Controlling the relaxation of magnetisation in magnetic nano-structures is key to optimising magnetic storage devices. Present day magnetic storage devices have what is known as a granular structure where the magnetic orientation of a section of grains (see the schematic) store the binary information (1’s and 0’s). At the nano-scale these grains can interact which affects how the magnetisation reacts to an external stimulus and therefore how the magnetisation is controlled.
In collaboration with experimental partners at Seagate Technology, in the Netherlands, as well as with, theoretical collaborators in the UK, our recently published article in Physical Review Bwe have shown that the effects of the exchange interaction between grains has a strong effect on the relaxation processes and time-scale of the dynamics. Experimentally a sample series with different intergrain exchange was measured using a pump-probe technique (optical ferromagnetic resonance) and showed that the damping decreased significantly with increasing interaction strength, confirmed by both (semi)-analytic and computational models, providing new insights into technologically relevant magnetic materials.
Without funding this work would not have been possible so the authors are gratefully to; the Marie Curie Incoming BeIPD-COFUND fellowship program at the University of Liège; the Advanced Storage Technology Consortium; and the European Commission under contract number 281043 (FEMTOSPIN). Thanks to Jamie Verwey for the schematic diagram.
Controlling magnetic order on ultrashort timescales is crucial for engineering the next-generation magnetic devices that combine ultrafast data processing with ultrahigh-density data storage. An appealing scenario in this context is the use of femtosecond (fs) laser pulses as an ultrafast, external stimulus to fully set the orientation and the magnetization magnitude of a spin ensemble. Achieving such control on ultrashort timescales, e.g., comparable to the excitation event itself, remains however a challenge due to the lack of understanding the dynamical behavior of the key parameters governing magnetism; the elemental magnetic moments and the exchange interaction.
In a new article published in the journal SPIN, we investigate the fs laser-induced spin dynamics in a variety of multi-component alloys and reveal a dissimilar dynamics of the constituent magnetic moments on ultrashort timescales. Moreover, we show that such distinct dynamics is a general phenomenon that can be exploited to engineer new magnetic media with tailor-made, optimized dynamic properties. Using phenomenological considerations, atomistic modeling and time-resolved X-ray magnetic circular dichroism (XMCD), we demonstrate demagnetization of the constituent sub-lattices on significantly different timescales that depend on their magnetic moments and the sign of the exchange interaction. The results can be used as a “recipe” for manipulation and control of magnetization dynamics in a large class of magnetic materials.
This work was lead by Ilie Radu (TU Berlin) and carried out in collaboration with a number of experimental and theoretical partners across Europe and Japan. The article is made publicly available through the journal’s open access format and was selected as the front cover highlight of the issue (see image above) and was in the top five most downloaded articles in 2015 in the journal SPIN. The work would not have been possible without the support of the European Community’s Seventh Framework Program (FP7/2007–2013) Grants No. NMP3-SL-2008-214469 (UltraMagnetron), No. 214810 (FANTOMAS) and No. 281043 (FEMTOSPIN) and ERC Grant No. 257280 (Femtomagnetism) as well as Grant No. 226716 and ERC-2013- AdG339813-EXCHANGE, the German Federal Ministry of Education and Research (BMBF) Grant No. 05K10PG2 (FEMTOSPEX), the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization for Scientic Research (NWO) is gratefully acknowledged.
As part of a collaboration with Diamond Light Source, The University of Nottingham and the University of York thisopen access article at Physical Review Letters demonstrates the possibility of low energy reversal of magnetic vortex core. The work, lead by Dr Stuart Cavill (The University of York) shows that by applying a time-varying strain to a ferroelectric layer that induces a strain in a magnetostrictive magnetic layer (Galfenol), vortex core dynamics are stimulated. The flux closure state is topologically symmetric and cannot be moved by simply applying a time-varying strain, therefore the symmetry must be broken. We achieved this by applying a gradient to the strain which moves one domain more than another in the vortex alternately. If the strain gradient is large enough the precession of the vortex core can be driven to force the vortex to reverse. Below is a short movie demonstrating the process.
The work was published on the 7th of August 2015 in Physical Review Letters as under the open access under a creative commons license. This was made available through the York open access fund. The work would have not been possible without the funding of the European Framework 7 project (FemtoSpin), the EPSRC, Diamond Light Source and industrial funding from Seagate Technology.