Table of contents

This special issue of Journal of Physics: Condensed Matter contains selected papers from the 1st International Conference on Nanospintronics Design and Realization (ICNDR 2004), which was held in Kyoto, Japan, 24--28 May 2004. This conference was organized by the Nanospintronics Design and Realization project members:

• Hideaki Kasai, Osaka (Chair of the Conference)

• Hisazumi Akai, Osaka

• Hajime Asahi, Osaka

• Wilson Agerico Diño, Osaka

• Hiroshi Harima, Kyoto

• Tomoyuki Kakeshita, Osaka

• Junjiro Kanamori, Kyoto

• Hiroshi Katayama-Yoshida, Osaka

• Koichi Kusakabe, Osaka

• Hiroshi Nakanishi, Osaka (Secretary)

• Tamio Oguchi, Hiroshima

• Teruo Ono, Osaka

• Naoshi Suzuki, Osaka

• Hitoshi Tabata, Osaka

under the auspices of the Japan Ministry of Education, Culture, Sports, Science and Technology (MEXT) Special Coordination Funds for Promoting Science and Technology, and the sponsorship of Osaka University and the International Institute for Advanced Studies (IIAS).

The conference is intended to provide an international forum for experimental and theoretical researchers, in the rapidly developing field of nanospintronics. It aims to:

• provide an overview of our current understanding of the physics of spin transport in (magnetic) semiconductors and hybrid magnetic/semiconductor structures;

• provide a venue to present and discuss the latest developments in using spin-dependent phenomena in nano-(opto-) electronics and computing applications;

• provide a venue for discussion and assessment of other possible means of exploiting the spin-dependent phenomena in future nano-(opto-) electronic and computing applications;

• address current (and foreseeable future) problems, of fundamental and applied nature, in an effort to bridge the physics and technology gap between semiconducting and magnetic materials.

All of these being geared towards bringing about the realization of a functioning nanospintronics.

A total of 127 delegates from 15 countries took part in ICNDR 2004, which was comprised of 62 invited oral presentations and 44 contributed posters.

The conference also has additional financial support from the Asahi Glass Foundation, the Foundation for Promotion of Material Science and Technology of Japan, the Izumi Science and Technology Foundation, the Kansai Research Foundation for Technology Promotion, the Kao Foundation for Arts and Sciences, the Murata Science Foundation, the Nanotechnology Researchers Network Center of Japan, and the Nippon Sheet Glass Foundation for Materials Science and Engineering.

The 2nd International Conference on Nanospintronics Design and Realization will be held in Germany, in 2007, and will be organized by Stefan Blügel, Patrick Bruno, and Dieter Weiss. We hope to see you there.

A review is given of the experimental situation and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductors, such as (Ga, Mn)As and p-(Cd, Mn)Te. It is emphasized that the Zener p–d model explains quantitatively the effect of strain on the easy axis direction as well as predicting correctly the presence of the reorientation transition, observed as a function of hole concentration and temperature. Possible suggestions put forward to explain the existence of in-plane uniaxial magnetocrystalline anisotropy are then quoted. Finally, magnetic stiffness computed within the same model of ferromagnetism is presented, and is shown to explain the domain width in perpendicular films. It is emphasized that a rather large magnitude of the stiffness accounts for both the excellent micromagnetic properties of ferromagnetic (III, Mn)V films and the quantitative applicability of the mean-field approximation to these materials.

The electronic structure and magnetism of selected diluted magnetic semiconductors (DMS) is reviewed. It is argued that the effect of antisite defects plays an important role in the magnetism of DMS materials and that these defects lower the saturation moment and ordering temperature. We also show that the interatomic exchange of these materials is short ranged. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we show that the observed critical temperatures of a broad range of diluted magnetic semiconductors, involving Mn-doped GaAs and GaN as well as Cr-doped ZnTe, are reproduced with good accuracy. We show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned on the Ga (or Zn) sites. This suggests that the ordering of DMS materials is heavily influenced by magnetic percolation and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.

On the basis of ab initio calculations for Mn-doped GaN, GaP, GaAs and GaSb, we discuss the origin of ferromagnetism in diluted magnetic semiconductors. The calculations use the Korringa–Kohn–Rostoker method in connection with the coherent potential approximation to describe the substitutional and moment disorder. By mapping the total energy results onto a Heisenberg model, the Curie temperature T_C is estimated in the mean field approximation (MFA). If impurity bands are formed in the gap, as is the case for (Ga, Mn)N, double exchange dominates leading to a characteristic dependence of T_C^MFA as a function of the Mn concentration c. On the other hand, if the d states are localized, as in (Ga, Mn)Sb, Zener's p–d exchange prevails, resulting in a linear c dependence of T_C^MFA. (Ga, Mn)As is an intermediate case, showing a -like behaviour in the local density approximation (LDA), but a nearly linear c dependence, if the more accurate LDA +U method with U = 4 eV is used.

