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.
I started looking at FeRh back in 2015 with Gino Hrkac in Exeter and Tom Thomson’s group in Manchester. This interesting material undergoes a change in its magnetic state from antiferromagnetic to ferromagnetic upon changing temperature (other stimuli are also possible). One of the original papers was by Fallot and Hocart and is regularly cited as such in the literature. However, I found that whilst it was well cited, I could not find a copy of the paper. It was often cited as – M. Fallot and R. Horcart, Rev. Sci. 77, 498 (1939). I tried for a while to find out which journal Rev. Sci. was by googling journal abbreviations (e.g. here) but to no avail.
Eventually, I tracked down the journal, which turned out to be “La Revue Scientifique”, which is a historic French journal founded in 1838 and, interestingly, also known as “La Revue Rose” due to the colour of its cover and in opposition to “La Revue Blue” (the nickname of “La Revue Politique et littéraire”. The particular edition (volume 77) from January 1939 has been scanned and can be found at the following link.
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.