Suche nach nicht kollinearen Spinstrukturen in reinen und mit seltenen Erden dotierten ultradünnen Ferriten
Theoretische Physik der kondensierten Materie
Zusammenfassung der Projektergebnisse
Ferrites which belong to the group of spinels are prototypes for ferrimagnetic materials, which are well-known and abundantly used in technology. In these materials, the magnetic moments (spins) of the ions residing in octahedral sites are anti-parallel to those ions residing in tetrahedral sites, leaving a finite resulting moment due to the different magnitudes of the magnetic moments. Recent advances in spintronic research have uncovered the importance of non-collinear (chiral) spin arrangements which can be used as information bits in novel information technology devices. Their stabilization requires the crystal structure to be acentric allowing the emergence of the chiral Dzyaloshinskii-Moriya interaction. Since many magnetic materials such as the ferrites are centrosymmetric, methods are needed to induce acentricity without destroying the crystal’s long-range order. Very recently is has been observed that the crystal structure of CoAl alloy films can be made acentric by epitaxial growth on a suitable substrate involving the formation of a strain gradient along the c-axis. However, this procedure is quite limited in that it requires a certain lattice mismatch between substrate and film as well as a certain film thickness range which severely limits the number of possible (magnetic) crystalline materials. Therefore, a more general procedure is required. In this project we have employed doping with a large ion, namely Dy3+ to induce non-collinearity. We found that in thin films of Dy-doped ferrite (concentration several percent), a helical spin texture of the Fe and the Ni magnetic moments develops which is attributed to the local strain gradient induced by the large Dy3+ ion residing in the octahedral site. Surprisingly, the magnetism of the Dy3+ plays no decisive role, which leads us to conclude that also non-magnetic ions are suitable for achieving a strain gradient induced emergence of the DMI and the stabilization of a helical spin texture as observed here. We believe that our results might open a new field in this particular branch of research. Our project was carried out in several steps including (a) different preparation methods, (b) structural analysis, (c) magnetic analysis and (d) theory We have carried out five experimental runs at Synchrotron radiation facilities [SOLEIL (1x), ALBA (3x), ELETTRA (1x)] using several state-for the art techniques (EXAFS, XMCD, SXRMS) in addition to in-house facilities (RBS, SQUID, MKE XRD) which were very demanding to carry out and to analyze. In addition, we had support from theory (A. Ernst, JKU Linz) providing insights into the magnetic film properties based on the structural data. This part of the project is not yet fully completed, because of the complexity to treat large atomic clusters. We are confident that this will be done in the very near future and the results will be published in a high-ranking journal.
