M. H. Siekman

Universiteit Twente, Enschede, Overijssel, Netherlands

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Publications (30)78.18 Total impact

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    ABSTRACT: Systems featuring large magnetoresistance (MR) at room temperature and in small magnetic fields are attractive owing to their potential for applications in magnetic field sensing and data storage. Usually, the magnetic properties of materials are exploited to achieve large MR effects. Here, we report on an exceptionally large (>2000%), room-temperature, small-field (few mT) MR effect in one-dimensional, nonmagnetic systems formed by molecular wires embedded in a zeolite host crystal. This ultrahigh MR effect is ascribed to spin blockade in one-dimensional electron transport. Its generic nature offers very good perspectives to exploit the effect in a wide range of low-dimensional systems.
    Science 07/2013; · 31.20 Impact Factor
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    ABSTRACT: We present the design, fabrication and characterization of an electrostatically actuated AFM probe. The probe consists of two cantilevers, a relatively long, stiff cantilever on which end a smaller and softer cantilever with a sharp tip is incorporated. The deflection of the small cantilever is controlled by an electrostatic gap-closing actuator, which is monolithically integrated on the cantilevers. A combination of a small and fast cantilever with an integrated electrostatic actuator allows for a high frequency operation and high speed AFM imaging. We have designed and fabricated a probe with the large cantilever having a stiffness of over 400 N/m and small cantilevers with a stiffness of up to 3.2 N/m and a resonance frequency up to 643 kHz. We have confirmed a proper operation of the probe using high-speed imaging (8 scan lines per second) in electrostatically driven tapping mode.
    Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS & EUROSENSORS XXVII), 2013 Transducers & Eurosensors XXVII: The 17th International Conference on; 01/2013
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    ABSTRACT: Cantilever arrays are employed to increase the throughput of imaging and manipulation at the nanoscale. We present a fabrication process to construct cantilever arrays with nanotips that show a uniform tip-sample distance. Such uniformity is crucial, because in many applications the cantilevers do not feature individual tip-sample spacing control. Uniform cantilever arrays lead to very similar tip-sample interaction within an array, enable non-contact modes for arrays and give better control over the load force in contact modes. The developed process flow uses a single mask to define both tips and cantilevers. An additional mask is required for the back side etch. The tips are self-aligned in the convex corner at the free end of each cantilever. Although we use standard optical contact lithography, we show that the convex corner can be sharpened to a nanometre scale radius by an isotropic underetch step. The process is robust and wafer-scale. The resonance frequencies of the cantilevers within an array are shown to be highly uniform with a relative standard error of 0.26% or lower. The tip-sample distance within an array of up to ten cantilevers is measured to have a standard error around 10 nm. An imaging demonstration using the AFM shows that all cantilevers in the array have a sharp tip with a radius below 10 nm. The process flow for the cantilever arrays finds application in probe-based nanolithography, probe-based data storage, nanomanufacturing and parallel scanning probe microscopy.
    Nanotechnology 03/2012; 23(13):135301. · 3.84 Impact Factor
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    ABSTRACT: We present a novel 3D nanowire pyramid as scanning microscopy probe for thermal imaging and atomic force microscopy. This probe is fabricated by standard micromachining and conventional optical contact lithography. The probe features an AFM-type cantilever with a sharp pyramidal tip composed of four freestanding silicon nitride nanowires with a diameter of 60 nm. The nanowires, which are made of silicon nitride coated by metal, form an electrical cross junction at the apex of the tip, addressable through the electrodes integrated on the cantilever. The cross junction on the tip apex can be utilized to produce heat and detect local temperature changes. Electrical and thermal properties of the probe were experimentally determined. The temperature changes in the nanowires due to Joule heating can be sensed by measuring the resistance of the nanowires. We employed the scanning probe in an atomic force microscope.