The effect of increasing the Fermi level by modulation doping on the incorporation of Mn into the III–V host lattice—and hence on the ferromagnetic properties of III_1−xMn_xV alloys—is investigated in Ga_1−xMn_xAs/Ga_1−yAl_yAs heterojunctions and quantum wells. Introducing Be acceptors into the Ga_1−yAl_yAs barriers leads to an increase of the Curie temperature T_C of Ga_1−xMn_xAs, from 70 K in undoped structures to over 100 K in modulation-doped structures. This increase is qualitatively consistent with multi-band mean field theory simulation of carrier-mediated ferromagnetism. Here the crucial feature is that the increase of T_C occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, and that T_C actually drops when the Be-doped layer is grown first. Using ion channelling techniques we provide direct evidence that this latter reduction in T_C is directly correlated with an increased formation of magnetically inactive Mn interstitials. This formation of interstitials is induced by the shift of the Fermi energy as the holes are transferred from the barrier to the quantum well during the growth.

Based on the electronic structure studies, we propose that the double perovskite LaV O₃/ARuO₃ superlattice (A = Ca,Sr and Ba) and thiospinel Mn(CrV)S₄ and Fe_0.5Cu_0.5(V_0.5Ti_1.5)S₄ are potential candidates for half-metallic antiferromagnets (HM-AFMs). We have also explored the effect of the Li intercalation on the electronic and magnetic properties of transition-metal (TM) doped anatase TiO₂. We propose a potential spintronic and electrochromic device made of Li intercalated Mn-doped TiO₂, which is controlled by the electric field. For (Fe, Cu) co-doped ZnO, the charge transfer between Fe and Cu leads to ferromagnetism via a type of the double-exchange mechanism.

The influence of intrinsic defects in half-metals is calculated in the case of NiMnSb. Of the 14 cases of intrinsic defects, five affect the half-metallic properties. They are energetically very unlikely to occur. Circumstances are discussed under which defects may even have a beneficial effect on the spin polarization of the conduction electrons. Non-intrinsic defects, like deliberate doping by rare-earth atoms, as well as the effect of nano-structured contacts may influence the magnon spectrum, improving the behaviour at finite temperature.

The influence of atomic disorder and heterointerfaces with III–V semiconductors on the electronic and magnetic properties of zinc-blende (ZB) CrAs is studied by ab initio calculations based on density-functional theory. Antisite Cr spins are coupled antiferromagnetically with the Cr spins at the ordinary sites, while the ferromagnetic coupling between the Cr spins at the ordinary sites is robust against defect formation. The degree of spin polarization is not reduced significantly by the impurity bands formed in the minority spin energy gap. In the ZB CrAs/GaAs junction, relatively high spin polarization is retained even at the interface in contrast to usual half-metal/semiconductor heterojunctions. Complete spin polarization is also preserved throughout ZB CrAs/GaAs multilayers and it is insensitive to the substitutional disorder between Cr and Ga sites.

Transparent magnetic semiconductors have been constructed by introducing transition metals such as Co or V into wurtzite ZnO crystals by a non-equilibrium thin film technique using laser molecular beam epitaxy. Co and V substituted for Zn have a divalent cationic state in the ZnO thin films formed epitaxially on the sapphire substrates. Raman measurements show that the wurtzite crystal structure can be maintained up to 15% and 5% for Co and V substitution, respectively. The results from photoemission spectra also suggest that Co does has not a metal state but a 2+ state. Magnetic circular dichroism (MCD), x-ray MCD, superconducting quantum interference device (SQUID) and nuclear magnetic resonance (NMR) measurements on (Zn, Co)O films indicate ferromagnetic properties with magnetizations of 0.1–0.3 μ_B/Co.

The magneto-optical effect in dilute magnetic semiconductors (DMSs) is directly related to the interaction between the d electrons of the transition metal ions and the s, p electrons of the host semiconductor. We show the advantages of the magneto-optical effect of DMSs as regards fabricating magneto-optical waveguide devices that can be integrated with other semiconductor optical devices. We also discuss the advantages of magneto-optical spectroscopy for characterizing DMSs. Intrinsic ferromagnetism of In_1−xMn_xAs, Ga_1−xMn_xAs and Zn_1−xCr_xTe is confirmed by using magnetic circular dichroism (MCD) spectroscopy. The MCD analyses also show that Zn_1−xTM_xO (TM = Mn, Fe, Co, Ni or Cu), Ga_1−xMn_xN and Ga_1−xCr_xAs are paramagnetic DMSs with the s, p–d exchange interaction. Ferromagnetic behaviours observed in some transition metal doped ZnO, GaN and GaAs samples are attributed to unidentified precipitations not detectable by means of x-ray diffraction analysis.

The epitaxial growth of zinc-blende CrAs/GaAs multilayers has been achieved by using the molecular-beam epitaxy method. The crystallographic quality was evaluated by reflection high-energy electron diffraction (RHEED) and cross-sectional transmission electron microscopy (TEM). The increase of the substrate temperature during growth up to 300 °C brings the RHEED pattern to a streak, in contrast to the case at 200 °C. TEM images show the atomically flat surface and interface of the multilayer.

