Table of contents

The development of magnetic Heusler compounds, specifically designed as materials for spintronic applications, has made tremendous progress in the very recent past [1–21]. Heusler compounds can be made as half-metals, showing a high spin polarization of the conduction electrons of up to 100% [1]. These materials are exceptionally well suited for applications in magnetic tunnel junctions acting, for example, as sensors for magnetic fields. The tunnelling magneto-resistance (TMR) effect is the relative change in the electrical resistance upon application of a small magnetic field. Tunnel junctions with a TMR effect of 580% at 4 K were reported by the group of Miyazaki and Ando [1], consisting of two Co₂MnSi Heusler electrodes. High Curie temperatures were found in Co₂ Heusler compounds with values up to 1120 K in Co₂FeSi [2]. The latest results are for a TMR device made from the Co₂FeAl_0.5Si_0.5 Heusler compound and working at room temperature with a TMR effect of 174% [3].

The first significant magneto-resistance effect was discovered in Co₂Cr_0.6Fe_0.4Al (CCFA) in Mainz [4]. With the classical Heusler compound CCFA as one electrode, the record TMR effect at 4 K is 240% [5]. Positive and negative TMR values at room temperature utilizing magnetic tunnel junctions with one Heusler compound electrode render magnetic logic possible [6].

Research efforts exist, in particular, in Japan and in Germany. The status of research as of winter 2005 was compiled in a recent special volume of Journal of Physics D: Applied Physics [7–20]. Since then specific progress has been made on the issues of (i) new advanced Heusler materials, (ii) advanced characterization, and (iii) advanced devices using the new materials.

In Germany, the Mainz and Kaiserslautern based Research Unit 559 `New Materials with High Spin Polarization', funded since 2004 by the Deutsche Forschungsgemeinschaft, is a basic science approach to Heusler compounds, and it addresses the first two topics in particular, with emphasis on new rational design of Heusler compounds and advanced characterization tools. This volume of Journal of Physics D: Applied Physics summarizes the latest research results obtained in the Research Unit and presents it to the scientific public as a cluster of refereed papers.

Half-metallic ferromagnets are an impressive example for the rational design of new materials based on computational physics. The paper of Kandpal et al demonstrates how a detailed understanding of the electronic structure, especially from the viewpoint of the properties of the minority band gap and the peculiar magnetic behaviour, enables us to predict new half-metallic compounds. A high interface quality and a well ordered compound are the preconditions to realize the predicted half-metallic properties. Wurmehl et al have carefully studied the surface and bulk structure of the classical Heusler compound CCFA using a combination of characterization methods. A deposition process of epitaxial thin films of CCFA was described by Conca et al. Kallmayer et al have correlated the structural properties of thin magnetron sputtered films determined by x-ray diffraction with details of the x-ray magnetic circular dichroism (XMCD) spectra. From the value of the magnetic moment located at the Cr atom and features of the Co absorption spectra, they conclude that the buffer layers lead to an improvement of local atomic order. A highlight of this Cluster Issue is the spin resolved photoemission result of Cinchetti and co-workers. A careful in situ preparation of the sample surface of CCFA leads to values for the room temperature spin polarization up to 45% at the Fermi level, the highest value measured so far at the surface region of a full Heusler compound at room temperature.

Co₂FeSi (CFS) is the half-metal Heusler compound with the highest Curie temperature reported so far [4]. Schneider et al report the deposition of well ordered thin Co₂FeSi films by RF magnetron sputtering. The thickness dependence of these thin epitaxial Co₂FeSi (110) films was investigated using XMCD by Kallmayer et al. The magnetic moment as a function of film thickness demonstrates the presence of dead layers, reducing the magnetization and the spin polarization of these films at all interfaces. The influence of Ga⁺ ion irradiation was studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect in a paper by Hamrle et al who, in a second paper, report an unusual huge quadratic magneto-optical Kerr effect in CFS films with L2₁ structure. The films exhibit a huge QMOKE signal, with its maxima of up to 30 mdeg, which is the largest QMOKE signal in reflection that has been measured thus far. Beside the half-metallicity and the high Curie temperature, an essential feature for such devices is the micro-magnetic domain structure. XMCD–PEEM has been used for a direct observation of the domain structure of single- and polycrystalline samples by Gloskowskii et al.

The spin polarization of Co₂FeSi films can be improved at room temperature, especially the temperature dependence of the magneto-resistance effect. For a TMR device with Co₂FeSi_0.5Al_0.5 Tezuka et al [3] have found a record TMR value for room temperature. Fecher et al have investigated the electronic structure of Co₂FeSi_{1 - x}Al_x. The series Co₂FeSi_{1 - x}Al_x is found to exhibit half-metallic ferromagnetism and it is shown that the electron-doping stabilizes the gap in the minority states for x = 0.5. This might be a reason for the exceptional temperature behavior of Co₂FeSi_0.5Al_0.5 TMR devices. Co₂Fe_0.5Mn_0.5Si is another candidate with Fermi energy in the middle of the minority states gap. Therefore Fecher et al have investigated the electronic structure of the series Co₂Fe_{1 - x}Mn_xSi by high energy, high resolution photoelectron spectroscopy. High energy photoemission is a new advanced method to study the electronic structure of bulk material, due to a large mean free path of the photo electrons. The high resolution measurements of the valence band close to the Fermi energy indicate the existence of the gap in the minority states for all investigated Co₂Fe_{1 - x}Mn_xSi compounds. Other Co₂ Heusler compounds are also possible candidates for magneto-electronic devices. Miura et al [21] have found that the disorder between Co and Y atoms correlates with the total valence electron charges around Y atom and have predicted that Ti-based compounds are better than Cr-, Mn- and Fe-based compounds in preventing the atomic disorder between Co and Y atoms. Kandpal et al have therefore investigated the electronic structure and disordering effects in Co₂TiSn using local probes, ¹¹⁹Sn Mössbauer spectroscopy and ⁵⁹Co nuclear magnetic resonance spectroscopy. They found that the sample possesses up to 10% of antisite (Co/Ti) disordering, a disorder that does not destroy the half-metallic character of this material.

We hope that this Cluster of papers will help to stimulate and push forward the research of materials with high spin polarization.