    01/2012;
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    ABSTRACT: We investigated structural, electronic, and magnetic properties of Co adsorbed on highly oriented pyrolytic graphite (HOPG). Distribution and atomic sites of 3 d transition-metal Co nanoislands and adatoms on HOPG were experimentally investigated by scanning tunneling microscopy with atomic resolution. In the very low thickness regime (≤0.6 Å), a strong nucleation mechanism and a preferred Co nanoisland diameter of ˜3.4 nm have been observed. Co adatoms were found to preferentially occupy β sites of the HOPG surface graphene layer and the atoms aggregated by further occupation of either α or overbond sites. This is in contrast to predictions based on density functional theory, which indicates that the hollow sites are the most energetically stable sites for Co adsorption. The presence of surface hydrocarbon contamination on graphite might be one possible cause of the observed active nucleation and stabilized nanoisland diameter of Co. The formation of Co carbide was evidenced by x-ray absorption spectroscopy. More importantly, the Co magnetic spin moment at the interface of Fe-capped ferromagnetic Co nanostructures and graphite, as determined by x-ray magnetic circular dichroism and sum-rule analysis, was found to be only 63% of the bulk value, implying a magnetically defective spin contact for carbon spintronics applications.
    Physical review. B, Condensed matter 08/2011; · 3.77 Impact Factor
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    ABSTRACT: A successful approach to drastically reduce or even completely eliminate friction and wear in scanning force microscopy is the use of electrostatic modulation of the normal force acting on the tip-sample contact. In this paper we have devised, fabricated and experimentally characterized a novel electrostatically actuated AFM probe. The probe consists of a flexible cantilever that has an electrostatic circular plate actuator with a built-in sharp tip monolithically integrated at its free end. This unique probe configuration will allow for the vibro-flexural mode operation in which vibration of the tip relative to the cantilever is generated and controlled by the integrated plate actuator, while the tip-sample interaction is resolved by deflection of the cantilever. We envision that this new operation mode will result in an efficient electrostatic force modulation, which in the end will enable us to control friction and wear during AFM imaging.
    Micro Electro Mechanical Systems (MEMS), 2011 IEEE 24th International Conference on; 02/2011
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    ABSTRACT: We present an optical technique to detect cantilever deflection of an array using Fraunhofer diffraction patterns. Application areas include probe-based data storage. Intensity profiles of different cantilever arrays are captured on a CCD camera and compared with our model. These measurements are in excellent agreement with the Fraunhofer theory, less than 3% deviation is found. Each cantilever can either be deflected by a fixed amount or undeflected. Based on noise measurements on our setup and intensity patterns simulations, we predict that this method allows the measurement of 1nm deflections in an array of six cantilevers with an SNR of 35dB.
    Logical Methods in Computer Science - LMCS. 01/2011;
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    ABSTRACT: In this paper, the scanning tunneling microscopy, scanning tunneling spectroscopy and X-ray absorption spectroscopy of cobalt adatoms and nanoislands were studied on a highly oriented pyrolytic graphite. Local electronic structure were observed by STS.
    Vacuum Electron Sources Conference and Nanocarbon (IVESC), 2010 8th International; 11/2010
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    ABSTRACT: We present a process for batch fabrication of a novel scanning microscopy probe for thermal and magnetic imaging using standard micromachining and conventional optical contact lithography. The probe features an AFM-type cantilever with a sharp pyramidal tip composed of four freestanding silicon nitride nanowires coated by conductive material. The nanowires form an electrical cross junction at the apex of the tip, addressable through the electrodes integrated on the cantilever. The cross junction on the tip apex can be utilized to produce heat and detect local temperature changes or to serve as a miniaturized Hall magnetometer enabling, in principle, thermal and magnetic imaging by scanning the probe tip over a surface. We have successfully fabricated a first probe prototype with a nanowire tip composed of 140 nm thick and 11 μ m long silicon nitride wires metallized by 6 nm titan and 30 nm gold layers. We have experimentally characterized electrical and thermal properties of the probe demonstrating its proper functioning. ©2010 IEEE.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2010;
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    ABSTRACT: Scanning rates of the atomic force microscope (AFM) could be significantly increased by integrating the force sensing probe with microelectromechanical systems (MEMS). We present a micromachining method for batch fabrication of in-plane AFM probes that consist of an ultra-sharp silicon nitride tip on a single crystal silicon cantilever. Our fabrication method is fully compatible with the silicon-on-insulator (SOI) micromachining allowing a straightforward monolithic integration of the AFM probes with high-aspect-ratio monocrystalline silicon MEMS. Scanning probes with a sharp tip having diameter of less then 10 nm are successfully realized and tested in a commercial AFM set-up demonstrating feasibility and the large innovation potential of this method.
