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Novel Graphene Nanodevices for Electronics and Magnetic Sensing- Vishal Panchal - National Physical Laboratory, in London Feb 28, 2013 from 02:30 PM to 04:00 PM Mott Seminar Room,
Graphene is a two-dimensional material comprised of only carbon atoms closely packed in a honeycomb crystal structure. Local layer thickness, substrate defects and environmental doping heavily influence electronic properties of graphene and need to be fully understood to make the material commercially viable. We present systematic and comprehensive studies of room temperature transport measurements and nanoscale functional mapping of micron-scale Hall devices fabricated out of nominally single-layer epitaxial graphene on 4H-SiC substrate. Transport measurements revealed the minimum detectable field of a typical 10-µm graphene sensor to be ~2.5 µT/√Hz, making them comparable with state-of-the-art semiconductor devices [1]. Surface potential measurements were used to study the graphene-electrode contact resistances and the effect of photochemical and substrate doping on the work function of 1-2 layer graphene [2]. The performance of graphene Hall sensors was tested by reliably detecting the stray magnetic field of a single 1-μm size magnetic particle [3] and magnetic force microscopy (MFM) probes [4]. Single particle detection is essential for counting magnetically labelled molecules in the biomedical industry, while the accurate determination of the MFM probes stray field is critical for quantitative MFM measurements. We demonstrate that epitaxial graphene Hall sensors exceed the performance of conventional semiconductor devices due to the robustness to larger biasing currents and increased coupling between the stray field and the active sensing layer of graphene. [1] V. Panchal, K. Cedergren, R. Yakimova, A. Tzalenchuk, S. Kubatkin, and O. Kazakova, “Small epitaxial graphene devices for magnetosensing applications,” Journal of Applied Physics, vol. 111, no. 7, p. 07E509, 2012. [2] V. Panchal, T. L. Burnett, R. Pearce, K. Cedergren, R. Yakimova, A. Tzalenchuk, and O. Kazakova, “Surface potential variations in epitaxial graphene devices investigated by Electrostatic Force Spectroscopy,” 2012 12th IEEE Conference on Nanotechnology (IEEE-NANO), pp. 1–5, 2012. [3] V. Panchal, D. Cox, R. Yakimova, and O. Kazakova, “Epitaxial Graphene Sensors for Detection of Small Magnetic Moments,” IEEE Transactions on Magnetics, Submitted, 2013. [4] V. Panchal, O. Iglesias-Freire, A. Lartsev, R. Yakimova, and A. Asenjo, “Magnetic scanning probe calibration using graphene Hall sensor,” IEEE Transactions on Magnetics, In Press, 2013.
"Understanding coherent oscillations of the electron in a SAW device". Mr. Aleksander Nikolic Feb 21, 2013 from 02:30 PM to 04:00 PM Mott Seminar Room,
The essence of the talk will be this. As this is work very much in progress no results will be reported; the primary focus will be on describing the experiment in question (performed by another Ph.D. student from the SP group), reporting some of its unusual/unexpected features, and the steps being to taken to simulate and explain those features (by me, using code written primarily by two more Ph.D. students from the SP group).
