The inorganic/molecular spinterface is an ideal platform for generating extraordinary spin effects. Understanding and controlling these spin-related effects is mandatory for the exploitation of such interfaces in devices. For this purpose we have investigated the adsorption of ?-sexithiophene (?-6T) on La0.7Sr0.3MnO3 (LSMO) as one of the prototypical material combinations used in organic spintronic devices. Atomic force microscopy (AFM), confocal photoluminescence, X-ray and utraviolet photoelectron spectroscopy, and metastable de-excitation spectroscopy unraveled the structure and the electronic configuration of 6T for various surface coverages. This data set allowed the determination of the characteristic features of occupied states: the band diagram and the work function. Finally, density functional theory enabled us to establish that the spin polarization in 6T molecular orbitals critically depends on the termination layer of LSMO, showing a substantial effect on the MnO-terminated one only. We believe that this research provides important hints for a comprehensive understanding of spinterface effects in general and offers key suggestions for the optimization of 6T/LSMO-based spin devices through the engineering of LSMO termination layer.

We assess the impact of the scattering physics assumptions on the thermoelectric properties of five Co-based p-type half-Heusler alloys by considering full energy-dependent scattering times vs the commonly employed constant scattering time. For this, we employ density functional theory band structures and a full numerical scheme that uses Fermi's golden rule to extract the momentum relaxation times of each state at every energy, momentum, and band. We consider electron-phonon scattering (acoustic and optical), as well as ionized impurity scattering, and evaluate the qualitative and quantitative differences in the power factors of the materials compared to the case where the constant scattering time is employed. We show that the thermoelectric power factors extracted from the two different methods differ in terms of (i) their ranking between materials, (ii) the carrier density where the peak power factor appears, and (iii) their trends with temperature. We further show that the constant relaxation time approximation smoothens out the richness in the band structure features, thus limiting the possibilities of exploring this richness for material design and optimization. These details are more properly captured under full energy/momentum-dependent scattering time considerations. Finally, by mapping the conductivities extracted within the two schemes, we provide appropriate density-dependent constant relaxation times that could be employed as a fast first-order approximation for extracting charge transport properties in the half-Heuslers we consider.

The quest for a spin-polarized organic light-emitting diode (spin-OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin-OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light-emitting device showing spin-valve effects at high voltages (up to 14 V). This allows the detection of a magneto-electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long-standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom.

We combine time-resolved pump-probe magneto-optical Kerr effect and photoelectron spectroscopy experiments supported by theoretical analysis to determine the relaxation dynamics of delocalized electrons in half-metallic ferromagnetic manganite La1-xSrxMnO3. We observe that the half-metallic character of La1-xSrxMnO3 determines the timescale of both the electronic phase transition and the quenching of magnetization, revealing a quantum isolation of the spin system in double-exchange ferromagnets extending up to hundreds of picoseconds. We demonstrate the use of time-resolved hard x-ray photoelectron spectroscopy as a unique tool to single out the evolution of strongly correlated electronic states across a second-order phase transition in a complex material.

Newly emerged materials from the family of Heuslers and complex oxides exhibit finite bandgaps and ferromagnetic behavior with Curie temperatures much higher than even room temperature. In this work, using the semiclassical top-of-the-barrier FET model, we explore the operation of a spin-MOSFET that utilizes such ferromagnetic semiconductors as channel materials, in addition to ferromagnetic source/drain contacts. Such a device could retain the spin polarization of injected electrons in the channel, the loss of which limits the operation of traditional spin transistors with non-ferromagnetic channels. We examine the operation of four material systems that are currently considered some of the most prominent known ferromagnetic semiconductors: three Heusler-type alloys (MnCoAl, CrVZrAl, and CoVZrAl) and one from the oxide family (NiFeO). We describe their band structures by using data from DFT (Density Functional Theory) calculations. We investigate under which conditions high spin polarization and significant I/I ratio, two essential requirements for the spin-MOSFET operation, are both achieved. We show that these particular Heusler channels, in their bulk form, do not have adequate bandgap to provide high I/I ratios and have small magnetoconductance compared to state-of-the-art devices. However, with confinement into ultra-narrow sizes down to a few nanometers, and by engineering their spin dependent contact resistances, they could prove promising channel materials for the realization of spin-MOSFET transistor devices that offer combined logic and memory functionalities. Although the main compounds of interest in this paper are MnCoAl, CrVZrAl, CoVZrAl, and NiFeO alone, we expect that the insight we provide is relevant to other classes of such materials as well.

The understanding of spin injection and transport in organic spintronic devices is still incomplete, with some experiments showing magnetoresistance and others not detecting it. We have investigated the transport properties of a large number of tris-(8-hydroxyquinoline)aluminum-based organic spintronic devices with an electrical resistance greater than 5 M? that did not show magnetoresistance. Their transport properties could be described satisfactorily by known models for organic semiconductors. At high voltages (>2 V), the results followed the model of space charge limited current with a Poole-Frenkel mobility. At low voltages (~0.1 V), that are those at which the spin valve behavior is usually observed, the charge transport was modelled by nearest neighbor hopping in intra-gap impurity levels, with a charge carrier density of n0 = (1.44 ± 0.21) × 1015 cm-3 at room temperature. Such a low carrier density can explain why no magnetoresistance was observed.

