Session K42: Simulating Magnetization Switching Across Multiple Time and Length Scales

Ab initio theory and multiscale modeling of ultrafast laser-induced magnetic processes

Ultrafast laser-induced demagnetization was discovered two decades ago,¹ however, the underlying fundamental processes of the fast magnetization decay continue to be debated. To elucidate the contributions of the various proposed microscopic mechanisms, we perform ab initio computational modeling, capable of distinguishing these different mechanisms, and compare to recent experiments.
Investigating thin Co films on an insulating substrate we quantify contributions stemming from exchange splitting reduction due to local spin-flip processes as well as spin-wave excitations, and find a surprisingly large magnon contribution to the ultrafast demagnetization.² Conversely, investigating Ni/Au bilayer films we predict a sizable fs spin current injected into the Au layer.³
A different, very interesting process is all-optical helicity-dependent magnetization switching where the magnetization of a material is manipulated with ultrashort circularly polarized laser pulses;⁴ its origin is still poorly understood. We develop materials’ specific ab initio theory for the magnetization induced by circularly polarized light, commonly referred to as inverse Faraday effect, applicable to absorbing materials.⁵ We show that the induced magnetization is strongly materials and frequency dependent and demonstrate a surprising difference between induced spin and orbital magnetizations. Combining effects of thermal laser heating with laser-induced magnetization in a multiscale modeling we predict all-optical switching in FePt nanoparticles with repeated circularly polarized laser pulses.⁶
[1] E. Beaurepaire et al, Phys. Rev. Lett. 76, 4250 (1996). [2] E. Turgut et al, Phys. Rev. B 94, 220408(R) (2016). [3] M. Hofherr et al, Phys. Rev. B 96, 100403(R) (2017) 100403. [4] C.-H. Lambert et al, Science 345, 1337 (2014). [5] M. Berritta et al, Phys. Rev. Lett. 117, 137203 (2016). [6] R. John et al, Sci. Rep. 7, 4114 (2017).   

Ab-initio description of all optical switching

By creating via pump laser pulse a non-equilibrium distribution of charge on sub-exchange time scales (i.e., faster than the time scale associated with spin flip in the ground state) we demonstrate that a precise control of magnetic structure is possible on ultra-short time scales, including switching of spin order from anti-ferromagnetic (AFM) to transient ferromagnetic (FM). The microscopic physics of this ultra-fast spin modulation is dominated by charge flows created by spin-preserving optical excitations, one of the fastest means of manipulating an electronic system by light. We demonstrate this mechanism to be universally applicable to AFM, FM, multilayers and bulk systems, and provide three rules that encapsulate the laser induced early time magnetization dynamics of multi-sub-lattice systems.   

Magnetophononics: ultrafast spin control through the lattice

The interplay between the electronic properties and crystal structure dictates the functional properties of materials. In this talk, I will discuss how one can use the selective excitation of phonons with light as a perturbative method to manipulate the magnetic properties of insulators. I will first introduce the general theoretical concept of phonon excitation towards the non-linear regime, also known as non-linear phononics [1], and its possible side effects onto crystal and electronic structure. In the following, I will present in detail two case studies which utilize this approach in magnetic materials.
I first will discuss the coherent modulation of the electric polarisation, which creates a magnetization even in materials with no existing spin structure [2]. This transient state of broken space and partiy odd symmetry is multiferroic and comprises unexpected outcomes like a phonon Zeeman effect and magnon excitation [3]. As a second example, I show how the phonon excitation and its related structural distortion modulates magnetic exchange interactions. For the specific case of Cr₂O₃, the right tuning of induced deformations [4] allows modulating the magnetic spin arrangement, which generates a new magnetic state. I will also speculate about where the field is going and provide my own perspectives.

[1] M. Först, et al., Nature Phys. 7, 854 (2011).
[2] D. M. Juraschek, et al., Phys. Rev. Mat. 1, 014401 (2017).
[3] T. F. Nova, et al. , Nature Phys. 13, 132 (2016).
[4] M. Fechner, et al., arXiv 1707.03216, (2017).   

Investigations of Spin Precession in Perpendicular Magnetic Materials Enabled by Time-Resolved Magneto-Optical Kerr Effect

Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE) is an all-optical method based on the ultrafast pump-probe technique that can be used to study the magnetization dynamics of materials, in addition to thermal and mechanical characterization. With optical excitation and the capability of reaching large magnetic fields, TR-MOKE can probe spin precession at high resonance frequencies (up to a few hundreds of GHz), beyond those achievable by conventional Ferromagnetic Resonance (FMR) methods. In this talk, we demonstrate the use of TR-MOKE to study the spin precession of two model material systems with large perpendicular magnetic anisotropy (PMA). The first model system is a series of tungsten (W) -seeded CoFeB thin films, capable of sustaining good PMA after post-annealling at temperatures of up to 400 °C. We measure the Gilbert damping (α) of W-seeded CoFeB films, and attribute the dependence of α on the annealing temperature to two competing effects: the enhanced crystallization of CoFeB and the dead-layer growth occurring at the CoFeB interfaces. The second model system of interest is composed of perpendicular ferromagnetic [Co/Pd]_n multilayers with varying anisotropy. We use ultrafast-laser heating to launch acoustic strain waves and capture their coupling with the spin precession in these [Co/Pd]_n multilayers. Understandings on such a strain-spin coupling may shed light on manipulating the precession and switching the magnetization in magnetoacoustic devices.   

Magnetization dynamics from an ab-intio perspective: from single atoms to skyrmions

One of the major challenges in information technology is to find new paradigms to increase computing speed and storage capacity. Today, several high-potential future bits are heavily explored across several length scales: from e.g. single atoms to large magnetic objects such as skyrmions. This requires concomitantly careful investigations and understanding on the mechanisms permitting their stability, their detection and manipulation, which are highly dynamical in nature leading to formidable computational challenges because of the complex interactions of the magnetic bits with the surrounding degrees of freedom.

I will present an overview of our investigations for different atoms deposited on several substrates. Combining time-dependent density functional theory and many-body perturbation theory, we showed that current-driven spin-state manipulation in nano-devices leads to rich transport patterns with new many-body features, resulting from the interaction of the electrons and the spin-excitations detectable with state of the art inelastic scanning tunneling spectroscopy. The intricate interplay between the intra- and interatomic exchange interactions and the spin-orbit interaction dramatically modify the dynamical behaviour of the nanomagnets and the lifetime of the newly produced state. Furthermore, I will address zero-point spin-fluctuations leading to a collapse of the magnetic anisotropy energy barrier even at absolute zero temperature and I will provide practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations. Finally, utilizing a multi-scale modelling approach, I will discuss the dynamics of sub-5 nm skyrmions heavily affected by the presence of single atomic-defects, which can act as pinning or repulsive centers depending on their chemical nature.