THERMAL ORDERING AND DEFECTS IN ARTIFICIAL MAGNETIC SQUARE ICE-Dr. Jason Morgan TFM Feb 14, 2013 from 02:30 PM to 04:00 PM Mott Seminar Room,
THERMAL ORDERING AND DEFECTS IN ARTIFICIAL MAGNETIC SQUARE ICE Artificial spin ices (ASIs) are 2D arrays of single domain nanomagnets, designed for the user-defined exploration of the physics of competing interactions and collective ordering [1-3]. A periodic lattice with strong local anisotropies captures the essence of geometrically frustrated materials such as pyrochlore spin ice and water ice [4], with Ising-like nanobar magnets converging at interlinked vertices. Furthermore, they are realisations of well known models in statistical mechanics. Crucially, the governing dipolar interactions can be tailored via nanopatterning and real-space observation of magnetic order is allowed via microscopy. Until recently, attention has largely focussed on the response of athermal systems to applied fields. For example, ac demagnetisation yields "icy" short-range correlated states however fails to access the true square ice ground state (GS) [1]. Furthermore, the generation and manipulation of magnetic "monopole" charge defects, analogous to ionic conduction defects in water ice [5], has also become a topic of intense interest. In this presentation I will discuss my previous work conducted at the University of Leeds, which reports the first experimental observations and subsequent studies of true thermal ordering in ASIs [2,3,6]. This is achieved via an one-shot early-fabrication-stage anneal process, which can allow for extensive GS ordering to be frozen into such systems. This picture is supported by the identification of a thermal distribution of magnetic excitations, within which evidence for charge-charge interactions can be identified. I will show how the strength of magnetic ordering can be controlled by the competing effects of dipolar coupling strength and quenched disorder, and parameterised using an effective temperature formalism. To close, future directions for this fascinating field will be discussed. References [1] R. F. Wang et al., Nature 439, 303 (2006); X. Ke et al., PRL 101, 037205 (2008) [2] J. P. Morgan, A. Stein, S. Langridge & C.H. Marrows, Nature Physics 7, 75 (2011) [3] Z. Budrikis, K. Livesey, J. P. Morgan, J. Akerman, A. Stein, S. Langridge, C. H. Marrows & R. L. Stamps, New Journal of Physics 14, 035014 (2012) [4] M. J. Harris et al., Phys. Rev. Lett. 79(13), 2554 (1997) [5] C. Castelnovo et al., Nature 451,42 (2008) [6] J. P. Morgan, J. Akerman, C. Phatak, A. Stein, S. Langridge & C. H. Marrows, Phys. Rev. B 87, 024405, (2013) Click here to Reply, Reply to all, or Forward
360º Domain Walls- Anibal Gonzalez-Oyarace, TFM Feb 07, 2013 from 02:30 PM to 04:00 PM Mott Seminar Room,
360º Domain Walls have been reported in the reversal processes of thin films as a result of pinning defects [1]. Recently they have also been observed during the reversal process of closed structures even without the presence of any apparent defects [2]. In terms of their application to technological devices, the 360º Domain Walls have been proposed to be used in the fabrication of as magnetic random access memory (MRAM) [3], spin wave generators [4], magnetic memory recording devices [5] and magnetic sensors [5]. In this talk I will introduce the main micromagnetic properties of a 360º Domain Walls and their role in the switching of nanorings driven by azimuthal fields. A short study on the resilience of this reversal mechanism under temperature will also be shown. Moreover, the results of a proposed magnetic memory recording devices based on 360º Domain Walls will be presented. Finally, initial results on the localized injection of 360º Domain Walls into stripes will be outlined. References: [1] P. Gaunt and C. K. Mylvaganam. Nucleation and pinning at 360◦ domain walls in SmCo_5 and related alloys. J. Appl. Phys., 48(6):2587–2590, 1977. [2] F. J. Casta no, C. A. Ross, C. Frandsen, and A. Eilez et al. Metastable states in magnetic nanorings. Phys. Rev. B, 67(18):184425, May 2003. [3]T. J. Hayward, J. Llandro, R. B. Balsod, J. A. C. Bland, D. Morecroft, F. J. Castano, and C. A. Ross. Switchingn behavior of individual pseudo-spin-valve ring structures. Phys. Rev. B, 74:134405, Oct 2006. [4] P. E. Roy, T. Trypiniotis, and C. H. W. Barnes. Micromagnetic simulations of spin-wave normal modes and the resonant field-driven magnetization dynamics of a 360◦ domain wall in a soft magnetic stripe. Phys. Rev. B, 82:134411, Oct 2010 [5] C. B. Muratov and V. V. Osipov. Bit storage by 360 domain walls in ferromagnetic nanorings. IEEE Trans. Magn., 45:3207–3209, 2009 [6] M. Diegel, R. Mattheis, and E. Halder. 360 ◦ domain wall investigation for sensor applications. IEEE Trans. Magn., vol. 40, no. 4,:2655–2657, 2004.
Spin gating electrical currents-Dr. Chiara Ciccareli, ME Group Dec 13, 2012 from 02:30 PM to 04:00 PM Mott Seminar,
In spin-orbit coupled magnetic materials the chemical potential depends on the orientation of the magnetisation. By making the gate of a field effect transistor magnetic, it is possible to tune the channel conductance not only electrically but also magnetically. Here, we demonstrate the spin-gating effect for an aluminium single electron transistor (SET) gated by GaMnAs. The conductance variation of the SET provides a direct probe of the magnetisation dependent change in the chemical potential of the magnetic gate with μeV resolution. In performing magnetic field sweeps in fixed directions we also observe the non-linearity of the chemical potential in GaMnAs. The control of a transistor conductance by the gate magnetisation demonstrates a new approach for constructing spin transistors: instead of spin-transport controlled by ordinary gates we spin-gate ordinary charge transport.