GaN-based magnetic semiconductors are expected to exhibit ferromagnetism even at high temperatures. We have studied the magnetic and optical properties of transition-metal- and rare-earth-doped GaN. GaCrN showed ferromagnetic characteristics at 7–400 K. Clear hysteresis and clear saturation were observed in the magnetization versus magnetic field (M–H) curves at all measuring temperatures. We observed photoluminescence (PL) emission from GaCrN layers. GaGdN also showed ferromagnetic characteristics at 7–400 K. Sharp PL emission was observed for GaGdN. X-ray diffraction and EXAFS measurements showed no phase separation, and that the Cr and Gd atoms substitute Ga sites. Applications to novel devices controlling charges (electrons and holes), spins and photons are expected.

Numerical results of electrical conductance and magnetoresistance of ferromagnetic tunnel junctions and semiconductor/semimetal junctions are presented, putting an emphasis on effects of disorder on the conductance. It is shown that the disorder strongly affects the tunnelling conductance and magnetoresistance. It is discussed how the interfacial resonant states appearing in semiconductor/semimetal junctions influence the conductance. Numerical simulation of diffusive conductivity of a two-dimensional electron gas with spin–orbit interaction is also performed, and the results are compared with the analytical result.

A first-principles approach to magnetic properties of diluted magnetic semiconductors (DMS) is presented that is based on the local spin-density approximation (LSDA) as implemented in the framework of the tight-binding linear muffin-tin orbital method, while the effect of randomness is described by the coherent potential approximation. Application of a real-space Green-function formalism yields the exchange pair interactions between distant magnetic atoms that are needed for quantitative studies of magnetic excitations including the Curie temperatures. We have found that the pair exchange interactions exhibit a strong directional dependence and are exponentially damped with increasing distance between magnetic atoms due to disorder and the half-metallic character of the DMS. As a case study we consider (Ga, Mn) As, (Ga, Mn)N, and (Zn, Cr)Te alloys. The calculations demonstrate that inclusion of disorder and, in particular, realistic distances among magnetic impurities, are needed to obtain critical temperatures which are in good agreement with available experiments.

We present a fully relativistic generalization of the Landauer–Büttiker formalism that has been implemented within the framework of the spin-polarized relativistic screened Korringa–Kohn–Rostoker Green function method.

This approach, going beyond the two-current model, supplies a more general description of the electronic transport. It is shown that the relativistic conductance can be split in terms of individual spin-diagonal and spin-off-diagonal (spin-flip) components, which allows a detailed analysis of the influence of spin–orbit-coupling-induced spin-flip processes on the spin-dependent transport.

We apply our method to calculate the ballistic conductance in Fe/GaAs/Fe magnetic tunnel junctions. We find that, by removing the spin selection rules, the spin–orbit coupling strongly influences the conductance, not only qualitatively but also quantitatively, especially in the anti-parallel alignment of the magnetization in the two Fe leads.

We discuss an application of the self-interaction-corrected local spin density (SIC-LSD) approximation to study electronic structure of some half-metallic ferromagnets and ferromagnetic insulators of current interest in spintronics. Both d- and f-electron materials are considered, and we concentrate on the nominal valence and ground state properties of these systems.

We find there are at least two different steady states for transport across noncollinear magnetic multilayers. In the conventional one there is a discontinuity in the spin current across the interfaces which has been identified as the source of current induced magnetic reversal; in the one advocated here the spin torque arises from the spin accumulation transverse to the magnetization of a magnetic layer. These two states have quite different attributes which should be discerned by current experiments.

The electronic structure and residual resistivity of diluted (Ga, Mn)As magnetic semiconductors are calculated from first principles using the linear muffin-tin orbital method, the coherent potential approximation, and the Kubo–Greenwood linear response theory. Particular attention is paid to the role of native compensating defects such as As antisites and Mn interstitials as well as to different magnetic configurations of the local Mn moments. The order of magnitude of the calculated resistivities compares reasonably well with available experimental data. The concentration variations of the resistivity reflect two basic mechanisms, namely the strength of the impurity scattering and the number of carriers. In agreement with a recent experiment, the calculated resistivities are strongly correlated with the alloy Curie temperatures evaluated in terms of a classical Heisenberg Hamiltonian.

We have investigated control of microstructure, that is, the rearrangement of martensite variants (crystallographic domain), in a disordered Fe–31.2Pd (at.%) single crystal and an ordered Fe₃Pt single crystal, exhibiting cubic to tetragonal martensitic transformation at 230 and 85 K, respectively. When a magnetic field is applied along the [001] direction to the specimen with the multi-variant state, the specimen expands along the field direction for Fe–31.2Pd and contracts for Fe₃Pt, because the specific variant which reduces the magnetocrystalline anisotropy most will be selected to grow by consuming other variants. The fraction of such variants reaches 100% for Fe–31.2Pd but does not for Fe₃Pt. In the field removal process, part of the field-induced strain recovers for Fe₃Pt but not for Fe–31.2Pd. The recoverable strain of Fe₃Pt depends on temperature and shows a maximum value of nearly 1% at 20 K. From the magnetization curve, the energy dissipated due to the rearrangement of variants by a magnetic field is obtained to be about 260 kJ m⁻³ for Fe–31.2Pd and about 180 kJ m⁻³ for Fe₃Pt. This is of the same order of magnitude as that evaluated from the stress–strain curve of Fe–31.2Pd, meaning that the rearrangement of variants by a magnetic field occurs by nearly the same path or process as that by external stress. On the basis of these results and magnetocrystalline anisotropy constants of martensite phases, the mechanism of rearrangement of variants under a magnetic field is discussed.