References

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[2] S Wurmehl, Fecher G H, Kandpal H C, Ksenofontov V, Felser C, and Lin H-J 2006 Investigation of Co₂FeSi: the Heusler compound with highest Curie temperature and magnetic moment  Appl. Phys. Lett.88 032503

[3] Tezuka N, Ikeda N, Sugimoto S and Inomata K 2006 175% TMR at room temperature and high thermal stability using Co2FeAl_0.5Si_0.5 full-Heusler alloy electrodes Appl. Phys. Lett.89 252508

[4] Block T, Felser C, Jakob G, Ensling J, Mühling B, Gütlich P, Cava R J 2003 Large negative magnetoresistance effects in Co₂Cr_0.6Fe_0.4Al  J. Solid State Chem.176 646

[5] Marukame T, Ishikawa T, Matsuda K I, Uemura T and Yamamoto M 2006 High tunnel magnetoresistance in fully epitaxial magnetic tunnel junctions with a full-Heusler alloy Co₂Cr_0.6Fe_0.4Al thin film Appl. Phys. Lett.88 262503

[6] Thomas A, Meyners D, Ebke D, Liu N-N, Sacher M D, Schmalhorst J, Reiss G, Ebert H, and Hütten A 2006 Inverted spin polarization of Heusler alloys for spintronic devices  Appl. Phys. Lett.89 012502

[7] Hillebrands B and Felser C 2006 Editorial: High-spin polarization of Heusler alloys J. Phys. D: Appl. Phys.39 issue 5 http://stacks.iop.org/0022-3727/39/i=5

[8] Galanakis I, Mavropoulos Ph and Dederichs P H 2006 Electronic structure and Slater–Pauling behaviour in half-metallic Heusler alloys calculated from first principles J. Phys. D: Appl. Phys.39 765 J. Phys. D: Appl. Phys.39 765

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[10] Kallmayer M, Elmers H J, Balke B, Wurmehl S, Emmerling F, Fecher G H and Felser C 2006 Magnetic properties of Co₂Mn_1-xFe_xSi Heusler alloys J. Phys. D: Appl. Phys.39 786

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[12] Leziac M, Mavropoulos Ph, Bihlmayer G and Blügel S 2006 Scanning tunnelling microscopy of surfaces of half-metals: an ab-initio study on NiMnSb(001) J. Phys. D: Appl. Phys.39 797

[13] Wurmehl S, Fecher G H, Kroth K, Kronast F, Dürr H A, Takeda Y, Saitoh Y, Kobayashi K, Lin H-J, Schönhense G and Felser C 2006 Electronic structure and spectroscopy of the quaternary Heusler alloy Co₂Cr_1-xFe_xAl J. Phys. D: Appl. Phys.39 803

[14] Inomata K, Okamura S, Miyazaki A, Kikuchi M, Tezuka N, Wojcik M and Jedryka E 2006 Structural and magnetic properties and tunnel magnetoresistance for Co₂(Cr,Fe)Al and Co₂FeSi full-Heusler alloys J. Phys. D: Appl. Phys.39 816

[15] Yamamoto M, Marukame T, Ishikawa T, Matsuda K, Uemura T and Arita M 2006 Fabrication of fully epitaxial magnetic tunnel junctions using cobalt-based full-Heusler alloy thin film and their tunnel magnetoresistance characteristics J. Phys. D: Appl. Phys.39 824

[16] Oogane M, Sakuraba Y, Nakata J, Kubota H, Ando Y, Sakuma A and Miyazaki T 2006 Large tunnel magnetoresistance in magnetic tunnel junctions using Co₂MnX (X = Al, Si) Heusler alloys J. Phys. D: Appl. Phys.39 834

[17] Bergmann A, Grabis J, Nefedov A, Westerholt K and Zabel H 2006 X-ray resonant magnetic scattering study of [Co₂MnGe/Au]_n and [Co₂MnGe/V]_n multilayers J. Phys. D: Appl. Phys.39 842

[18] Picozzi S, Continenza A and Freeman A J 2006 Magneto-optical properties of Heusler compounds from a first-principles approach J. Phys. D: Appl. Phys.39 851

[19] Ślebarski A 2006 Electron-correlation effects in a disordered Fe₂TiSn Heusler alloy J. Phys. D: Appl. Phys.39 856

[20] Entel P, Buchelnikov V D, Khovailo V V, Zayak A T, Adeagbo W A, Gruner M E, Herper H C and Wassermann E F 2006 Modelling the phase diagram of magnetic shape memory Heusler alloys J. Phys. D: Appl. Phys.39 865

[21] Miura Y, M Shirai and Nagao K 2006 Ab initio study on stability of half-metallic Co-based full-Heusler alloys J. Appl. Phys.99 08J112

In this paper, results of ab initio band structure calculations for A₂BC Heusler compounds that have A and B sites occupied by transition metals and C by a main group element are presented. This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap, the peculiar transport properties and magnetic behaviour found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both A and B atoms to the total magnetic moment. The magnitude of the total magnetic moment shows a trend consistent with the Slater–Pauling type behaviour in several classes of these compounds. The total magnetic moment also depends on the kind of C atoms although they do not directly contribute to it. In Co₂ compounds, a change of the C element changes the contribution of the t_2g states to the moment at the Co sites. The localized moment in these magnetic compounds resides at the B site. Other than in the classical Cu₂-based Heusler compounds, the A atoms in Co₂, Fe₂ and Mn₂-based compounds may contribute significantly to the total magnetic moment. It is shown that the inclusion of electron–electron correlation in the form of LDA + U calculations helps to understand the magnetic properties of those compounds that already exhibit a minority gap in calculations where it is neglected. Besides the large group of Co₂ compounds, half-metallic ferromagnetism was here found only in such compounds that contain Mn.

The quarternary substitutional series Co₂Cr_1−xFe_xAl was investigated by means of surface and bulk sensitive techniques in order to exploit its structural and compositional properties. Both bulk and powder samples of the alloy series were investigated to obtain specific information about this material.

The long range order was determined by means of x-ray diffraction and neutron diffraction, while the site specific (short range) order was proved by extended x-ray absorption fine structure spectroscopy. The magnetic structure was investigated by Mössbauer spectroscopy in transmission and scattering modes in order to compare and separate powder and bulk properties. The chemical composition was analysed by means of x-ray photo emission spectroscopy combined with Auger electron spectroscopy depth profiling. The results from these methods are compared to get an insight into the differences between surface and bulk properties and the appearance of disorder in such alloys.

The material shows an extremely high sensitivity to oxygen. In particular, powder materials show a high amount of oxygen contamination. Therefore, an additional oxide-mediated tunnel magneto-resistance may always contribute to measurements of magneto-resistive effects because the oxide layers will provide natural tunnelling barriers. In addition, the results suggest that thin films have to be produced under ultra-high vacuum conditions.

