Surface chemistry of porphyrins and phthalocyanines

Perpendicular magnetic tunnel junctions based on MgO/CoFeB structures are of particular interest for magnetic random-access memories because of their excellent thermal stability, scaling potential, and power dissipation. However, the major challenge of current-induced switching in the nanopillars with both a large tunnel magnetoresistance ratio and a low junction resistance is still to be met. Here, we report spin transfer torque switching in nano-scale perpendicular magnetic tunnel junctions with a magnetoresistance ratio up to 249% and a resistance area product as low as 7.0 Ω µm2, which consists of atom-thick W layers and double MgO/CoFeB interfaces. The efficient resonant tunnelling transmission induced by the atom-thick W layers could contribute to the larger magnetoresistance ratio than conventional structures with Ta layers, in addition to the robustness of W layers against high-temperature diffusion during annealing. The critical switching current density could be lower than 3.0 MA cm−2 for devices with a 45-nm radius. Perpendicular magnetic tunnel junctions with large tunnel magnetoresistance and low junction resistance are promising for the magnetic random access memories. Here the authors achieve the spin-transfer-torque switching in perpendicular magnetic tunnel junctions with 249% tunnel magnetoresistance and low resistance-area product.

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Current-induced magnetization switching in atom-thick tungsten engineered perpendicular magnetic tunnel junctions with large tunnel magnetoresistance.

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing $\ensuremath{\pi}({p}_{z})$ electrons onto a magnetic surface. The hybridization of the out-of-plane ${p}_{z}$ atomic-type orbitals with the $d$ states of the metal leads to the inversion of the spin polarization at the organic site due to a ${p}_{z}\mathrm{\text{\ensuremath{-}}}d$ Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

Design of the Local Spin Polarization at the Organic-Ferromagnetic Interface

Spintronics holds the promise for future information technologies. Devices based on manipulation of spin are most likely to replace the current silicon complementary metal-oxide semiconductor devices that are based on manipulation of charge. The challenge is to identify or design materials that can be used to generate, detect, and manipulate spin. Since the successful isolation of graphene and other two-dimensional (2D) materials, there has been a strong focus on spintronics based on 2D materials due to their attractive properties, and much progress has been made, both theoretically and experimentally. Here, we summarize recent developments in spintronics based on 2D materials. We focus mainly on materials of truly 2D nature, that is, atomic crystal layers such as graphene, phosphorene, monolayer transition metal dichalcogenides, and others, but also highlight current research foci in heterostructures or interfaces. In particular, we emphasize roles played by computation based on first-principles methods which has contributed significantly in the designs of spintronic materials and devices. We also highlight challenges and suggest possible directions for further studies. WIREs Comput Mol Sci 2017, 7:e1313. doi: 10.1002/wcms.1313

For further resources related to this article, please visit the WIREs website.

Prospects of spintronics based on 2D materials

Making effective usage of quantum size effect is a foregrounded strategy to design and fabricate excellent electromagnetic microwave absorption materials. In this research, ZnFe2O4 quantum dots were coated by hybrid amorphous carbon to form a sea islands structure by a facile electrostatic self-assembly synthetic technology. Simultaneously, the rejection of heterogeneous charges leads to the formation of quantum dots, by which the quantum size effects on dielectric and magnetic characteristic were investigated. Consequently, multiple hetero-interface and interfacial polarization was originated from polycrystalline feature of ZnFe2O4 with spinel and inverse spinel structures. In particular, the electromagnetic microwave absorption properties of ZnFe2O4 were greatly optimized, as the minimized reflection loss reached −40.68 dB at the frequency 11.44 GHz and thickness 2.5 mm, while the effective bandwidth corresponding was 3.66 GHz (from 9.87 to 13.52 GHz). The largest effective bandwidth was 4.16 GHz (from 8.08 to 12.24 GHz) with a thickness of 3 mm. It is suggested that high performance of microwave absorption of ZnFe2O4 quantum dots was well guided by the optimized impedance matching and attenuation constants.

Electrostatic self-assembly synthesis of ZnFe2O4 quantum dots (ZnFe2O4@C) and electromagnetic microwave absorption

By constructing asymmetric polar interfaces, all-oxide ferroelectric tunnel junctions (FTJs) are proposed that can achieve a sizable tunneling electroresistance (TER) effect. Based on first-principles quantum transport calculations on a prototypical LaNiO3/BaTiO3/LaNiO3 junction, we predict that TER reaches 103% under a finite bias. Driven by the asymmetric polar interfaces, the resultant intrinsic electric field causes a highly asymmetric electrostatic potential in comparison to that of the FTJ with symmetric polar interfaces. As a result, the tunneling resistance changes significantly upon polarization reversal leading to sizable TER. The physical origin of the TER effect can be well understood in terms of local density of states, transport in momentum space, real-space scattering states and a free-electron tunneling model. Our results provide an insight into the understanding of ferroelectricity and the TER mechanism in FTJs and will be useful for FTJ-based devices design.

