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Different models have been used to evaluate the interfacial Dzyaloshinskii-Moriya interaction (DMI) from the asymmetric bubble expansion method using magneto-optics. Here we investigate the most promising candidates over a range of different magnetic multilayers with perpendicular anisotropy. Models based on the standard creep hypothesis are not able to reproduce the domain wall (DW) velocity profile when the DW roughness is high. Our results demonstrate that the DW roughness and the interface roughness of the sample layers are correlated. Furthermore, we give guidance on how to obtain reliable results for the DMI value with this popular method. A comparison of the results with Brillouin light scattering (BLS) measurements on the same samples shows that the BLS approach often results in higher measured values of DMI.

We thank the Engineering and Physical Sciences Research Council (EPSRC UK) for support via Grants No. EP/M000923/1, No. EP/K036408/1, No. EP/I004483/1, No. EP/S031081/1, and No. EP/S030263/1. L.L., S.S., D.J. and G.T. acknowledge also support from STFC-ISIS Neutron and Muon Source and Ada Lovelace Centre at STFC-SCD. We acknowledge use of the ARCHER (via the U.K. Car–Parrinello Consortium, EP/P022618/1 and EP/P022189/2), U.K. Materials and Molecular Modelling Hub (EP/P020194/1), and STFC Scientific Computing Department's SCARF HCP facilities. We acknowledge support from the Henry Royce Institute. This work was also supported financially through the EPSRC Grant Nos. EP/ P022464/1, and EP/R00661X/1. Muonium is a combination of first- and second-generation matter formed by the electrostatic interaction between an electron and an antimuon (μ+). Although a well-known physical system, their ability to form collective excitations in molecules had not been observed. Here, we give evidence for the detection of a muonium state that propagates in a molecular semiconductor lattice via thermally activated dynamics: a muonium polaron. By measuring the temperature dependence of the depolarization of the muonium state in C60, we observe a thermal narrowing of the hyperfine distribution that we attribute to the dynamics of the muonium between molecular sites. As a result of the time scale for muonium decay, the energies involved, charge and spin selectivity, this quasiparticle is a widely applicable experimental tool. It is an excellent probe of emerging electronic, dynamic, and magnetic states at interfaces and in low dimensional systems, where direct spatial probing is an experimental challenge owing to the buried interface, nanoscale elements providing the functionality localization and small magnitude of the effects. Publisher PDF

Using scanning thermal microscopy, we have mapped the spatial distribution of temperatures in an operating nanoscale device formed from a magnetic injector, an Ag connecting wire, and a magnetic detector. An analytical model explained the thermal diffusion over the measured temperature range (2–300 K) and injector-detector separation (400–3000 nm). The characteristic diffusion lengths of the Peltier and Joule heat differ remarkably below 60 K, a fact that can be explained by the onset of ballistic phonon heat transfer in the substrate.

Combining magnetic and superconducting functionalities enables lower energy spin transfer and magnetic switching in quantum computing and information storage, owing to the dissipationless nature of quasi-particle mediated supercurrents. Here, we put forward a system where emergent spin-ordering and diffusion of Cooper pairs are achieved at a non-intrinsically magnetic nor superconducting metallo-molecular interface. Electron transport, magnetometry and low-energy muon spin rotation are used to probe time-reversal symmetry breaking in these structures. By comparing the Meissner expulsion in a system including a Cu/C60 spin-converter interface to one without, we observe a paramagnetic contribution that can be explained due to the conversion of spin-singlet Cooper pair states into odd-frequency triplet states. These results demonstrate the potential of metallo-molecular interfaces to achieve singlet to triplet Cooper pair conversion, a capability not present in either metal or molecule separately that could be used in the generation and controlled diffusion of spin polarised dissipationless currents. Magnetic molecules deposited on a metallic substrate constitute a method to engineer the spin properties of the molecule and has potential application in low-power information storage devices. Here, the authors investigate a superconductor/molecule/normal metal heterostructure and demonstrate spin-ordering and proximity induced superconducting properties at the metallo-molecular interface.