Epitaxial thin films of the theoretically predicted half metal Co₂Cr_0.6Fe_0.4Al were deposited by dc magnetron sputtering on different substrates and buffer layers. The samples were characterized by x-ray and electron beam diffraction (RHEED) demonstrating the B2 order of the Heusler compound with only a small fraction of disorder on the Co sites. Magnetic tunnelling junctions with Co₂Cr_0.6Fe_0.4Al electrode, AlO_x barrier and Co counter electrode were prepared. From the Jullière model a spin polarization of Co₂Cr_0.6Fe_0.4Al of 54% at T = 4 K was deduced. The relation between the annealing temperature of the Heusler electrodes and the magnitude of the tunnelling magnetoresistance effect was investigated and the results are discussed in the framework of morphology and surface order based on in situ scanning tunnelling microscopy (STM) and RHEED investigations.

For the fully ordered Heusler alloy Co₂Cr_0.6Fe_0.4Al half-metallic ferromagnetism has been predicted. Local disorder other than the Al–Cr/Fe (B2)-type disorder is known to destroy the half-metallic bandgap. The usage of appropriate buffer layers improves the structural quality of thin films. We correlate the structural properties of thin magnetron sputtered films determined by x-ray diffraction with details of the x-ray magnetic circular dichroism spectra. From the value of the magnetic moment located at the Cr atom and features of the Co absorption spectra we conclude that the buffer layers lead also to an improvement in the local atomic order. The atomic ordering gradually approaches the level of local B2-type disorder achieved for bulk materials. We discuss these results in view of theoretical predictions for partly disordered systems.

In this paper we investigate the surface spin polarization in a 100 nm Co₂Cr_0.6Fe_0.4Al film grown ex situ epitaxially on MgO(1 0 0) with a 10 nm Fe buffer layer by means of spin-resolved photoemission. We show that a careful in situ preparation of the sample surface leads to values for the room temperature spin polarization up to 45% at the Fermi level. To our knowledge, this is the highest value measured so far at the surface region of a full Heusler alloy at room temperature.

We report the deposition of thin Co₂FeSi films by RF magnetron sputtering. Epitaxial (1 0 0)-oriented and L2₁ ordered growth is observed for films grown on MgO (1 0 0) substrates. (1 1 0)-oriented films on show several epitaxial domains in the film plane. Investigation of the magnetic properties reveals a saturation magnetization of 5.0 µ_B/fu at low temperatures. The temperature dependence of the resistivity ρ_xx(T) exhibits a crossover from a T^3.5 law at T < 50 K to a T^1.65 behaviour at elevated temperatures. ρ_xx(H) shows a small anisotropic magnetoresistive effect. A weak dependence of the normal Hall effect on the external magnetic field indicates the compensation of electron and hole like contributions at the Fermi surface.

Element-specific magnetic properties of ultrathin epitaxial Co₂FeSi(110) films were measured using x-ray magnetic circular dichroism (XMCD). The epitaxial Heusler films were grown by RF magnetron sputtering on substrates. The magnetization of thicker films as determined by XMCD is smaller than expected for a half-metallic material. In addition, the magnetization decreases considerably for films thinner than 10 nm. The thickness dependence of the magnetic moment can be described by introducing a certain number of dead layers representing a deficiency of magnetization at the interfaces. Quantitative evaluation results in a dead layer thickness of 0.8 nm at room temperature, consisting of a temperature induced size effect of 0.1 nm and a surface effect of 0.15 nm at the top and 0.55 nm at the bottom interface.

A Co₂FeSi (CFS) film with L2₁ structure was irradiated with different fluences of 30 keV Ga⁺ ions. Structural modifications were subsequently studied using the longitudinal (LMOKE) and quadratic (QMOKE) magneto-optical Kerr effect. Both the coercivity and the LMOKE amplitude were found to show a similar behaviour upon irradiation: they are nearly constant up to ion fluences of ≈6 × 10¹⁵ ion cm⁻², while they decrease with further increasing fluences and finally vanish at a fluence of ≈9 × 10¹⁶ ion cm⁻², when the sample becomes paramagnetic. However, contrary to this behaviour, the QMOKE signal nearly vanishes even for the smallest applied fluence of 3 × 10¹⁴ ion cm⁻². We attribute this reduction of the QMOKE signal to an irradiation-induced degeneration of second or higher order spin–orbit coupling, which already happens at small fluences of 30 keV Ga⁺ ions. On the other hand, the reduction of coercivity and LMOKE signal with high ion fluences is probably caused by a reduction of the exchange interaction within the film material.

Co₂FeSi(1 0 0) films with L2₁ structure deposited onto MgO(1 0 0) were studied using both the longitudinal and quadratic (QMOKE) magneto-optical Kerr effect. The films exhibit a huge QMOKE signal with a maximum contribution of up to 30 mdeg, which is the largest QMOKE signal in reflection that has been measured thus far. This large value is a fingerprint of an exceptionally large spin–orbit coupling of second or higher order. The Co₂FeSi(1 0 0) films exhibit a rather large coercivity of 350 Oe and 70 Oe for film thicknesses of 22 nm and 98 nm, respectively. Despite the fact that the films are epitaxial, they do not provide an angular dependence of the anisotropy and the remanence in excess of 1% and 2%, respectively.

The Heusler compound Co₂FeSi is a promising material for magneto-electronic devices. With a Curie temperature of 1100 K and a saturation magnetization of 6 Bohr magnetons and a high spin polarization at the Fermi edge it fulfils the essential requirements for magnetic sensors or spin valve structures. An essential feature for such devices is the micro-magnetic domain structure. X-ray magnetic circular dichroism–photo emission electron microscopy has been used for a direct observation of the domain structure of single- and polycrystalline samples. The polycrystalline material exhibits a micro-magnetic ripple structure, as it is well known for pure Co and other polycrystalline Heusler compounds. The (1 1 0)-oriented surface of the single crystal exhibits a multi-domain pattern characteristic for systems with an easy axis that might point out of the surface. Asymmetric 180° Bloch walls with changing sense of rotation are observed between oppositely magnetized domains. Spin polarized photo emission from a single domain of the single crystal shows a spin polarization of 16% at the Fermi energy and up to 35% in the d-bands, at room temperature.

