Scanning tunneling microscopy based point-contact measurements of nanoscale magnetic systems

Abstract

In this thesis, the method of scanning tunneling microscopy (STM) based pointcontacts is presented together with measurements on nanonscale magnetic systems. The method allows point-contacts to be positioned with nm lateral resolution and a subsequent injection of high current densities due to the small size of the contacts. This may further be used to investigate how shape- and size effects in nanoscale magnetic systems correlate with their transport properties. Specifically, STM based point-contacts are found to be well suited for the characterization of the local magnetoresistance and spin dependent transport effects in nanomagnets. In addition, the high spatial resolution allows point-contact spectroscopy investigations of nanostructured systems.

The work is divided into three parts: the investigation of the local magnetoresistance of spin-valve rings, the design and construction of a custom STM and the study of iron islands on graphite. Relevant backgrounds on magnetism in nanoscale systems, point-contact measurements and scanning tunneling microscopy have also been summarized.

In the first part, the local magnetoresistance of Co (20nm)/ Cu (5nm)/Fe19Ni81(2.5nm) spin-valve rings is investigated with point-contact measurements and micromagnetic simulations. Lateral variations in the magnetoresistance are found to correlate with the location of magnetic domain walls in the rings. Measurements with varying currents further indicate that current induced effects leads to offsets in the magnetic fields required for magnetic switching. The offsets are attributed to the spin-transfer torque effect in the magnetically soft Fe 19Ni81 layer and the Oersted field effect in the hard Co layer.

In the second part, the design and performance of the “compact concentric scanning tunneling microscope” (CCSTM) is described. The CCSTM is designed to operate in ultra high vacuum, at variable temperatures and in external magnetic fields. The custom electronics- and data acquisition setup provides well controlled point-contacts and a number of different measurement modes, including direct and indirect measurement of the resistance through I-V curves and lock-in amplifier methods. The modular design of the system easily allows new methods and procedures to be added as they are developed. The performance is tested with topographic images and point-contact measurements of graphite.

In the third part, thin films and islands of iron deposited on graphite are investigated with the CCSTM. A transition from large clusters at room temperature to monolayer thin films at 170 K are observed with STM images. A decreasing dynamic resistance (dV/dI) with increasing bias in point-contact measurements of iron islands and cluster on graphite are found to be consistent with the addition of electronic states on graphite due to adsorption of iron. Finally, an unusually large magnetoresistance, of the order of 10 - 40 % is observed in both the direct (V/I) and indirect (dV/dI) resistance of point-contacts to iron films with a nominal thickness of 8 ˚A of iron on graphite.