C. Brombacher

Perpendicular FePt-based exchange-coupled composite media

Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design.

L10 FePt based exchange coupled composite bit patterned films

We demonstrate a 2.5-fold coercivity reduction in FePt based exchange coupled composite bit patterned media (ECC-BPM) by coupling a lower anisotropy Co/Pd–Co/Ni-multilayer system to the top of a high anisotropy FePt L10 film. Furthermore the tight switching field distribution (SFD) of the lower anisotropy system reduces the SFD of the ECC-BPM composite system compared to a single layer FePt film. The relative amount of switching field and SFD reduction in these ECC-BPM arrays agree with corresponding micromagnetic simulations.

FePtCu alloy thin films: Morphology, L10 chemical ordering, and perpendicular magnetic anisotropy

Rapid thermal annealing was applied to transform sputter-deposited Fe51Pt49/Cu bilayers into L10 chemically ordered ternary (Fe51Pt49)100−xCux alloys with (001) texture on amorphous SiO2/Si substrates. It was found that for thin film samples, which were processed at 600 °C for 30 s, the addition of Cu strongly favors the L10 ordering and (001) texture formation. Furthermore, it could be revealed by transmission electron microscopy and electron backscatter diffraction that the observed reduction of the ordering temperature with Cu content is accompanied by an increased amount of nucleation sites forming L10 ordered grains. The change of the structural properties with Cu content and annealing temperature is closely related to the magnetic properties. While an annealing temperature of 800 °C induces strong perpendicular magnetic anisotropy (PMA) in binary Fe51Pt49 films, the addition of Cu systematically reduces the PMA. However, due to the enhancement of both the A1-L10 phase transformation and the developm...

L10 FePtCu bit patterned media

Chemically ordered 5 nm-thick L1₀ FePtCu films with strong perpendicular magnetic anisotropy were post-patterned by nanoimprint lithography into a dot array over a 3 mm-wide circumferential band on a 3 inch Si wafer. The dots with a diameter of 30 nm and a center-to-center pitch of 60 nm appear as single domain and reveal an enhanced switching field as compared to the continuous film. We demonstrate successful recording on a single track using shingled writing with a conventional hard disk drive write/read head.

Tailoring particle arrays by isotropic plasma etching: an approach towards percolated perpendicular media

Plasma etching of densely packed arrays of polystyrene particles leads to arrays of spherical nanostructures with adjustable diameters while keeping the periodicity fixed. A linear dependence between diameter of the particles and etching time was observed for particles down to sizes of sub-50 nm. Subsequent deposition of Co/Pt multilayers with perpendicular magnetic anisotropy onto these patterns leads to an exchange-decoupled, single-domain magnetic nanostructure array surrounded by a continuous magnetic film. The magnetic reversal characteristic of the film-particle system is dominated by domain nucleation and domain wall pinning at the particle locations, creating a percolated perpendicular media system.

Nanopatterned CoPt alloys with perpendicular magnetic anisotropy

CoPt alloy films with perpendicular magnetic anisotropy were grown on SiO2 nanoparticle arrays with particle sizes as small as 10 nm. In order to induce perpendicular magnetic anisotropy in the CoPt film, a MgO seed layer was sputter deposited. Despite the fact that neighboring CoPt film caps are interconnected, individual caps appear as single domain and for most of them their magnetization orientation can be reversed individually. This behavior might be caused by domain wall nucleation and pinning preferentially at the rim of each cap. Thus, arrays of magnetic caps with defined pinning sites can be considered as a percolated perpendicular medium.

Contribution of the easy axis orientation, anisotropy distribution and dot size on the switching field distribution of bit patterned media

A magnetic nanostructure array was fabricated by post-patterning of a L10 ordered 5-nm-thick FePtCu film revealing a rather broad switching field distribution (SFD). The individual contributions to the SFD of the dot array were investigated by micromagnetic simulations. Based on transmission electron microscopy results, the dots show a truncated cone shape which was directly used for the finite element model. The influence of single parameters, i.e., easy axis distribution, magnetic anisotropy, and dot size, on the SFD was estimated quantitatively and compared. Furthermore, the influence of damage induced during the nanofabrication process was analyzed and correlated with experimental results.

Template-directed self-assembled magnetic nanostructures for probe recording

We employ a template-directed self-assembly process to arrange nanospheres as small as 20 nm in a polymer resist template with regular hole arrays with periods down to 42 nm produced using extreme ultraviolet interference lithography. To demonstrate magnetic probe recording employing magnetic tips, an array of magnetic caps with perpendicular magnetic anisotropy was then created by depositing Co/Pt multilayer films on 60 nm nanospheres, arranged on a square lattice with 100 nm period. The magnetic nanocaps can be switched individually in a controlled fashion, with recognition of a successful switching event realized by measurement of a force-distance curve.

Scaling dependence and tailoring of the pinning field in FePt-based exchange coupled composite media

We studied exchange coupled composite (ECC) media with an out-of-plane easy axis consisting of hard magnetic L1(0) chemically ordered FePtCu alloy films and magnetically softer [Co/Pt](N) multilayer stacks. By tailoring the structural properties of the ternary FePtCu alloy and [Co/Pt](N) multilayers, we can tune the magnetic parameters of the composite in a wide range. This allowed us to address experimentally one of the most crucial properties determining the performance of ECC media, namely the pinning field of the magnetic domain wall present at the interface between the hard and soft layers. We demonstrate that the pinning field is proportional to the difference of the magnetic anisotropy constants of the hard and soft layers, which confirms the theoretical predictions. Furthermore, we show that the pinning field can be efficiently decreased after an additional annealing step. Transmission electron microscopy investigations indicated that the origin of the observed effect is due to a heat-induced phase transformation of iron oxide present at the interface between the hard and soft layers. This study reveals that tailoring the properties of the hard/soft interface is another efficient tuning knob for optimization of the performance of ECC media.

Magnetization Reversal in Arrays of Magnetic Nanoperforations

Nanoperforated ZrO2 membranes, produced via organic/inorganic self-assembly using block copolymer micelles, serve as template for a Co/Pt multilayer stack with out-of-plane magnetic anisotropy. The deposition of the magnetic film results in an array of magnetic nanodots surrounded by a continuous magnetic film. These magnetic dots are found to be in a single-domain state and appear as magnetically exchange isolated from the surrounding film. The latter observation can be related to the specific morphology of the nanoperforations. In addition, strong domain-wall pinning of the continuous magnetic film at the locations of the nanoperforations was observed. Therefore, this approach is promising to create a percolated magnetic medium, which is considered as future recording media.