Conduction Electron Polarization Research Articles

SPIN TRANSFER TORQUE SWITCHING AND PERPENDICULAR MAGNETIC ANISOTROPY IN FULL HEUSLER ALLOYCo2FeAl-BASED TUNNEL JUNCTIONS

Some of Co -based full Heusler alloys have remarkable properties in spintronics, that is, high spin polarization of conduction electrons and low magnetic damping. Owing to these properties, magnetic tunnel junctions (MTJs) using Co -based full Heusler alloys are potentially of particular importance for spintronic application such as magnetoresistive random access memories (MRAMs). Recently, we have first demonstrated spin transfer torque (STT) switching and perpendicular magnetic anisotropy (PMA), which are required for developing high-density MRAMs, in full-Heusler Co2FeAl alloy-based MTJs. In this review, the main results of the experimental demonstrations are shown with referring to related issues, and the prospect of MTJs using Heusler alloys is also discussed.

Ru-NQR Study for Novel Phase Transition in CeRu2Al10

We have performed Ru-NQR measurements on CeRu2Al10 exhibiting novel phase transition at an abnormally high temperature T0 = 27.3 K and on NdRu2Al10 with a magnetic transition temperature Tm = 2.4 K as a reference RKKY system. The splitting of the NQR line due to internal fields below T0 shows a mean-field-like monotonic increase, indicating no change in the magnetic structure below T0. The internal field strength is one order larger than those at Al sites in CeRu2Al10, and is comparable to that at the Ru site in NdRu2Al10 despite the sevenfold smaller magnitude of the 4f moment, being indicative of an enhanced conduction electron polarization at the Ru site. One of the causes of the high T0 might be the enhanced exchange coupling through the Ce–Ru–Ce path mediated by the enhanced conduction electron polarization. Being similar to that in the Al site, the nuclear spin–lattice relaxation rate 1/T1 shows a gap-like decrease below T0 without enhancement owing to a critical slowing down at T0, in contrast to t...

Transport characteristics of ferromagnetic metal - insulator - superconductor: determination of spin polarization of conduction electrons

New method for determination of the spin polarization of conducting electrons of a ferromagnetic metal using a tunnel junction with a thin superconducting counter-electrode is proposed. An effect of the symmetry of the superconducting order parameter on the contact resistance has been analyzed. Reference 10, figures 3.

Hyperfine fields and the polarization of conduction electrons in semimetallic Heusler alloys

The distribution of hyperfine fields in half-metallic Heusler alloys is measured using the nuclear spin-echo method. The results from studying a great many series of samples of different composition reveal the enormous problems in obtaining materials with increased polarization of conduction electrons via the usual method of varying their composition. Other alternatives are proposed.

A differential spin current detection scheme

We provide detailed calculations for modeling an alternative scheme to detect spin polarization of conduction electrons injected into a nonmagnetic metal or degeneratively doped semiconductor using transport to two oppositely polarized ferromagnetic metal contacts. We show that, as in the well-known spin injection problem, detection efficiency can be amplified by the addition of spin-selective tunneling barriers. Considering the appropriate geometry and achievable injection rates, we estimate that the differential current can be as high as 1-10 nA for reasonable design parameters.

Spin polarization of conduction electrons and electronic structure of gadolinium

The structure of magnetic moment in metallic gadolinium is studied with special attention paid to the conduction electron spin-polarization. The theoretical electronic structure, Fermi surface topology, neutron magnetic form-factor and magnetic Compton profiles of ferromagnetic gadolinium are analysed. The calculated band structure is in good agreement with the results of a photoemission experiment, especially in the position of the 4f band. Some tendency to parallelization behaviour in the Fermi surface was observed. The maximum entropy reconstruction of the magnetization density from neutron diffraction data is also presented and compared with the theoretically predicted distribution. It is shown that in contrast to the old interpretation of the magnetic form factor data, the negative spin polarization in the interstitial regions may contain contribution coming from d-type electrons. The negative magnetization within the muffin-tin should characterize semi-core electrons 5s and 5p. Presence of this magnetization can hardly be found in neutron measurements, yet it is possible to observe it in accurate magnetic Compton profiles. The latter are calculated and discussed.

Long-range spin current driven by superconducting phase difference in a josephson junction with double layer ferromagnets.

We theoretically study spin current through ferromagnet (F) in a Josephson junction composed of s-wave superconductors and two layers of ferromagnets. Using quasiclassical theory, we show that the long-range spin current can be driven by the superconducting phase difference without a voltage drop. The origin of this spin current is due to spin-triplet Cooper pairs (STCs) formed by electrons of equal spin, which are induced by the proximity effect inside the F. We find that the spin current carried by the STCs exhibits long-range propagation in the F even where the Josephson charge current is practically zero. We also show that this spin current persists over a remarkably longer distance than the ordinary spin current carried by spin polarized conduction electrons in the F. Our results thus indicate the promising potential of Josephson junctions based on multilayer ferromagnets for spintronics applications with long-range propagating spin current.

