Time-resolved imaging with HERALDO.

The aim of this proposal is to develop a soft x-ray hologrphic imaging technique for time-resolved measurements of magnetisation dynamics in the GHz regime. PI has recently demonstrated the first application of Holography with Extended Reference by Autocorrelation Linear Differential Operator (HERALDO) that was used to image both 'in-plane' and 'out-of-plane' magnetised magnetic domains in submicroscopic magnetic elements with high spatial resolution. A vortex structure of permalloy thin film elements with diameters within 500 nm was imaged with a resolution of 30 nm. As a lensless technique it was shown to be immune to mechanical vabration and drift of the x-ray beam, as well as being element specific. Here we would like to extend the capability of this technique for studies of dynamic properties of vortex oscillators based on spin-torque transfer devices. In particular we would like to explore the gyration mechanisms of spin-torque vortex oscillators (STVO) based on nano-contact elements. The main objection here is to add picosecond temporal resolution to the technique in order to image magnetisation configuration of the NC STVO at different phases of the gyration cycle. The information gained will provide increased understanding of the operation of STVOs and lead to increased frequency and increased power output through increased amplitude of gyration and phase-locking of multiple STVO devices.

In order to implement and apply the time-resolved measurments by HERALDO we propose to carry out experiments on the beamline SEXTANTS, at the synchrotron SOLEIL. The PI has recently been awarded 18 shifts of beam-time on this beam-line, which is scheduled for operation in June 2014. SEXTANTS is a recently opened beamline offering a high brilliance polarised soft x-ray beam for measurements using elastic and non-elastic scattering. As part of the equipment suit SEXTANTS provides a dedicated set-up for imaging using Fourier Transform Holography (FTH). As a diffraction technique HERALDO require similar experimental arrangements as FTH, thus will require minimum modification. In the first stage of the experiments (June 2014) the aim is to test the technique in time-averaged mode, when the vortex configurations will be measured in a continuous mode. In the second stage of the experiments, in the next round of proposals, the intention is to apply the technique in fully time-resolved mode.

The proposed program is an excellent development opportunity for both teams participating in the experiments. PI and his collaborators will have access to one of the best experimental facilities that currently exist for measurements in soft x-ray holography. This will allow them to learn new techniques which will significantly improve the application of HERALDO. So far the method was applied on instrumentation that was not purposely designed, and the efficiency of measurements and their practicality was minimal due to the errors or time spent on optimisation of the processes involved. For example, a design of the beamstop or significance of the beamline shutter performance, may seem insignificant but can result in substantial difference in the image reconstruction, something that the team has learned from personal experience. Having access to the equipment that has not only been specifically designed and built for the holography experiments, but has already been demonstrated in a number of well publicised research works, will allow the team to advance the technique and eventually built a dedicated instrument at the UK synchrotron facilities at Diamond. Similarly, for the team at SOLEIL, this collaboration will bring the opportunity to learn a new imaging technique that can potentially increase the inflow of users. The development of HERALDO as a high resolution time-resolved imaging technique will greatly benefit scientists working in the area of magnetisation dynamics. HREALDO offers superior spatial resolution that is not accessible to other x-ray microscopy tools (e.g. STXM, XPEEM) due to the nature of the synchrotron beamline environment (e.g. beam drift, vibration, x-ray aberration). As a lens-free technique, X-ray holography is less sensitive to such effects. In addition, it offers a wider range of measurement conditions. In contrast with XPEEM, experiments may easily be performed with an applied magnetic field, while the x-rays are also able to access buried layers, enabling the measurement of devices with overlaid electrical contacts. As a wide field imaging technique, measurement times are greatly reduced compared to STXM. Access to this new microscopy tool will allow scientists to explore high frequency phenomena, that have previously been studied by spectroscopic techniques that either have no access to spatial information, or with spatial resolution bounded by the diffraction limit for visible radiation. This field of research is directly related to applications of nanotechnology within hard disk recording and magnetic random access memory (MRAM). The ability to switch magnetisation at high speed within a nanoscale region is one of the central problems for these technologies. In this project we will begin by applying holographic imaging to a system that exhibits large magnetic contrast and is easily perturbed by a modest RF magnetic field. However the technique can be extended for studies of large amplitude dynamics in other systems such as domain wall propagation and rotational/precessional switching processes, where the imaging procedure will be identical but additional work is required to generate a suitable stimulus e.g. larger pulsed field amplitude. HERALDO will extend the capability of SOLEIL and then eventually at Diamond and so become available to a large number of potential users.