The magnetocrystalline anisotropy (MCA) of bulk and thick films of FePt is calculated from a 'first-principles' theory. The starting point is a description from electronic density functional theory for systems of interacting electrons moving in lattices of ions. Relativistic effects such as spin–orbit coupling are included. FePt readily transforms into a CuAu-type (L 1₀) ordered phase and this coincides with the material's high anisotropy. Here we describe how to calculate the MCA of a partially ordered alloy and to extract its dependence on the long range chemical order parameter η. We present calculations of the MCA of FePt as a function of η and find excellent agreement with the experimental data of Okamoto et al (2002 Phys. Rev. B 66 024413) and others with respect to the magnetic easy axis, the magnitude of the MCA and its trend with decreasing η. We also study the paramagnetic phase of the ordered alloy using the 'disordered local moment' picture of metallic magnetism at finite temperatures. We calculate a Curie temperature of 935 K in reasonable agreement with experiment (710 K) and find the easy axis for the onset of ferromagnetic order to coincide with the magnetic easy axis found at low temperatures.

We present a study of total energy as a function of volume in the cubic phase of LaMnO₃. A state of charge disproportionation into planes of Mn³⁺O₂/Mn⁴⁺O₂ was found. It is argued that the pressure driven localization/delocalization transition might go smoothly through a region of Mn³⁺ and Mn⁴⁺ coexistence.

New candidates for half-metallic materials were theoretically designed recently by Geshi et al. The materials are calcium pnictides, i.e. CaP, CaAs and CaSb. When the zinc-blende structure was assumed, these compounds showed half-metallic electronic band-structure, in which a curious flat band was found. To explain this magnetism, we investigated characters of orbitals on this flat band of CaAs. The hybridization of p states of As with d states of Ca is shown to be essential for formation of a flat band made of localized orbitals. The appearance of complete spin polarization in the flat band suggests that the flat-band mechanism is relevant for the ferromagnetism. A connection from the first-principles result to a solvable Hubbard model with a flat band is discussed.

We review our recent work on ferromagnet/semiconductor hybrid structures. In particular we focus on magnetotransport experiments performed on Co/InAs/Co hybrid structures fabricated on the cleaved edge of an InAs/InGaAs heterostructure. By modulation doping we inserted a high-mobility two-dimensional electron system (2DES) between the Co source and drain contacts which were separated by about 0.2 µm. This separation was smaller than the mean free path in the 2DES. Two additional metallic gate electrodes were integrated, thus forming a prototype spin field-effect transistor on a cleaved (110) surface of an InAs/InGaAs heterostructure. Intriguingly, we observe two characteristic magnetotransport signals at 4.2 K: (a) a hysteretic spin-valve-like signal and (b) a large positive magnetoresistance. We attribute the latter to the extraordinary magnetoresistance effect, i.e. the magnetic-field induced current redistribution between the Co contacts and the 2DES.

Structural properties of GaN and ZnO layers doped with magnetic impurities were investigated by Raman scattering. Long-range lattice ordering and local atomic arrangement around magnetic impurities were analysed, and their solubility limit was considered.

For this study, GaN layers doped with Mn and Cr, and ZnO layers doped with Co and V were prepared by molecular beam epitaxy and pulsed laser deposition, respectively. ZnO layers codoped with Ga and N were also studied for the purpose of p-type activation. These samples were observed using a Raman microprobe using visible and deep UV lasers for excitation. The main results are as follows:

In Ga_1−xMn_xN layers, a uniform solid solution was formed for Mn concentration up to x = 1–2%. An impurity mode was observed at 585 cm⁻¹ and assigned to a local vibrational mode of Mn substituting the Ga site. At higher Mn concentrations, rapid deterioration in lattice ordering occurred. Ga_1−xCr_xN layers showed good lattice ordering up to x = 3–5%. The samples showed resonance enhancement of LO-phonon signals when excited by a UV laser at 266 nm (4.7 eV). This indicates the photo-injection of free carriers to a diluted magnetic semiconductor.

ZnO layers codoped with Ga and N showed many impurity modes due to host lattice defects. A strong signal at 580 cm⁻¹ showed a characteristic broadening at high concentrations of N and Ga. This suggested the formation of complex centres with N or related defects. Zn_1−xCo_xO and Zn_1−xV_xO layers formed uniform solid solutions up to , but precipitation of the secondary phase was observed at . These samples presented common defect modes as observed in codoped samples. Our result suggests that the impurity modes in ZnO-based materials can be used as a sensitive probe of host lattice defects induced by the impurity incorporation process.