"Novel Spintronics effects based on the Spin-orbit Coupling: Spin-gating and spintronics with antiferromagnets'"-Dr. Joerg Wunderlich, Hitachi Lab Dec 06, 2012 from 02:30 PM to 04:00 PM Mott Seminar ,
Magnetic properties of EuO-Mr. Pedro Monterio, TFM Nov 29, 2012 from 02:30 PM to 04:00 PM Mott Seminar,
The ferromagnetic semiconductor EuO, with Curie temperature Tc = 69 K has long been considered a model system since the magnetic moment is highly localized and the exchange interactions are well approximated by the Heisenberg model. Renewed interest has developed because of the possibility of using it in thin-film form as a spin-filter in contact with Si, Ge, or GaAs. Spin injection from EuO into Si has been demonstrated with an extremely high polarization (above 90 %). Despite these highly desirable properties, the low Curie temperature of EuO currently keeps it from technological development in spintronics applications. In this talk I will discuss how the magnetic moment and Tc change with the film thickness. I will also explore the physics underlying the Tc enhancement of oxygen deficient EuO and La-doped EuO showing some insights from muon spin rotation and relaxation measurements.
Oxide interface; LaAlO3/SrTiO3, Dr. Kiyoung Lee, TFM Nov 22, 2012 from 02:30 PM to 04:30 PM Mott Seminar,
Since the first observation of high mobility electron gas at LaAlO3 (LAO) and SrTiO3 (STO) interface in 2004, the research on complex oxide heterostructures have been fertile for its potential electronic applications. It has also been diverging to other physical phenomena to electric field tunable superconductivity, metal-insulator transition, and coexistence of superconductivity and ferromagnetism. In this talk, I will introduce what we have recently observed in a LAO/STO heterostructure via persistent photoconductivity effect following a brief overview of this research area. We are able to stablise the high mobility two dimensional HOLE gas at the surface in coexistence with an electron gas at the interface. A series of Shubnikov-de Haas oscillations in various gate voltages allows to see the hole gas with a density of 3x10^11 cm^-2 and a mobility of 10,000 cm^2/Vs.
Prof. Mark Blamire, Materials Science Dep. Cambridge Nov 15, 2012 from 02:30 PM to 04:00 PM Mott Seminar,
Currents flowing in circuits containing ferromagnets transport spin as well as charge; this is the fundamental origin of effects such as giant magnetoresistance (GMR). In contrast, the superconducting state is created by the pairing of electrons with antiparallel spins; this means that a supercurrent transports charge but not spin. Until very recently this would have made the idea of superconducting spin electronics completely paradoxical. The possibility that the alternative triplet pairing state could be induced in ferromagnets had been theoretically predicted and convincing evidence for long-range supercurrents in ferromagnets has now been obtained by several groups, working on different materials systems. Because triplet supercurrents are expected to be spin polarised, form the starting point for a programme aiming to merge the exciting fields of superconductivity and spintronics. This talk describes the developments which have led to this point and the current research taking place within the Materials Department.
Design of an electronic circuit for controlling magneto-rheological fluids in shock absorbers-Angadjit Singh Nov 08, 2012 from 02:30 PM to 11:15 PM Mott Seminar,
I will be giving a talk on Magneto-Rheological fluids (MR) and their applications. MR fluids were first invented in 1940 but have started gaining increased interest recently. Smart fluids such as MR fluids have lately been used in the automotive industry e.g. as shock absorbers and in seismic dampers in earthquake prone areas. Audi also recently introduced their version of a shock absorber in their 2'nd generation Audi TT cars. I will be presenting how these smart fluids are fabricated in the laboratory and talk about their structural and magnetic properties as measured using Electron microscopy (SEM, TEM) and VSM magnetometry. MR fluids are dispersions of fine magnetically soft, multi-domain particles. On applying an external magnetic field on the fluid, properties such as the 'Yield stress' can change in milliseconds. This rapid change in behavior of the fluid can be exploited using an electronic feedback circuit mechanism, which I have built and will be discussing in detail along with many other auxiliary experiments.
