Advocacy for femto-phono-magnetism in ultrafast time

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Scientific advances (2022). DOI: 10.1126/sciadv.abq2021″ width=”800″ height=”398″/>

Phonon spectra (along the Γ−X−Δ−Λ direction) for FePt with EPC strength indicated by the width of the spectral line. (A) Phonon energies are given in electron volts on the left with corresponding values ​​shown in terahertz on the right. As can be seen (width of the orange curve), the EPC is strongest at point X (for more details on the nature of these modes, see the legend), (B) the L10 structure (two unit cells in the x direction) with Fe atoms in red and Pt atoms in gray; (C) out-of-plane Fe-Pt mode at point Γ, which has a period of 165 fs; (D) In-plane Pt mode at point X, which has a period of 191 fs. Credit: Scientists progress (2022). DOI: 10.1126/sciadv.abq2021

The magnetic material can be regulated by ultrafast laser pulses in the field of ferromagnetism. In a new report now published in Scientists progressSangeeta Sharma and a team of scientists from the Max-Planck Institute in Germany have developed a powerful new method to facilitate magnetic ordering at ultrafast times by coupling the phonon excitations of spinning and charging nuclei to create femto-phono -magnetism.

The team used state-of-the-art theoretical simulations of coupled spin, charge and lattice dynamics to identify coupled spin-phonon non-adiabatic patterns, which dominated spin dynamics in early time. The results showed how physicists and materials scientists can selectively pre-excite the nuclear system to regulate femtosecond spin dynamics in materials.

Plaidoyer pour le femto-phono-magnétisme en temps ultrarapide

Time-normalized atomic-resolution spin momentum (in femtoseconds) in laser-pumped FePt, with the vector potential of the laser pulse shown in gray. Spin dynamics calculations are performed for both full nuclear dynamics (i.e. including both phonon pre-excitation and the forces generated on the nuclei by momentum transfer from the excited electron system ) and in the absence of nuclear dynamics. The displacement of the atoms during the phonon modes is represented by black dotted lines. Results are presented for the two most strongly coupled phonon modes at point X: (A) the in-plane Pt mode (see Fig. 1D) and (B) the in-plane Fe mode. Credit: Scientists progress (2022). DOI: 10.1126/sciadv.abq2021

Information storage via femto-phono-magnetism

Information can be stored and processed at a rate determined by fundamental time scales at which external fields can influence matter. The researchers determined the fastest route by having matter interact with the electromagnetic field of light, where ultrafast lasers can regulate magnetic order as a key pathway for determining microscopic order. This process can occur via a range of methods either by spin transfer between magnetic sublattices or by regulating the spin-orbit coupling. Of all the processes, the lattice acts as a reservoir of energy and momentum that holds the demagnetized angular momentum.

In this work, the team investigated the role lattice phonon excitation plays in the dynamics of magnetic ordering at ultrafast time scales. They used iron platinum (FePt) in the study to show how selectively exciting the grating before applying the laser pulse resulted in significantly improved demagnetization. However, they did not observe any discernible change in the magnetic moment in the absence of the laser pulse.

Plaidoyer pour le femto-phono-magnétisme en temps ultrarapide

The system is pumped with a laser pulse at different times during the phonon mode. The displacement of atoms during the phonon mode (solid black line) and the vector potential of the pump laser pulse (red) are shown from (A) to (E). The corresponding magnetic moment (in bohr magneton) as a function of time (in femtoseconds) for Fe atoms in FePt is shown in (F) to (J). Credit: Scientists progress (2022). DOI: 10.1126/sciadv.abq2021

Electron-phonon coupling and femto-phono-magnetism

Sharma and colleagues regulated spin dynamics via selected phonon modes and observed phonon spectra and electron-phonon coupling for iron and platinum samples. They used a double-pump setup where the phonons were pre-excited, then they included an optical laser pump to regulate the spins in the presence of the excited phonon modes. The team observed iron-platinum spin dynamics under the influence of a pump pulse, in the presence of two strongly coupled phonon modes. The in-plane platinum mode had a significant effect on the spin dynamics.

The results highlighted the ability to pre-excite nuclear dynamics to influence ultrafast demagnetization for faster spin regulation. Also, although the large electron phonon coupling is useful as a guide, it did not result in a large spin-phonon coupling. Researchers explored the logic of increased demagnetization resulting from non-adiabatic coupled spin-nuclear dynamics; that is, in which the nuclear motion has been affected by more than one electronic state.

They noted spin current flow from iron to platinum atoms for terahertz generation – a critically important radiative effect, which they intend to explore in the future. The researchers also plan to study light emissions resulting from phonomagnetism. The main results of this work showed how the minority spin current between sublattices regulates the physics of spin-phonon coupling.

Outlook

In this way, Sangeeta Sharma and colleagues demonstrated a multi-component magnet for information storage via femto-phono-magnetism. In this study, nuclear degrees of freedom not only functioned as an energy sink for pumped spins, but also facilitated the enhancement of spin dynamics at femtosecond timescales. The team explored the nature of spin-phonon coupling and the microscopic mechanisms underlying the enhanced degaussing effect.

The results will facilitate a pathway to spin regulation via small-amplitude coherent phonons in multicomponent metallic magnets. Scientists expect future studies to explore the effect of these currents on light emission, where the results will provide a novel mechanism for regulating magnetic ordering at femtosecond timescales, for generalized applications in condensed matter physics.


The magnetization dynamics of rare earth metals and the role of ultrafast magnon generation


More information:
Sharma et al, Case for femto-phono-magnetism with FePt, Scientists progress (2022). DOI: 10.1126/sciadv.abq2021

DN Basov et al, Towards on-demand properties in quantum materials, Natural materials (2017). DOI: 10.1038/nmat5017

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Quote: Making a case for femto-phono-magnetism in ultrafast times (October 18, 2022) retrieved October 19, 2022 from https://phys.org/news/2022-10-case-femto-phono-magnetism-ultrafast.html

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