We present calculations of the electronic and magnetic structure of the cubic phases of Mn–Ga–As. We find that for high coordination numbers, as for a bcc and fcc environment, the Ga/As atoms form broad metallic-like sp bands which interact with Mn. For most of these systems antiferromagnetic structures are the most stable configurations. For AsMn in the ZnS structure we find a half metallicity both in the ferromagnetic and the [001] anti ferromagnetic modification. Calculations of Mn–GaAs supercells allow us to formulate a possible mechanism to enhance the magnetic ordering temperature. Since T_c is found to depend strongly on the number of excess electrons in the system, alloying with constituents with fewer electrons than As could help to stabilize the FM state and thus might increase the magnetic ordering temperature.

The successful growth of zinc-blende half-metallic compounds, namely CrAs and CrSb, in thin film forms offers a new direction to search for novel spintronic materials. By using a well documented first-principles algorithm, the VASP code, we predict the electronic and magnetic properties of superlattices made of these exciting half-metallic materials. Not only are the superlattices constructed with two of the half-metallic compounds (CrAs/MnAs) but also they are modelled to combine with both a III–V (GaAs–MnAs/CrAs/GaAs) and a IV–IV (MnC/SiC) semiconductor. We investigate variable thicknesses for the combinations. For every case, we find the equilibrium lattice constant as well as the lattice constant at which the superlattice exhibits the half-metallic properties. For CrAs/MnAs, the half-metallic properties are presented and the magnetic moments are shown to be the sum of the moments for MnAs and CrAs. The half-metallic properties of GaAs–MnAs/CrAs/GaAs are found to be crucially dependent on the completion of the d–p hybridization. The magnetic properties of MnC/SiC are discussed with respect to the properties of MnC.

The electronic structure, magnetic and electric properties, and lattice stability of multiferroic BiMnO₃ as a typical system in perovskite Bi transition-metal oxides (BiMO₃) are studied from first principles. It is demonstrated theoretically for the first time that the orbital ordering within the Mn e_g orbitals is actually realized in BiMnO₃, being consistent with crystallographic data, and plays a crucial role in the appearance of ferromagnetism. Total-energy calculation shows the ferromagnetic state is indeed stabilized. Electrical polarization of BiMnO₃ is also estimated based on the Berry phase theory. Lattice instability to off-centred displacement, which is driven by strong covalent bonding between Bi 6p and O 2p states, is found to be rather common in the BiMO₃ series.

Electronic structures of La-doped Ba₂FeMoO₆ double perovskite oxides have been investigated using photoemission spectroscopy and soft x-ray absorption spectroscopy. The ground states of Ba_2−xLa_xFeMoO₆ are found to be in the Fe²⁺–Fe³⁺ mixed-valent states and the Fe valence state increases toward 3+ with increasing x. The states close to E_F consist of the degenerate Mo–Fe states. The LSDA +U calculations for Ba_2−xLa_xFeMoO₆ (x = 0, 1) show a larger occupied bandwidth of the Mo states for x = 1 than for x = 0, which is consistent with the higher T_C value predicted in a type of the double exchange mechanism.

Edge effects on possible magnetism appearing in finite hydrogenated zigzag nanotubes are investigated using the DFT–LSDA calculations. Our result shows that two types of edges, i.e. a mono-hydrogenated zigzag edge (MHZE) and a di-hydrogenated zigzag edge (DHZE), can give magnetic moments in finite nanotubes. Due to curvature of the tube, however, the total magnetic moment S_tot is smaller than an expected value given by a counting rule of S_tot, which holds for magnetic nanographite ribbons. In addition to magnetic nanotubes found by Okada and Oshiyama, we show that a (7,0) tube and an (8,0) tube with both MHZE and DHZE are magnetic. The reactions to create hydrogenated (7,0) and (8,0) tubes are exothermic.

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.

We have investigated the electronic structures and magnetic properties of Li-intercalated Co-doped rutile TiO₂. For non-intercalated Ti_0.9375Co_0.0625O₂, the half-metallic and low-spin () ground state is obtained. By Li intercalation, Ti_0.9375Co_0.0625O₂ becomes a paramagnetic insulator at the concentration of Li/Ti = 0.067. At the higher concentration of Li/Ti = 0.133, it becomes a paramagnetic metal. Hence, as in the transition metal doped anatase TiO₂ case, we expect that the magnetic and transport properties of Co-doped rutile TiO₂ can be controlled by an electric field.

We have searched for a new highly spin-polarized ferromagnet which has a higher spin moment than that of known half-metallic transition metal pnictides with the zinc-blende structure by first-principles calculations. To generate the high spin moment we focus on Gd compounds. Our calculation shows that a (GdN)₁/(CrAs)₁ structure is a ferromagnetic material. The total magnetic moment of this ferromagnet is over 9.9 µ_B per chemical formula.

Based on ab initio calculations of electronic structures of Mn-doped GaN, a carrier codoping method with size compensation to enhance the Curie temperature (T_C) of GaN-based dilute magnetic semiconductors is proposed. Beryllium (Be) is chosen as a codopant to reduce the volume expansion caused by Mn doping, and also to enhance the 3d-density of states at Fermi level by hole doping. It is found that the mid-gap impurity band is strongly broadened due to strong p–d hybridization, resulting in a considerable increase of the hole density. With Be substituting both the cations and anions, the reduction of the antiferromagnetic super-exchange interaction in the regime of high Mn-concentrations and the highest value of T_C due to the ferromagnetic double exchange interaction are strongly expected.

