Category Archives: Papers

New Paper on Multiscale Modeling of Fe/Ir Interfaces

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.

Schematic of an Fe/Ir/Fe Sandwich

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.

 

 

Strain Effects on the Metamagnetic Phase Transition in FeRh on MgO

In a collaboration between the Universities of Manchester and Exeter a new article has been  published today in Scientific Reports. Lead by Dr Craig Barton at the University of Manchester we investigated the role of strain in FeRh layers on MgO.

FeRh goes from antiferromagnetic to ferromagnetic in bulk at around 70°C. However, in thin films this temperature can change depending on the thickness and the material it is interfaced with. In this new article we measured the effect of film thickness on the transition temperature and correlated the effects with changes in the structural properties. We corroborate a number of observations using theoretical models that account for the strain at the interface. The article is open access so anyone can download and read the article here.

Interaction effects in granular media

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.

Schematic of an optical pump probe simulation on granular media.
Schematic of an optical pump probe setup granular media. The grains (represented as individual magnetic moments) have a given configuration (giving an initial magnetisation M’) and upon laser excitation relax to a new magnetic state. This can be used to probe the relaxation time-scales of the material.

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 B we 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.

BeIPD COFUND