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Current Induced Magnetization Dynamics in Nanostructures
Uhlíř, Vojtěch ; Thiaville, André (oponent) ; Ravelosona, Dafiné (oponent) ; Šikola, Tomáš (vedoucí práce)
This thesis deals with the study of current-induced magnetization dynamics and domain wall (DW) motion in NiFe/Cu/Co nanowires, induced by the so-called spin-transfer torque effect. Prior to this work, transport measurements had proven that in this trilayer system, DWs in NiFe can be moved with relatively low current densities, suggesting a particularly high spin-torque efficiency. The aim of this study has been to use photoemission electron microscopy combined with x-ray magnetic circular dichroism at synchrotron radiation sources to observe directly the magnetic configurations in the trilayers and their evolution during and after the application of nanosecond current pulses. An important step of the work has been to optimize the growth of the NiFe/Cu/Co layers, in the view of increasing interface quality and minimize interlayer coupling. The process of nanowire patterning by e-beam lithography has also been optimized. Two kinds of measurements have been carried out: i) quasi static measurements, where the domain configuration is observed before and after the application of current pulses and ii) dynamic measurements, where the magnetic configuration has been observed during the application of current pulses. The first measurements have allowed us to study the statistical behaviour of DWs under the application of current pulses: on one hand, the domain wall velocities reach extremely high values for relatively low current densities (up to 600 m/s for 5x10^11 A/m^2). On the other hand, DW motion over distances larger than 2-3 microns is strongly hindered by pinning. Time-resolved measurements during the current pulses, carried out for the first time by our team, have allowed us to demonstrate that the NiFe magnetization is strongly tilted in the direction transverse to the nanowire direction, due to the presence of a transverse Oersted field. This effect might contribute to the enhancement of DW velocities in the NiFe layers.
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Current Induced Magnetization Dynamics in Nanostructures
Uhlíř, Vojtěch ; Thiaville, André (oponent) ; Ravelosona, Dafiné (oponent) ; Šikola, Tomáš (vedoucí práce)
This thesis deals with the study of current-induced magnetization dynamics and domain wall (DW) motion in NiFe/Cu/Co nanowires, induced by the so-called spin-transfer torque effect. Prior to this work, transport measurements had proven that in this trilayer system, DWs in NiFe can be moved with relatively low current densities, suggesting a particularly high spin-torque efficiency. The aim of this study has been to use photoemission electron microscopy combined with x-ray magnetic circular dichroism at synchrotron radiation sources to observe directly the magnetic configurations in the trilayers and their evolution during and after the application of nanosecond current pulses. An important step of the work has been to optimize the growth of the NiFe/Cu/Co layers, in the view of increasing interface quality and minimize interlayer coupling. The process of nanowire patterning by e-beam lithography has also been optimized. Two kinds of measurements have been carried out: i) quasi static measurements, where the domain configuration is observed before and after the application of current pulses and ii) dynamic measurements, where the magnetic configuration has been observed during the application of current pulses. The first measurements have allowed us to study the statistical behaviour of DWs under the application of current pulses: on one hand, the domain wall velocities reach extremely high values for relatively low current densities (up to 600 m/s for 5x10^11 A/m^2). On the other hand, DW motion over distances larger than 2-3 microns is strongly hindered by pinning. Time-resolved measurements during the current pulses, carried out for the first time by our team, have allowed us to demonstrate that the NiFe magnetization is strongly tilted in the direction transverse to the nanowire direction, due to the presence of a transverse Oersted field. This effect might contribute to the enhancement of DW velocities in the NiFe layers.
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