Ultrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet

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Ultrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet

E. Golias, I. Kumberg, I. Gelen, S. Thakur, J. Gördes, R. Hosseinifar, Q. Guillet, J. K. Dewhurst, S. Sharma, C. Schüßler-Langeheine, N. Pontius, and W. Kuch

Phys. Rev. Lett. 126, 107202 – Published 10 March 2021

Abstract

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in $Co / Mn$ multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn $L_{3}$ resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.

Received 26 June 2020
Revised 15 January 2021
Accepted 25 January 2021

DOI:https://doi.org/10.1103/PhysRevLett.126.107202

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Ultrafast magnetic effects Ultrafast magnetization dynamics

Magnetic multilayers Metals

Reflectivity X-ray magnetic circular dichroism

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

E. Golias ^1,*,†, I. Kumberg ¹, I. Gelen ¹, S. Thakur ¹, J. Gördes¹, R. Hosseinifar¹, Q. Guillet ¹, J. K. Dewhurst², S. Sharma³, C. Schüßler-Langeheine ⁴, N. Pontius ⁴, and W. Kuch ^1,‡

¹Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
²Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany
³Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Strasse 2A, 12489 Berlin, Germany
⁴Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein Straße 15, 12489 Berlin, Germany

^*evangelos.golias@gmail.com
^†Present address: MAX IV Laboratory, Lund University, Fotongatan 2, 22484 Lund, Sweden.
^‡kuch@physik.fu-berlin.de

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Issue

Vol. 126, Iss. 10 — 12 March 2021

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Images

Figure 1
(a) Schematic representation of our experimental geometry. The intensity of the reflected x rays is probed by a photodiode in a $Θ − 2 Θ$ specular geometry with incidence angle $θ = 8.7 5 °$ . Oscillating red and spiraling blue arrows indicate the linearly polarized infrared laser and the circularly polarized soft-x-ray pulses, respectively. (b) The underlying principle of the OISTR mechanism: the electric field of light can, above a threshold value, transfer electrons coherently between atoms. In an AFM material, majority states from the first atom are transferred to the minority of the second (and vice versa). When an asymmetry is present, such as the interface between Mn and Co, a charge filling imbalance is introduced and a transient FM alignment emerges. (c) Calculation of the differential laser power absorption on each layer of the $Co / Mn$ multilayer. Red and blue lines correspond to the absorption from Co and Mn layers, respectively. The gold line corresponds to the absorption from the Cu(001) substrate, here, only a small part close to the interface with the multilayer is shown. The absorption in the substrate slowly becomes virtually zero within 40 nm from the interface. On the upper part, a schematic of our sample is displayed. Red oscillating arrows represent the incoming and reflected laser light, while red, blue, and gold blocks represent the Co, Mn layers and the Cu(001) substrate, respectively.
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Figure 2
Time-resolved magnetic contrast from the $L_{3}$ edge of (a) Mn and (b) Co. Blue bars in (a) and red shaded regions in (b) correspond to the statistical errors for the measurements based on Poisson statistics. The zero time delay is defined here at the maximum of the laser pump pulse. The red oscillating and the dashed lines represent the pump pulses and their full width at half maximum, respectively, while the light-red shaded area indicates the experimental time resolution.
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Figure 3
TDDFT calculations for a five-atomic-layer stack composed of 3 ML Co and 2 ML Mn. (a) Time-dependent magnetic moment of each layer. Note that the stacking order of the layers in the simulation corresponds to the atomic order of the legend and time zero is defined here at the beginning of the excitation pulse, as shown in panel (b). (b) Time-dependent total magnetic moment of all Mn (blue line) and all Co (red line) atoms in the stack. The gray oscillating line corresponds to the temporal profile of the vector potential of the pump pulse. (c),(d) Partial DOS (PDOS) of the Mn atoms at the first (Mn1), second (Mn2) Mn layer, respectively, in states/eV/spin. Black dashed lines represent the full (i.e., occupied and unoccupied) partial DOS of the ground state. Solid blue and red lines represent the occupied and transiently populated part of the PDOS at $t = 14.5 fs$ and $t = 24.2 fs$ , respectively. The zero of the energy scale corresponds to the Fermi energy of the ground state. Up- and down-pointing arrows mark the PDOS for the spin-up (positive values) and spin-down (negative values) states, respectively. (e)–(h) Differential partial density of occupied states between $t = 24.2 fs$ and $t = 0 fs$ (equal to the occupied ground state PDOS) for (e) spin up in Co layers, (f) spin down in Co layers, (g) spin up in Mn layers and (h) spin down in Mn layers, respectively.
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