Real-time analysis of the spinmotive force due to domain wall motion

Using numerical simulations, we study the spinmotive force induced by domain wall motion in a high magnetic field region far above the Walker breakdown field. We find that the DC component of the spinmotive force scales with the applied magnetic field even in a field range where the wall motion is no longer associated with periodic angular rotation of the wall magnetization. As the field is increased, spikes in the voltage signals start to appear, which are mainly attributed to vortex core collisions, nucleation, and annihilation, and this tendency is enhanced with further increases in the field. At high fields, the slope of the generated DC voltage vs. applied field curve is expected to depend only on the spin polarization of conduction electrons and, thus, can be used to accurately determine the degree of spin polarization in various materials.

Charge transfer processes and magnetoresistance in strontium ferromolybdate with dielectric barriers

The Sr2FeMoO6−δ compound attracts the attention of researchers due to a high Curie temperature, large values of negative magnetoresistance at room temperature and a practically 100% spin polarization of conduction electrons. We have studied the role of grain boundaries on the electrical transport in Sr2FeMoO6−δ in applied magnetic fields up to 8 T. The compound was synthesized out of partially reduced SrFeO3−x, SrMoO4−y precursors (sample I). At the first oxidation stage upon annealing at 700 K in argon with a partial oxygen pressure p(O2) = 10 Pa for 15 h (sample II), the internal structure of the Sr2FeMoO5.82 grains did not change. Sample II exhibits a mixed type of electrical conduction. In a magnetic field B its resistivity decreases without changing the mixed regime of charge transport, only shifting the temperature of minimum resistivity (TmB) to lower values. At temperatures above TmB the conductivity is predominantly metallic, whereas below TmB down to 4.2 K it is of semiconductor type. The increase of the annealing time up to 30 h (sample III) brings about an increase of the resistivity and the appearance of semiconductor-type conductivity at T = 300–4.2 K, which indicates the formation of a continuous insulating interlayer between the grains. In a magnetic field, the resistivity decreases, with the conductivity changing from the semiconducting to a mixed one. In the latter case the charge transport can occur both through point metallic contacts and by means of spin-dependent tunneling across dielectric interlayers between Sr2FeMoO5.82 grains.

Anomalous electronic and magnetic properties of the Eu2Ru2O7 pyrochlore

Structural, 99Ru Mössbauer, dc and ac susceptibility magnetization, and magneto-transport properties of the polycrystalline Eu2Ru2O7 pyrochlore are reported in this paper. From the experimental data, we deduce that the ruthenium cations Ru4+ (S = 1) are surrounded by an unusual electronic environment, involving conduction electron polarization and extrinsic Eu3+ ions at low temperature. This situation leads to an anomalous spin-glass transition at 23 K.

Magnetic and transport properties of selected YbxGd1−xNi5 intermetallics

We report the magnetic and transport properties of the YbxGd1−xNi5 (x ≤ 0.5) intermetallic compounds. According to the X-ray diffraction (XRD) this series crystallize in the hexagonal CaCu5-type structure. The magnetic properties in the studied system strongly depends on Yb concentration. Moreover, the so-called field cooled–zero field cooled (FC–ZFC) curves reveal an unexpected existence of a negative magnetization M in a certain temperature range. The M(T) dependences exhibit two compensation points in which the value of M equals zero. The origin of the observed behaviour may be due to the possible switching effect in spin (M4fS) and orbital (M4fL) moment within 4f sublattice. Near the compensation points where the opposing contribution from 4f local magnetic moments are nearly equals the conduction electron polarization become significant. Additionally, the observed exchange bias in the vicinity of the compensation point can be attributed to the anisotropy between the coupling within spin (M4fS) and orbital (M4fL) moments. The observed unique behaviour is the most evident in the case of x = 0.4 ytterbium concentration. The transport properties confirm the results obtained by the magnetic studies.

Detection of Conduction Electron Spin Polarization in n-GaAs by Negative Muonium

Abstract In our ZF μ + SR studies of the spin-dependent response to the polarized conduction electrons produced by circularly polarized 830 nm laser light in a semi-metallic and strain-free n- type GaAs (3×10 16 cm -3 Si), among possible states of positive muon, Mu − ( μ + e − e − ) had been found to take a spin-dependent exchange scattering, while muonium (Mu 0 ) at bond-center was found to be insensitive. In order to understand this surprising spin-dependent Mu − response, the LF (up to 0.4 T) μ + SR studies were carried out and data is suggesting a contribution of bound electron(s) of Mu − . Possible mechanisms of the Mu − response are given.

Theory of Half-Metallic Ferrimagnetism in Double Perovskites

Double perovskites such as Sr(2)FeMoO(6) are rare examples of materials with half-metallic ground states and a ferrimagnetic T(c) above room temperature. We present a comprehensive theory of the temperature and disorder dependence of their magnetic properties by deriving and validating a new effective spin Hamiltonian for these materials, amenable to large-scale three-dimensional simulations. We show how disorder, ubiquitous in these materials, affects T(c), the magnetization, and the conduction electron polarization. We conclude with a novel proposal to enhance T(c) without sacrificing polarization.