A ferromagnetic/nonmagnetic semiconductor DyN/GaN superlattice sample showed a large coercivity H_c of about 1800 Oe at 7 K, which originates from DyN layers. Ferromagnetic order was also observed at 300 K, which may come from GaDyN alloy layers formed at GaN/DyN interfaces. For this superlattice sample, spin tunnel resistance characteristics were observed at low temperatures. GaCrN/GaN/GaCrN trilayer structures were grown by radio frequency molecular beam epitaxy. Clear hysteresis and clear saturation characteristics were observed in the magnetization versus magnetic field curves at all measuring temperatures. A hysteresis loop was observed in the magnetic field dependence of vertical electrical resistance.

The spin-polarized electron momentum distribution (magnetic Compton profile: MCP) of ferromagnetic Sr₂FeMoO₆ has been measured using the magnetic Compton scattering technique at room temperature. The experimental MCP is in good agreement with that obtained by a first-principles band-structure calculation. The experiment and calculation reveal a hollow around p_z = 0, which support the conclusion that there exists a negatively spin-polarized Fe 3d–Mo 4d mixed state.

We investigate and discuss spin polarization effects on cyclohexane (C₆H₁₂) dehydrogenation using a Ni atom as a test catalyst, by performing total energy calculations based on the density functional theory (DFT). We compare the results with those of the well known catalyst Pt. We consider the process where cyclohexane approaches a transition metal M (M: Ni and Pt), and determine the reaction paths from the calculated potential energy surfaces (PESs) for singlet cyclohexane/M and triplet cyclohexane/Ni systems. Unlike the singlet cyclohexane/Ni, no energy is required to separate cyclohexyl intermediate (C₆H₁₁) from the H–Ni system for the triplet cyclohexane/Ni. Our results suggest that the catalytic reactivity of spin-polarized Ni becomes close to that of Pt, which is considered to be, up to now, the best catalyst for cyclohexane dehydrogenation.

We investigate the structural and electronic properties of Co₂CrAl/GaAs (100) and (110) interfaces using density functional theory within the generalized gradient approximation. Adhesion energy is calculated for various interfacial structures, and it is found that the (100) CrAl-terminated surface, which was theoretically predicted to be highly spin polarized, shows weaker adhesion to GaAs than the (100) Co-terminated surface. We then examine the spatial behaviour of local spin polarization at the Fermi level for the (100) and (110) interfaces, choosing the interfacial structures which give rise to relatively large adhesion energy. The (110) interface turns out to have very high spin polarization at the interface, while such behaviour is not observed in the (100) interface.

Zn_0.4Mn_0.6Se thin films were grown on GaAs(100) substrates by HWE. The evolution of the crystal structure from the β-phase to the α-phase is characterized by x-ray diffraction patterns. The variation of the lattice constants for both the α-phase and the β-phase in the films is also investigated. The surface morphology is gradually roughened as the phase is changed. In the photoluminescence spectrum intra-Mn²⁺ transitions are observed and found to be substrate temperature dependent.

We perform first-principles density functional calculations for Mn doped group-IV (Ge, Si) based diluted magnetic semiconductors. It is found that the ferromagnetic spin configuration is stabilized when two Mn atoms are located along the [110] direction in Ge and Si hosts due to the strong ferromagnetic exchange interaction between Mn impurities. Furthermore, our results show that a ferrimagnetic Mn cluster is more stable than the ferromagnetic Mn configuration along the [110] direction, indicating that Mn impurities tend to gather and create undesired magnetic clusters in Ge and Si hosts. We conclude that the vicinal Ge(111) and Si(111) surfaces play an important role in the growth process of Mn doped Ge and Si in obtaining a ferromagnetically stable Mn configuration.

We investigate magnetic properties of the nanoscale ferrimagnetic ring [Mn(hfac)₂NITPh]₆ that is suggested to have a tendency of frustration. The spin densities of the candidate models are discussed. The spin density of the model with frustration shows characteristic behaviour different from those of the other models. Except for the quantum fluctuation which is a little bigger for the frustrated model, the difference in overall behaviour of the spin density is explained by the spin alignment in corresponding classical spin systems. The spin alignment is determined by competition of the Zeeman energy and the Mn–radical exchange interaction, when a canted spin alignment of Mn spins from the axis of the external magnetic field appears in the magnetization process for the other models, while it is absent for our frustrated model.

Optical properties of GaCrN, GaDyN and GaGdN have been studied by means of photoluminescence spectroscopy having spectral and time resolution. Photoluminescence (PL) emission at around 3.29 eV was determined to be the band-to-band transition of GaCrN. The PL decay time of GaCrN depends on emission energy. For GaDyN, Dy³⁺ ions in host GaN exhibited several sharp peaks originating from the intra-4f orbital transitions of the series of ⁴F_9/2−⁶H_J (J = 15/2,13/2,...), and the lifetime of the ⁴F_9/2−⁶H_13/2 transition was as long as 87 µs at 13 K. The broad defect level for GaDyN provides an efficient pathway for the carrier-mediated energy transfer between Dy³⁺ ions and the GaN host. However, no such evidence is found between Gd ions and GaN in the GaGdN layer.

