On February, 13, we have the pleasure to welcome Aurore FINCO, from university of Hamburg in Spintec.
At 13H30, she will give a seminar in room 434 A.
Various ways to tune non-collinear magnetism in ultrathin films
In order to develop spintronic devices, a precise control of the magnetic state in ultrathin films is necessary. Starting from the Fe/Ir interface, known to generate a large Dzyaloshinskii-Moriya interaction, we show how to obtain and tune spin spirals or magnetic skyrmions using spin-polarized scanning tunneling microscopy (SP-STM).
Since the lattice mismatch between Fe and Ir is very large, the epitaxially grown double Fe atomic layer on Ir(111) exhibits a network of dislocation lines along high-symmetry directions. These lines allow to relieve the epitaxial strain in the film and fix the propagation direction of spin spirals with a period about 1.5 nm.
When atomic hydrogen is incorporated in the double layer film, the dislocation lines network disappears and is replaced by a p(2×2) hexagonal superstructure. The propagation direction of the spin spirals is thus not fixed anymore and their period increases to 3.5 nm.
Furthermore, they transform into skyrmions under application of a magnetic field of 3 T whereas, this is not the case in the pristine double layer. Ab initio calculations indicate that the H atoms must be located between the two Fe layers and allow to reproduce the observed results.
Another possibility to modify the magnetic state is to add one more atomic Fe layer. As well as in the double layer film, spin spirals propagate along dislocation lines in the triple layer film. In addition, the spacing between the lines is varying over the sample surface which means that the strain relief is also not uniform. Our measurements revealed a clear correlation between the dislocation line spacing and the magnetic period. Based on a simple micromagnetic model, we attribute this observation to a modulation of the effective exchange coupling by the epitaxial strain relief.
Although the effect of strain relief was studied at low temperature, we also investigated the temperature dependence of the spin spirals.
Whereas no spiral could be observed in the double layer above 150 K, the spirals are still stable in the triple layer Fe up to room temperature.
Moreover, their period increases drastically and becomes 15 times larger than at 8 K. In order to explain this unexpected temperature-dependent behaviour, we propose a classical spin model based on different magnetic parameters in the three Fe layers. This model allows us to reproduce the period increase using both a mean-field and a Monte Carlo approach.
Under application of a magnetic field, the low-temperature spin spirals split up into skyrmions with a typical size about 5 nm. We demonstrate that they can reliably be written and deleted locally by the STM tip.
The strong-bias polarity dependence and the linear behavior of the threshold voltage with the tip-sample distance shows that electric field plays the dominant role in the switching mechanism.