On the role of spin polarization of electrons in the effect of giant injection magnetoresistance in the Ni-polymer-Cu system

The effect of spin polarization of electrons injected from a ferromagnet on the giant injection magnetoresistance was investigated for a ferromagnet-polymer-nonmagnetic metal experimental structure. The degree of spin polarization was varied by introducing a depolarizing nonmagnetic metal (Cu) layer between the ferromagnet and the organic transport layer. It was established that the coefficient of giant injection magnetoresistance depends significantly on the thickness of the depolarizing layer. In particular, the effect was not observed at a thickness larger than 12 nm and decreased exponentially at a smaller thickness as the thickness increased. The conclusion was drawn that the spin polarization of electrons plays a decisive role in the effect of giant injection magnetoresistance.     

Possible path to a new class of ferromagnetic and half-metallic ferromagnetic materials

We introduce a path to a possibly new class of magnetic materials whose properties are determined entirely by the presence of a low concentration of specific point defects. Using model Hamiltonian and ab initio band structure methods we demonstrate that even large band gap nonmagnetic materials as simple as CaO with a small concentration of Ca vacancies can exhibit extraordinary properties. We show that such defects will initially bind the introduced charge carriers at neighboring sites and depending on the internal symmetry of the clusters so formed, will exhibit "local" magnetic moments which for concentrations as low as 3% transform this nonmagnetic insulator into a half-metallic ferromagnet.     

Effect of different conductivity between the spin polarons on spin injection in a ferromagnet/organic semiconductor system

Spin injection across ferromagnet/organic semiconductor system with finite width of the layers was studied theoretically considering spin-dependent conductivity in the organic-semiconductor. It was found that the spin injection efficiency is directly dependent on the difference between the conductivity of the up-spin and down-spin polarons in the spin-injected organic system. Furthermore, the finite width of the structure, interfacial electrochemical-potential and conductivity mismatch have great influence on the spin injection process across ferromagnet/organic semiconductor interface.     

Dynamical Theory of X-Ray Diffraction for Restricted Beams: I. Coherent Scattering by a Porous Crystal

The processes of electron spin dynamics in a hybrid nonresonance structure, which includes a layer of a diluted magnetic II–Mn–VI semiconductor and an asymmetric quantum well (QW) of a nonmagnetic III–V semiconductor, are experimentally studied. The nonresonance of the structure is determined by the fact that the level of the ground state of the magnetic layer falls into the range of the excited states of the nonmagnetic QW. The electron polarization in the ground thermalized state of QW is found not to depend on the magnetic part of the structure. However, the magnetic part affects the electron polarization in the excited state via spin injection from the magnetic semiconductor and the mixing of the electronic states of the magnetic and nonmagnetic subsystems of the structure. The possibility of controlling the polarization of an electron spin by carrier excitation toward the region of mixed states along with the absence of depolarizing influence of the magnetic semiconductor on carriers in the thermalized state of QW can be applied to design new spintronic devices along with those that use spin injection, optical orientation, and depolarization.     

Influence of Magnetization on the Spin Pumping Efficiency in a Ferromagnet–Normal Metal Bilayer Structure

The problem of spin current generation and transformation into electric signals in thin-film ferromagnet/nonmagnetic metal bilayer structures is investigated. This direction is of considerable scientific interest and promising for applications in spintronics. An LSMO/Pt structure consisting of an epitaxial film of ferromagnetic manganite La2/3Sr1/3O3 grown on a single-crystal NdGaO3 substrate and coated with a platinum film has been studied experimentally. The spin current was generated by the spin pumping method upon the excitation of a ferromagnetic resonance in the ferromagnetic layer and was detected by the electric voltage USP arising in the nonmagnetic metal layer due to the inverse spin Hall effect. Owing to its relatively low Curie temperature (~350 K), using LSMO allowed the influence of ferromagnetic-layer magnetization on the spin current generation to be studied in detail in the temperature range 100–350 K. In this case, the influence of the shape of the ferromagnetic resonance line, which is the convolution of homogeneous (Lorentzian) spin packets and inhomogeneous Gaussian broadening (Voigt model), was consistently taken into account. As a result of our analysis of all the parameters defining USP, we have obtained the temperature dependence of the mixed spin conductance, which has turned out to be approximately proportional to the ferromagnet magnetization squared. This result is compared with existing theoretical models.     