Local magnetic moments in benzenes of a multiple-decked sandwich cluster, namely, Mn_n(C₆H₆)_n+1 (n = 1,2) are investigated using density functional theory. A significant increase in the magnetic moments of benzenes was found in the 2p_z orbitals. From the values of the local magnetic moments of Mn(C₆H₆)₂ and Mn₂(C₆H₆)₃, we find that the local magnetic moment of benzene in the centre of the chain is larger than that at the edge position. These results indicate a promising possibility of changing the magnetic properties of benzenes using a magnetic material.

We investigate the electronic and magnetic properties of a metallic (3, 3) single-walled carbon nanotube (SWCNT) filled with a linear Fe nanowire, based on the ab initio spin-polarized density functional theory. We find that in the stable structure the Fe-filled (3, 3), (5, 0) SWCNTs exhibit semiconducting properties, and the Fe nanowires lose their magnetic moment, while the (4, 4), (5, 5), (6, 6), (6, 0) SWCNTs having larger radius with linear Fe nanowires are metallic, and show magnetic moments.

The band gap of half-metallic ferromagnets can be affected by the spin–orbit coupling, which introduces there a small, but non-vanishing, density of states. We study this effect in the case of Heusler alloys. We find that, as a rule, the spin polarization in the middle of the gap decreases for compounds of heavier elements.

We investigate the magnetic properties for Fe adatoms on Si(001) on the basis of first-principles calculations. For the Si(001) surface, we consider systems having buckling dimers and H-terminated symmetric dimers. We find that the magnetic moment of the Fe adatom on the latter surface is larger than that of the Fe adatom on the former surface. The results positively indicate potential applications in the realization/fabrication of nanospintronics devices.

We have measured the electrical resistance and the high-field magnetoresistance (MR) up to 30 T for nano-scale misfit layered compounds Ca_3−xY_xCo₄O₉ (x = 0, 0.1, 0.2). The resistance shows the characteristic behaviour of the variable range tunnelling at low temperatures. The negative MR ratio increases remarkably with decreasing temperature and with increasing x at temperatures below 20 K. We propose that the MR in the cobaltite can be explained by the magnetic tunnelling effect.

The magnetic and structural properties of MBE-grown films of a novel diluted magnetic semiconductor (DMS), Zn_1−xCr_xTe, were investigated. The magnetization versus magnetic field (M–H) measurement exhibited a clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T_C) estimated from the Arrott-plot analysis increased almost linearly with the Cr concentration (x) up to 275 K at x = 0.17. In the magnetization versus temperature (M–T) measurement, irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This behaviour, which is typically observed in a magnetic random system such as the spin-glass or superparamagnetic phase, is considered to be related to the local structural inhomogeneity observed by high resolution transmission electron microscopy.

We investigated electron transport in ladder-structured Kondo lattice models with antiferromagnetic coupling between localized S = 1/2 spins. In a system where two spins on the ladder rung dimerize, conductivity shows an anomalous non-monotonic field dependence in contrast with ordinary magnetoresistance. This is attributed to a spin configuration that is represented by a product of singlets and triplets on the ladder rungs. However, when the spin exchange interactions of the Heisenberg-type are considered between two spins on diagonals in the ladder, such a spin configuration is destroyed and the non-monotonic field dependence is dissolved.

X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) have been used to study self-assembled Co nanoclusters on Si(111)-7  × 7 substrate. The multiplet fine structures of the XAS spectrum indicate that there is a contribution of Co³⁺ atoms in the cluster due to the Co 3d–Al 3p hybridization at the Co–Al interface. The very small and positive L₂ peak compared to the negative peak at the L₃ edge appearing in the XMCD spectrum indicates a localized atomic-like character of the 3d electrons. Estimation based on the sum rules gives a very large value of m_L/(m_S+7m_T) = 0.37 for the Co nanoclusters.

We present a theoretical method for studying diluted magnetic semiconductors, extending the dynamical cluster approximation. The precursor effects of the localization and the direct exchange interaction between magnetic impurities can be considered in the method. We apply the method to the two-dimensional square lattice system with random localized spins. We show that the strong antiferromagnetic superexchange interaction between nearest-neighbour sites suppresses the polarization of the carrier spin.

We investigate the transport properties of Fe thin films on Cu(111) based on first principles calculation. We calculate the electron current through these Fe thin films, which can be observed by using a double-tipped scanning tunnelling microscope. We find that the conductance is majority spin polarized. On the basis of the band structures for this system, we discuss the origin of these interesting transport properties.

We have observed the site-resolved (Co, Mn and Ge) magnetic moments of Co₂MnGe with the use of 2p core absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The evaluated Co and Mn 3d spin magnetic moments are consistent with the values given by the neutron scattering experiment. We have found that the orbital magnetic moment is large for the Co 3d states and is recognizable even for the Mn 3d state, suggesting that a spin–orbit coupling should be reconsidered in the energy band structure in order to verify a half-metallic nature of this alloy. Moreover, we have revealed that the magnetic moment is induced on the Ge site in Co₂MnGe.