This work reports on high resolution photoelectron spectroscopy for the valence band of Co₂Mn_1−xFe_xSi (x = 0,0.5,1) excited by photons of about 8 keV energy. The measurements show good agreement with calculations of the electronic structure using the LDA + U scheme. It is shown that the high energy spectra reveal the bulk electronic structure better compared with low energy x-ray photoelectron spectroscopy spectra. The high resolution measurements of the valence band close to the Fermi energy indicate the existence of the gap in the minority states for all the three alloys.

This work reports electronic structure calculations for the Heusler compound Co₂FeAl_1−xSi_x. Particular emphasis was put on the role of the main group element in this compound. The substitution of Al by Si leads to an increase in the number of valence electrons with increasing Si content and may be seen as electron-doping. Self-consistent electronic structure calculations were performed to investigate the consequences of the electron-doping on the magnetic properties. The series Co₂FeAl_1−xSi_x is found to exhibit half-metallic ferromagnetism and the magnetic moment follows the Slater–Pauling rule. It is shown that the electron-doping stabilizes the gap in the minority states for x = 0.5.

Polycrystalline samples of the Heusler compound Co₂TiSn have been prepared and studied using bulk techniques (x-ray diffraction and magnetization) as well as local probes (¹¹⁹Sn Mössbauer spectroscopy and ⁵⁹Co nuclear magnetic resonance spectroscopy) in order to determine how disorder affects the half-metallic behaviour and also to establish the joint use of Mössbauer and NMR spectroscopies as a quantitative probe of local atom ordering in these compounds. Additionally, density functional electronic structure calculations on ordered and partially disordered Co₂TiSn compounds have been carried out at a number of different levels of theory in order to simultaneously understand how the particular choice of DFT scheme as well as disorder affects the computed magnetization. Our studies suggest that a sample which seems well ordered by x-ray diffraction and magnetization measurements can possess up to 10% of antisite (Co/Ti) disordering. Computations similarly suggest that even 12.5% antisite Co/Ti disorder does not destroy the half-metallic character of this material. However, the use of an appropriate level of non-local DFT is crucial.

Catalytic chemical vapour deposition (CCVD) is currently the most promising technique to produce carbon nanotubes (CNTs) on a specific site as well as on a large-scale. Here, we review our recent experimental studies on CNTs grown by CCVD. The yield as well as the structural characteristics of CNTs can strongly be affected by the choice of the catalyst as well as the catalyst support. In particular, CaCO₃ is found to be an excellent support material which actively contributes to the CNT growth. Our systematic study of the elastic measurements of multi-walled CNTs grown by CCVD indicates that Young's modulus is generally low, independent from their precise growth conditions. This behaviour is attributed to the high density of structural defects typically present in CCVD grown CNTs, which cannot be healed by additional heat treatment. However, Young's modulus of about 1 TPa is found for double-walled CNTs indicating that CCVD grown CNTs can also have a high strength as arc-discharge tubes as long as their diameter is small. Furthermore, CNTs are mechanically attached to scanning probe tips and tested in contact as well as in tapping mode. The scans reveal that the contact between the probe and the CNT is the major problem. Finally, we compare the cytotoxicity of CNTs with that of carbon nanoparticles as well as nanofibres. Our results indicate that toxicity strongly depends on the number of chemically active sites on the surface.

Fine particles of cobalt ferrite were synthesized by the sol–gel method. Subsequent heat treatment at different temperatures yielded cobalt ferrites having different grain sizes. X-ray diffraction studies were carried out to elucidate the structure of all the samples. Dielectric permittivity and ac conductivity of all the samples were evaluated as a function of frequency, temperature and grain size. The variation of permittivity and ac conductivity with frequency reveals that the dispersion is due to Maxwell–Wagner type interfacial polarization in general, with a noted variation from the expected behaviour for the cold synthesized samples. High permittivity and conductivity for small grains were explained on the basis of the correlated barrier-hopping model.

Ba-doped multiferroic BiFeO₃thin films were successfully prepared on Pt/TiO₂/SiO₂/Si(1 0 0) substrates by pulsed laser deposition. X-ray diffraction showed that the Bi_0.75Ba_0.25FeO₃thin film was single phase with (1 0 1) preferential polycrystalline orientation. Both ferroelectricity and ferromagnetism of these films were observed at room temperature by P–E and M–H loop measurements, respectively. The magnetoelectric coupling effect was demonstrated by measuring the dielectric constant in a varying magnetic field. The dielectric constants measured at 10 kHz increased with an increase in the applied magnetic field, giving a coupling coefficient (ε_r (H) − ε_r (0))/ε_r (0) of 1.1% at H = 8 kOe at room temperature, which shows potential application.

The annealing effects on the microstructure and magnetic properties of Zn_0.92Co_0.08O films annealed at different temperatures (200, 400 and 600 °C) in air and vacuum have been systematically investigated. The as-grown and annealed films are all highly textured with (002) orientation and all show room temperature ferromagnetism. The as-grown film is in a state of compressive stress and the film becomes almost stress-free after an annealing treatment at 400 °C in vacuum, while the film annealed at 600 °C goes into a state of tension. The magnetic moment increases after annealing in vacuum and decreases after annealing in air at the same temperature, compared with that of as-grown film. Moreover, the magnetic moment increases with the annealing temperature up to 400 °C and then shows a rapid decline at an annealing temperature of 600 °C in vacuum. The experimental results indicate that the enhancement of ferromagnetism is strongly correlated with the increase in oxygen vacancies and that the existence of interstitial Zn atoms plays an important role in mediating the ferromagnetism in Co-doped ZnO films.

The critical phase transition temperatures of the ferroelectric (FE) phase and the ferromagnetic (FM) phase in epitaxial 1–3 type multiferroic thin films were obtained based on the thermodynamic model. Analytic expressions of the para–ferro transition temperatures were derived as functions of the volume fraction of the FM phase by considering the effect of the coupled elastic stresses arising from the FE/FM and the film/substrate interfaces. Our results show that the critical temperatures are significantly affected by the induced stresses and can be controlled by adjusting the volume fractions of the different phases within the thin film.

The magnetic and magnetocaloric properties of the intermetallic compounds Tb_1−xTm_xCo₂ (with x = 0, 0.2 and 0.5) have been studied. It is found that partial replacement of Tb by Tm in TbCo₂ leads to a reduction in the ordering temperature, which is attributed to the decrease in the exchange strength due to the lower spin value of Tm³⁺ as compared with that of Tb³⁺. The analysis of the zero-field heat capacity data at low temperature shows that the coefficient of electronic heat capacity increases with increase in Tm content and is attributed to the presence of local spin fluctuations. The variation of the magnetocaloric effect (MCE) has been explained on the basis of the magnetic properties. Temperature dependence of the MCE shows that this system may be useful for magnetic refrigeration applications in a sub-room temperature regime.

