In this paper we discuss current-driven magnetic domain wall (DW) dynamics in ferromagnetic nanowires in the current-perpendicular-to-plane geometry. We show that spin transfer torque from direct spin-polarized current applied parallel to a magnetic domain wall induces DW motion in a direction independent of the current polarity. This unidirectional response of the DWto spin torque enables DWpumping - long-range DWdisplacement driven by alternating current. Our numerical simulations reveal that DW pumping can be resonantly amplified through excitation of internal degrees of freedom of the DW by the current. We also experimentally study the interactions of spin-torque with a domain wall. We resonantly excite high-speed motion of a domain wall trapped at a pinning site and observe velocities over 800 m/s at current densities below 10⁷ A/cm².

Domain walls in ferromagnetic nanowires are important for proposed devices in recording, logic, and sensing. The realization of such devices depends in part on the ability to quickly and accurately control the domain wall from creation until placement. Using micromagnetic computer simulation we demonstrate how a combination of externally applied magnetic fields is used to quickly inject, move, and accurately place multiple domain walls within a single wire for potential recording and logical operations. The use of a magnetic field component applied perpendicular to the principle domain wall driving field is found to be critical for increased speed and reliability. The effects of the transverse field on the injection and trapping of the domain wall will be shown to be of particular importance.

We overview the recent developments in spin current generation mechanisms and study the spin pumping effect and diffusive spin current in detail based on a microscopic theory. The spin-charge conversion using the inverse spin Hall effect is also discussed. Spin chemical potential describing the diffusive spin current is calculated by linear response theory and spin injection effect is discussed based on the result.

The synthesis of colloidal Cr³⁺-doped In₂O₃ NCs with the body-centered cubic bixbyte-type crystal structure, and Cr³⁺-doped SnO₂ NCs with the rutile crystal structure was described. Ligand-field electronic absorption spectroscopy suggests that Cr³⁺ dopants have quasi-octahedral coordination in both In₂O₃ and SnO₂ NC host lattices. Unlike free-standing nanocrystals, the nanocrystalline films fabricated from colloidal Cr³⁺-doped In₂O₃ and SnO₂ nanocrystals exhibit room temperature ferromagnetism. Analogous magnetic behavior suggests the same origin of ferromagnetic ordering in both materials. The observed ferromagnetism has been related to the existence of extended structural defects, formed at the interfaces between nanocrystals in nanocrystalline films. These structural defects are likely responsible for the formation of charge carriers which mediate the dopant magnetic moment ordering.

We predict that strong coupling is feasible between photons and a ferromagnetic nanomagnet, due to exchange interactions that cause very large numbers of spins to coherently lock together with a significant increase in oscillator strength while still maintaining very long coherence times. The interaction of a ferromagnetic nanomagnet with a single photonic mode of a cavity is analyzed in a fully quantum-mechanical treatment. Exceptionally large quantum-coherent magnet-photon coupling with coupling terms in excess of several THz are predicted to be achievable in a spherical cavity of ~ 1 mm radius with a nanomagnet of ~ 100 nm radius and ferromagnet resonance frequency of ~ 200 GHz. This should substantially exceed the coupling observed in solids between orbital transitions and light. Eigenstates of the nanomagnet-photon system correspond to entangled states of spin orientation and photon number over 105 values of each quantum number. Initial coherent state of definite spin and photon number evolve dynamically to produce large coherent oscillations in the microwave power with exceptionally long dephasing times of few seconds. In addition to dephasing, several decoherence mechanisms including elementary excitation of magnons and crystalline magnetic anisotropy are investigated and shown to not substantially affect coherence upto room temperature. The optimal nanomagnet size is predicted to be just below the threshold for failure of the macrospin approximation.