Avenues of Metrology with Quantum Dots Mete Atature Jun 13, 2012 from 02:30 PM to 04:00 PM Mott Seminar,
Self-assembled semiconductor quantum dots display discrete electronic energy levels coupled by optical transitions and they are governed by spin-dependent optical selection rules. This opens an optical channel to control and detect a single spin in a quantum dot and significant progress was witnessed based on this property in the field of quantum information science. In the quest to suppress the detrimental effects of the environment, quantum metrology emerged as a recent topic of research with potential applications. Excitonic transitions of self-assembled quantum dots can depend strongly on the ambient electric and magnetic fields, as well as strain. We will discuss how this susceptibility can be utilized in return for electric field sensing with sensitivity better than 1 V/m/√Hz and we will continue on to other avenues of quantum metrology using self-assembled quantum dots.
Spin blockade and Kondo physics in carbon nanotube double quantum dots- Dr. Mark Buitelaar -Semiconductor Physics group Jun 07, 2012 from 02:30 PM to 04:00 PM Mott Seminar Room,
In this talk, I will discuss recent measurements in which we investigate spin blockade and Kondo physics in carbon nanotube double quantum dots. Spin blockade is observed in weakly coupled double quantum dots when electron transitions between the dots are forbidden by spin conservation. As such, this phenomenon is of considerable importance in spin-based quantum information processing schemes as a way to convert the spin degree of freedom to a much easier detectable charge state or current. Here I discuss results on spin blockade in carbon nanotube double quantum dots and coupling to impurity spins in the nanotube environment. The ability to control the tunnel couplings in the nanotube devices also allows us to investigate carbon nanotube double quantum dots which are much more strongly coupled to their leads. In this case, we observe pronounced Kondo features. Of particular interest here is the competition between Kondo correlations and exchange coupling between the spins, when both quantum dots contain an odd number of electrons. These effects have been studied as a function of tunnel coupling, temperature and magnetic field and will be discussed during the second part of the talk, as well as recent radio-frequency reflectometry measurements on the nanotube double quantum dots.
Frequency generation by a magnetic vortex-antivortex dipole in spin-polarized current- Prof. Stavros Komineas from the Dept. of Applied Mathematics, Univ. of Crete, Greece May 31, 2012 from 02:30 PM to 04:00 PM Mott Seminar Room, Mott Building,
A rotating vortex-antivortex dipole may be generated due to spin-polarized current flowing through a nano-aperture in a magnetic element. The vortex dipole dynamics is analyzed using the Landau-Lifshitz equation including a Slonczewski spin-torque term for in-plane spin-current polarization. The spin-torque acts to stabilize the vortex dipole at a definite vortex-antivortex separation. We establish that the vortex dipole is set in steady state rotational motion. An external in-plane magnetic field can be used to tune the frequency of rotation. We derive analytically a relation for the frequency of rotation and we find numerically steady-state rotating vortex-antivortex pairs in a more realistic model. The rotating pair under a spin-polarized current is an attractor of the motion, therefore it is expected to be a stable state.
Overview of two-dimensional electron gases based on the LaAlO3/SrTiO3 interface- Dr Thomas Fix -Device Materials Group May 24, 2012 from 02:15 PM to 04:00 PM Mott Seminar Room,
T. Fix, F. Schoofs, J. L. MacManus-Driscoll and M. G. Blamire. Department of Materials Science, University of Cambridge, Cambridge, United Kingdom The origin of the free charge layer which forms at the LaAlO3/SrTiO3 interface [1] is still uncertain. Still the impact of defects on the conduction properties can provide useful information to understand this system. The defects that we will consider here can be induced by the substrate steps, by the deposition parameters, or can be voluntarily introduced in the form of dopants. By varying the dopant distance from the interface, the doping concentration and the nature of the dopant in SrTiO3 at the LaAlO3/SrTiO3 interface we show the extreme sensitivity of the system and confirm that the underlying phenomenon accounting for the conduction is most likely an electronic reconstruction, with the majority of the carriers confined within 1 unit cell of the interface [2,3]. This talk will give an overview of all the work on LaAlO3/SrTiO3 in our group. [1] Ohtomo A., Hwang, H. Y., Nature 427, 423 (2004). [2] T. Fix, J.L. MacManus-Driscoll, M.G. Blamire, Appl. Phys. Lett. 94, 172101 (2009). [3] T. Fix, F. Schoofs, J.L. MacManus-Driscoll, and M.G. Blamire, Phys. Rev. Lett. 103, 166802 (2009).