The track density increase in a perpendicular magnetic recording system is limited by the adjacent-track interference (ATI). In this work, a composite micromagnetic simulation model of the read/write process is developed to analyse ATI by the symmetry of signal and noise in two adjacent W = 60 nm tracks with the track pitch of the order of 100 nm. Based on the two-dimensional medium noise distribution of dibit recording, it is found that the noise in the first and later recorded tracks start to be asymmetric when the track pitch is lower than 2 W; if the read width is limited within 2/3 of the write width, the asymmetry of noise appears when the track pitch is less than 1.5 W. At higher recording densities, the signal-to-noise ratio degradation is mainly due to the noise caused by the interference from the signal of the adjacent track. Side writing can be effectively eliminated by the use of a guard band whose width is at least half the track width.

We have studied the growth temperature dependence of the interfacial reaction of Heusler-alloy Co₂FeSi/GaAs(0 0 1) hybrid structures. The reaction proceeds dominantly by Co in-diffusion, resulting in the formation of isolated grains in the GaAs substrate starting at the growth temperature T_G of 200–250 °C. The interfacial reaction is classified into two stages: (i) intermediate T_G regime (250–300 °C), where a highly oriented single-crystalline phase, most likely ternary Co₂GaAs, is formed in the topotaxial relationship of ; and (ii) high T_G regime (≥350 °C) where binary CoAs is formed in the asymmetric topotaxial relationship of CoAs[0 0 1](2 1 0) || GaAs[1 1 0](0 0 1).

Dry etching and its effect on the characteristics of submicron feature-size PbZr_1−xTi_xO₃ (PZT) capacitors with PtO_x top electrode were investigated. The photoresist (PR)-masked PtO_x films were etched by an Ar/(20%)Cl₂/O₂ helicon wave plasma. A fence-free pattern with a significantly high etch rate and sidewall slope was obtained by the addition of O₂ into the etching gas mixture, due to the chemical instability of PtO_x and the formation of a PtO₂ passivation layer to suppress redeposition of the etch by-products on the etched surface. The patterned PtO_x electrode can be further used as a hard mask for etching the PZT film, subsequently, with the gas mixture of Ar, CF₄ and O₂. A high etching rate of PZT and a good etching selectivity to PtO_x can be obtained at 30% O₂ addition into the Ar/(50%)CF₄ plasma. The etched capacitors have a steep, 72°, sidewall angle with a clean surface. Moreover, the addition of O₂ into the etching gas can well preserve the properties and the fatigue endurance of PtO_x/PZT capacitors.

During the coherent bicolour optical treatment of Eu₂O₃ doped telluride glasses at temperatures near the glassing points, an efficient second-order susceptibility equal to about 2.1 pm V⁻¹ at fundamental wavelength 1907 nm was achieved. Bicolour optical treatment was performed by two coherent laser beams originating from 50 ps Nd-YAG laser (λ = 1.32 µm) exciting the high pressure hydrogen laser cell emitting at 1907 nm. The non-centrosymmetric grating of the medium was formed by coherent superposition of the fundamental laser at 1907 nm and the doubled frequency one at 953.5 nm. The maximal optical second harmonic generation (SHG) signal occurs for 0.5% of Eu₂O₃ (in wt%). It was discovered that the photoinduced SHG achieves saturation during the treatment at about 6–8 min. The ratio between the fundamental beam with power density of about 1.3 GW cm⁻² and the writing beam of about 0.024 GW cm⁻² corresponds to the optimal photoinduced SHG. The remaining value of the photoinduced SHG is larger for the samples with higher second-order susceptibility. It is necessary to emphasize that the efficient grating process is possible only within the narrow temperature range corresponding to the glassing phase transitions and the effect disappeared for nondoped glasses and for glasses doped with 5% of Eu₂O₃ (in wt%). Possible physical mechanisms of the phenomenon observed are discussed.

Organic field-effect transistors (FETs) have been fabricated by using poly(p-phenylenevinylene) (PPV) films prepared from a water soluble precursor polymer and p-channel FET conduction was obtained by using platinum source-drain electrodes. When the conversion temperatures for preparing PPV films changed between 180 and 280 °C, the field-effect hole mobilities changed between 4.3 × 10⁻⁴ and 8.4 × 10⁻⁶ cm² V⁻¹ s⁻¹. The highest field-effect hole mobility was seen on the PPV thermally converted at 180 °C. The decreases in field-effect mobilities in the PPV films with further extended average π-conjugation length converted at higher temperatures were ascribed to morphological defects owing to crystallization.

The influences of Casimir and van der Waals forces on the nano-electromechanical systems (NEMS) electrostatic torsional varactor are studied. A one degree of freedom, the torsional angle, is adopted, and the bifurcation behaviour of the NEMS torsional varactor is investigated. There are two bifurcation points, one of which is a Hopf bifurcation point and the other is an unstable saddle point. The phase portraits are also drawn, in which periodic orbits are around the Hopf bifurcation point, but the periodic orbit will break into a homoclinic orbit when meeting the unstable saddle point.

The carrier effective masses of CaO in the cubic phase are estimated by ab initio calculations, which are used for the simulation of Si/CaO metal–oxide–semiconductor (MOS) devices by solving Schrödinger and Poisson equations self-consistently. The possibility of using CaO as a gate dielectric material for MOS device applications is then discussed. The theoretical simulations point to the possibility of using CaO as a gate dielectric, but thin films of CaO on silicon still present roughness that precludes its actual use as a gate dielectric material.

Devices for the direct detection of the spin current, based on the anomalous Hall effect (AHE), are fabricated on n-type GaAs bulk semiconductor materials. The AHE is observed in the device when the photoinduced spin-polarized electrons are injected into it, and it is found that the effect depends on the applied electric field. The origin of the field-dependent observed Hall effect is discussed based on the D'yakonov–Perel' (DP) spin relaxation mechanism. The spin-dependent Hall effect is also found to be enhanced with increasing doping concentration. The present experimental results might have potential applications in semiconductor spintronic devices since the effect is closely related to the spin Hall effect.

