Wolfgang Raberg

Temperature Compensation in Silicon-Based Micro-Electromechanical Resonators

In this paper, we present passive temperature compensation by means of silicon dioxide. Using an oxide refilling technique it avoids gap distance reduction and therefore prevents degradation of electromechanical coupling and motional resistance of the micro-electromechanical resonator. Samples are fabricated and electrically characterized to demonstrate the feasibility of the process concept. A constant quality factor (Q) and only a slight increase of the series resistance value (Rm) are achieved, while the frequency inaccuracy due to temperature variation is reduced. ANSYS simulations are carried out to evaluate the potential of the technique, resulting in a remaining inaccuracy of less than 40ppm.

Controlled data storage for non-volatile memory cells embedded in nano magnetic logic

Among the beyond-CMOS technologies, perpendicular Nano Magnetic Logic (pNML) is a promising candidate due to its low power consumption, its non-volatility and its monolithic 3D integrability, which makes it possible to integrate memory and logic into the same device by exploiting the interaction of bi-stable nanomagnets with perpendicular magnetic anisotropy. Logic computation and signal synchronization are achieved by focus ion beam irradiation and by pinning domain walls in magnetic notches. However, in realistic circuits, the information storage and their read-out are crucial issues, often ignored in the exploration of beyond-CMOS devices. In this paper we address these issues by experimentally demonstrating a pNML memory element, whose read and write operations can be controlled by two independent pulsed currents. Our results prove the correct behavior of the proposed structure that enables high density memory embedded in the logic plane of 3D-integrated pNML circuits.

Diffusion based degradation mechanisms in giant magnetoresistive spin valves

Spin valve systems based on the giant magnetoresistive effect as used, for example, in hard disks and automotive applications consist of several functional metallic thin film layers. We have identified by secondary ion mass spectrometry two main degradation mechanisms: one is related to oxygen diffusion through a protective cap layer and the other one is interdiffusion directly at the functional layers of the giant magnetoresistive stack. By choosing a suitable material as cap layer (TaN), the oxidation effect can be suppressed.

Influence of reference layer stability on the switching performance of sub-micron-sized magnetic tunnel junctions

Arrays of sub-micron-sized, magnetic tunnel junctions are patterned and the switching field (Hc) of the free layer (FL) and its distribution (σ) are measured by Kerr magnetometry. A correlation of Hc and σ with the magnetic stiffness (stability against external magnetic fields) of the reference system (RS) is observed. For simple pinned RSs the stiffness is varied by different materials (CoFe vs CoFeB) and its thicknesses, while for artificial antiferromagnetic pinned systems the thickness of the exchange coupling layer of Ruthenium is modified. It is shown that less stiffness causes a lower Hc and larger σ. This effect is assumed to be due to magnetic mirror charges induced in RS by the magnetic dipoles of the patterned FL elements. These mirror charges screen the dipole stray field of the FL resulting in lower effective shape anisotropy, which goes along with reduced switching fields. Micromagnetic simulations performed for different pinning strengths of a single layer RS clearly support this observatio...

Vortex magnetization state in a GMR spin-valve type field sensor

Micromagnetic sensors, viz., Hall elements, fluxgate, magnetoresistance and magnetoimpedance sensors, play a major role towards the miniaturization in the industrial society.

Thermally activated switching of small magnetic tunnel junctions

Small magnetic tunnel junctions were prepared and the magnetization reversal of their free layers was studied by applying high-speed reversal fields driven by pulsed currents. From the decay of the switching field with the number of pulses or the pulse length the thermal activation energy was deduced. Large cells (width>150 nm) showed a significant lower activation energy than expected from single-domain-theory, indicating non-uniform switching. Small cells (width /spl sim/ 90 nm and length 150 to 200 nm) could be well described by Stoner-Wohlfarth single domain model and single step switching with Arrhenius activation energy. For scaling of magnetic random access memories down to 100 nm this would indicate that a smaller free layer volume is to a large degree compensated by higher relative activation energy due to more uniform switching.