5d metals are used in electronics because of their high spin-orbit coupling (SOC) leading to efficient spin-electric conversion. When C60 is grown on a metal, the electronic structure is altered due to hybridization and charge transfer. In this work, we measure the spin Hall magnetoresistance for Pt/C60 and Ta/C60, finding that they are up to a factor of 6 higher than those for pristine metals, indicating a 20-60% increase in the spin Hall angle. At low fields of 1-30 mT, the presence of C60 increased the anisotropic magnetoresistance by up to 700%. Our measurements are supported by noncollinear density functional theory calculations, which predict a significant SOC enhancement by C60 that penetrates through the Pt layer, concomitant with trends in the magnetic moment of transport electrons acquired via SOC and symmetry breaking. The charge transfer and hybridization between the metal and C60 can be controlled by gating, so our results indicate the possibility of dynamically modifying the SOC of thin metals using molecular layers. This could be exploited in spin-transfer torque memories and pure spin current circuits.

We report on the electrical transport properties of Nb based Josephson junctions with Pt/Co\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{68}$$\end{document}68B\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{32}$$\end{document}32/Pt ferromagnetic barriers. The barriers exhibit perpendicular magnetic anisotropy, which has the main advantage for potential applications over magnetisation in-plane systems of not affecting the Fraunhofer response of the junction. In addition, we report that there is no magnetic dead layer at the Pt/Co\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{68}$$\end{document}68B\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{32}$$\end{document}32 interfaces, allowing us to study barriers with ultra-thin Co\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{68}$$\end{document}68B\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{32}$$\end{document}32. In the junctions, we observe that the magnitude of the critical current oscillates with increasing thickness of the Co\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{68}$$\end{document}68B\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{32}$$\end{document}32 strong ferromagnetic alloy layer. The oscillations are attributed to the ground state phase difference across the junctions being modified from zero to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi $$\end{document}π. The multiple oscillations in the thickness range \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.2~\leqslant ~d_\text {CoB}~\leqslant ~1.4$$\end{document}0.2⩽dCoB⩽1.4 nm suggests that we have access to the first zero-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi $$\end{document}π and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi $$\end{document}π-zero phase transitions. Our results fuel the development of low-temperature memory devices based on ferromagnetic Josephson junctions.

We report on the electrical transport properties of Nb based Josephson junctions with Pt/Co$_{68}$B$_{32}$/Pt ferromagnetic barriers. The barriers exhibit perpendicular magnetic anisotropy, which has the main advantage for potential applications over magnetisation in-plane systems of not affecting the Fraunhofer response of the junction. In addition, we report that there is no magnetic dead layer at the Pt/Co$_{68}$B$_{32}$ interfaces, allowing us to study barriers with ultra-thin Co$_{68}$B$_{32}$. In the junctions, we observe that the magnitude of the critical current oscillates with increasing thickness of the Co$_{68}$B$_{32}$ strong ferromagnetic alloy layer. The oscillations are attributed to the ground state phase difference across the junctions being modified from zero to $\pi$. The multiple oscillations in the thickness range $0.2~\leqslant~d_\text{CoB}~\leqslant~1.4$~nm suggests that we have access to the first zero-$\pi$ and $\pi$-zero phase transitions. Our results fuel the development of low-temperature memory devices based on ferromagnetic Josephson junctions., Comment: Main text: 11 pages, 3 figures, 1 table. Supplementary information: 9 pages, 5 figures

Hybrid interfaces form a spin capacitor for the generation and storage of spin angular momentum via optical or electronic stimuli., We show that hybrid MnOx/C60 heterojunctions can be used to design a storage device for spin-polarized charge: a spin capacitor. Hybridization at the carbon-metal oxide interface leads to spin-polarized charge trapping after an applied voltage or photocurrent. Strong electronic structure changes, including a 1-eV energy shift and spin polarization in the C60 lowest unoccupied molecular orbital, are then revealed by x-ray absorption spectroscopy, in agreement with density functional theory simulations. Muon spin spectroscopy measurements give further independent evidence of local spin ordering and magnetic moments optically/electronically stored at the heterojunctions. These spin-polarized states dissipate when shorting the electrodes. The spin storage decay time is controlled by magnetic ordering at the interface, leading to coherence times of seconds to hours even at room temperature.

We thank the Engineering and Physical Sciences Research Council (EPSRC) in the United Kingdom for support through Grants EP/P001556/1, EP/J01060X/1, EP/I004483/1, and EP/M000923/1. R.S. wishes to acknowledge EPSRC for a scholarship via the Grant EP/L015110/1. XAS/XMCD experiments were performed in the BOREAS beamline at the Alba synchrotron (Proposals ID2014071101 and ID2015091530). M.V. acknowledges Mineco Grant FIS2013-45469-C4-3-R. Use of the N8 High Performance Computing (HPC) (EPSRC EP/K000225/1) and ARCHER (via the UK Car–Parrinello Consortium, EP/K013610/1 and EP/P022189/1) Charge transfer at metallo–molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc–C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo–carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2−π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz–π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices. Postprint

