R. Rick

Multiple reference Fourier transform holography with soft x rays

The authors demonstrate multiple reference source Fourier transform holography with soft x rays. This technique extends the detection limit of high resolution lensless imaging by introducing spatial multiplexing to coherent x-ray scattering. In this way, image quality is improved without increasing the radiation exposure to the sample. This technique is especially relevant for recording static images of radiation sensitive samples and for studying spatial dynamics with pulsed light sources. Applying their technique in the weak illumination limit they image a nanoscale test object by detecting ∼2500 photons. The observed enhancement in the signal-to-noise ratio of the image follows the square root of the number of reference sources.

Origin of magnetic switching field distribution in bit patterned media based on pre-patterned substrates

Using a combination of synchrotron radiation based magnetic imaging and high-resolution transmission electron microscopy we reveal systematic correlations between the magnetic switching field and the internal nanoscale structure of individual islands in bit patterned media fabricated by Co/Pd-multilayer deposition onto pre-patterned substrates. We find that misaligned grains at the island periphery are a common feature independent of the island switching field, while irregular island shapes and misaligned grains specifically extending into the center of an island are systematically correlated with a reduced island reversal field.

Extended field of view soft x-ray Fourier transform holography: toward imaging ultrafast evolution in a single shot

Panoramic full-field imaging is demonstrated by applying spatial multiplexing to Fourier transform holography. Multiple object and reference waves extend the effective field of view for lensless imaging without compromising the spatial resolution. In this way, local regions of interest distributed throughout a sample can be simultaneously imaged with high spatial resolution. A method is proposed for capturing multiple ultrafast images of a sample with a single x-ray pulse.

Steplike versus continuous domain propagation in Co/Pd multilayer films

We investigate the microscopic reversal behavior in perpendicular-anisotropy magnetic thin films using an integrated mask-sample design that allows probing a nanoscale sample region with soft x-rays. Local hysteresis loops and spectroholography images are obtained from the transmitted signal exploiting x-ray magnetic circular dichroism. Our data provide direct evidence of microscopic spin-flip avalanches, such as responsible for Barkhausen noise. In comparison with macroscopic magnetometry measurements we find evidence for the sputter pressure dependent introduction of local defects that prevent a continuous domain wall motion but are not strong enough to introduce the appearance of microscopic return point memory.

Magnetic design evolution in perpendicular magnetic recording media as revealed by resonant small angle x-ray scattering

We analyze the magnetic design for different generations of perpendicular magnetic recording (PMR) media using resonant soft x-ray small angle x-ray scattering. This technique allows us to simultaneously extract in a single experiment the key structural and magnetic parameters, i.e., lateral structural grain and magnetic cluster sizes as well as their distributions. We find that earlier PMR media generations relied on an initial reduction in the magnetic cluster size down to the grain level of the high anisotropy granular base layer, while very recent media designs introduce more exchange decoupling also within the softer laterally continuous cap layer. We highlight that this recent development allows optimizing magnetic cluster size and magnetic cluster size distribution within the composite media system for maximum achievable area density, while keeping the structural grain size roughly constant.

Optimal signal-to-noise ratios for soft x-ray lensless imaging.

We propose and demonstrate a method to gauge and optimize the signal-to-noise ratios (SNRs) in lensless imaging using partially coherent sources. Through spatial filtering we tuned the coherence width of an incoherent soft x-ray undulator source, and we deduce that there exists an optimal spatial filter setting for imaging micrometer-sized objects, while high-resolution imaging is best executed without spatial filtering. Our SNR analysis, given spatial coherence, allows for an estimation of the required exposure time at synchrotron sources and pulse fluence at x-ray laser sources.

Phase retrieval in x-ray lensless holography by reference beam tuning

We show the ability to determine the relative phase between the object and a reference scatterer by tuning the overall intensity and phase of the reference wave. The proposed reference-guided phase retrieval algorithm uses the relative phase as a constraint to iteratively reconstruct the object and the reference simultaneously, and thus does not require precisely defined reference structures. The algorithm also features rapid and reliable convergence and overcomes the uniqueness problem. The method is demonstrated by a soft-x-ray coherent imaging experiment that utilizes a large micrometer-sized reference structure that can be turned on and off, yielding an object image with resolution close to the reconstruction pixel size of 21 nm.