Two-for-One in Solar Cells-Mr. Bruno Ehrler May 10, 2012 from 02:30 PM to 03:30 PM Mott Seminar Room,
Conventional solar cells are limited to a maximum achievable power conversion efficiency of about 34%. They are made from a single semiconductor material that determines the amount of light that is absorbed in the solar cell. The more light a solar cell absorbs, the less energy can be extracted per photon. This trade-off is called the Shockley-Queisser limit. We are working on a new type of hybrid organic/inorganic solar cell, pairing two materials with different absorption properties. The organic material has the property of converting one high-energy photon into two low-energy excited states, a process called singlet-exciton fission. Using this process it is possible to design solar cells in such a way that they are not limited by the Shockley-Queisser limit. We have made the first solar cell of that kind and have already reached efficiencies that are comparable with the best hybrid organic/inorganic solar cell efficiencies.
'Characteristics of mesoscopic ferromagnetic rings in magnetic fields and their potential use in phase contrast electron-microscopy'- Gerard Higgins May 02, 2012 from 02:30 PM to 04:00 PM Mott Seminar Room,
Flux-closed vortex states of mesoscopic rings can be used to introduce a phase difference between electrons passing through the ring and those passing outside it, by the Aharonov-Bohm effect. Thus they are candidates for use as phase plates in transmission electron-microscopy. The stability of the vortex state is of critical importance. Micromagnetic simulations have been carried out using OOMMF on rings with different shapes (circular and D-shaped), magnetic anisotropies (including grains), widths, diameters, thicknesses and cellsizes.
Magnetic ratchet for 3-dimensional data storage based on perpendicularly magnetized Pt/CoFeB- Dr Reinoud Lavrijsen Apr 26, 2012 from 03:00 PM to 04:00 PM Mott Seminar Room,
Magnetic ratchet for 3-dimensional data storage based on perpendicularly magnetized Pt/CoFeB The use of (perpendicularly magnetized) synthetic-antiferromagnets (AFM) formed by ferromagnetic (FM) layers coupled anti-ferromagnetically, is highly suitable for a wide field of applications due to the high control achieved in the properties of these artificial structures. Specifically, these synthetic-AFMs have a high potential to be used in spintronic devices [1] . A full understanding of the reversal of such structures under external fields and currents is therefore of high interest. In particular, the perpendicularly magnetized archetypal [[Pt/Co]X/Ru]N with X,N > 1 system has been studied extensively due to its excellent properties for data storage applications and fundamentally for the nano-scale phase-formations it exhibits [2,3]. In this presentation we will discuss our latest study of the tunability of the RKKY-interlayer exchange coupling (IEC) in Pt/CoFeB/Pt/Ru/Pt/CoFeB/Pt stacks with perpendicular magnetic anisotropy (PMA) [4]. By inserting a Pt layer between Ru and CoFeB, the PMA of the ultrathin CoFeB layers is stabilized and the IEC can be tuned. In particular, we show that the IEC versus Pt thickness exhibits a simple exponential decay with a decay length of 0.16 nm. This allows us to create multilayers (N>>1) using single CoFeB layers with PMA. By carefully tuning the IEC and CoFeB thickness we will show how a sharp magnetic kink soliton can be injected and propagated using a unidirectional ratchet scheme. This paves the way for a novel 3-dimensional shift register, where the data is stored in the 3rd dimension. This potentially holds the promise for a thousand-fold increase in data storage density. [1] B. G. Park, J. Wunderlich, X. Marti, V. Holy, Y. Kurosaku, M. Yamada, H. Yamamoto, A. Nishide, J. Hayakawa, H. Takahashi, A. B. Shick, and T. Jungwirth, Nature Mat. 10, 347 (2011). [2] O. Hellwig, A. Berger, K. J. B., and F. E. E., J. Magn. Magn. Mater. 319, 13 (2007). [3] O. Hellwig, T. Hauet, T. Thompon, E. Dobisz, J. D. Risner-Jamtgaard, D. Yaney, B. D. Terris, and E. E. Fullerton, Appl. Phys. Lett. 95, 232505 (2009). [4] R. Lavrijsen, A. Fernández-Pacheco, D. Petit, R. Mansell, J. H. Lee, and R. P. Cowburn, Appl. Phys. Lett., 100, 052411, (2012)
Synthesis and applications of magnetic nanoparticles for bio-recognition and point of care medical diagnostics- Prof. Adarsh Sandhu from the "Electronics-Inspired Interdisciplinary Research Institute (EIIRIS), Toyohashi University of Technology Mar 12, 2012 from 02:00 PM to 03:30 PM TCM Seminar,