We demonstrate a Josephson junction with a weak link containing two ferromagnets, with perpendicular magnetic anisotropy and independent switching fields in which the critical current can be set by the mutual orientation of the two layers. Such pseudospin-valve Josephson junctions are a candidate cryogenic memory in an all superconducting computational scheme. Here, we use Pt/Co/Pt/CoB/Pt as the weak link of the junction with $d_\text{Co} = 0.6$ nm, $d_\text{CoB} = 0.3$ nm, and $d_\text{Pt} = 5$ nm and obtain a $60\%$ change in the critical current for the two magnetization configurations of the pseudospin-valve. Ferromagnets with perpendicular magnetic anisotropy have advantages over magnetization in-plane systems which have been exclusively considered to this point, as in principle the magnetization and magnetic switching of layers in the junction should not affect the in-plane magnetic flux., Comment: 5 pages, 3 figures

Many devices, such as lateral spin valves, depend critically on the quality of interfaces formed between different materials, and hence require the entire device to be fabricated within an ultra-high vacuum environment. This is possible using angled deposition with a suspended mask such that, by depositing from specific angles, different patterns form on the substrate beneath. We use a bi-layer of MMA(8.5)MAA copolymer and PMMA patterned by electron-beam lithography (EBL) to form such a mask. It is necessary, though, to perform proximity effect correction (PEC) in EBL to achieve the correct spatial distribution of electrons, and hence produce the desired pattern in the developed resist. For bi-layer processes this is a three-dimensional (3D) correction since we must optimise for two different critical doses (one for the copolymer, the other for the PMMA) at defined 3D positions within the resist stack. We perform this 3D correction using the commercial software BEAMER produced by GenISys GmbH. We show that by applying manual shape segregation and modulation to the exposure pattern, prior to the “3D-PEC” algorithm, it is possible to achieve much higher contrasts in the spatial distribution of absorbed energy and hence significantly increase the processing window, and yield in the fabrication of suspended masks.

The response of ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}\text{/}\mathrm{Cu}\text{/}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ trilayer structures to excitation by a femtosecond laser pulse has been studied in optical pump-probe experiments and compared with the response of ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (YIG) and ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ reference samples. The optical pump induces a partial demagnetization of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$, a large thermal gradient within the YIG, and temperature differences across the interfaces within the sample stack. When a moderate magnetic field is applied close to normal to the sample plane, so as to quasialign the YIG magnetization with the field and cant the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ magnetization from the plane, ultrafast demagnetization initiates precession of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ magnetization. The transient temperature profile within the samples has been modeled using a one-dimensional finite-element computational model of heat conduction, while the magnetization dynamics are well described by a macrospin solution of the Landau-Lifshitz-Gilbert equation. The precessional response of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ layers within the trilayers and the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ reference sample are very similar for pump fluences of up to $1.5\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^{2}$, beyond which irreversible changes to the magnetic properties of the films are observed. These results suggest that the spin Seebeck effect is ineffective in modifying the precessional dynamics of the present $\mathrm{YIG}\text{/}\mathrm{Cu}\text{/}{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ samples when subject to ultrafast optical excitation.

THz coplanar waveguides were fabricated on quartz wafers with integrated epitaxially transferred low temperature grown gallium arsenide photoconductive switches. THz radiation was excited on-chip and transmitted through a tapering of the coplanar waveguide structure where it was focused down to ∼ 1.66μm. Theoretical modelling of the device confirms high E-field confinement and concentration.

Perpendicularly magnetized nanowires exhibit distinct domain wall types depending on the geometry. Wide wires contain Bloch walls, narrow wires N\'eel walls. Here, the transition region is investigated by direct imaging of the wall structure using high-resolution spin-polarized scanning electron microscopy. An achiral intermediate wall type is discovered that is unpredicted by established theoretical models. With the help of micromagnetic simulations, the formation of this novel wall type is explained., Comment: 6 pages, 4 figures

