Magnetic Fe-Pt alloy thin films


The demand of L10 ordered FePt-based thin films with tailored structural and magnetic properties for envisioned applications in data storage devices, magnetic sensors, or permanent magnets has triggered intensive research activities. In particular, the ternary FeCuPt alloys are considered to have high technological relevance as the addition of Cu allows tailoring of the magnetic anisotropy, the Curie temperature, and the crystallographic orientation, as well as lowering the ordering temperature during post-annealing of initially chemically disordered FePt films.
In previous studies, it was demonstrated that rapid thermal annealing (RTA) of FePt/Cu multilayers does not only lead to chemical ordering, but also to the development of a (001) texture on amorphous substrates, which reveals the high potential of the RTA process for the fabrication of future FePt-based devices.

Amorphous ferrimagnetic rare-earth-transition-metal (RE-TM) thin films


Coupling phenomena:
Crystallinity is not a pre-requisite for ferromagnetism - amorphous metals like Gd-Fe and Tb-Fe show a variety of unique magnetic properties. The most striking differences to crystalline ferromagnets are the absence of grain boundaries and the inherent frustration terms. The former becomes of great importance for devices and nanostructures, when the lenghtscale are comparable to typical grain sizes, whilst the latter yields rich magnetic phase diagrams. Furthermore ferrimagnetic systems exhibit a compensation temperature where the magnetic sublattices of Fe and Tb compensate each other and the net magnetization is zero.
Here our research focus is on exchange coupled ferri-/ferromagnetic heterostructures. The magnetic coupling in such systems consists of two types of pair interactions, an antiparallel exchange between the rare-earth (RE) and
transition-metal (TM) moments and a parallel exchange of the TM moments themselves. Therefore, the exchaneg bias effect can arise by both interactions simultaneously and occurs even for fully compensated interfaces.


All-optical switching (AOS):
Stanciu et al. (Phys. Rev. Lett. 99, 047601 (2007)) recently demonstrated that a circularly polarized femtosecond laser pulse not only demagnetizes the ferrimagnet GdFeCo, but also switches the magnetic state to the opposite direction without any external magnetic field. Recently, we demonstrated this effect also in Tb-Fe alloy films and showed that a magnetic compensation point is not a necessary prerequisite for all-optical helicity dependent switching. A laser-induced increase of sample temperature up to the Curie point was verified to be important for AOS. Furthermore, AOS is intimately linked to a low remanent magnetization.

Magnetic Nanostructures – Coupling Phenomena and Scaling

A highly innovative approach to form large area nanostructure arrays is the deposition of material onto self-assembled nanoparticle monolayers. Deposition of Co/Pt multilayers in particular leads to exchange isolated, quasi single-domain nanocap structures with a locally varying anisotropy orientation pointing perpendicular to the particle surface. Such anisotropy distribution has a pronounced impact on the magnetization reversal mechanism. Furthermore soft magnetic films like permalloy can also be deposited onto particle arrays resulting in the formation of a magnetic vortex state, which is characterized by the chirality or handedness of the vortex as a product of the in-plane flux closure (circulation) which can rotate clockwise or counterclockwise (c = ±1), and the direction of the out-of-plane vortex core polarization (p = ±1) which can point up or down. These objects are of considerable technological interest due to their low magnetic stray field, resulting in a high magnetic stability and high integration density and may offer new routes for spintronic applications in sensorics or data storage.    

Magnetic recording
magnetic_recording   In cooperation with Hitachi GST (San Jose, USA) a Scanning Magneto-Resistive Microscope (SMRM) was set up. This device is used to study the recording performance of magnetic storage material including nanostructure arrays (patterned media) by write-read tests, employing state-of-the-art recording heads.