Influence of polarization of conduction electrons in semiconductor on their light absorption

Influence of polarization of conduction electrons in semiconductor on their light absorption

ChemInform Abstract: Spin‐Transfer Magnetization Switching in Ordered Alloy‐Based Nanopillar Devices

AbstractReview: 55 refs.

Geometrical structure and spin order of Gd13 cluster

The spin-polarized generalized gradient approximation to the density-functional theory has been used to determine the lowest energy structure, electronic structure, and magnetic property of Gd(13) cluster. Our results show that the ionic bonding is combined with the covalent characteristics in stabilizing the Gd cluster. The ferrimagnetic icosahedron is found to be the lowest energy configuration, in which the centered Gd atom couples antiferromagnetically with the rest Gd atoms surrounding it. No spin non-collinear evidence has been detected in our calculations. It is identified that the local magnetic moments of Gd atom are about 8 μ(B) regardless of geometrical structure. Finally, the comprehensive electronic structure analyses show that the indirect long-range magnetic coupling between the polarized 4f is mediated by the polarization of 5d, 6s, and 6p conduction electrons, which is the typical Ruderman-Kittel-Kasuya-Yosida interactions.

Spin-transfer magnetization switching in ordered alloy-based nanopillar devices

This paper reviews spin-transfer magnetization switching in ordered alloy-based nanopillar devices. L10-ordered FePt was used for one of the earliest demonstrations of spin-transfer switching in perpendicularly magnetized systems. The behaviour of magnetization switching deviates from the predictions based on a macro-spin model, suggesting incoherent magnetization switching in the system with a large perpendicular magnetic anisotropy. The effect of a 90° spin injector on spin-transfer switching was also examined using L10-ordered FePt. Full-Heusler alloys are in another fascinating material class for spin-transfer switching because of their high-spin polarization of conduction electrons and possible small magnetization damping. A B2-ordered Co2FeAl0.5Si0.5-based device showed a low intrinsic critical current density of 9.3 × 106 A cm−2 for spin-transfer switching as well as a relatively large current-perpendicular-to-plane giant-magnetoresistance (CPP-GMR) up to ∼9%. The specific physical properties of ordered alloys may be useful for fundamental studies and applications in spin-transfer switching.

Recent experimental results on the superconductor/ferromagnet proximity effect

The recent experimental results on the proximity effect in heterostructures composed of superconducting and ferromagnetic thin films are reviewed. First, the experimental observation and investigation of the spin screening effect, i.e., a spin polarization in the V layer developing in the superconducting state under the influence of a spin polarization of conduction electrons in the ferromagnetic layer are discussed. This effect was predicted theoretically by Bergeret et al. [F. S. Bergeret, A. F. Volkov, and K. B. Efetov, EPL 66, 111 (2004); Phys. Rev. B 69, 174504 (2004)]. Then, the progress concerning the experimental realization of the superconducting spin switch device based on the superconductor/ferromagnet proximity effect is presented.

Dynamic nuclear self-polarization with circularly polarized light

Lattice nuclei of III-V type semiconductors can be self-polarized through the contact hyperfine interaction with artificially excited conduction electrons at extremely low temperatures. In the self-polarization process, a positive feedback between the nuclear magnetization and the expansion of the energy splitting of the conduction electron states due to this nuclear magnetization takes an essential role. Extending the original theoretical work on the self-polarization with linearly polarized (LP) light by Dyakonov and Perel,1 we examined the effect of the circularly polarized (CP) light on the phenomenon of the nuclear self-polarization. While no polarization is obtained with LP light above the critical temperature, the nuclear polarization is enhanced also above the critical temperatures with CP light. More important finding in the application of CP light is that the direction of the nuclear polarization can be controlled by the orientation of the excited conduction-electron polarization, which is easily manipulated by changing the degree of polarization of the excitation light. Related to this polarization change, it is also remarked that the nuclear polarization shows a hysteresis phenomenon below the critical temperature when the degree of the electron polarization is swept up and down.

First-principles study on half-metallic properties of CuHg2 Ti-type alloys FeV2 Z (Z = Si, Ge, As, Sb)

The first-principles calculation within density-functional theory is used to investigate the electronic structures and magnetic properties in CuHg2Ti-type alloys FeV2Z (Z = Si, Ge, As, Sb). FeV2Sb is predicted to be half-metal and the energy gap lies in the majority-spin bands. Though FeV2Si, FeV2Ge, and FeV2As show no half-metallicity, their spin polarization of conduction electrons is around 99.7, 99.0, and 97.8%, respectively. The investigation of volume distortion shows that over 99% spin-polarization can be retained in the range of 5.70–6.05 A for FeV2Si, 5.85–6.05 A for FeV2Ge, and 5.90–6.00 A for FeV2As, respectively. FeV2Sb can keep half-metallicity from 6.00 to 6.15 A. We studied the effects of defects at transition-metal sites and Ge, As, and vacancy substitution for Sb atom on the electronic structures in FeV2Sb, and the results indicate that half-metallicity in FeV2Sb is not sensitive to substitutions for Sb sites.