We report measurements on yttrium iron garnet (YIG) thin films grown on both gadolinium gallium garnet (GGG) and yttrium aluminium garnet (YAG) substrates, with and without thin Pt top layers. We provide three principal results: the observation of an interfacial region at the Pt/YIG interface, we place a limit on the induced magnetism of the Pt layer and confirm the existence of an interfacial layer at the GGG/YIG interface. Polarised neutron reflectometry (PNR) was used to give depth dependence of both the structure and magnetism of these structures. We find that a thin film of YIG on GGG is best described by three distinct layers: an interfacial layer near the GGG, around 5 nm thick and non-magnetic, a magnetic bulk phase, and a non-magnetic and compositionally distinct thin layer near the surface. We theorise that the bottom layer, which is independent of the film thickness, is caused by Gd diffusion. The top layer is likely to be extremely important in inverse spin Hall effect measurements, and is most likely Y2O3 or very similar. Magnetic sensitivity in the PNR to any induced moment in the Pt is increased by the existence of the Y2O3 layer; any moment is found to be less than 0.02 uB/atom., 10 pages, 4 figures

Yttrium iron garnet has a very high Verdet constant, is transparent in the infrared and is an insulating ferrimagnet leading to its use in optical and magneto-optical applications. Its high Q-factor has been exploited to make resonators and filters in microwave devices, but it also has the lowest magnetic damping of any known material. In this article we describe the structural and magnetic properties of single crystal thin-film YIG where the temperature dependence of the magnetisation reveals a decrease in the low temperature region. In order to understand this complex material we bring a large number of structural and magnetic techniques to bear on the same samples. Through a comprehensive analysis we show that at the substrate -YIG interface, an interdiffusion zone of only 4–6 nm exists. Due to the interdiffusion of Y from the YIG and Gd from the substrate, an addition magnetic layer is formed at the interface whose properties are crucially important in samples with a thickness of YIG less than 200 nm.

The relaxation of Fe/MgO/Fe tunnel junctions grown epitaxially on (001) MgO substrates has been measured by in-situ grazing incidence in-plane X-ray diffraction during the thermal annealing cycle. We find that the Fe layers are fully relaxed and that there are no irreversible changes during annealing. The MgO tunnel barrier is initially strained towards the Fe but on annealing, relaxes and expands towards the bulk MgO value. The strain dispersion is reduced in the MgO by about 40% above 480 K post-annealing. There is no significant change in the “twist” mosaic. Our results indicate that the final annealing stage of device fabrication, crucial to attainment of high TMR, induces substantial strain relaxation at the MgO barrier/lower Fe electrode interface.

We have investigated single electron spin transport in individual single crystal bcc Co30Fe70 nanoparticles using scanning tunnelling microscopy with a standard tungsten tip. Particles were deposited using a gas-aggregation nanoparticle source and individually addressed as asymmetric double tunnel junctions with both a vacuum and a MgO tunnel barrier. Spectroscopy measurements on the particles show a Coulomb staircase that is correlated with the measured particle size. Field emission tunnelling effects are incorporated into standard single electron theory to model the data. This formalism allows spin-dependent parameters to be determined even though the tip is not spin-polarised. The barrier spin polarisation is very high, in excess of 84%. By variation of the resistance, several orders of magnitude of the system timescale are probed, enabling us to determine the spin relaxation time on the island. It is found to be close to 10 μs, a value much longer than previously reported.

The magnetic properties of exchange biased systems deposited on top of fullerene-based materials have been studied. An enhanced exchange bias field has been observed for Si/SiO2/C60/Co/Cu/Co/IrMn/Ta and Si/SiO2 /FeC60 /Co/Cu/Co/IrMn/Ta in comparison to a typical structure of Si/SiO2/Ta/Co/Cu/Co/IrMn/Ta. Here the magnetic properties of the devices are changed significantly with an organic underlayer or seed layer. The magnetic ordering is well maintained, and C60 seeding results in an exchange bias of (19.5 ±0.5) mT, some 7 mT higher than that of Ta seeded spin valves (12.5 ±0.5 mT). A combination of MOKE (magneto-optical Kerr effect), X-ray diffraction and Raman spectroscopy have been used to characterize the materials and further understand the interactions between the Co and fullerene-based materials.

The strong link between the precise molecular structure and electronic properties of single wall carbon nanotubes is well known, however the experimental investigation of these links remains a challenge. This is in part due to the difficulty in determining the structure, or chirality of individual carbon nanotubes. Performing diffraction studies within a transmission electron microscope has become arguably the most powerful technique for the accurate determination of the chirality of, not only individual single wall, but also multi-wall and small bundles of carbon nanotubes. In this paper we give an up to date review of the analytical approaches to chirality assignment. For comparison we perform each of the discussed techniques on the same diffraction pattern, obtained using standard electron diffraction techniques, and discuss the ability of each to allow for the unambiguous determination of chirality within experimental uncertainties. The aim of this review is to give guidance as to the most pragmatic approach to extracting carbon nanotube chirality from electron diffraction patterns without the need for performing complicated and time consuming simulations.

