These results indicate that the uniformity of the thickness of the etched BG area is good enough to allow the study of 2D–2D tunneling spectroscopy.

Reflection high-energy electron diffraction (RHEED) was used for in situ monitoring of the growth behavior of GaAs at various temperatures. The growth was performed on both singular and 2° off (001) GaAs substrates. The temperature dependence as well as the As/Ga beam equivalent pressure ratio dependence of the RHEED specular beam intensity oscillation characteristics were studied. Strong RHEED oscillations were observed at low temperatures under As/Ga ratios far from the stoichiometric condition.

We have grown (In,Mn)As quantum dots (QDs) on GaAs (100), (211)B and (311)B substrates. The observation of reflection high energy electron diffraction pattern and atomic force microscopy measurement confirmed the formation of the (In,Mn)As QDs. The structure grown on GaAs (100) showed a broad range of dot sizes with irregular shape. For the structure grown on GaAs (311)B, (In,Mn)As QDs with bimodal size distribution were observed. The (In,Mn)As QDs grown on GaAs (211)B showed improved size uniformity compared to those grown on GaAs (100) and (311)B. The effect of Mn as a surfactant on InAs nanostructures was also studied.

Preparation and properties of a new class of diluted magnetic semiconductors (DMS's) based on III-V compounds are reviewed. Epitaxial films of (In,Mn)As and (Ga,Mn)As, the new III-V based DMS's, are now possible by low temperature molecular beam epitaxial growth (< 300°C). The magnetic properties of the epitaxial layers were revealed by the use of the anomalous Hall effect with the assistance of SQUID magnetization measurements. The epitaxial films showed antiferromagnetic, superparamagnetic, and ferromagnetic behaviors depending on the Mn-Mn interaction, which was modified by the presence of carriers and strain in the layers. These new III-V based DMS's will open up new fields in semiconductor technology, where both semiconducting and magnetic properties play critical roles.

We have studied magnetic and transport properties of a series of Ga 1−x Mn x As GaAs samples with different Mn concentrations (x = 0.015−0.071. For Mn content higher than about 0.02, carrier(hole)-induced ferromagnetism is observed. Samples with x = 0.035 and 0.043 behave as ferromagnetic dirty metals. With further increase of Mn content above x ∼ 0.05, the zero-field resistivity turns a semiconducting temperature dependence. Very large negative magnetoresistance is observed in non-metallic samples near the metal-nonmetal transitions both in the low and the high Mn content regimes.

A new III–V diluted magnetic semiconductor, (Ga, Mn)As, was prepared by low-temperature molecular beam epitaxy. The lattice constant of (Ga, Mn)As films determined by X-ray diffraction showed a linear increase with increase of Mn composition, suggesting homogeneous incorporation of Mn in the film. Magnetization measurements revealed ferromagnetic order at low temperature, while magnetotransport measurements showed the anomalous Hall effect and negative magnetore-sistance associated with the ferromagnetic order in the (Ga, Mn)As films.

The effects of applied electric fields on intersubband (ISB) transitions in modulation-doped n-i-n and p-i-n type GaAs AlGaAs single quantum well (SQW) structures are studied experimentally using Fourier Transform InfraRed (FTIR) spectroscopy. The n-i-n SQW devices exhibit a red shift of 0.7 meV in the ISB transition energy and a 12% decrease in the integrated absorbance at the maximum applied bias of 3.2 V with reference to their zero bias values, most of which is attributed to a rise in the device temperature due to Joule heating owing to the current flowing through the device. The p-i-n SQW devices exhibit strong quenching of the absorption when a negative bias is applied, while there was no observable shift in the ISB transition energy.

Background/purpose In air-conditioned rooms, dry air exacerbates some skin diseases, for example, senile xerosis, atopic dermatitis, and surface roughness. Humidifiers are used to improve air dryness, which often induces excess humidity and thermal discomfort. To address this issue, we investigated the effects of water nanodroplets (mist) on skin hydration, which may increase skin hydration by penetrating into the interstitial spaces between corneocytes of the stratum corneum (SC) without increasing air humidity. Methods We examined biophysical parameters, including skin conductance and transepidermal water loss (TEWL), and biomechanical parameters of skin distension/retraction before and after suction at the forehead, lateral canthus, and cheek, with or without mist, in a testing environment (24°C, 35% relative humidity) for 120 min. Results In the group without mist, TEWL values significantly decreased at all the sites after 1 h compared with the initial values. However, in the presence of mist, TEWL values were maintained at the initial values through the test, yielding significant differences vs. the group without mist. There were no significant differences between mist and mist-free groups in terms of skin conductance. Skin distension was significantly increased in the group with mist compared with that in the group without mist at the forehead and cheek, suggesting a softening effect of mist. Conclusion Skin deformation of the face was improved by mist, suggesting hydration of the SC by mist. The change in TEWL was influenced by mist, suggesting supply of water to the skin, particularly the SC, by mist. These data indicated that a mist of water nanodroplets played an important role in softening skin in an air-conditioned room without increasing excess humidity.

We show that the magnetic characteristics of Ta|CoFeB|MgO magnetic heterostructures are strongly influenced by doping the Ta underlayer with nitrogen. In particular, the saturation magnetization drops upon doping the Ta underlayer, suggesting that the doped underlayer acts as a boron diffusion barrier. In addition, the thickness of the magnetic dead layer decreases with increasing nitrogen doping. Surprisingly, the interface magnetic anisotropy increases to ~1.8 erg/cm2 when an optimum amount of nitrogen is introduced into the Ta underlayer. These results show that nitrogen doped Ta serves as a good underlayer for Spintronics applications including magnetic tunnel junctions and domain wall devices.

The local structures around Zr atoms such as coordination number of oxygen and cations, and the Zr O and Zr Ln (Ln: La, Nd and Ce) interatomic distances in the (La1−xMx)2Zr2O7 (M Nd or Ce with x = 0–1.0) pyrochlore compounds, where Nd and Ce are simulation of actinides, were studied by means of extended X-ray absorption fine structure (EXAFS) spectrometry. The local structure and lattice constant determined by EXAFS and X-ray diffraction, respectively in this study, are discussed in relation to the dissolution behavior in HCl solution (pH = 1) at 363 K reported previously. The change in the local structure was also investigated with a sample before and after leaching test in HCl solution (pH = 1) at 363 K for 14 days.

Scanning tunneling microscopy was used to study surface structures and doping properties of molecular beam epitaxy (MBE)-grown Si-doped GaAs(001)-(2 × 4) and GaAs(001)-c(4 × 4) surfaces on the atomic scale. It was found that on the Si-doped (2 × 4) surface, arsenic dimers formed several energetically different configurations besides the most stable two-dimer configuration. The doping properties of the c(4 × 4) surface were found to be significantly different from the (2 × 4) case. The Si dopants tended to segregate on top of the c(4 × 4) surface and formed small asymmetric clusters oriented along the [110] direction.

Energy-dispersive X-ray diffraction experiments of molten KCl under high pressure have been carried out by using synchrotron radiation. The diffraction profiles of molten KCl were acquired just above the melting temperature of KCl up to 4 GPa. The reduced structure factor S(Q)'s for molten KCl do not show any change in their primary features, except for a gradual increase in the first peak intensity with increasing pressure. This implies that molten KCl does not show a first-order phase transition, such as the B1-B2 transition, found in solid KCl, but that the local structure in molten KCl must be changed by compression. According to a molecular-dynamics simulation, this change of S(Q) can be explained by a continuous increase in the coordination number of the nearest-neighbor ions in molten KCl with pressure.