Functionalized magnetic nanoparticles are important components in bio-recognition and medical diagnostics. Here, we present a review of our contribution to this interdisciplinary research field. We start by describing a simple one-step process for the synthesis of highly uniform ferrite nanoparticles (d = 20–200 nm) and their functionalization with amino acids via carboxyl groups. For real-world applications, we used admicellar polymerization to produce 200 nm diameter ‘FG beads’, consisting of several 40 nm diameter ferrite nanoparticles encapsulated in a co-polymer of styrene and glycidyl methacrylate for high throughput molecular screening. The highly dispersive FG beads were functionalized with an ethylene glycol diglycidyl ether spacer and used for affinity purification of methotrexate—an anti-cancer agent. We synthesized sub-100 nm diameter magnetic nanocapsules by exploiting the self-assembly of viral capsid protein pentamers, where single 8, 20, and 27 nm nanoparticles were encapsulated with VP1 pentamers for applications including MRI contrast agents. The FG beads are now commercially available for use in fully automated bio-screening systems. We also incorporated europium complexes inside a polymer matrix to produce 140 nm diameter fluorescent-ferrite beads (FF beads), which emit at 618 nm. These FF beads were used for immunofluorescent staining for diagnosis of cancer metastases to lymph nodes during cancer resection surgery by labeling tumor cell epidermal growth factor receptor (EGFRs), and for the detection of brain natriuretic peptide (BNP)—a hormone secreted in excess amounts by the heart when stressed—to a level of 2.0 pg ml−1 . We also describe our work on Hall biosensors made using InSb and GaAs/InGaAs/AlGaAs 2DEG heterostructures integrated with gold current strips to reduce measurement times. Our approach for the detection of sub-200 nm magnetic bead is also described: we exploit the magnetically induced capture of micrometer sized ‘probe beads’ by nanometer sized ‘target beads’, enabling the detection of small concentrations of beads as small as 8 nm in ‘pumpless’ microcapillary systems. Finally, we describe a ‘label-less homogeneous’ procedure referred to as ‘magneto-optical transmission (MT) sensing’, where the optical transmission of a solution containing rotating linear chains of magnetic nanobeads was used to detect biomolecules with pM-level sensitivity with a dynamic range of more than four orders of magnitude. Our research on the synthesis and applications of nanoparticles is particularly suitable for point of care diagnostics.
Surface Dynamics on Picosecond Timescales- Dr. Andrew Jardine Mar 08, 2012 11:55 AM
The dynamical behaviour of individual atoms and molecules on surfaces underpins a huge range of fundamental and technological processes. On the scale of individual atoms, events occur extremely quickly. For example, the characteristic period of vibration for an atom adsorbed onto a surface is about 1 ps. These 'atomic' scales mean is has traditionally been extremely difficult to follow the microscopic detail of surface transport. I will describe how new experimental techniques - in particular helium spin-echo - have made it possible to follow and understand a wide range of dynamical processes in this new and otherwise unstudied physical regime.
Gas Adsorption on Magnetic Surfaces-Shin-Liang Chin Jan 19, 2012 from 03:00 PM to 04:30 PM Mott Seminar Room,
This talk aims to give a brief introduction to the CASTEP software on how to use it and what it can do. I will also present some of my recent calculations on C and CO adsorption on the Co (110) surface using CASTEP. Structurally, both the C and CO cause the first two surface layers to expand whereas the second and third layers contract. When CO adsorbs on the 3-fold-hollow site, the CO adsorbate tilts at an angle of about 18o to minimize the energy. Both the C and CO adsorbate clearly suppress the surface magnetic moment. At 0.5 ML coverage, the C adatom suppresses up to 47% of the magnetic moment in the surface layer compared to a clean Co (110), whereas the CO adsorbate only suppresses up to 16%. For the 1.0 ML coverage case, both the C and CO adsorbates suppress almost equivalently well at 68% and 63% respectively. Comparing the density of states (DOS) before and after the adsorption reveals that the suppression of the magnetic moment is mainly due to the reduction in the spin asymmetry of the 3d-states of the Co (110) surface atoms. Finally, a correlation between the amount of charge transfer and the degree of suppression of the surface magnetic moment was also observed.