The growth of metallic nanoparticles formed on chemically modified graphene (CMG) by physical vapor deposition is investigated. Fine control over the size (down to ∼1.5 nm for Au) and coverage (up to 5 × 10(4) μm(-2) for Au) of nanoparticles can be achieved. Analysis of the particle size distributions gives evidence for Au nanocluster diffusion at room temperature, while particle size statistics differ clearly between metal deposited on single- and multilayer regions. The morphology of the nanoparticles varies markedly for different metals (Ag, Au, Fe, Pd, Pt, Ti), from a uniform thin film for Ti to a droplet-like growth for Ag. A simple model explains these morphologies, based only on consideration of 1) the different energy barriers to surface diffusion of metal adatoms on graphene, and 2) the ratio of the bulk cohesive energy of the metal to the metal-graphene binding energy. Understanding these interactions is important for controlling nanoparticle and thin-film growth on graphene, and for understanding the resultant charge transfer between metal and graphene.

Conventional microelectronics exploits only the charge degree of freedom of the electron. Bringing the spin degree of freedom to bear on sensing, radio frequency, memory and logic applications opens up new possibilities for ‘more than Moore’ devices incorporating magnetic components that can couple to an external field, store a bit of data or represent a Boolean state. Moreover, the electron spin is an archetypal two-state quantum system that is an excellent candidate for a solid-state realization of a qubit.

International audience; We report the measurement of reflected neutron intensity ''hysteresis loops'' from Co/Ru multilayers that have both antiferromagnetically and ferromagnetically coupled regions. We show that by measuring the four neutron spin-resolved reflectivities at a particular value of wavevector transfer, the normalised value of the relevant magnetic order parameter may be determined. The response of that order parameter to an applied magnetic field may hence be tracked. We have benchmarked our results against conventional magnetometry and magnetotransport measurements.

A modification of the Blonder–Tinkham–Klapwijk (BTK) model is proposed for the description of the ferromagnet-superconductor interface. Modelling the contact potential with a rectangular barrier, we investigate the influence of the barrier width at the interface on the value of spin polarisation measured by point contact Andreev reflection. Results suggesting that neglecting the width of the barrier at the interface can lead to an overestimation of the spin polarisation using the original BTK model are presented. This effect is particularly strong for low values of polarization and vanishes for high polarisation. The impact on analysis of the experimental data is also discussed.

Tapered coplanar waveguides with integrated photoconductors were designed, fabricated, and measured, with pulsed transmission results comparing well with High Frequency Structure Simulator simulations which predict increased confinement and electric field concentration in the tapered region. Devices made with titanium/gold metallisation were used to demonstrate transmission and confinement, while the magnetoresistive properties of devices with cobalt/copper multilayers were used to demonstrate the field concentration. In the latter case, a mathematical framework was developed to understand the relationship between tapering effects and the picosecond magnetoresistance response.

Andreev reflection (AR) is a charge transfer process occurring at a normal metal-superconductor interface whereby an incident electron (hole) of energy less than the superconducting gap may be converted to supercurrent with reflection of a hole (electron) of opposite momentum and spin. Recent interest has focused on nonlocal or crossed Andreev reflection (CAR) in a nanoscale device consisting of two normal metal leads contacting a superconductor with spatial separation of the order of the superconducting coherence length. CAR presents a possible source of solid state quantum entanglement, via the reflection of a hole (electron) produced by AR of an electron (hole) at one N/S junction at the second, forming an entangled Andreev pair and equivalent to transmission of two opposite spin quasiparticles into the separate leads. By use of ferromagnetic leads strongly spin dependent CAR and competing electron cotunelling (EC) may be controlled in a practical device. Based on design studies, micromagnetic simulation and computer modeling we have fabricated such nanoscale devices consisting of sputter deposited Fe or Co leads patterned by electron beam lithography ranging in size from 100 nm-3 mum, with in-plane separation of 100-300 nm, in point contact with an optically patterned 5 mum Al superconducting electrode. These devices were cooled to a minimum of 500 mK using an adiabatic demagnetization refrigerator to detect nonlocal voltages characteristic of CAR and differentiable under applied magnetic field from competing EC. Here we present our initial results.

