Domain wall spintronics in novel magnetic nanostructures

Research output: ThesisDoctoral Thesis

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Magnetic domain walls (DWs) could form the basis of potential novel magnetic based memory, logic, and Lab-On-a-Chip devices, with a great potential to increase performance and enable new processes. Being DW detection and manipulation the main obstacle to overcome to achieve integration into new devices. The aim of this thesis is to investigate the manipulation and tracking of DWs through electrical measurements, and study the effect of localized magnetic moments on DW dynamics.
Study of DW dynamics inside of in-plane magnetization devices is performed using L-shaped magnetic nanostructures. The L-shaped devices are used as a DW trap to study DW tracking using magnetoresistance (MR) measurements. It is shown, through magnetic force microscopy (MFM) imaging, that the L-shaped nanostructures of certain dimensions geometrically constrain the magnetization to four specific remanent states (i.e. referring to the magnetization at the corner of the L-shape: head-to-head DW, tail-to-tail DW, tail-to-head and head-to-tail magnetization), thus allowing the use of L-shaped devices as DW traps. Due to these simple magnetization states and using anisotropic magnetoresistance effect (AMR), which links magnetization with electrical resistance, we demonstrate that it is possible to track the magnetization and identify the magnetic state of the devices by performing MR measurements.
To ease detecting the effect of external influences on the DW dynamics, variations of the design of the L-shaped DW traps are tested. MR measurements, in combination with in situ MFM and micromagnetic simulations, demonstrate that square corner L-shaped nanostructures with circular disks at the end of the nanowires show a larger difference between DW pinning/depinning fields, and have a more symmetric behaviour when magnetic field is applied at different angles.
Magnetic bead (MB) detection experiments are performed by placing a single superparamagnetic bead near the DW pinning position on top of the L-shaped nanostructure. The placement is done using micromanipulation. The results demonstrate detectable influence of the MB for L-shaped devices with widths below 200 nm. Additionally, simulations demonstrate existence of two DW depinning mechanisms, which appear at different field orientations. In the first depinning mechanism, the DW at the corner remains pinned and is annihilated by another moving DW, hence, no influence of the MB is detected. In the second
depinning mechanism, the original DW depins by moving from the corner and, hence, it is affected by the MB.
To complement the detection experiments, atomic force microscopy (AFM) probes modified with a MB attached are used. Magnetic scanning gate microscopy (mSGM) is used to move the MB around the L-shaped nanostructure at different heights, and measure the sensing volume of the L-shaped devices.
Planar Hall effect (PHE) measurements, perform on hybrid Py/Au junctions demonstrate larger changes in resistance than the analogous AMR-based magnetization tracking techniques. Moreover, by performing mSGM in the PHE geometry, we demonstrate that PHE type of measurements can also be used to detect single MB, with a sensing volume similar that of the L-shaped nanostructures.
Study of DW dynamics in out-of-plane magnetization nanostructures is carried out on CoFeB nanostructures. We study the anomalous Hall effect (AHE) and the anomalous Nernst effect (ANE) and how these two effects can be used to track magnetization in nanostructures and potentially be implemented to detect MBs. Additionally, the experiments with CoFeB devices measuring the AHE and ANE, in combination with differential phase contrast imaging, allow to measure the stray field of the MB used for measuring the sensing volume in in-plane devices.
MFM imaging complements MR measurements when studying magnetization evolution. However, when studying DW dynamics, probe-sample interaction has to be carefully analysed. In the last part of this thesis we study custom-made MFM probes with nanostructures lithographically defined onto the probe’s tip and compare them with standard MFM probes. Electron holography experiments in combination with in situ MFM imaging demonstrate that the magnetization states of the custom-made probes can be controllably switched into two high magnetic moment states, and two low moment states, both exhibiting high switching fields. This enables the use of custom-made probes to study samples that exhibit appreciable probe/sample interaction, and therefore require a probe with high coercivity and low moment.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Antonov, Vladimir, Supervisor
Thesis sponsors
Award date1 Feb 2018
Publication statusPublished - 2018


  • nanomagnetism
  • MFM
  • Magnetic bead
  • Biomedical Engineering
  • Sensor
  • domain wall
  • magneoresistance
  • SPIN

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