Quantum Dot Biosensors- Christian Cimorra (TFM) Dec 15, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
A novel sensing approach for the detection of analytes based on the photonic properties of Quantum Dots (QD) will be described with particular reference to medical applications. Photoluminescence (PL) from the QD nanocrystal lattice is contained and stabilised by capping agents, which can be designed to react to microenvironmental molecules thus conferring a quantifiable photo-physical effect. The functional agent of choice is the biomolecule Glutathione (GSH) due to its reducing potential and metal-ion complexation abilities. In addition, the relatively simple and facile synthesis methodology delivers controllable size-dependant QD architectures with tunnable emissions (480nm-410nm) offering multiplexing potential. The performance of a glucose sensing platform will be the model for the proof-of-concept.
Resistivity of Metal Oxides- Lakshay Jethi (TFM) Dec 08, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
Cobalt ferrite nanoparticles prepared by chemical co-precipitation route were synthesized and characterized using XRD, infra-red spectroscopy and electron paramagnetic resonance spectroscopy. These nanoparticles can be used for sensing ethanol vapours. The physical properties relevant to sensing aspects are investigated. The resistance, responsiveness, sensitivity, optimum working temperature and response time of cobalt ferrite nanoparticle based semiconductor ethanol vapour sensor are obtained from the response and recovery curves. They are practically the same with minor deviations in the studied temperature range. The results show that the largest resistance increase was observed in the 25ºC measurements. On varying the alcohol vapour concentration, the resistance of the sensor varies slightly in a particular temperature operating range. The mechanism for sensing ethanol will also be described.
Electronically Driven Major-Reversal with No Major Field-Dr Massimo Ghidini (Materials Sciences) Dec 01, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
The electrical reversal of magnetization is one of the key goals in the study of multiferroic and magnetoelectric materials, and was recently demonstrated in ferromagnetic films using strain1 or exchange bias2 from multiferroic single-crystal substrates. However, each reversal event required the reversal of an applied magnetic field, and so repeatable magnetic reversal under purely electrical control remains an outstanding goal. Here we perform microscopic investigations of commercially manufactured multilayer capacitors (MLCs) which serendipitously display strain-mediated magnetoelectric coupling between magnetostrictive Nielectrodes and piezoelectric BaTiO3 (BTO) dielectric layers. Magnetic force microscopy (MFM) reveals perpendicularly magnetized features in the electrode layers that exhibit electrically driven repeatable magnetization reversals with no applied magnetic field. Micromagnetic modelling supports our interpretation that this non-volatile magnetization reversal may be achieved via a dynamic process that is associated with a temporary reduction in perpendicular uniaxial magnetic anisotropy driven by strain from fast and reversible voltage-driven ferroelectric-domain switching. I will also report on the reversible control of perpendicular magnetic anisotropy in polycrystalline nickel films evaporated on single-crystal ferroelectric BTO substrates via thermal and electrical treatments. We have employed both MFM and photoemission electron microscopy with magnetic contrast from x-ray magnetic circular dichroism (XMCD-PEEM), to study the effects of thermally and electrically driven changes in the BTO substrate on the magnetic domain structure of the nickel films. I will show that the perpendicular component of the magnetization can be erased and reset by thermally cycling through the structural transitions of the substrate and more importantly, it can be reversibly electrically controlled at room temperature.
III-IV Semiconductors- Dr Ian Farrer (SP) Nov 24, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
MBE technology has played a crucial role in the development of nanostructures enabling the study of electron interactions at low temperatures. This talk will present an overview of Molecular Beam Epitaxy with particular reference to III-V materials in a research setting. The operation of a III-V growth system will be described with emphasis on the production of very high purity structures and the constraints that this imposes. The procedure for access to either existing wafer material or new sample requests will also be outlined.
Avenues of Metrology with Quantum Dots- Dr Mete Atatüre (AMOP) Nov 17, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
Self-assembled semiconductor quantum dots display discrete electronic energy levels coupled by optical transitions and they are governed by spin-dependent optical selection rules. This opens an optical channel to control and detect a single spin in a quantum dot and significant progress was witnessed based on this property in the field of quantum information science. In the quest to suppress the detrimental effects of the environment, quantum metrology emerged as a recent topic of research with potential applications. Excitonic transitions of self-assembled quantum dots can depend strongly on the ambient electric and magnetic fields, as well as strain. We will discuss how this susceptibility can be utilized in return for electric field sensing with sensitivity better than 1 V/m/√Hz and we will continue on to other avenues of quantum metrology using self-assembled quantum dots.