Spin resonance associated with itinerant electrons affected by the injected spin current

The effect of spin injection on the spin resonance associated with itinerant electrons in a nonmagnetic semi-conductor has been studied within the phenomenological approach. The power absorbed under the resonance conditions has been analyzed as a function of the frequency, effective g-factor, and magnetic field direction. It is shown that the power absorbed by the spin-polarized charge carriers under the resonance conditions can provide a tool for studying the spin injection.     

Spin incommensurability and two phase competition in cobaltites

The perovskite LaCoO3 evolves from a nonmagnetic Mott insulator to a spin cluster ferromagnet (FM) with the substitution of Sr2+ for La3+ in La1-xSrxCoO3. The clusters increase in size and number with x and the charge percolation through the clusters leads to a metallic state. Using elastic neutron scattering on La1-xSrxCoO3 single crystals, we show that an incommensurate spin superstructure coexists with the FM spin clusters. The incommensurability increases continuously with x, with the intensity rising in the insulating phase and dropping in the metallic phase as it directly competes with the commensurate FM, itinerant clusters. The spin incommensurability arises from local order of Co3+-Co4+ clusters but no long-range static or dynamic spin stripes develop. The coexistence and competition of the two magnetic phases explain the residual resistivity at low temperatures in samples with metalliclike transport.     

Coherent spin dynamics of electrons and excitons in nanostructures (a review)

The studies of spin phenomena in semiconductor low-dimensional systems have grown into the rapidly developing area of the condensed matter physics: spintronics. The most urgent problems in this area, both fundamental and applied, are the creation of charge carrier spin polarization and its detection, as well as electron spin control by nonmagnetic methods. Here, we present a review of recent achievements in the studies of spin dynamics of electrons, holes, and their complexes in the pump-probe method. The microscopic mechanisms of spin orientation of charge carriers and their complexes by short circularly polarized optical pulses and the formation processes of the spin signals of Faraday and Kerr rotation of the probe pulse polarization plane as well as induced ellipticity are discussed. A special attention is paid to the comparison of theoretical concepts with experimental data obtained on the n-type quantum well and quantum dot array samples.     

Giant magnetoresistance in ferromagnet/superconductor superlattices

We show magnetoresistance in excess of 1000% in trilayers containing highly spin-polarized La0.7Ca0.3MnO3 and high-Tc superconducting YBa2Cu3O7. This large magnetoresistance is reminiscent of the giant magnetoresistance (GMR) in metallic superlattices but with much larger values, and originates at spin imbalance due to the injection of spin-polarized carriers. Furthermore, in contrast to ordinary GMR, the magnetoresistance is intimately related to the superconductivity in the YBa2Cu3O7 layer and vanishes in the normal state. This result, aside from its fundamental importance, may be of interest for the design of novel spintronic devices based on ferromagnet/superconductor structures.     

Perpendicular giant magnetoresistance in magnetic multilayered nanowires

We present giant magnetoresistance (GMR) measurements performed on electrodeposited Co/Cu multilayered nanowires. The variation of the GMR with the thicknesses of the Cu and Co layers over wide ranges is discussed in the framework of the Valet-Fert model for perpendicular GMR. The interface and bulk spin-dependent scattering parameters as well as the spin diffusion lengths in the nonmagnetic and ferromagnetic layers are extracted from this analysis.     

Effect of codoping with C or N on electronic structures and magnetic properties of ZnO:Cu

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of ZnO:Cu. The results indicate that Cu-doped ZnO prefers a ferromagnetic ground state and behaves like a half-metallic ferromagnet. The magnetic moment mainly localizes at Cu atom and the rest mainly comes from the spin polarized O atoms. It has been found that the ferromagnetic stability can be enhanced slightly by substituting an oxygen atom with one N atom; while the ferromagnetic stability can be weakened by replacing one O atom with a C atom. Due to absence of magnetic ion and the 100% spin polarization of the carriers in ZnO:Cu, one can expect that Cu-doped ZnO would be a useful half-metallic ferromagnet both in practical application and in theoretical studies.     