The authors report a new tandem cell structure for improving the short-circuit performance of a dye-sensitized solar cell. Two dye-sensitized nanocrystalline TiO₂ films were placed face-to-face as electrodes. As a counter electrode, a platinum mesh sheet with transmittance was inserted between the electrodes. Two TiO₂ anodes were connected in parallel with each other and in series with the Pt-mesh sheet cathode. Because the current density of the two TiO₂ anodes was added, the conversion efficiency was improved from 1.8% to 3.9%. The light energy absorption model in the front and back electrodes well described the output of each electrode. The front electrode thickness was increased according to the model; therefore, the efficiency of the novel tandem cell was improved by up to 4.7% for the thickness of 7.8 µm.

Spiral microdiscs with waveguide based on a conducting polymer have been fabricated on quartz substrates and the emission properties have been studied. The directivity and the polarization properties of laser emission from the spiral microcavities with waveguide based on a poly(p-phenylenevinylene) derivative have been investigated by pulsed photo-pumping. The red laser emission from the spiral microdisc with waveguide has been observed, and the directivity of this microdisc geometry has been discussed taking the mathematical simulation into consideration.

The nonvolatile memory characteristics of metal–oxide–semiconductor (MOS) structures containing Au nanocrystals in the Al₂O₃/SiO₂ matrix were studied. In this work, we have demonstrated that the use of Al₂O₃ as control oxide prepared by atomic-layer-deposition enhances the erase speed of the MOS capacitors. A giant capacitance–voltage hysteresis loop and a very short erase time which is lower than 1 ms can be obtained. Compared with the conventional floating-gate electrically erasable programmable read-only memories, the erase speed was promoted drastically. In addition, very low leakage current and large turn-around voltage resulting from electrons or holes stored in the Au nanocrystals were found in the current–voltage relation of the MOS capacitors.

Changes of the electron dynamics during the mode transition (E- to H-mode) in a hydrogen radio-frequency (rf) inductively coupled plasma are investigated using space and phase resolved optical emission spectroscopy. The E-mode is characterized through relatively weak optical emission which is strongly modulated on a nanosecond time scale during the rf-cycle, with one pronounced maximum per cycle. The modulation in H-mode, with twice the rf-frequency, is significantly weaker while the emission intensities are about two orders of magnitude higher. In particular the transition between these two modes is studied under variations of rf-power input and gas pressure. Characteristic spatio-temporal structures are observed and can be understood in the frame of a simple model combining both coupling mechanisms in the transition regime.

This paper is aimed at investigating how the positive dc corona performance of conductor-to-plane gaps is influenced by grounded and negatively stressed metallic grids located underneath the coronating conductor. A systematic experimental study is made to investigate the corona current–voltage characteristics and the current density distribution at the ground plane with and without grid. The wire-to-wire spacing, number, lateral distribution and height of grid wires as well as the conductor diameter are varied. The current density distribution at the ground plane is correlated with Warburg law that was proposed for point-plane coronas.

The flowing afterglow of an N₂–O₂ discharge in the 0.6–10 Torr range is examined in the perspective of achieving sterilization of medical devices (MDs) under conditions ensuring maximum UV intensity with minimum damage to polymer-based MDs. The early afterglow is shown to be responsible for creating strong erosion damage, requiring that the sterilizer be operated in a dominant late-afterglow mode. These two types of afterglow can be characterized by optical emission spectroscopy: the early afterglow is distinguished by an intense emission from the 1st negative system (band head at 391.4 nm) while the late afterglow yields an overpopulation of the v' = 11 ro–vibrational level of the N₂(B) state, indicating a reduced contribution from the early afterglow N₂ metastable species. We have studied the influence of operating conditions (pressure, O₂ content in the N₂–O₂ mixture, distance of the discharge from the entrance to the afterglow (sterilizer) chamber) in order to achieve a dominant late afterglow that also ensures maximum and almost uniform UV intensity in the sterilization chamber. As far as operating conditions are concerned, moving the plasma source sufficiently far from the chamber entrance is shown to be a practical means for significantly reducing the density of the characteristic species of the early afterglow.

Using the NO titration method, we obtain the (absolute) densities of N and O atoms in the afterglow at the NO injection inlet, a few cm before the chamber entrance: the N atom density goes through a maximum at approximately 0.3–0.5% O₂ and then decreases, while the O atom density increases regularly with the O₂ percentage. The spatial variation of the N atom (relative) density in the chamber is obtained by recording the emission intensity from the 1st positive system at 580 nm: in the 2–5 Torr range, this density is quite uniform everywhere in the chamber. The (relative) densities of N and O atoms in the discharge are determined by using the actinometry method: the density of N atoms decreases from its maximum value at 0% O₂ as the percentage of O₂ is increased while the density of O atoms increases, almost linearly, as a function of the percentage of O₂, as in the afterglow. The intensity variation of the NO_β UV emission as a function of the percentage of O₂ is characterized by a maximum around 0.6% O₂ (2 Torr) followed by an approximately exponential decay. We observe that, in the 0–1% O₂ range, the UV emission is limited by the availability of O atoms. Beyond this point, the decrease of the UV intensity follows the decrease in the N atom density, while on the average, the O atom density keeps on increasing with O₂%. Erosion of polymer microspheres is found to be strongest at the chamber axis when no O₂ is present, implying a dominant early afterglow. Adding even only 1% O₂ causes a strong quenching of the N₂ metastable species, leading to a dominant late afterglow and therefore considerably reducing the etching rate at the axis. In contrast, at 5 cm from the axis under the same operating conditions, a dominant late afterglow prevails; in the absence of oxygen, erosion is negligible, but it increases regularly as O₂ is introduced, following approximately the increase in the O atom density.

On an AIRIX facility, a high-intensity electron beam is used to generate an x-ray pulse for radiographic purposes. The measured electron beam spot size is found to be much larger than its computed value. It is shown that this discrepancy is removed considering ion emission at the target, under electron beam interaction. A model has been developed to simulate the plasma produced during the interaction of the electron beam with the target. The predictions have been compared with ion identification results developed on the PIVAIR prototype accelerator as well as with the x-ray spot size and dose measurements obtained on AIRIX. The simulations show that the ion effect can be reduced by placing a thin foil (100 µm thickness) at an appropriate distance from the target to catch the ions emitted and to stabilize the electron beam spot size. The X-spot size is then reduced, as found experimentally on AIRIX. Moreover, for a smaller thickness barrier foil (5 µm), the ion effect can even be suppressed, producing an X-spot size close to that obtained without ion emission.

