We present a simple design for a very-low temperature STM for the investigation of mesoscopic superconductors. The nonmagnetic microscope operates in a conventional MOTIVATIONUp to now only a small number of scanning tunnelling microscopes (STM) world-wide are operating at temperatures below 1 K, in high magnetic fields, and achieve high energy resolution E < 1 mV. Some of them combine UHV and very low temperatures 1-5 while other designs operate in conventional cryostats. 6-8 The STMs for non-UHV conditions are mostly specialized instruments for investigating heavy fermion superconductors 9 or the spatial dependence of the superconducting proximity effect. [10][11][12] However, a compact STM that combines very low temperatures, operation in magnetic field, and very high energy resolution with robustness, versatility and easy handling is still lacking. Our new setup that is described in the present article fulfills these requirements. It is very small, matching with most commercial low-temperature facilities, does not require elaborate vibration damping and uses only the very modest amount of seven cables. Furthermore, it is already designed for lower temperatures than presented here, is in principle UHV compatible and can be combined with more complex positioning systems. The special physical project, for which purpose our STM was designed, is to investigate the 525 0022-2291/07/0500-0525/0
The deposition of Co/Pd multilayers onto self-assembled spherical particles provides a system with unique magnetic properties. The magnetic caps have high perpendicular magnetic anisotropy, are single-domain, and strongly exchange decoupled, but in electrical contact with each other, thus enabling magnetotransport measurements. By applying an external magnetic field, the caps can be switched individually. We systematically studied the magnetoresistance on a two-dimensional cap array consisting of Co/Pd multilayers deposited on particles with a diameter of 200 nm. In the vicinity of the coercive field, a hysteretic resistance peak occurs. It can be explained with the random magnetization configuration of the magnetic caps leading to an increased spin-dependent scattering of the conduction electrons. The underlying mechanism might be comparable to the one causing giant magnetoresistance in granular alloys. For temperatures above 77 K, additional resistivity contributions with high saturation fields are observed, which are tentatively explained by the decreasing size of magnetically ordered parts of the caps with increasing temperature, resulting finally in superparamagnetic behavior in the contact area between neighboring caps.
Electrical contacts of the width of only one atom can be realized by the break‐junction technique. The conductance decreases stepwise due to structural reconfigurations when tearing a nano‐bridge in the few‐atom range. Transport is described by an ensemble of channels with possibly quite high transmission probabilities. For a single break‐junction the last one‐atom contact consists of a material‐specific channel ensemble, determined by the chemical valance as verified for quite a number of metals. d‐electrons in half‐metals and spin‐effects in magnetic materials will complicate this simple model. Break‐junctions also provide ideal contacts to investigate transport through freely suspended clusters or molecules like DNA.
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