Spin transport in molecular systems has been attracting many people, because a weak spin-orbit interaction in molecules allows us to expect good spin coherence. Although spin injection and spin transport in molecules were not easily achieved at room temperature, graphene, which is one of the most attractive materials in condensed matter physics since 2004, provided an ideal platform to realize and discuss spin injection and transport at room temperature. We present our study on spin injection into graphene and important findings of unique spin transport properties in graphene.kwave

The electrical injection and detection of spin-polarized carriers in semiconductors at room temperature has been one of the key challenges in spintronics. Exploiting spin functionality in silicon, the dominant electronic material, is particularly crucial in order to realize the next generation of information processing devices based on spin. Here we present our recent demonstration of electrical spin injection into n-type and p-type silicon from a ferromagnetic tunnel contact, the spin manipulation via the Hanle effect, and the electrical detection of the induced spin accumulation, all at room temperature. A control experiment that makes use of a non-magnetic nanolayer inserted between the ferromagnet and the tunnel barrier supports the data, proving spin injection and excluding any spurious signals. We also report Hanle effect measurements in two-terminal geometry and show that in this configuration the Hanle signal is always dominated by spin accumulation below the two individual contacts, rather than spin transport from injector to detector through the semiconductor channel. The results provide many new insights and open a platform for further exploration of spin functionality in complementary silicon devices operating at ambient temperature.

We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic magnetic multilayer, in which two strongly ferromagnetic layers are exchange-coupled through a weakly ferromagnetic spacer. We show theoretically that the system allows a spin-thermoelectronic control of the relative orientation of the outer layers. Supporting experimental evidence of thermally controlled switching from parallel to anti-parallel magnetization orientations in the sandwich is presented. We show magneto-resistance oscillations may take place with frequencies up to GHz. We discuss in detail an experimental realization of a device that can operate as a thermo-magneto-resistive switch or oscillator.

The appearance of high mobility electrons at the LaAlO₃/SrTiO₃ (LAO/STO) interface has raised strong interest in the material science community and a lively debate on the origin of the phenomenon. In particular, in view of the large band gaps of the two bulk single crystals constituting this heterostructure, the realization of a conducting system was totally unexpected. A possible explanation is an electronic reconstruction of the interface, realizing a transfer of electrons from the LaAlO₃ surface to SrTiO₃ near the interface, thereby avoiding the polarization catastrophe associated with the alternating polar layers of the LaAlO₃ film. The predictions of theoretical models based on this idea are quite peculiar and need to be verified by specific experiments able to address the electronic properties of the LAO/STO buried interface. Here, by using x-ray spectroscopy techniques, we show that the appearance of an electron system is correlated to the removal of the degeneracy of the titanium 3d states, and doped electrons appear in a band preferentially created by the hybridization between 3d_xy states of titanium and oxygen 2p_x,y states. This splitting is consistent with an ordering of the Ti 3d_xy orbital belonging to the TiO₆ octahedra close to the interface, as theoretically proposed. However, the valence of titanium ions remains prevalently 4+, therefore other mechanisms should be also considered for the stabilization of the system.

In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors. In this work, time resolved spectroscopy of InSb-based parabolic multi-quantum wells and narrow gap ferromagnetic alloys grown by MOVPE, have been pursued. In addition, in this study, we report on CR experiments carried out on the ferromagnetic InMnAs film, on which clear resonance signals have been successfully observed in high magnetic fields. Investigation of the electronic structure of III-Mn-V alloys by techniques such as the cyclotron resonance can shed important light on the origin of ferromagnetism and the p-d exchange interaction in III-Mn-V systems. Our results are important for understanding the electronic and magnetic states in these material systems.

The spin-orbit interaction constitutes a weak but essential perturbation to the Hamiltonian of magnetic systems. Linking spins with atomic structure, spin-orbit coupling assumes a prominent role in structures of reduced dimensionality, where it defines the internal anisotropy fields. In this paper, we discuss interface-enhanced spinorbit effects that arise in metallic multilayers in the presence of an electric current. We demonstrate that a novel type of spin torque can be induced in ferromagnetic metal films lacking structure inversion symmetry through the Rashba effect. Owing to the combination of spin-orbit and exchange interactions, we show that electrons flowing in the plane of a Co layer with asymmetric Pt and AlO_x interfaces produce an effective transverse magnetic field of 1 T per 10⁸ A/cm² of applied current. This torque does not require a current flowing through noncollinear magnetic structures, opening new perspectives for room temperature applications in spintronics.

