Martin Herman Siekman

Stripe domains in Fe-Zr-N nanocrystalline films

We report on the transition between a magnetic stripe domain structure and in-plane orientation of the spins, as a function of nitrogen content, for 500nm thick Fe-Zr-N films prepared by DC reactive sputtering on glass substrates. The saturation field decreases and the saturation magnetization increases with decreasing nitrogen content. For 4at% N, the magnetic behavior of the films becomes specific for a soft magnetic material. The magnetic spin distribution was investigated by transmission Mossbauer spectroscopy (TMS) to probe the entire sample and Magnetic Force Microscopy to image the surface.

Tailoring particle arrays by isotropic plasma etching: an approach towards percolated perpendicular media

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.

Ballistic hole emission microscopy on metal-semiconductor interfaces

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.

A read and write element for magnetic probe recording

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.

Wafer-scale fabrication of scanning thermal probes with integrated metal nanowire resistive elements for sensing and heating

Scanning Thermal Microscopy (SThM) and micro-thermal analysis allow the study of thermal phenomena at micro- and nanoscale. We present a novel scanning resistive probe aimed for thermal imaging and localized thermal analysis. The probe features an AFM cantilever with a sharp pyramidal tip. Metal nanowires are integrated at the inner edges of the pyramidal tip forming an electrical cross-junction at the apex. The nanometer-sized cross-junction, addressable through microelectrodes, can be utilized both as a local temperature sensor and a heater. We have fabricated a first prototype of the probe with a 150 μm long, 36 μm wide and 0.5 μm thick silicon nitride cantilever. Platinum nanowires, 300 nm wide and 100 nm thick, are successfully integrated using a wafer-scale fabrication process based on corner lithography. We have experimentally characterized electrical and thermal properties of the probe demonstrating its proper functioning.

Nanoscale magnetic hysteresis of Ni80Fe20/Au/Co trilayers using ballistic electron magnetic microscopy

Ballistic electron magnetic microscopy is used to study spin-dependent hot-electron transport and local magnetic switching of ferromagnetic thin films grown on a Au/Si(100) collector. For Ni80Fe20 films, the collector current is a factor of 2 larger than for Co, consistent with the shorter hot-electron attenuation length of Co. For Ni80Fe20/Au/Co spin valves, the collector current is reduced by a factor of 5 when the relative magnetization of the ferromagnetic layers changes from parallel to antiparallel. By sweeping the applied magnetic field, we obtain nanoscale hysteresis loops, where the hot electrons are collected from an area of about 10 nm.

A novel electrostatically actuated AFM probe for vibroflexural mode operation

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.

Displacement Sensing by Field Emission with Nanometer Resolution

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.

Pyramidal nanowire tip for atomic force microscopy and thermal imaging

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.

Track edges in metal-evaporated tape and thin metal-particle tape

MFM images of tracks written in ME and MP tape have been obtained. The analysis of the images concentrated on the track edges. A track written with signals of 0.5 μm wavelength overwrites a part of a track written with a wavelength of 1 μm. The sharpness of the edges was derived from MFM results. It can be seen that the MP sample shows smaller changes in sharpness of the edge with an increasing write current than ME tape. In ME tape, the region between the λ=0.5 μm and the λ=1 μm parts of the track is much wider than the original λ=1 μm edge.