Holger Stillrich

Magnetic anisotropy and the cone state in Co/Pt multilayer films

The magnetic anisotropy of Co/Pt multilayers is investigated. The perpendicular uniaxial anisotropy is discussed in second order approximation under a variation in Co and Pt layer thicknesses. The evolution of anisotropy constants is shown in the phase diagram of first and second order anisotropy constants. A thickness driven spin reorientation via the canted phase is observed for a single Co layer as well as for Co/Pt multilayer films.

Soft x-ray holographic microscopy

We present a new x-ray microscopy technique based on Fourier transform holography (FTH), where the sample is separate from the optics part of the setup. The sample can be shifted with respect to the holography optics, thus large-scale or randomly distributed objects become accessible. As this extends FTH into a true microscopy technique, we call it x-ray holographic microscopy (XHM). FTH allows nanoscale imaging without the need for nanometer-size beams. Simple Fourier transform yields an unambiguous image reconstruction. We demonstrate XHM by studying the magnetic domain evolution of a Co/Pt multilayer film as function of locally varied iron overlayer thickness.

Magnetic soft x-ray holography study of focused ion beam-patterned Co/Pt multilayers

We report on Fourier transform holography (FTH) experiments on nanostructured Co/Pt multilayer films with 40 nm spatial imaging resolution. The films have been nanostructured by means of focused ion beam (FIB) milling. Applying the ion beam through the supporting membrane with controlled and homogeneous dosing allows for higher resolution magnetic structuring of the ion-sensitive film compared to direct FIB patterning. Nanostructured samples with magnetic stripes exposed to different ion doses and magnetic arrays with 200 nm lattice constant were successfully prepared and imaged by FTH. We present image-processing routines for artifact-free image reconstruction. With this, we could investigate the FIB-induced anisotropy modulation and the perpendicular domain structure in the nanostructured samples, showing how to control the domain size and configuration by applying the appropriate ion dose either homogeneously or concentrated in single spots.

Overcoming the field-of-view restrictions in soft x-ray holographic imaging

We present a new concept for imaging by soft x-ray holography. Microscopylike imaging capabilities were achieved by the separation of mask and sample. The use of two independent silicon nitride membranes, one for the field-of-view-defining mask and the reference beam, and the other for the sample, allows to image different areas on the sample. The movement of the field-of-view across the sample is realized by a piezomotor-driven sample stage that permits relative and stable positioning with nm-precision. We demonstrate the capabilities of the x-ray holographic microscopy (XHM) technique by showing images with 60 nm spatial resolution of an artificially structured 100 nm thick gold film with a lateral size of 19 × 4 μm2.

Magneto-optic properties of electron cyclotron resonance ion beam sputtered and magnetron sputtered Co/Pt multilayers

An investigation of the magneto-optic properties of single Co films sandwiched by Pt films is presented. The films are grown via electron cyclotron resonance ion beam sputtering and magnetron sputtering. Kerr rotation and ellipticity are measured for the multilayers, varying Pt cap, Co, as well as Pt buffer layer thickness. The set of data allows the separation of optic and magneto-optic contributions to the Kerr signals. The magneto-optic constants (Voigt constants) are determined and compared with literature values. The striking result is the strong difference of the magneto-optic constants for ultrathin and thick Co films. The analysis of the data reveals that for ultrathin films, the Kerr signals have contributions that come presumably from the interfaces, indicative of polarized Pt.

Nanostructured Ferromagnetic Systems for the Fabrication of Short-Period Magnetic Superlattices

A new method to fabricate arrays of ferromagnetic nanostructures is presented which is based on copying the morphology of self-assembled organic layers via ion milling into ferromagnetic Co/Pt multilayers. The self-assembly of diblock copolymer micelles is used. A very flexible tuning of the magnetic properties is possible via the variation of the multilayer composition. The impact of the growth method on the magnetic properties of the multilayer is described and the spin reorientation in Co/Pt discussed. It is demonstrated that arrays of ferromagnetic and superparamagnetic particles can be fabricated with particle sizes < 20 nm. The proposed method gives direct access to the tailoring of magnetic properties of nanosized objects.

Shallow HEMTs For Lateral Magnetic Superlattices

We study two‐dimensional electron gases subjected to modulated potentials and magnetic fields. With mask techniques it is possible to prepare lateral magnetic superlattices with periods below 100 nm. For a sufficiently strong modulation of the magnetic field at the location of the 2DEG at such small periods, optimized heterostructures with extremely shallow 2DEGs are required. Here, we report on the development of shallow HEMTs containing 2DEGs less than 20 nm below the surface and compatible deposition techniques for the magnetic layers.