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Details of Grant 

EPSRC Reference: EP/P008550/1
Title: Optical detection of magnetisation dynamics induced by spin-orbit torques
Principal Investigator: Hicken, Professor R
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Brown University HGST University of Gothenburg
Department: Physics
Organisation: University of Exeter
Scheme: Standard Research
Starts: 27 February 2017 Ends: 31 December 2020 Value (£): 522,335
EPSRC Research Topic Classifications:
Magnetism/Magnetic Phenomena
EPSRC Industrial Sector Classifications:
Electronics Information Technologies
Related Grants:
Panel History:
Panel DatePanel NameOutcome
21 Jul 2016 EPSRC Physical Sciences Physics - July 2016 Announced
Summary on Grant Application Form
The modern world is completely dependent upon electronic devices that operate through the flow of charged particles called electrons i.e. electric current. However the electron also carries 'spin' angular momentum, and has an associated magnetic moment, like a tiny bar magnet. The aim of Spintronics is to use the spin of an electron to control its motion and how it interacts with magnetic materials. The most celebrated spintronic device is the 'spin-valve', a trilayer structure in which two ferromagnetic (FM) layers are separated by a non-magnetic spacer layer. The spin-valve is engineered so that the magnetic moment of one FM layer is fixed, while that of the other is free to align with an applied magnetic field, like a compass needle. As the relative orientation of the two magnetic moments varies, a large change in electrical resistance of the trilayer is observed. Since the resistance is easily measured, the spin-valve can act as a magnetic field sensor. In fact a spin-valve sensor is used to read back information in every hard disk that is sold today.



When current is passed between the fixed and free FM layers an inverse effect can be observed. The flow of electrons transfers angular momentum from one FM to the other, and, by Newton's 2nd Law, exerts a spin transfer torque (STT). This torque can act upon the magnetic moment of the free layer, causing it to change its orientation. The spin-valve can also be designed to have two stables states, with different electrical resistance, that can be used to store digital information. Arrays of such devices are used in magnetic random access memory (MRAM). Alternatively, in a spin transfer oscillator (STO), the free layer magnetization oscillates at microwave frequency when DC current is applied. Since the resistance also oscillates, microwave voltage oscillations are generated. The STO is unusual in that its frequency can be tuned through multiple octaves by varying the DC current. Multiple STOs can be defined at chip level, as circuit components, or in arrays for increased power output.

In recent years it has been realized and demonstrated that the spin-orbit interaction, a relativistic effect, may also be used to manipulate the electron spin. The spin can in turn be used to generate a STT, which has been termed spin-orbit torque (SOT) in light of its origin. SOTs are generated by the spin Hall effect (SHE) and the Rashba effect, but the separation of these torques from each other, and from the torque generated by the flow of charge (Oersted torque), is still being debated. The optimization of SOT for use in MRAM has attracted enormous interest because it removes the need to pass large electric currents through fragile insulating layers that conduct electricity by quantum mechanical tunneling.

In this project we will use time resolved scanning Kerr microscopy (TRSKM) to explore, understand and optimize SOTs in device structures of the highest quality supplied by HGST, Brown University and the University of Gothenburg, all of whom are leaders in their respective fields. Crucially we will modify our TRSKM so that a magnetic field can be applied with any orientation in 3 dimensional space, while high frequency electrical probes are connected to the device, and a focused optical probe is used to determine the instantaneous orientation of the magnetization vector. This internationally unique instrument will allow us to determine the SOTs from the static and dynamic response of the magnetization, rather than the electrical resistance, as different electrical stimuli are applied. Furthermore the sub-micron spatial resolution of TRSKM will allow us to separate different torques through their spatial variation, and understand how SOTs interact with dynamic magnetic modes in a confined geometry. Finally, we will use this same instrument to understand how SOTs induce magnetic precession in STOs and switching in candidate MRAM devices.

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