The layer structure of a series of Fe/Au multilayers has been studied by grazing incidence specular and diffuse X-ray scattering. In particular, the evolution of the layer and interface characteristics has been followed as a function of the number of bilayers grown. Torque magnetometry measurements showed that the easy axis of magnetization of all samples lay in the film plane, the effective anisotropy constant per unit volume remaining constant as the bilayer number increased. These data contrast with previous studies showing that Fe/Au multilayers exhibited perpendicular anisotropy when the Fe layer is thinner than 10 A. Fitting of model structures to the grazing incidence specular and diffuse X-ray scattering data showed that the r.m.s. roughness was high, being between 10 and 20 A for different samples within the series. There was no systematic trend in the amplitude as a function of bilayer repeat number but the in-plane length scale of the roughness increased with layer thickness. The fractal parameter h fell linearly with bilayer repeat number. We suggest that the high level of roughness results in the in-plane anisotropy in these layers. A superlattice sample with 100 bilayers displayed a significantly lower roughness of 8 (±0.3) A and perpendicular anisotropy.

The temperature dependence of the spin diffusion length typically reflects the scattering mechanism responsible for spin relaxation. Within nonmagnetic metals it is reasonable to expect the Elliot-Yafet mechanism to play a role and thus the temperature dependence of the spin diffusion length might be inversely proportional to resistivity. In lateral spin valves, measurements have found that at low temperatures the spin diffusion length unexpectedly decreases. By measuring the transport properties of lateral Py/Cu/Py spin valves, fabricated from Cu with magnetic impurities of $l1$ ppm and $\ensuremath{\sim}4$ ppm, we extract a spin diffusion length which shows this suppression below 30 K only in the presence of the Kondo effect. We have calculated the spin-relaxation rate and isolated the contribution from magnetic impurities. We find the spin-flip probability of a magnetic impurity to be 34%. Our analysis demonstrates the dominant role of Kondo scattering in spin relaxation, even in low concentrations of order 1 ppm, and hence illustrates its importance to the reduction in spin diffusion length observed by ourselves and others.

Exchange bias is commonly manifested as the hysteresis-loop shift observed when a ferromagnet is in contact with an antiferromagnet. Here, we report observations of exchange bias with unusual features of a ferromagnet in contact with a spin glass, demonstrating that this is a phenomenon of greater generality. The easily measured properties of the ferromagnet allow access to the internal magnetic degrees of freedom of the glass to which they are coupled. Our results show that a Co/CuMn bilayer system exhibits all the rich phenomena of coercivity enhancement, bias-field shifts and training effects associated with a conventional ferromagnet/antiferromagnet system. Nevertheless, striking differences arise, such as an orientation reversal of the bias field in a small temperature range just below the blocking temperature. We argue that all features can be understood within the context of a random-field model for long-ranged oscillatory Ruderman-Kittel-Kasuya-Yosida (RKKY) coupled spins.

International audience; A phenomenological approach is presented for the giant magnetoresistance (GMR) of magnetic multilayers oriented in the CPP mode (current perpendicular to the plane of the layers). New results are found for the dependence of the GMR on the number of repeats of the multilayer.

We report on the findings of a study conducted on Co/Cu giant magnetoresistance multilayers with a wide variation in the number of bilayer repeats at temperatures from 4.2 to 300 K. The total film thickness varied from 77 to 2700 A. When the data were treated within a Fuchs–Sondheimer approximation for finite size effects at any one temperature an excellent agreement with the observed variation in giant magnetoresistance ratio was found. Over ~50 bilayers were required before the full giant magnetoresistance is observed. Unexpectedly, the length scale on which the GMR rises was found to be the same at all temperatures measured. This leads to unphysical temperature dependences in the Fuchs–Sondheimer mean free paths. Subsequent structural characterization showed that this length scale is related to the vertical coherence length of the grains within the sample. The same length scale was found for other related multilayer systems grown in the same chamber. The breakdown of the Fuchs–Sondheimer model is discussed in terms of modern theories of phase-coherent transport in the giant magnetoresistance: full giant magnetoresistance is only obtained when electrons can coherently sample a large enough number of layers.

We present transient ellipsometry measurements made upon Au, Cu, Ag, Ni, Pd, Ti, Zr, and Hf thin films under identical experimental conditions. Using an elliptically polarized pump beam, we have simultaneously observed the specular inverse Faraday effect (SIFE) and specular optical Kerr effect (SOKE) contributions to the optical polarization response, in order to extract the real and imaginary parts of the non-vanishing components of the third-order optical susceptibility tensor. The signal magnitudes and the extracted tensor components show a systematic variation that reflects the underlying band structure of the different metals. A time delay observed between the SIFE and SOKE signals is interpreted in terms of lifetimes for the transient polarization that may be of comparable magnitude to the pulse width. This suggests that formulae derived in the continuous wave limit may not be applicable in the description of ultrafast nonlinearities in metallic samples. The implications of our results for different areas of applied optics are discussed.