A detailed study of the interaction between high-speed gas flows and surface dielectric barrier discharges (DBD) is presented. In the present paper, it is demonstrated that a DBD can be sustained in transonic airflows, up to isentropic Mach numbers of 1.1. The plasma is characterized electrically, as well as optically with a CCD camera and a photomultiplier tube. Different airflow velocities, plasma excitation frequencies and voltages are investigated. The airflow has a significant influence on the plasma characteristics: the glow component is reduced, the discharge becomes more filamentary and most importantly, the light emission duration from individual microdischarges is reduced by more than a factor of ten at high flow velocities. Large edge effects play a key role in the interaction between the flow and the plasma. These results offer new perspectives for the use of dielectric barrier discharges in transonic and supersonic gas flows and their applications to airflow control and to plasma-assisted combustion.

Experimental data are presented on the electrical explosion of wires of micrometre diameters with a 10 kA current having a rise time rate of up to 50 A ns⁻¹. The influence of circuit and wire parameters and properties of the wire material on the process of possible rupture of current in the circuit and resulting overvoltage at the discharge gap is evaluated. A large amount of experimental data on electrical explosion of wire in various media and in vacuum in the micro- and nanosecond range is analysed. The data are compared with theoretical estimations.

The aim is first to calculate and validate, from an ion swarm unfolding technique, the sets of collision cross sections of several ions (O⁻, NO⁻, He⁺, N₂O⁺, , and ) colliding with N₂O and He gases and their mixtures. The aim is then to calculate the corresponding ion swarm parameters in these pure gases and their mixtures in a large E/N range. These ion interactions and swarm data are very useful more particularly for the modelling of the electrical and energetic behaviour of RF discharge in N₂O–He mixtures with a small admixture of SiH₄ used in PECVD applications for thin film depositions. The sets of collision cross sections for these different ion/gas systems involve both elastic and inelastic processes in the case of an ion energy range varying from thermal energy up to about 100 eV. The elastic ion–gas interaction has been described by using a rigid core potential model which is well adapted for both polar and non-polar systems and also symmetric and asymmetric systems. Momentum transfer cross sections are then determined using a semi-classical JWKB approach while inelastic cross sections are taken from the literature and completed from empirical laws. Then by using these different cross section sets, it is possible to obtain the ion swarm data for different gas mixtures involving N₂O and He whatever their relative proportions. These ion swarm data are obtained from an optimized Monte Carlo method well adapted for the ion transport in pure gas as well as gas mixtures. The specific behaviour of these swarm data (ion mobility or drift velocity, diffusion coefficient and reaction rates) is analysed in the case of each ion in different N₂O–He mixtures over a quite large E/N range (1–1000 Td) showing more particularly the non-validity of the classical linear approximations such as Blanc law.

At atmospheric pressure and room temperature, dielectric barrier discharge induced plasma oxidation for achieving supported TiO₂ photocatalysts derived from TiCl₄ adsorbed onto γ-Al₂O₃ pellets was studied. The supported TiO₂/γ-Al₂O₃photocatalysts prepared by a cyclic 'adsorption–discharge' approach, without requirement of heat treatment, exhibit high activity in the photocatalytic degradation reaction of formaldehyde. The mass spectra and optical emission spectra during O₂/Ar discharge for oxidizing the adsorbed-state TiCl₄ were measured. The mechanism for the TiO₂ formation from adsorbed-state TiCl₄ by plasma oxidation was discussed.

An analysis of the opening and closing of a crack in a viscoelastic solid has been made assuming a Lennard-Jones law of force acting between the crack faces. The results are compared with those of an earlier analysis in which a simple Dugdale model of the surface forces was assumed.

The approximate 'reciprocal rule' between the apparent surface energies for opening and closing cracks is confirmed. At low speeds a linear relation between the apparent surface energy and the crack speed is found. The lengths of the process zone for opening and closing cracks are found to be very similar and so presumably are not responsible for the very different values of the apparent surface energy.

Lead iodide nanoparticles are synthesized in reverse micelle solution of AOT/H₂O/n-heptane. Optical absorption spectra and TEM analysis indicated the formation of crystalline particles with an average radius of 1.5 nm, which is less than the Bohr radius of the exciton (1.9 nm) in bulk PbI₂. Using theoretical models and optical spectra of quantum confined PbI₂ nanoparticles, a radius of 1.5 nm and a thickness of 1.7 nm was calculated, which are in full agreement with the TEM results. Particles were isolated from the dispersed medium and were analysed by powder XRD and Raman spectroscopy, indicating the formation of a predominantly 2H-PbI₂ polytype. This work presents the first case of fully isolated, fully characterized solid nanoparticles of PbI₂. It also presents XRD and Raman spectrum for the first time for PbI₂ nanoparticles of intermediate quantum confinement.

Indium zinc oxide (IZO) has attracted much attention recently for use in transparent oxide films compared with the ITO film. We carried out the deposition of IZO on a polyethylene terapthalate (PET) substrate at room temperature by a low-frequency (LF) magnetron sputtering system. These films have amorphous structures with excellent electrical stability, surface uniformity and high optical transmittance. The effects of LF applied voltage and O₂ flow rate were investigated. The electrical and optical properties were studied. At optimal deposition conditions, thin films of IZO with a sheet resistance of 32 Ω/sq and an optical transmittance of over 80% in the visible spectrum range were achieved. The IZO thin films fabricated by this method do not require substrate heating during the film preparation of any additional post-deposition annealing treatment. The experimental results show that films with good qualities of surface morphology, transmittance and electrical conduction can be grown by the LF magnetron sputtering method on PET which is recommendable.

The technique of surface doping is used to reduce the contact resistance between Au and poly(3-hexylthiophen-2,5-diyl) (P3HT) in Au(bottom)/P3HT/Au(top) sandwich type cells. To implement this technique, dodecyl benzene sulfonic acid (DBSA) is found to be an effective bulky dopant of P3HT as confirmed by four probe conductivity measurements, absorption and photoluminescence spectra. Sandwich cells treated with DBSA showed electrical short due to diffusion of DBSA across the P3HT film in Au(bottom)/DBSA/P3HT/DBSA/Au(top) sandwich cells, which confirms that DBSA is not immobilized at the surface. To restrict DBSA primarily at the surface, an aqueous solution of poly(ethylenedioxy thiophene) stabilized in poly(styrene sulfonic acid) (PEDOT : PSS) is utilized to make an emulsion with DBSA. The application of this emulsion at the top and bottom Au/P3HT interface has resulted in a decrease of contact resistance by nearly four orders of magnitude.

