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978-3-8439-0032-4, Reihe Physik
Heat assisted spin-transfer torque manipulation on the nanoscale using a spin-polarized scanning tunneling microscope
110 Seiten, Dissertation Universität Hamburg (2011), Softcover, A5
Spin-polarized scanning tunneling microscopy (SP-STM) is a powerful tool to image in real space on the atomic scale, and to manipulate atoms and molecules mechanically on a surface. Within the framework of this Ph.D. thesis, SP-STM is demonstrated to be further capable of controlled magnetization manipulation by the local injection of an elevated tunnel current, i.e. heat assisted spin-transfer torque manipulation.
The experiments presented are performed on Fe/W(110) monolayer nanoislands, a system exhibiting uniaxial anisotropy. As a first step, the intrinsic thermal induced switching behavior of several individual nanoislands is investigated as a function of the temperature. The energy barrier and the Arrhenius prefactor for the magnetization reversal are determined and analyzed as function of the island size and shape. The reversal is found to occur via domain wall nucleation and propagation through the nanostructure.
Elevated spin-polarized tunnel currents are utilized to manipulate the thermally activated magnetization reversal. The local current injection from the magnetic probe tip leads to a distinct modification of the intrinsic switching behavior. The contributions of spin torque and Joule heating are identified and quantified. Spatially resolved measurements to analyze the modified switching behavior as function of the lateral position of current injection reveal the role of the Oersted field.
Further, heat assisted spin-torque magnetization switching of individual, quasi-stable nanostructures is demonstrated. The effect of high spin-polarized current pulses on static magnetization is investigated at fixed bias polarity. Experiments with varying pulse parameters reveal that the spin torque depends on the current polarization. Spin-polarized current pulses and ramps at alternating bias polarity are utilized to reliably switch the magnetization direction of a nanoisland back and forth. The evaluation of the switching efficiency as function of the pulse parameters allows for the discrimination and quantification of spin torque and Joule heating. Finally, critical currents for magnetization reversal are determined by the application of triangular current sweeps at different sweep rates. The analysis allows for a comparison of spin torque and Joule heating found by the different manipulation procedures. The results of this work open the pathway to a wide range of new experiments and manipulation possibilities on the local scale.