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FemtoSpin Mid Term Review

FemtoSpin Mid Term Review

 

The FemtoSpin project is making rapid progress in a field of enormous scientific and technical importance. The scientific basis of the project is research in ultrafast magnetisation processes induced by the interaction of light beams from femtosecond lasers. Experimentally, a light beam from a powerful-pulsed laser with a pulse width of around 50 femtoseconds is split into 2 beams. The first, high intensity (pump) beam is incident on the magnetic material, in the form of a thin film of thickness around 10nm, causing rapid heating. The second, low intensity (probe) beam is sent around a delay line, arriving at the sample with a measured delay and used to measure the magnetisation of the film using a technique known as the Magneto-Optical Kerr Effect. By means of such pump-probe experiments it has become possible to measure the magnetic response of a material on a timescale of tens of femtoseconds. This is a topic at the cutting edge of condensed matter and materials physics. The FemtoSpin participants are at the forefront of the field. FemtoSpin is predominantly aimed at developing models of the ultrafast magnetisation dynamics, but also has a core of leading experimental groups. The proven strong collaboration between the participants has led to a number of significant advances, including the discovery and verification of a phenomenon known as Thermally Induced Magnetisation Switching (TIMS) in which magnetisation switching can be achieved by a heat pulse alone in the absence of an applied field. This astonishing result has triggered a major worldwide effort to provide a full understanding of the phenomenon and its translation into practical applications. The technical implications for information storage technology are extremely exciting, giving the possibility of increased data rates along with reduction of device complexity and power requirements.

 

FemtoSpin is taking place within a rapidly evolving industrial context. In terms of magnetic information storage, the drive to higher recording densities is based on Heat Assisted Magnetic Recording (HAMR), which uses laser pulses to heat the storage medium so as to allow reversal of the magnetisation. This relies on a combination of laser heating coupled with standard technology to generate a localised magnetic field to induce the magnetisation switching. In the write-field transducer, a large current is passed through conducting coils, inducing a field in the magnetic storage medium. Because of materials limitations, this field is limited in magnitude: a factor which will ultimately limit areal storage densities. Technologically, the complexity of manufacture of the write transducer is already slowing down the pace of development. The use of optical switching would remove the requirement for the inductive write transducer, significantly reducing both manufacturing costs and power requirements. At the same time, the field of spin electronics (or Spintronics), in which device functionality is dependent on the spin of the electron rather than simply the charge, is developing rapidly. Spintronics is a strong candidate to replace conventional electronics as this technology reaches its physical limitations. Again, optical reversal is a potential candidate for switching the magnetisation in spintronic devices.

 

Against this background, FemtoSpin is making excellent progress. The physical understanding of ultrafast magnetisation dynamics relies on the development of advanced models of the fundamental properties of magnetic materials and their response to ultrafast laser excitation. This requires the development of new and powerful ‘multiscale’ magnetic models linking the different length scales associated with electronic structure calculations, atomistic models and mesoscopic approaches. In parallel with this, FemtoSpin partners are carrying out state-of-the-art experimental measurements and working closely with the theoretical groups to ensure rapid progress in the understanding of the fundamental physics in relation to the underlying materials properties. In particular we have identified the probable mechanism of TIMS, leading to the explanation of the fact that the phenomenon has been observed only in ferrimagnetic alloys of rare earth and transition metals. This led to the theoretical prediction of TIMS in synthetic ferrimagnets consisting of two ferromagnetic layers coupled antiferromagnetically: experimental testing of this prediction is under way.

 

Summary of objectives

  • Obtain fundamental knowledge of dynamic processes on the fs timescale; this requires the development of new approaches to treat non-equilibrium electron dynamics, utilizing Density Functional Theory and applying these to understand the fundamental mechanisms underlying ultrafast spin dynamics.
  • Advanced atomistic models; this includes spin models with equations of motion beyond Langevin dynamics; new approaches to induced spins and transport; integration of thermodynamic and quantum approaches
  • Mesoscopic model development; this requires mesoscopic modelling using a generalised Landau-Lifshitz-Bloch (LLB) Equation; formulation for ferrimagnets and determination of LLB parameters from SDFT calculations and atomistic models.
  • Multiscale calculations and link to experiments; verification of models against experiment; feedback from experiments to model development; material studies; large-scale calculations and device simulations.
  • Detailed materials studies; candidate materials with especially promising properties on the femtosecond timescale will be investigated. This will encompass single-phase materials and alloys in addition to novel structured materials with engineered properties.