Vertical crossbar devices based on manganite and cobalt injecting electrodes and a metal-quinoline molecular transport layer are known to manifest both magnetoresistance (MR) and electrical bistability. The two effects are strongly interwoven, inspiring new device applications such as electrical control of the MR and magnetic modulation of bistability. To explain the device functionality, we identify the mechanism responsible for electrical switching by associating the electrical conductivity and the impedance behavior with the chemical states of buried layers obtained by in operando photoelectron spectroscopy. These measurements revealed that a significant fraction of oxygen ions migrate under voltage application, resulting in a modification of the electronic properties of the organic material and of the oxidation state of the interfacial layer with the ferromagnetic contacts. Variable oxygen doping of the organic molecules represents the key element for correlating bistability and MR, and our measurements provide the first experimental evidence in favor of the impurity-driven model describing the spin transport in organic semiconductors in similar devices.

In the rapidly growing field of spintronics, simultaneous control of electronic and magnetic properties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevant effects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depth-dependent analysis of two paradigmatic systems, namely La(1-x)Sr(x)MnO"3 and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface, respectively, for (Ga,Mn)As and La(1-x)Sr(x)MnO"3, for fully restoring bulk properties.

A novel functionalization of a ferromagnetic electrode employed in spintronic devices is reported. Self-assembling monolayer technique has been used to chemisorb a paramagnetic phosphonate functionalized nitronyl-nitroxide radical (NitPO) on the ferromagnetic La0.7Sr0.3MnO3 (LSMO) manganite surface. This interfacial layer causes clearly detectable modifications of the behavior in prototypical LSMO/NitPO/Gaq3/AlOx/Co vertical spintronic devices at temperatures below the ferromagnetic alignment (estimated by density functional theory) of the magnetic moments of NitPO and LSMO. This behavior can be justified by a significant spin filtering effect at the engineered interface, with a carrier selection (spin-up) opposite to that of the LSMO/Gaq3 interface (spin-down). It is proposed that the engineering of spin injecting interfaces with molecules having magnetic moment enables additional mechanisms to control and manipulate the spin polarization of currents in spintronic devices.

Ultrathin manganite films are widely used as active electrodes in organic spintronic devices. In this study, a scanning tunnelling microscopy (STM) investigation with atomic resolution revealed previously unknown surface features consisting of small non-stoichiometric islands. Based upon this evidence, a new mechanism for the growth of these complex materials is proposed. It is suggested that the non-stoichiometric islands result from nucleation centres that are below the critical threshold size required for stoichiometric crystalline growth. These islands represent a kinetic intermediate of single-layer growth regardless of the film thickness, and should be considered and possibly controlled in manganite thin-film applications.

Organic materials are attractive for building spintronics devices owing to their expected long spin lifetimes. Moreover, the ability to control their properties by changing their composition and molecular structure makes them easier to tailor to given tasks than inorganic materials. However, most studies of candidate organic spintronics materials focus on their bulk spin transport characteristics. Here we investigate the equally important process of spin injection and how it is influenced by interface coupling in the prototype organic semiconductor, Alq(3). We fabricate nanometre-scale (La, Sr) MnO(3)/Alq(3)/Co magnetic tunnel junctions that exhibit a magnetoresistive response of up to 300%. Furthermore, we develop a spin transport model that describes the role of interfacial spin-dependent metal/molecule hybridization on the effective polarization allowing enhancement and even sign reversal of injected spins. We expect such insights to lead towards the molecular-level engineering of metal/organic interfaces to tailor spin injection and bring new electrical functionalities to spintronics devices.

The electronic structures of the 8-hydroxyquinoline-aluminum (Alq(3))/Al(2)O(3)/Co interfaces were studied by photoelectron spectroscopy. A strong interface dipole was observed, which leads to a reduction in the electron injection barrier. The x-ray photoelectron spectroscopy spectra further indicate that the Al(2)O(3) buffer layer prevents the chemical interaction between Alq(3) molecules and Co atoms. X-ray magnetic circular dichroism results demonstrate that a Co layer deposited on an Al(2)O(3) buffered Alq(3) layer shows better magnetic ordering in the interface region than directly deposited Co, which suggests a better performance of spin valves with such a buffer layer. This is consistent with the recent results from [Dediu , Phys. Rev. B 78, 115203 (2008)].

With the general objective of studying interfaces between ferromagnetic materials and organic semiconductors, we report ac impedance investigations on La0.7Sr0.3MnO3 (LSMO)/tris(8-hydroxyquinoline)aluminium (Alq3)/Al and Indium Tin Oxide (ITO)/Alq3/Al heterostructures, in the frequency range between 20 Hz and 1 MHz. The comparison of the equivalent circuits deduced to fit the experimental ac responses allows isolating a specific RC contribution which can be attributed to the LSMO/Alq3 interface region. Using the information obtained from our ac measurements, we propose a model which fits the temperature dependence of the magnetoresistance in spin valves combining LSMO electrodes and Alq3 layers. (C) 2008 Elsevier B.V. All rights reserved.