Fe/Au(001) multilayers have been grown by molecular beam epitaxy using a new technique for the preparation of Au buffer layers on MgO(001) substrates. Scanning tunnelling microscopy and grazing incidence x-ray scattering reveal that the Au buffers have extremely low interface roughness and a long in-plane correlation length. X-ray scattering and diffraction show that multilayers grown on the new buffers, although having little difference in their crystalline structure, have much smoother interfaces than those grown on previous buffers. Compared with earlier multilayers with rougher interfaces, the giant magnetoresistance has increased by 20% due to more complete antiferromagnetic exchange coupling. Measurement of the saturation conductivity shows that the number of specular reflections occurring during electron channelling remains unchanged even though the interfacial roughness is reduced by a factor of six. It is postulated that the cause of this is either that spin-dependent scattering at the interface is being driven solely by the difference in the band structure, not by diffuse scatter at the interfaces, or that the wavelength of the transport electrons is such that the change in the roughness has no effect upon the specular scattering.

We have constructed an apparatus to evaporate ultra thin films of Copper Phthalocyanine (CuPc) on ultra thin gold epitaxial film grown by MBE, which thicknesses are in the range of 30 A. We present STM pictures of CuPc on ultra thin gold which show a columnar alignment perpendicular to the substrate and preliminary in-situ resistance measurement of the hybrid device. Using theoretical modeling on a simpler system, we calculate the electronic properties and estimate the expected changes in the quantum and temperature dependent resistance of the thin gold layer, when it is covered by a discotic overlayer.

A new class of hybrid organic/inorganic nanostructures, comprising self-organised discotic liquid crystal layers deposited on ultrathin metal films, has been investigated both experimentally and theoretically. Calculations show that the periodic self-organised molecular layer gives rise to a new, hybrid electronic bandstructure, resulting in modulation of the metal film conductivity. In situ conductivity measurements during deposition of such self-organised layers confirm that the metal film conductivity is altered. Theoretical modeling also shows that the AC conductivity should show structure related to the carrier trapping and one-dimensional transport features of the self-organised layer.

A novel hybrid organic/inorganic heterostructure is described, comprising a highly ordered discotic liquid crystal layer, self-organised on an ultrathin Au film. The electronic properties of the hybrid structure have been investigated using a range of theoretical techniques. The strong bonding of the molecules onto the metal surface, combined with the high degree of periodicity across the molecular layer, results in the formation of a hybrid metal/molecular band structure which has been calculated using a periodic Newns-Anderson model. The in-plane conductivity was calculated using the Kubo-Greenwood formula, and electron trapping in the molecular layer was considered by use of a diffusion model. The effect of trapping is apparent in the frequency domain conductivity. The sensitivity of the frequency domain response to variations in the trapping parameters was studied, and interpreted in terms of potential gas sensing functionality.

We present experimental and theoretical results for the propagation of 2D electrons through simple magnetic barriers. We show that it is not possible to use a ballistic picture to understand the barrier conductance even when the width of the barrier inthe directionof curren t propagationis small compared to the electronmeanfree path. ? 2002 Elsevier Science B.V. All rights reserved.

This work was supported by the Engineering and Physical Sciences Research Council through grants EP/K00512X/1, EP/K036408/1, EP/J01060X/1 and EP/I004483/1. Only three elements are ferromagnetic at room temperature: the transition metals iron, cobalt and nickel. The Stoner criterion explains why iron is ferromagnetic but manganese, for example, is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: according to this criterion, the product of the density of states and the exchange integral must be greater than unity for spontaneous spin ordering to emerge. Here we demonstrate that it is possible to alter the electronic states of non-ferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, to overcome the Stoner criterion and make them ferromagnetic at room temperature. This effect is achieved via interfaces between metallic thin films and C60 molecular layers. The emergent ferromagnetic state exists over several layers of the metal before being quenched at large sample thicknesses by the materiala € s bulk properties. Although the induced magnetization is easily measurable by magnetometry, low-energy muon spin spectroscopy provides insight into its distribution by studying the depolarization process of low-energy muons implanted in the sample. This technique indicates localized spin-ordered states at, and close to, the metal-molecule interface. Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer. This mechanism might allow for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors. Charge transfer at molecular interfaces may thus be used to control spin polarization or magnetization, with consequences for the design of devices for electronic, power or computing applications (see, for example, refs 6 and 7). Postprint