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Ferroelectricity and tunneling electroresistance effect driven by asymmetric polar interfaces in all-oxide ferroelectric tunnel junctions

In this paper, we combine first-principles calculations and experiments to investigate the magnetic properties of aluminum-doped TiO2 films of rutile structure. Density-functional theory with generalized gradient approximation based calculations were carried out for three cases, where the TiO2 lattice contains oxygen vacancies VO only, an oxygen is substituted by a fluorine atom, or a Ti is substituted by an aluminum. Magnetic moments associated with the formation of Ti3+ ions are found in all cases but they couple differently resulting in different magnetic states. Al-doped samples prepared in our labs exhibit ferromagnetism at room temperature with a TC near 340 K. The experimental results are consistent with the first principles calculations, and the magnetism is associated with the VO defect electrons induced by the Al doping. The defect electron occupies nearby Ti sites giving rise to the Ti3+ moments and, at the same time, has spatially extended wavefunctions assuring overlapping between neighbors.

Origin of ferromagnetism in aluminum-doped TiO2 thin films: Theory and experiments

The effects of epitaxial strain on the ferroelectric, structural properties of KTaO3 are studied by means of first-principles calculations. We show that the ferroelectric polarization magnitude as well as the orientation can be tuned by an in-plane strain: the c-phase is energetically more stable than the aa-phase at a large compressive strain while a phase transition from c- to aa-phase is observed at a large tensile strain, owing to the significant polarization-strain coupling. More importantly, based on relativistic first-principles calculations, we demonstrate a large Rashba spin splitting in the strained KTaO3. Interestingly, the spin textures in momentum space can be controlled and switched via polarization switching. Our tight-binding analysis indicates that the combination of spin-orbit coupling and ferroelectric distortion plays a key role for the observed Rashba spin splitting. Our results present some fundamental understanding of the interplay between Rashba effect and ferroelectricity in oxide...

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Strain-tunable ferroelectricity and its control of Rashba effect in KTaO3

We report a first-principles theoretical investigation of spin-polarized quantum transport in organic magnetic tunnel junctions (OMTJs) to provide a microscopic understanding on the sign of the tunnel magnetoresistance ratio (TMR). We consider two different OMTJs, formed by sandwiching 1-stearic acid radicals (1-SAR) or 1,18-stearic diacid radicals (1,18-SDR) between two Ni electrodes. Even though the main difference between them is only on one of the Ni/molecule contacts, such a structure difference is found to induce a significant sign change of the TMR. The TMR is negative for 1-SAR at -19.6%, but is positive for 1,18-SDR at 13.7%. By investigating the concept of scattering density of states (SDOS), we found that scattering processes of p electrons at the Ni/molecule interface determines the sign of TMR. Based on spin polarization of the SDOS, we extend the Julliere model to explain both the sign and the value of the TMR qualitatively and semiquantitatively. It is concluded that understanding spin-polarized quantum transport in organic magnetic tunnel junction requires a comprehensive knowledge of the electronic structures of the molecule, the metal electrode, and the metal-molecule contacts. DOI: 10.1103/PhysRevB.86.224419

Organic magnetic tunnel junctions: The role of metal-molecule interface

We report the investigation on the ferroelectricity and tunneling electroresistance (TER) effect in PbTiO3 (PTO)-based ferroelectric tunnel junctions (FTJs) using first-principles calculations. For symmetric FTJs, we have calculated the average polarizations of PTO film and effective screening lengths of different metal electrodes for a number of FTJs, which is useful for experimental research. For asymmetric FTJs, significant asymmetric ferroelectric displacements in PTO film are observed, which is attributed to the intrinsic field generated by the two dissimilar electrodes. Moreover, by performing quantum transport calculations on those asymmetric FTJs, a sizable TER effect is observed. It is found that the asymmetry of ferroelectric displacements in PTO barrier, which is determined by the difference of work functions of the electrodes, controls the observed TER effect. Our results will help unravel the TER mechanism of asymmetric FTJs in most experiments and will be useful for the designing of FTJ-based devices.

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L. L. Tao

Papers

Ferroelectricity and tunneling electroresistance effect driven by asymmetric polar interfaces in all-oxide ferroelectric tunnel junctions

Origin of ferromagnetism in aluminum-doped TiO2 thin films: Theory and experiments

Strain-tunable ferroelectricity and its control of Rashba effect in KTaO3

Organic magnetic tunnel junctions: The role of metal-molecule interface

Ferroelectricity and tunneling electroresistance effect in asymmetric ferroelectric tunnel junctions