Voltage generation by ferromagnetic resonance at a nonmagnet to ferromagnet contact

A ferromagnet can resonantly absorb rf radiation to sustain a steady precession of the magnetization around an internal or applied magnetic field. We show that, under these ferromagnetic resonance conditions, a dc voltage is generated at a normal-metal electric contact to a ferromagnet with spin-flip scattering. The spin dynamics in the nonmagnetic region is accounted for by a frequency-dependent renormalization of the interface conductances. This mechanism allows sensing of time-dependent magnetizations by established dc electronic techniques.     

Theoretical study on spin-polarized injection of electrical currents into polymer semiconductors

Different from electrons and holes in traditional inorganic semiconductors, the charge carriers in polymer semiconductors are spin polarons and spinless bipolarons. In this paper, a theoretical model is presented to describe the spin-polarized injection of electrical currents from a ferromagnetic contact into a nonmagnetic polymer semiconductor. In this model, a new relation of conductivity to concentration polarization for polymer semiconductors is introduced based on a three-channel model to describe the spin-polarized injection of electrical currents under large electrical current densities. The calculated results of the model reveal the effects of the polaron ratio, the carrier concentration polarization, the interfacial conductance, the bulk conductivity of materials, and the electrical current density, etc. on the spin polarization of electrical currents. As conclusions, the large and matched bulk conductivity of materials, the small spin-dependent interfacial conductance, the thin polymer thickness and the large enough electrical current are critical factors for upgrading the spin polarization of electrical currents in polymer semiconductors. Particularly, when the polaron ratio in polymer semiconductors approaches the concentration polarization of the ferromagnetic contact, a modest concentration polarization is sufficient for achieving a nearly complete spin-polarized injection of electrical currents.     

Advanced techniques for all-electrical spectroscopy on spin caloric phenomena

Spin-dependent properties of nanomagnets and magnetic/nonmagnetic hybrid systems have gained a renewed interest after the discovery of spin caloric transport phenomena. To explore such properties in detail, advanced techniques need to be developed. In this paper we report novel approaches to study both the magneto–thermal and magneto–mechanical characteristics of hybrid systems. These techniques involve in particular broadband spectroscopy of spin dynamics and surface acoustic waves in the GHz frequency regime. By these means we investigate ferromagnet/semiconductor hybrid systems to explore spin pumping and the Seebeck effect as well as ferromagnet/piezoelectric hybrid systems to address magneto–mechanical coupling at high frequencies.     

New method of the spin-polarization detection in tunnel junctions ferromagnet-insulator-charge density wave metal

A tunnel junction between a metal partially gapped by charge density waves metal (CDWM) and a ferromagnet (FM) in an external magnetic field is considered. Only the Zeeman paramagnetic effect is taken into account. It is shown that the peaks in the dependence of differential conductance versus voltage, induced by the CDW gap, split, with each peak having a predominant spin polarization. This effect makes it possible to electrically measure the polarization of current carriers in FMs.     

Spin asymmetry and s-electron itinerancy in Co/X (X=Co,Cu, V and Ta) multilayers: their dependence on the electronegativity of non-magnetic layers

The electronic structure and magnetic properties of the Co at the Co/X (X=Co, Cu, V and Ta) interfaces have been studied by first-principle discrete variational method. We have found that the spin asymmetry and the s-electron itinerancy of the Co interface layer in the Co/X systems are strongly dependent on the electronegativity of the non-magnetic layers. A large difference in the electronegativity between the non-magnetic and Co layers is unfavorable both for s-electron itinerancy and for the spin exchange split of DOS at the Fermi level. Further study on charge density has revealed that a bond is formed across the Co/V and Co/Ta interfaces.     

