Perpendicular Reading of Single Confined Magnetic Skyrmions

Thin-film sub-5 nm magnetic skyrmions constitute an ultimate scaling alternative for future digital data storage. Skyrmions are robust non-collinear spin-textures that can be moved and manipulated by small electrical currents. We show here an innovative technique to detect isolated nanoskyrmions with a current-perpendicular-to-plane geometry, which has immediate implications for device concepts. We explore the physics behind such a mechanism by studying the atomistic electronic structure of the magnetic quasiparticles. We investigate how the isolated skyrmion local-density-of-states which tunnels into the vacuum, when compared to the ferromagnetic background, is modified by the site-dependent spin-mixing of electronic states with different relative canting angles. Local transport properties are sensitive to this effect, as we report an atomistic conductance anisotropy of over 20% for magnetic skyrmions in Pd/Fe/Ir(111) thin-films. In single skyrmions, engineering this spin-mixing magnetoresistance possibly could be incorporated in future magnetic storage technologies

Transport properties of copper phthalocyanine based organic electronic devices

Ambipolar charge carrier transport in Copper phthalocyanine (CuPc) is studied experimentally in field-effect transistors and metal-insulator-semiconductor diodes at various temperatures. The electronic structure and the transport properties of CuPc attached to leads are calculated using density functional theory and scattering theory at the non-equilibrium Green's function level. We discuss, in particular, the electronic structure of CuPc molecules attached to gold chains in different geometries to mimic the different experimental setups. The combined experimental and theoretical analysis explains the dependence of the mobilityand the transmission coefficient on the charge carrier type (electrons or holes) and on the contact geometry. We demonstrate the correspondence between our experimental results on thick films and our theoretical studies of single molecule contacts. Preliminary results for fluorinated CuPc are discussed.Comment: 18 pages, 16 figures; to be published in Eur. Phys. J. Special Topic

Defect-implantation for the all-electrical detection of non-collinear spin-textures

The viability of past, current and future devices for information technology hinges on their sensitivity to the presence of impurities. The latter can reshape extrinsic Hall effects or the efficiency of magnetoresistance effects, essential for spintronics, and lead to resistivity anomalies, the so-called Kondo effect. Here, we demonstrate that atomic defects enable highly efficient all-electrical detection of spin-swirling textures, in particular magnetic skyrmions, which are promising bit candidates in future spintronics devices. The concomitant impurity-driven alteration of the electronic structure and magnetic non-collinearity gives rise to a new spin-mixing magnetoresistance (XMRdefect). Taking advantage of the impurities-induced amplification of the bare transport signal, which depends on their chemical nature, a defect-enhanced XMR (DXMR) is proposed. Both XMR modes are systematised for 3d and 4d transition metal defects implanted at the vicinity of skyrmions generated in PdFe bilayer deposited on Ir(111). The ineluctability of impurities in devices promotes the implementation of defect-enabled XMR modes in reading architectures with immediate implications in magnetic storage technologies

Tailoring the chiral magnetic interaction between two individual atoms

Chiral magnets are a promising route towards dense magnetic storage technology due to their inherent nano-scale dimensions and energy efficient properties. Engineering chiral magnets requires atomic-level control of the magnetic exchange interactions, including the Dzyaloshinskii–Moriya interaction, which defines a rotational sense for the magnetization of two coupled magnetic moments. Here we show that the indirect conduction electron-mediated Dzyaloshinskii–Moriya interaction between two individual magnetic atoms on a metallic surface can be manipulated by changing the interatomic distance with the tip of a scanning tunnelling microscope. We quantify this interaction by comparing our measurements to a quantum magnetic model and ab-initio calculations yielding a map of the chiral ground states of pairs of atoms depending on the interatomic separation. The map enables tailoring the chirality of the magnetization in dilute atomic-scale magnets

Polaron Effect on the Binding Energy of Shallow Donor in Cylindrical Quantum Dot

In the framework of the effective-mass approximation and the modified Lee-Low-Pines variational method, we present a theoretical study of the effect of the confined longitudinal-optical phonon and two types of surface-optical phonon (top and side mode) on the binding energy of shallow donor in cylindrical quantum dot. The effect of quantum confinement is described by an infinitely deep potential well. The impact of these different phonon modes is important and depends on the dimension of the quantum dot

Binding Energy of a Bound Polaron in a Quantum Well Wire

Theoretical study of the binding energies of an off-center donor hydrogenic impurity in a cylindrical quantum well wires semiconductor is presented. Calculations are performed in the framework of the effective mass approximation using the variational approach. We describe the effect of the quantum confinement by an infinitely deep potential well and we take into consideration the interaction between the charge carrier (electron and ion) and the optical phonons (confined longitudinal optical and surface optical). Our results show that the impurity binding energy depends strongly on the spatial confinement, the impurity position and the polaronic corrections

Absence of a spin-signature from a single Ho adatom as probed by spin-sensitive tunneling

Whether rare-earth materials can be used as single-atom magnetic memory is an ongoing debate in recent literature. Here we show, by inelastic and spin-resolved scanning tunnelling-based methods, that we observe a strong magnetic signal and excitation from Fe atoms adsorbed on Pt(111), but see no signatures of magnetic excitation or spin-based telegraph noise for Ho atoms. Moreover, we observe that the indirect exchange field produced by a single Ho atom is negligible, as sensed by nearby Fe atoms. We demonstrate, using ab initio methods, that this stems from a comparatively weak coupling of the Ho 4f electrons with both tunnelling electrons and substrate-derived itinerant electrons, making both magnetic coupling and detection very difficult when compared to 3d elements. We discuss these results in the context of ongoing disputes and clarify important controversies

Carrier-density and field-dependent charge-carrier mobility in organic semiconductors with correlated Gaussian disorder

Recently, it has been demonstrated that for organic semiconductors with a Gaussian density of states (DOS) and with on-site energies that are spatially uncorrelated the hopping mobility of charge-carriers can be strongly carrier-density-dependent (extended Gaussian disorder model, EGDM). In the literature, it has been argued that for some materials, the on-site energies are actually spatially correlated. In this paper, we develop a full description of the mobility in a correlated Gaussian DOS (extended correlated disorder model, ECDM), using a master-equation method. We show that the mobility is less strongly carrier-density-dependent than in the EGDM, but that the field dependence is more pronounced. The field dependence is found to be described by a Poole-Frenkel factor, as has been deduced from empirical analyses of experimental data, but only in a limited field range. As an example of an application, we present a comparison between analyses of the current-voltage-temperature J(V,T) characteristics of a poly-phenylene-vinylene (PPV) based hole-only device using the EGDM and the ECDM. For both cases, excellent fits can be obtained, but with the EGDM a more realistic value of the intersite distance is found than in the case of the ECDM. We view this as an indication that site-energy correlations do not play an important role in PPV. © 2009 Elsevier B.V. All rights reserved