Time-Resolved XUV Absorption Spectroscopy and Magnetic Circular Dichroism at the Ni M2,3-Edges

Marcel Hennes,Renaud Delaunay,Benedikt Rösner,Denys Naumenko,Vitaliy A Guzenko,Boris Vodungbo,Valentin Chardonnet,Romain Jarrier,Emanuele Pedersoli,Benjamin Watts,Aladine Merhe,Maxime Lebugle,Michel Hehn,Ignacio Lopez-Quintas,Tatiana Savchenko,Armin Kleibert,Emmanuelle Jal,Marco Zangrando,Florian Döring,Flavio Capotondi,J Lüning ,Grégory Malinowski ,Ivaylo Nikolov ,S G Chiuzbăian ,Christian Dávid

doi:10.3390/app11010325

Abstract

Ultrashort optical pulses can trigger a variety of non-equilibrium processes in magnetic thin films affecting electrons and spins on femtosecond timescales. In order to probe the charge and magnetic degrees of freedom simultaneously, we developed an X-ray streaking technique that has the advantage of providing a jitter-free picture of absorption cross-section changes. In this paper, we present an experiment based on this approach, which we performed using five photon probing energies at the Ni M2,3-edges. This allowed us to retrieve the absorption and magnetic circular dichroism time traces, yielding detailed information on transient modifications of electron and spin populations close to the Fermi level. Our findings suggest that the observed absorption and magnetic circular dichroism dynamics both depend on the extreme ultraviolet (XUV) probing wavelength, and can be described, at least qualitatively, by assuming ultrafast energy shifts of the electronic and magnetic elemental absorption resonances, as reported in recent work. However, our analysis also hints at more complex changes, highlighting the need for further experimental and theoretical studies in order to gain a thorough understanding of the interplay of electronic and spin degrees of freedom in optically excited magnetic thin films.

Highlights

The evolution of the minority electron (H −, light green) absorption with time depends on the probing energy. Note that these changes result from a modification of available states before and after optical excitation
As we will explain in the discussion section, we believe that this is the signature of a magnetic circular dichroism (MCD) spectrum shift
Our results show that the absorption and MCD dynamics both depend on the probing

Summary

Introduction

The problem is interesting from a fundamental perspective—to date, it is not clear how angular momentum is transferred between the spin and electron systems and the crystalline lattice on sub-picosecond time scales—but it is technologically relevant, as using light to steer magnetization on sub-ps timescales might pave the way for ultrafast all-optical spintronics and data storage [8,9,10]. Recent developments illustrating the application potential of ultrafast optical excitations include, for example, the creation of artificial neural networks [11], or the demonstration that focusing grating couplers can be used for photonic–electronic integration with magnetoresistive random access memory technology [12].

Results

Discussion

Conclusion