 

Work performed and major results achieved

Electronic structure calculations: The modelling activities here are aimed at the development of fundamental understanding of the properties underlying ultrafast magnetisation dynamics. In particular, theoretical models of the optomagnetic field associated with the laser pulse have been developed. The role of spin transport has also been investigated. Specifically a model of the contributions of superdiffusive spin transport as a mechanism for magnetisation changes following a laser pulse has been developed. The model shows that a thin Fe layer in contact with Ni can initially increase in magnetisation after a laser pulse. Central to the multiscale problem is the link between electronic structure calculations and atomistic spin models. This is carried out by means of a coarse graining approach, in which the fundamental properties are mapped onto a classical atomistic spin Hamiltonian assuming a fixed magnitude for the atomic spin value. We have developed techniques for the determination of magnetic parameters and applied them to studies of numerous materials, including bilayer films where the interface has an important effect on the magnetic parameters.

Atomistic calculations: We have developed a spin model, which gives an improved treatment of the properties of GdFeCo. The model treats separately the 5d and 4f electrons of the Gd. The laser excites the former, while the latter carry the atomic spin. We have also developed highly efficient GPU-based code, which enables the calculation of dynamic structure factors, leading the calculation of magnon dispersion relations and intensities across the Brillouin zone.

 

Macrospin models. These models represent a further coarse graining, being based on macrospins having dimensions of nm. The equation of motion of the macrospin must allow for changes in the magnitude of the spin at elevated temperatures. Model development within FemtoSpin is based on the Landau-Lifshitz-Bloch (LLB) equation of motion. The coarse graining associated with the multiscale approach is completed by using atomistic models, parameterised using first principle calculations, to calculate the important input parameters for the LLB model, specifically the temperature dependent anisotropy, exchange and longitudinal susceptibility. These models will form an important part of the planned comparison with experimental data.

 

Major results achieved

 

Importantly, we find that the phenomenon of heat-driven reversal seems has its origin in the presence of a 2-magnon bound state at the g point of the Brillouin zone, which transfers angular momentum between sublattices. This is an important factor in the investigation of heat-driven reversal and in the design of future materials exhibiting the phenomenon. We have also used an

atomistic model to investigate the dynamics of a Synthetic Ferrimagnet (SFiM) comprised of FePt and Fe layers separated by a Ru layer. The calculations predict heat-driven reversal to take place in such SFiM systems  (Patent submitted). Our goal of validating the theoretical and computational approaches is well under way, in particular in relation to the origin of the differential spin dynamics.

 

From the model development viewpoint we have made significant progress. In terms of the multiscale model we have succeeded in the development of techniques for mapping ab-initio information onto classical spin Hamiltonians. An initial application to the properties of FePt/Fe interfaces has been made since this will form an important part of the comparison with experiment. Ab-initio and atomistic models feed into the large scale macrospin models which form an important link to experiments and which are also expected to evolve into future design tools. These are generally based on the Landau-Lifshitz-Bloch (LLB) equation of motion. Our development of multi-sublattice and quantum LLB equations are important developments: the multi-sublattice version has given important insight in conjunction with atomistic calculations.

 

Expected final results

 

The completion of a strong multiscale theoretical and computational framework for research into ultrafast spin manipulation in nanostructured magnetic materials validated by parallel state of the art experimental programmes. The collaboration between advanced theory and modelling of realistic systems and novel, cutting edge experiments, will result in a deeper understanding of the fundamental physics of spin ordered materials and will lead to the development of advanced computational tools for the design of a new generation of materials for applications in ultrafast devices. This is an important and difficult challenge, as it spans multiple timescales (from femtoseconds to nanoseconds) and it involves the creation of interfaces between electronic structure calculations, atomistic models and mesoscopic simulations that jointly form the required multiscale approach. The understanding achieved, and the computational tools developed will provide a vital platform for the development of novel devices for magnetic information storage and processing. This progress toward realisation of all-optical reversal via the heat driven switching process was given a considerable boost by the prediction that the phenomenon can occur in synthetic ferrimagnet structures consisting of FePt coupled to Fe. This brings into play the high anisotropy of FePt, which is necessary for stability of writes information at high densities

 

The socio-economic benefits are expected to arise from the potential contribution to the increasing information storage and processing requirements of today’s society. Practical realisation of all-optical storage would result in numerous benefits. In particular the large effective fields involved would assist the progress to higher storage densities. At the same time, the complexity of write transducers in magnetic recording would be significantly reduced, leading to a reduction in processing costs and material wastage. It is also expected that all optical write transducers would give rise to a considerable reduction in the power required to store individual bits.