The influence of grain boundaries on the magnetoresistance in the perovskite manganites La_0.7A_0.3MnO₃ (A = Ca,Sr) and the layered perovskite manganites La_1.36Sr_1.64Mn₂O₇ has been investigated by means of ac and dc magnetic susceptibility and magnetoresistance measurements. The following characteristics were found. (i) The magnetoresistance (MR) of single-crystal La_0.7Ca_0.3MnO₃ is almost independent of the magnetic field at low temperatures, and the MR of polycrystalline La_0.7A_0.3MnO₃ (A = Ca,Sr) and La_1.36Sr_1.64Mn₂O₇ decreases drastically with the initial increment of the magnetic field up to 2.5 × 10⁻¹ T. (ii) The resistivity and the absolute value of the MR of polycrystalline La_0.7Sr_0.3MnO₃ increase with decreasing grain size at 4.2 K. (iii) The resistivity of these polycrystalline manganites shows a time-dependent nature, but the single-crystal form does not.

ZnO layers doped simultaneously with Ga and N (codoping), and magnetic elements (V, Co) were characterized by Raman scattering to study their structural stability. Five impurity modes were observed in range 200–900 cm⁻¹ in the doped samples, and showed characteristic variation with the doping level. It is shown that these modes can be used as a good measure of lattice defects induced by doping. The Raman spectra showed that the magnetic elements were incorporated up to 5 mol% without serious deterioration in crystallinity.

Structural properties of GaN layers doped with magnetic impurities (Mn, Cr) were investigated by Raman scattering. In the case of Mn-doping, an impurity mode was observed at around 585 cm⁻¹ with concentrations of Mn up to 1–2%. This mode was assigned to a local vibrational mode of Mn substituting the Ga site, and interpreted as spectral evidence of a uniform solid solution in the samples. For Cr-doping, good crystalline quality was also confirmed up to [Cr] = 3–5%. Cr is more miscible in GaN than Mn. Resonance enhancement of LO phonon signal was observed in GaCrN layers when excited by a deep UV laser at 266 nm (4.7 eV). This indicates generation of photo-carriers.

We investigated the stability of the ferromagnetism of CrAs and CrSb in the zinc-blende structure against the lattice distortion, systematically. A calculation within the generalized gradient approximation using a full potential linearized augmented plane wave method was performed. We compared the ferromagnetic state and the antiferromagnetic state assuming tetragonal distortion with the lattice constants a and c changing independently and determined the spin polarization ratio in the ferromagnetic phase. The result shows that complete spin polarization (half-metallic ferromagnetism) remains stable even in the presence of large tetragonal distortion. On the other hand, our calculation shows that two monolayers of CrAs is enough to produce a half-metallic state in the CrAs/GaAs multilayer. Thus, the present result suggests that the half-metallic nature persists in various atomic-scale superlattices made of distorted CrAs or CrSb.

The tunnel-magneto-resistance (TMR) effect has been extensively studied in the last couple of years. While some experiments showed a strong dependence on the metal–insulator interface in these systems, most theoretical work gave little emphasis to the complicated interplay between electronic structure, atomic structure and the tunnelling process. We present calculations of the atomic structure of Fe/MgO/Fe(001) interfaces and its influence on the electronic structure. The tunnel current is calculated using the Landauer approach to describe the electron transport.

We investigate transport through 5–10 nm thin epitaxial GaAs(001) and (110) barriers sandwiched between polycrystalline iron films. It turns out that tunnelling is the dominant transport mechanism; this is indicated by a nonlinear I–V-characteristic, an exponential dependence of the tunnelling current on the barrier thickness and the temperature dependence of the current. We observe a pronounced tunnelling magnetoresistance (TMR) effect at low magnetic fields. Annealing decreases the TMR effect drastically and indicates the sensitivity of this effect to layer intermixing. At high magnetic fields we observe a distinct negative magnetoresistance (MR) at low temperatures and a positive MR at higher temperatures. This negative MR contribution is only observed for ferromagnetic iron contacts and is absent if iron is replaced by copper or gold electrodes.

By using ab initio methods on different levels we study the magnetic ground state of (finite) atomic wires deposited on metallic surfaces. A phenomenological model based on symmetry arguments suggests that the magnetization of a ferromagnetic wire is aligned either normal to the wire and, generally, tilted with respect to the surface normal or parallel to the wire. From a first-principles point of view, this simple model can be best related to the so-called magnetic force theorem calculations being often used to explore magnetic anisotropy energies of bulk and surface systems. The second theoretical approach that we use to search for the canted magnetic ground state is the first-principles adiabatic spin dynamics technique extended to the case of fully relativistic electron scattering. First, for the case of two adjacent Fe atoms on a Cu(111) surface we demonstrate that the reduction of the surface symmetry can indeed lead to canted magnetism. The anisotropy constants and consequently the ground state magnetization direction are very sensitive to the position of the dimer with respect to the surface. We also performed calculations for a seven-atom Co chain placed along a step edge of a Pt(111) surface. As far as the ground state spin orientation is concerned, we obtain excellent agreement with experiment. Moreover, the magnetic ground state turns out to be slightly non-collinear.