Metal-Oxide Interfaces in Magnetic Tunnel Junctions

Metal-oxide interfaces play an important role in spintronics—a new area of microelectronics that exploits spin of electrons in addition to the traditional charge degree of freedom to enhance the performance of existing semiconductor devices. Magnetic tunnel junctions (MTJs) consisting of spin-polarized ferromagnetic electrodes sandwiching an insulating barrier are such promising candidates of spintronic devices. The paper reviews recent results of first-principle density-functional studies of the atomic and electronic structure of metal-oxide interfaces in Co/Al₂O₃/Co and Co/SrTiO₃/Co MTJs. The most stable interface structures, O-terminated for fcc Co (111)/-alumina(0001) and TiO₂-terminated with oxygens on top of Co atoms for fcc Co (001)/SrTiO₃(001) were identified based on energetics of metal-oxide cohesion at the interface. The covalent character of bonding for both the Co/alumina and Co/SrTiO₃ interface structures has been determined based on the pattern of electron distribution across the interface. The Al-terminated Co/alumina interface that corresponds to an under-oxidized MTJ exhibits a metallic character of bonding. The unusual charge transfer process coupled with exchange interactions of electrons in Co results in quenching of surface magnetism at the interface and substantial reduction of work of separation. The electronic structure of the O-terminated Co/Al₂O₃/Co MTJ exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 Å. The Co/SrTiO₃/Co MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen, which results in an antiparallely aligned induced magnetic moment on Ti atoms. This may lead to a negative spin polarization of tunneling across the SrTiO₃ barrier from the Co electrode. The results illustrate the important fact that spin-polarized tunneling in magnetic tunnel junctions is not determined entirely by bulk density of states of ferromagnet electrodes, but is also very sensitive to the nature of the insulating tunneling barrier, as well as the atomic structure and bonding at the ferromagnet/insulator interface.     

Effects of electric field and magnetic induction on spin injection into organic semiconductors

Spin-polarized injection and transport into ferromagnetic/organic semiconductor structure are studied theoretically in the presence of the external electric field and magnetic induction. Based on the spin-drift-diffusion theory and Ohm's law, we obtain the charge current polarization, which takes into account the special carriers of organic semiconductors. From the calculation, it is found that the current spin polarization is enhanced by several orders of magnitude by tuning the magnetic induction and electric fields. To get an apparent current spin polarization, the effects of spin-depended interfacial resistances and the special carriers in the organic semiconductor, which are polarons and bipolarons, are also discussed.     

Spin-injection mechanism of magnetization reversal and hysteresis of current in magnetic junctions

A novel mechanism is proposed for magnetization reversal by the current of magnetic junctions with two metallic ferromagnetic layers and thin separating nonmagnetic layer. The spin-polarized current flows perpendicularly to the interfaces between the ferromagnetic layers, in one of which the spins are pinned and in the other they are free. No domain structure is formed in the ferromagnetic layers. The current breaks spin equilibrium in the free layer, which manifests itself in the injection or extraction of spins. The nonequilibrium spins interact with the magnetization of the lattice due to the effective field of s-d exchange, which is current dependent. At currents exceeding a certain threshold value, this interaction leads to magnetization reversal. Two threshold currents for magnetization reversal have been obtained theoretically, which are reached as the current increases or decreases, respectively. Thus, the phenomenon of current hysteresis is found. The calculated results are in good agreement with experiments on magnetization reversal by current in three-layer junctions of composition Co(I)/Cu/Co(II) prepared in a pillar form.     

Tunnel barrier enhanced voltage signal generated by magnetization precession of a single ferromagnetic layer

We report the electrical detection of magnetization dynamics in an Al/AlOx/Ni80Fe20/Cu tunnel junction, where a Ni80Fe20 ferromagnetic layer is brought into precession under ferromagnetic resonance conditions. The dc voltage generated across the junction by the precessing ferromagnet is enhanced about an order of magnitude compared to the voltage signal observed when the contacts in this type of multilayered structure are Ohmic. We discuss the relation of this phenomenon to magnetic spin pumping and speculate on other possible underlying mechanisms responsible for the enhanced electrical signal.