It has been known for over two decades that it is possible to manipulate magnetic order using femtosecond laser pulses. Many groups have demonstrated that single laser pulses can demagnetise a magnet on the picosecond time-scale, with a subsequent recovery of the order proceeding as the sample cools. Such measurements are routinely made these days using a pump-probe setup and detecting a change in the magnetic order through the Kerr effect. However, the pump beam is often micrometres across, hence what is measured is a spatial average. Just how the magnetic structure changes on the nanometre length-scale has, until now, not been well understood.
In a recent article in Nature Communications, in collaboration with several groups across the world, we have recently demonstrated how these magnetic structures vary across the nano-metre to micrometre length-scale and from the femto-second to nano-second time-scale. We have found “evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism”.
This has been very well explained in this article on the University of Colorado Boulder’s website. The article is open access and hence free for anyone to download and read.
Today sees the publication of my latest article, published in Scientific Reports. This work involves the use of first principles methods and atomistic spin dynamics to study the magnetic properties of Fe/Ir/Fe sandwiches. Such magnetic systems with interfaces are extremely difficult to model accurately, but by using first and second principles models we have been able to obtain layer-by-layer equilibrium and dynamic properties, which are even trickier to determine experimentally.
By using the SIESTA code to structurally relax the interfaces (see schematic) of different Ir, the ground state atomic structure can be found. We then used the Budapest SKKR code to determine an extended Heisenberg Hamiltonian. This complex Hamiltonian has a complete lack of translational invariance perpendicular to the plane, essentially meaning that each Iron plane is in its own environment which interact differently with the others. Our spin dynamics results show that this has important consequences for the equilibrium magnetic properties, as well as the dynamics. We find that the spinwaves are stiffened with increasing temperature, which goes against the thermal effects that usually result in a decrease. This is due to the frustration arising from the exchange at the interface with Ir. Finally, our results reveal a plane-wise dependence of the demagnetisation process.
The work was done in collaboration with international groups including ICN2 (Barcelona), Budapest University of Technology and the Universities of Exeter and York. The work is Open Access meaning that it is free for all to view (see this link). This was made possible due to the Sheffield Hallam University Open Access Fund. I would also like to thank Eddy Verbaan and the Library Research Support Team for their help in obtaining funding to make this article Open Access.
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.