Final Report Summary - JANUS DYNAMICS (Magnetization dynamics in anisotropic magnetic nanoparticles investigated using time-resolved X-ray and neutron scattering techniques)
The magnetization relaxation of nanosized magnetic objects plays an important role for their technological applicability. For applications in data storage, a large magnetic anisotropy is required in order to retain the magnetization state in the required relaxation time and temperature range. For application in the fields of magnetic imaging or magnetic hyperthermia, however, the electromagnetically induced relaxation of superparamagnetic nanoparticles is desired. Magnetization relaxation effects depend on the interplay of magnetic anisotropy and volume and are thus largely influenced by the nanoparticle composition, shape, interface effects, and interparticle interactions. The Marie-Curie IEF project “Magnetization dynamics in anisotropic magnetic nanoparticles investigated using time-resolved X-ray and neutron scattering techniques” was dedicated to the quantitative investigation of the magnetization relaxation in magnetic nanoparticles and their assemblies. In particular, the effects of shape anisotropy, exchange bias, and dipolar interparticle interactions were to be explored. Janus nanoparticles, consisting of two epitaxially aligned hemispheres of different composition, allow for a controlled variation of anisotropic shape and interface effects in nanoparticles, and thus were thus chosen as model systems for this project along with shape anisotropic iron oxide nanoparticles. We investigated the static magnetization and dynamic magnetization relaxation effects by application of advanced X-ray and neutron scattering techniques including stroboscopic small-angle scattering and the pulsed TISANE technique. As a result, we aimed at precise information on the influence of exchange bias and interparticle interactions on the Néel relaxation.
In the framework of this project, we have successfully studied the temperature-dependent magnetization in FePt nanoparticles. This is an important prerequisite for our ongoing studies of the impact of exchange bias exerted by MnO subunits on the nanoparticle magnetization in FePt@MnO Janus nanoparticles. Moreover, we have investigated the static magnetization distribution in iron oxide nanoparticles of cubic shape and varying edge lengths. Our combined static polarized SANS and GISANS study on the non-interacting nanoparticles as well as mesocrystalline arrangements thereof will significantly contribute to the understanding of the magnetic nanoparticle morphology and dipolar interparticle interactions in the nanosized regime.
Whereas the projected time-resolved studies on magnetization relaxation and Brownian relaxation in shape anisotropic nanoparticles were delayed significantly for technical reasons, first experiments carried out so far promise interesting results with high impact for further studies. This includes insight into the structural reorientation of highly shape anisotropic iron oxide nanoparticles in rotating and alternating magnetic fields, as well as the directionally resolved magnetization relaxation in oriented assemblies of iron oxide nanocubes. These projects will be continued in the near future, as soon as the TISANE technique is available in combination with a polarized incident neutron beam. While still ongoing, these objectives have the potential to significantly advance the understanding of magnetization dynamics in anisotropic nanoparticles and thus to contribute to the exploration of further suitable nanomaterials for technological applications.