Nanocrystalline diamonds, varying in size from 40 to 400 nm, with random faceting were grown without the help of initial nucleation sites on nickel substrates as seen by scanning electron micrographs. These carbonaceous films were deposited in a microwave plasma reactor using hexane/nitrogen based chemical vapour deposition. The substrate temperatures during deposition were varied from 400 to 600 °C. The morphological investigations obtained by scanning electron micrographs and atomic force microscopy revealed the presence of nanocrystallites with multifaceted structures. Micro Raman investigations were carried out on the deposited films, which conclusively inferred that the growth of nanodiamond crystallites seen in the scanning electron micrographs correlate with clear Raman peaks appearing at 1120 and 1140 cm⁻¹. Nanoindentation analysis with atomic force microscopy has revealed that the carbonaceous deposition identified by the Raman line at ∼1140 cm⁻¹, in fact, is related to nanodiamond on account of its hardness which was ∼30 GPa. X-ray diffraction data supported this fact.

The structure, and the optical and electrical properties of CN_x films, deposited by facing-target reactive sputtering at various N₂ fractions (P_N) in the gas mixture, have been studied systematically. XPS analyses indicate that N concentration increases with rising P_N and reaches 28 at.% at a P_N of 5%. The pure C films are mainly composed of sp³ C atoms. Increasing the N content in CN_x films results in an increase of the number of sp² C atoms and the size of aromatic sp²-hybridized C clusters, while the sp² C remains relatively low content and rather small sized. Optical and electrical transport measurements show that all the films are semiconducting and the carrier transport is dominated by variable-range hopping between local electron states, due to the large number of defects in the films. With increasing P_N, the optical band gap and room temperature resistivity increase due to the passivation of some defects in the tail states.

Nominally undoped and N-doped ZnO thin films were grown by plasma-assisted metalorganic chemical vapour deposition. P-type conductivity was confirmed by Hall-effect measurements, not only in the N-doped but also in the nominally undoped ZnO. The zinc vacancy and extrinsic nitrogen acceptor states were identified by low-temperature photoluminescence, with the energy level located at 270 meV and 180 meV above the valence-band maximum, respectively. An evident increment in the oxygen as well as nitrogen concentration in the p-type ZnO : N layer was well confirmed by secondary ion mass spectroscopy.

The effect on the polarization of antiferroelectric (AFE) PNZST ((Pb,Nb)(Zr,Sn,Ti)O₃) thin films by ε–E (dc bias field) cycles was studied. It was shown that in these films the AFE ordering is destroyed by the application of a dc electrical field bias along the surface normal direction. After removing the dc bias the film relaxes slowly back to the initial AFE state. This phenomenon is dependent on the film thickness. The relaxation time decreases with increasing film thickness. With increasing storage time of the sample after removing the dc bias at room temperature or heat treatment above the Curie temperature, the AFE ordering can return. From the characteristics of hysteresis loops and ε–E behaviours, we can ascertain that this phenomenon could be attributed to the difference in the poled volume at the interfaces between the electrode and the film.

Nd₂Fe₁₄B/Nd-O thin films with high-energy product (BH)_max = 33.9 MGOe are fabricated by a one-step sputtering process, i.e. dc magnetron sputtering on a Si substrate heated at 600–650 °C, without a post-deposition annealing process. The highly anisotropic Nd-Fe-B/Nd-O thin film with high-energy product is realized immediately after the deposition, owing to the anisotropic growth of both the Nd-O and the Nd₂Fe₁₄B phases. The coexistence of the nanoscale Nd₂Fe₁₄B and Nd-oxide phases in the thin films is confirmed by means of x-ray diffraction, high-resolution transmission electron microscopy (HRTEM) and energy-filtered transmission electron microscopy. The boundaries between the buffer layer and the Nd-Fe-B layers and the different phases are investigated by HRTEM.

We have studied the resonant modes in a planar cavity resonator containing a bilayer of non-dissipative anisotropic metamaterials. Different from the isotropic case, the resonance in such a resonator is closely related to the dispersion relation of the anisotropic medium. Three cases of different combinations of materials are discussed and it is found that sub-wavelength resonant modes may occur in all cases. Three kinds of resonant modes with different electromagnetic field distributions have been revealed and analysed. Requirements of the material and geometry parameters to construct a sub-wavelength resonator are revealed by an approximate analysis and verified by a rigorous solution of the characteristic equation, which demonstrate that this kind of sub-wavelength resonator brings more design flexibility and tolerance.

Nano-barium titanate (BT) was prepared by a sol–gel method. The prepared powders were characterized by x-ray powder diffraction and transmission electron microscopy. The complex relative dielectric permittivity (ε = ε' − jε'') and magnetic permeability (μ = μ' − jμ'') of the BT powders were measured in the frequency range 8 ∼ 18 GHz. The BT/epoxide resin (EP) composite with different volume contents was investigated. The effects of thickness on the BT/EP composite were studied. It was found that an optimum thickness and contents of the absorber can yield the maximum reflection loss which could be obtained over a broad frequency region in the X and Ku bands. Our results indicate that BT could be a promising microwave absorption material.

This paper presents a numerical analysis of the membrane voltage induced on biological cells, under the influence of an externally applied field in such processes as electroporation or electrofusion. We focus on the configurations in which an insulator plate with an orifice is used and study the cases of (a) a cell placed on the orifice or (b) two cells in contact at the orifice, when a stepwise voltage is applied across the plate. Formulation based on the boundary element method is made assuming that a biological membrane is an infinitesimally thin insulator. Results of the calculation show that, due to the field constriction created by the orifice plate, almost all the externally applied voltage is imposed on the membrane at the orifice, and the membrane voltage outside the orifice is virtually zero; field tailoring with the use of the orifice plate enables control over the magnitude and localization of the membrane voltage. It is also shown that we can induce breakdown exclusively around the contact point of a pair of cells for high-yield electrofusion with such a geometry.

It has come to the attention of the Institute of Physics that this article should not have been submitted for publication owing to its substantial replication of an earlier paper (Yu G H, Zhao H C, Li M H, Zhu F W and Lai W Y 2002 Interface reaction of Ta/Ni₈₁Fe₁₉ or Ni₈₁Fe₁₉/Ta and its suppression Applied Physics Letters80 455). Consequently this paper has been retracted by the Institute of Physics and by the authors.