    01/2010;
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    ABSTRACT: Plasma etching of densely packed arrays of polystyrene particles leads to arrays of spherical nanostructures with adjustable diameters while keeping the periodicity fixed. A linear dependence between diameter of the particles and etching time was observed for particles down to sizes of sub-50 nm. Subsequent deposition of Co/Pt multilayers with perpendicular magnetic anisotropy onto these patterns leads to an exchange-decoupled, single-domain magnetic nanostructure array surrounded by a continuous magnetic film. The magnetic reversal characteristic of the film-particle system is dominated by domain nucleation and domain wall pinning at the particle locations, creating a percolated perpendicular media system.
    Nanotechnology 04/2009; 20(10):105304. · 3.84 Impact Factor
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    ABSTRACT: This paper presents a technique to determine the Young’s modulus and residual stress of thin films using a simple micromachined silicon cantilever as the test structure. An analytical relation was developed based on the shift in resonance frequency caused by the addition of a thin film on the cantilever. FEM simulations were performed which confirmed the validity of assumptions made in our analytical model. Resonance frequency measurements both before and after the deposition of the thin film improve the accuracy of the results. Experiments were performed on PZT deposited by pulsed laser deposition and on evaporated Co80Ni20 thin films.
    Journal of Statistical Planning and Inference - J STATIST PLAN INFER. 01/2009;
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    ABSTRACT: The possibility of magnetic probe recording into a continuous perpendicular medium is discussed. By applying a current from the tip to the medium, a very localized area can be heated and bits as small as 80 nm in diameter could be written. This value is close to the calculated minimum diameter of reversed cylindrical domains in our perpendicular medium. A current can be injected directly from the tip to the medium by means of a current source, or one can use capacitive currents. We prefer the first method, since the current, and therefore the heating process, can be controlled more precisely. The energy required to write a bit is in the order of 1 nJ. Calculations show that most of the heat is dissipated at the tip end. Demagnetizing fields of the surrounding material play an important role and are so strong that bits can be written without applying an external field. By decreasing the film thickness, the demagnetizing fields are reduced, and selective overwriting of previously written bits could be demonstrated.
    Journal of Physics D Applied Physics 07/2008; 41(15):155008. · 2.53 Impact Factor
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    ABSTRACT: Field emission is used as a displacement sensing method, exploiting the exponential relation between field emission current and electrode gap. Atomic force microscopy (AFM) probes have been used as field emission source to measure I/V characteristics which were found to correspond well to theory. The field emission sensor was operated in a more linear regime by using feedback on the position of the probe in order to maintain a constant current. The sensitivity of the sensor for displacement was found to be 0.26 V/nm at a range of ~100 nm. From the experimental data, typical parameters for the Fowler-Nordheim equation were deduced and used to model the sensor performance. The measurements confirm that field emission can be applied to sense the distance between a probe tip and sample with <20 nm resolution.
    Solid-State Sensors, Actuators and Microsystems Conference, 2007. TRANSDUCERS 2007. International; 07/2007
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    ABSTRACT: The thermal dependence and distribution of the switching fields of arrays of magnetic Co50Ni50/Pt nanodots has been studied. These dots, with a diameter of 90nm, are arranged in a hexagonal pattern with a periodicity of 300nm. Field-dependent magnetic force microscopy was used to measure the switching field distribution of the array, which was found to range from 80 to 192kA/m, a value which is confirmed by vibrating sample magnetometer measurements. Additionally, the temperature dependence on the collective behaviour of the switching fields of the array has been investigated. The energy barrier at zero field was estimated to have a value in between 1.8×10−19 and 2.1×10−19J. Combining this value with the effective anisotropy determined by torque measurements, the switching volume can be estimated to lie in between 1.2×103 and 1.4×103nm3.
