O. Hellwig

Lensless imaging of magnetic nanostructures by X-ray spectro-holography

Our knowledge of the structure of matter is largely based on X-ray diffraction studies of periodic structures and the successful transformation (inversion) of the diffraction patterns into real-space atomic maps. But the determination of non-periodic nanoscale structures by X-rays is much more difficult. Inversion of the measured diffuse X-ray intensity patterns suffers from the intrinsic loss of phase information, and direct imaging methods are limited in resolution by the available X-ray optics. Here we demonstrate a versatile technique for imaging nanostructures, based on the use of resonantly tuned soft X-rays for scattering contrast and the direct Fourier inversion of a holographically formed interference pattern. Our implementation places the sample behind a lithographically manufactured mask with a micrometre-sized sample aperture and a nanometre-sized hole that defines a reference beam. As an example, we have used the resonant X-ray magnetic circular dichroism effect to image the random magnetic domain structure in a Co/Pt multilayer film with a spatial resolution of 50 nm. Our technique, which is a form of Fourier transform holography, is transferable to a wide variety of specimens, appears scalable to diffraction-limited resolution, and is well suited for ultrafast single-shot imaging with coherent X-ray free-electron laser sources.

Bit-Patterned Magnetic Recording: Theory, Media Fabrication, and Recording Performance

Bit-patterned media (BPM) for magnetic recording provides a route to thermally stable data recording at >1 Tb/in2 and circumvents many of the challenges associated with extending conventional granular media technology. Instead of recording a bit on an ensemble of random grains, BPM comprises a well-ordered array of lithographically patterned isolated magnetic islands, each of which stores 1 bit. Fabrication of BPM is viewed as the greatest challenge for its commercialization. In this paper, we describe a BPM fabrication method that combines rotary-stage e-beam lithography, directed self-assembly of block copolymers, self-aligned double patterning, nanoimprint lithography, and ion milling to generate BPM based on CoCrPt alloy materials at densities up to 1.6 Td/in2. This combination of novel fabrication technologies achieves feature sizes of <;10 nm, which is significantly smaller than what conventional nanofabrication methods used in semiconductor manufacturing can achieve. In contrast to earlier work that used hexagonal arrays of round islands, our latest approach creates BPM with rectangular bit cells, which are advantageous for the integration of BPM with existing hard disk drive technology. The advantages of rectangular bits are analyzed from a theoretical and modeling point of view, and system integration requirements, such as provision of servo patterns, implementation of write synchronization, and providing for a stable head-disk interface, are addressed in the context of experimental results. Optimization of magnetic alloy materials for thermal stability, writeability, and tight switching field distribution is discussed, and a new method for growing BPM islands from a specially patterned underlayer-referred to as templated growth-is presented. New recording results at 1.6 Td/in2 (roughly equivalent to 1.3 Tb/in2) demonstrate a raw error rate <;10-2, which is consistent with the recording system requirements of modern hard drives. Extendibility of BPM to higher densities and its eventual combination with energy-assisted recording are explored.

Separating dipolar broadening from the intrinsic switching field distribution in perpendicular patterned media

A critical requirement for bit patterned media applications is the control and minimization of the switching field distribution (SFD). Here, we use the ΔH(M,ΔM) method to separate dipolar interactions due to neighbor islands from the intrinsic SFD by measuring a series of partial reversal curves of perpendicular anisotropy Co∕Pd based multilayer films deposited onto prepatterned Si substrates. For a 100-nm-period island array the dipolar broadening contributes 22% to the observed SFD. For a 45-nm-period array this value increases to 31%. These results highlight the importance of quantifying long-range dipolar interactions for determining the intrinsic SFD of patterned media.

Bit patterned media based on block copolymer directed assembly with narrow magnetic switching field distribution

Electron-beam (E-beam) directed assembly, which combines the long-range phase and placement registration of e-beam lithography with the sharp dot size and spacing uniformity of block copolymer self assembly, is considered highly promising for fabricating templates that meet the tight magnetic specifications required for write synchronization in bit patterned media magnetic recording systems. In our study, we show that this approach also yields a narrower magnetic switching field distribution (SFD) than e-beam patterning or block copolymer self-assembly alone. We demonstrate that the pattern uniformity, i.e., island diameter and placement distributions are also important for achieving tight magnetic SFDs.

Coercivity tuning in Co/Pd multilayer based bit patterned media

In order to adjust the reversal field of high anisotropy [Co(2.8 A)/Pd(9 A)]8 bit patterned media (BPM), one may increase the individual Co thickness to change the overall anisotropy or alternatively combine the high anisotropy multilayer with a lower anisotropy material, thus creating a heterogeneous laminated system. In the current study, we find that [Co/Pd]8−N[Co/Ni]N hard-soft laminated BPM allows tuning the coercivity while maintaining a narrow normalized switching field distribution (σSFD/HC), whereas homogeneous structures with increased individual Co thickness show a more pronounced increase in σSFD/HC. Possible reasons for the different behavior, such as changes in strain and texture, are discussed.

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.

Magnetic dot arrays with multiple storage layers

An approach to increasing the data storage density of magnetic recording was investigated wherein dot arrays are combined with multiple magnetic storage layers. The latter consists of two magnetically decoupled perpendicular Co∕Pd multilayer stacks. As a result of the difference in the coercivity between the two stacks, the orientation of the remanent magnetization of each stack can be set independent of the orientation of the other layer. Therefore, each dot allows storing four different magnetization states, which give rise to four different readback signal levels. Thus, the investigated media structure allows doubling the storage density in magnetic recording applications. It was found that even for thick spacer layers a weak magnetostatic coupling of the storage layers is present preferring ferromagnetic alignment of the storage layers.

Disorder-induced microscopic magnetic memory

Using coherent x-ray speckle metrology, we have measured the influence of disorder on major loop return point memory (RPM) and complementary point memory (CPM) for a series of perpendicular anisotropy Co/Pt multilayer films. In the low disorder limit, the domain structures show no memory with field cycling--no RPM and no CPM. With increasing disorder, we observe the onset and the saturation of both the RPM and the CPM. These results provide the first direct ensemble-sensitive experimental study of the effects of varying disorder on microscopic magnetic memory and are compared against the predictions of existing theories.

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.

Role of boron on grain sizes and magnetic correlation lengths in recording media as determined by soft x-ray scattering

We have measured the chemical grain sizes and magnetic correlation lengths in CoCr-based magnetic recording media films using resonant soft x-ray small-angle scattering. We find that the addition of boron, while leading to slightly smaller physical grains, dramatically reduces the magnetic correlation length. These results show that B additions effectively act to suppress intergranular magnetic exchange via segregation to the grain boundaries.