Much effort has been devoted recently to designing systems exhibiting simultaneous magnetic and ferroelectric order (multiferroics) with a strong magnetoelectric coupling, which could enable the electrostatic control of magnetism in the solid state. One approach consists of exploring interfacial couplings between magnetic and ferroelectric phases of composite systems, where magnetoelectric couplings larger than those typical of single-phase multiferroics have been achieved. Here, we overview our recent work on epitaxial Pb(Zr_0.2Ti_0.8)O₃/La_0.8Sr_0.2MnO₃ (PZT/LSMO) heterostructures tailored to display a large magnetoelectric coupling, which relies on the sensitivity of the magnetic properties of the doped manganites to charge. The magnetoelectric response in this system is hysteretic, displaying abrupt switching between two magnetic states for the two states of the ferroelectric polarization. The microscopic origin of this effect, which is studied using advanced spectroscopic techniques, arises from changes of the valence state of Mn in LSMO induced by the electrostatic modulation in the charge carrier density. Hence, the magnetoelectric coupling in these multiferroic heterostructures is charge-based and electronic in origin. From a quantitative comparison between the measured change in valency and magnetic moment, we conclude that the interfacial spin ordering is modified upon charge doping. This ability to control spin via electric fields opens a new pathway for the development of novel spin-based technologies.

We calculate transport properties of HgTe quantum wells that exhibit the quantum spin Hall effect. We concentrate on the ballistic bulk contribution as a function of aspect ratio and Fermi energy. We show that the conductance and the shot noise are distinctively different for the so-called normal regime (the topologically trivial case) and the so-called inverted regime (the topologically non-trivial case). Thus, it is possible to verify the topological order of two-dimensional topological insulators such as HgTe quantum wells not only via observable edge properties but also via observable bulk properties. In addition, we show that the bulk contribution can even exceed the edge contribution for certain parameter regimes (and in all regimes for the case of the shot noise). We test the validity of our analytical approach against a tight-binding model that allows us to include random disorder numerically which is shown to have only a minor effect as long as its strength does not exceed the bulk gap.

Recent advances in spin response of organic semiconductors include long polaron spin coherence time measured by optically detected magnetic resonance (ODMR); substantive room-temperature magneto-electroluminescence and magneto-conductance obtained in organic light emitting diodes (OLED); and spin-polarized carrier injection from ferromagnetic electrodes in organic spin valves (OSV). Although the hyperfine interaction (HFI) has been foreseen to play an important role in organic spin response, clear experimental evidence has been lacking. Using the chemical versatility advantage of the organics, we studied and compared spin responses in films, OLED and OSV devices based on π-conjugated polymers made of protonated, H-, and deuterated, D-hydrogen having a weaker HFI strength. We demonstrate that the HFI plays a crucial role in all three spin responses. OLEDs and films based on the D-polymers show substantial narrower magneto-electroluminescence, magneto-conductivity and ODMR responses; whereas due to the longer spin diffusion, OSV devices based on D-polymers show substantially larger magnetoresistance that reaches ~330% at small bias voltage and low temperatures.

In this work, we successfully fabricate Fe3O4 nanoparticles self-assembled with molecules to explore a new approach of studying the molecular spintronics. Fourier transform infrared spectroscopy measurements indicate that one monolayer molecules chemically bonds to the Fe3O4 nanoparticles and the physically absorbed molecules do not exist in the samples. The magnetoresistance (MR) of molecule fully coated ~10 nm size nanoparticles is up to 7.3% at room temperature and 17.5% at 115 K under a field of 5.8 kOe. And the MR ratio is more than two times larger than that of pure Fe₃O₄ nanoparticles. This enhanced MR is likely arising from weak spin scattering while carriers transport through the molecules. Moreover, a very large low field magnetoresistance is also observed with ~500nm ferromagnetic Fe₃O₄ nanoparticles coated with acetic acid molecules. Those features open a door for the development of future spin-based molecular electronics.