    Microsystem Technologies 12/2006; 13(2):177-180. · 0.83 Impact Factor
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    ABSTRACT: A hybrid structure of a ferromagnetic Co/Au/Ni81Fe19 trilayer on p-type silicon is used to probe the excitation of electron-hole pairs in a ferromagnet during inelastic decay of hot electrons and the subsequent spin-dependent transport of the excited holes into the valence band of the p-type Si collector. The hole current is remarkably sensitive to magnetic fields, with a magnetocurrent of up to 250% and, unexpectedly, with a positive sign. We determine effective attenuation lengths and their magnetic field and bias voltage dependence.
    Applied Physics Letters 01/2006; · 3.79 Impact Factor
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    ABSTRACT: The transport of hot holes across metal-semiconductor interfaces is studied using ballistic hole emission microscopy. From the tip of a scanning tunneling microscope nonequilibrium holes are injected into a thin metallic overlayer on a p-type Si semiconductor, inducing a current of holes into the Si valence band. We have studied hole transport across interfaces between p-type Si and different metals (Au, Cu, and Al). It is found that the magnitude of the transmitted hole current depends strongly on the type of metal, the Schottky barrier height, and the energy distribution of the holes. In addition, we show that a significant yet smaller hole current can be induced in the reverse case where the tip is used to inject hot electrons, generating holes during inelastic decay in the metal overlayer. The results are compared to recent results on spin-dependent hole transmission in ferromagnet/p-type semiconductor structures.
    IEEE Transactions on Magnetics 11/2005; · 1.42 Impact Factor
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    ABSTRACT: A technique to study nanoscale spin transport of holes is presented: ballistic hole magnetic microscopy. The tip of a scanning tunneling microscope is used to inject hot electrons into a ferromagnetic heterostructure, where inelastic decay creates a distribution of electron-hole pairs. Spin-dependent transmission of the excited hot holes into an underlying p-type semiconductor collector induces a hole current in the valence band of the semiconductor, with magnetocurrent values up to 180%. The spin-filtering of holes is used to obtain local hysteresis loops and magnetic imaging with spatial resolution better than 30 nm.
    Applied Physics Letters 02/2005; 86(8):082502-082502-3. · 3.79 Impact Factor
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    ABSTRACT: Spin-dependent transport of nonequilibrium holes in ferromagnetic thin films and trilayers is investigated using ballistic hole magnetic microscopy. For Co, the hole attenuation length is short and increases from 6 to 10 A in the energy range 0.8 to 2 eV. The hole transmission of a Ni(81)Fe(19)/Au/Co trilayer is clearly spin dependent, resulting in a surprisingly large current change by a factor of 2.3 in a magnetic field. The energy and spin dependence of the hole transmission cannot be explained by the phase space available for inelastic decay of the hot holes.
    Physical Review Letters 02/2005; 94(2):027204. · 7.73 Impact Factor
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    ABSTRACT: We present our results on the development of magnetic sensors for application in magnetic probe recording. Successful writing experiments on a magnetic medium with perpendicular anisotropy show that magnetic domains of 130 nm can be reversed in a heat-assisted process. For reading purposes we propose a magnetoresistive sensor. The optimization of the shape of the sensor was performed using micromagnetic simulations with the requirement that the sensor has to be capable of both read and write operations. At this stage, the experimental realization of the sensor was carried out at a wafer-base level. The fabrication technique consists of a combination of optical lithography and focused ion beam etching.
    Journal of Physics D Applied Physics 01/2005; 38(3):363. · 2.53 Impact Factor