Tuneable interface exchange for optical magnetisation control- Dr Theo Trypiniotis (TFM) Nov 10, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
Beyond studies of spin dependent transport which have traditionally dominated the field of spintronics, an increasing number of investigations of more ‘exotic’ spin related phenomena are being reported in the last few years. In this talk I will present the extension of our work in spin transport, into the use of spin accumulation for the optical control of magnetisation. We can routinely use optical spin orientation in III -V semiconductors as a reliable spin source for spin transport investigations. We have recently successfully combined the above with ferromagnetic resonance (FMR) measurements on epitaxial ferromagnetic-metal/semiconductor interfaces. The optically controlled spin polarised carrier population at the interface affects the ferromagnet magnetisation via an interface exchange interaction the signature of which is measured in the FMR response. We have demonstrated an effective exchange field of the order of tens of Gauss at room temperature, which offers a new method for the optical control of magnetization.
The Spin Hall Effect- Dr Jörg Wunderlich (Hitachi Lab) Nov 03, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
We realized an all semiconductor spin Hall effect transistor by combining the Spin transistor concept proposed by Datta and Das with the (inverse) spin Hall effect (iSHE) detection. We first discuss the Spin Hall effect in a device where spins are optically generated in the depletion layer of a reverse biased pn-junction and injected into a 2-dimensional electron gas (2DEG). In the 2DEG, the spin-orbit interaction (SOI) can be modulated with electrostatic gates to control locally the spin-polarziation. This concept of spin-manipulation is tested by using the SHE. The SHE is also used to demonstrate a spin AND logic function in a 2D channel with two gates. Finally, we demonstrate the inverse SHE detection in a semiconductor combined with an electrical spin injection and manipulation, and to perform a quantitative analysis of the measured iSHE voltages. In our GaAs microchannel with an Fe injection electrode, the spin current in a lateral semiconductor channel is detected by the iSHE and the spin polarization is simultaneously measured by the non-local spin valve effect using an additional Fe electrode. The spins in the channel are manipulated via the Hanle spin precession induced by an applied magnetic eld and via a drift of electrons induced by an applied electrical bias. The output spin signal is suppressed or enhanced depending on the applied drift, rendering the device an electrically controlled spin amplifier.
Spin Injection into Germanium- Chen Shen (TFM) Oct 27, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
In this study, we generate spin polarised electrons in GaAs near an epitaxial Fe/GaAs interface using optical spin orientation. By applying high forward bias, we remove or invert the Schottky barrier. A spin filtering current was obtained as a function of bias for opposite magnetic fields. The results reveal a clear spin filtering in the absence of a tunnelling barrier and suggest a successful experimental demonstration of ballistic spin transport across an epitaxial Fe/GaAs interface. Using a simple transport model based on a transfer matrix approach within an effective mass approximation, we semi-quantitatively obtain the polarisation of the interfacial density of states by fitting our experimental data. By comparing with the theoretical results based on a tight-binding approach by Honda* et al.*[1], our experimental results suggest the interface is a mixture of Fe-As and Fe-Ga terminated structure.
Electrodeposition with Bitrex- Jos Cooper (TFM) Oct 20, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room,
We investigate the effects of denatonium benzoate (Bitrex) on the electrodeposition of copper films from a boric acid bath using scanning electron microscopy, cyclic voltammetry, electrochemical quartz microbalance (EQCM) measurements and corrosion studies. In the absence of Bitrex, pure copper films grown by this method are optically black owing to the appearance of complex surface nanostructures. The addition of Bitrex acts as a levelling agent preventing the formation of these nanostructures even for concentrations as low as 0.02mM producing a lustrous film with low surface roughness. Bitrex is also found to improve the corrosion resistance by up to a factor of 20 and increase the current efficiency by over a factor of two. Bitrex is hypothesised to act directly on the cathode, partially inhibiting the growth or lowering the deposition current.
MISM Conference Overview Magnetotactic Bacteria- Dr Adrian Ionescu (TFM) Oct 13, 2011 from 02:30 PM to 03:30 PM Mott Seminar Room ,
Dr Adrian Ionescu (TFM)