The contact between a molecule and a metallic electrode contributes to or even determines the characteristics of organic devices, such as their electronic properties. This is partly due to the charge transfer that takes place when two materials with different chemical potentials are put together. In the case of magnetic electrodes, the transfer can be accompanied by the transmission of a net spin polarization or spin doping. In nanocarbon systems, hybridization and spin doping can suppress the moment of a transition metal ferromagnet through the loss of majority spin electrons to the organic. Here, ${\mathrm{C}}_{60}$ is shown to become ferromagnetic as a result of spin doping from cobalt with an induced moment of 1.2 ${\ensuremath{\mu}}_{\mathrm{B}}$ per cage while suppressing the moment of the ferromagnet by up to 21%. Polarized neutron reflectivity and x-ray magnetic circular dichroism reveal the presence of an antiferromagnetic coupling of the interfacial layers of cobalt and ${\mathrm{C}}_{60}$, and weakly coupled induced magnetism propagating into the bulk organic. Thus, it is shown that the deposition of molecules with high electron affinity can be used to induce zero-voltage spin injection.

We report on the temperature dependence of the recently discovered spin Hall magnetoresistance in a yttrium iron garnet (YIG)/platinum (Pt) thin film. The YIG/Pt layers are an ideal choice as the combination of an insulating magnetic material and the high spin-orbit interaction in Pt gives a relatively large magnetoresistance and no electrical conduction occurs in the YIG. The temperature dependence of the magnetoresistance was measured between 1.4 K and 280 K from which the temperature dependence of the spin diffusion length in Pt has been extracted. We found that the best agreement between our data and the recently published [Chen, Phys. Rev. B 87, 144411 (2013)PRBMDO1098-012110.1103/ PhysRevB.87.144411] theory of the spin Hall magnetoresistance is given by an assumed Elliot-Yafet mechanism of spin relaxation with temperature-independent spin Hall angle and spin mixing conductance. The best estimate for the spin diffusion length returns values between 0.57 and 3.85 nm.

Time-resolved scanning Kerr microscopy (TRSKM) has been used to examine the effect of nonlocal spin transfer torque (NL-STT) within a two-terminal current perpendicular-to-plane nonlocal spin valve. A combination of Oersted field and NL-STT was used to excite and control the magnetization dynamics. By comparison with a macrospin model, the strength of the NL-STT was quantified and found to be comparable to that achieved by direct injection of spin-polarized current. The sensitivity of the magneto-optical probe to small out-of-plane deflections of the magnetization also allows the NL-STT to be quantified in quasi-dc measurements, greatly simplifying the investigation of STT phenomena.

We present transport and structural data from epitaxial (100) and (111) Au/Fe superlattices grown by molecular beam epitaxy. From their analysis, we conclude that an electron channeling mechanism, due to strong specular reflection of the minority spin carrier at the Au/Fe interfaces, is responsible for the high conductivity in the (100) superlattices.

The effects of four different kinds of impurity δ-layer inserted into the Co layer of Co/Cu multilayers were investigated. Two completely different behaviours were found. Very small fractions of a monolayer of Ta or Cr increased the remanence of our multilayers up to almost 100%, destroying completely the giant magnetoresistance (GMR). This does not happen when an even larger amount of Cu or Pd is embedded in the Co layer, good antiferromagnetic coupling is maintained. In this case the remanence of the structure only increases by 20% for roughly an order of magnitude higher quantity of material, and naturally this still gives good values of the GMR. The decrease of the GMR is entirely explained in terms of a loss of antiferromagnetic alignment due to weaker indirect exchange coupling over the Cu spacer—even though this spacer layer remains pure Cu. These effects have been explained in terms of the magnetic properties of the different elements used as impurity affecting the spin-dependent reflectivity for electrons of the Co/Cu interfaces.

International audience; We show how the transport of properties of electrons in disordered solids can be simulated taking into account atomic motion. The time-dependent Schrödinger equation and the molecular dynamics equation are solved in tandem for electronic diffusion and as a test of the methods we obtain metallic behaviour and variable range hopping in appropriate circumstances. We also describe a stable algorithm which introduces dissipation into the Schrödinger equation via imaginary components for the energy levels enabling electrons to be removed from a system as is necessary in device simulation. This is important because the often used leap frog method becomes unstable in this situation.