Making a case for femto-phono-magnetism in ultrafast times

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

Magnetic matter can be regulated by ultrafast laser pulses in the field of ferromagnetism. In a new report now published in Science Advances, Sangeeta Sharma and a team of scientists at the Max-Planck Institute in Germany developed a powerful new method to facilitate magnetic order at ultrafast times by coupling phonon-excitations of the nuclei to spin and charge to create femto-phono-magnetism.

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

Making a case for Femto-Phono-Magnetism in ultrafast times
Normalized atom-resolved spin moment as a function of time (in femtoseconds) in laser-pumped FePt, with the vector potential of the laser pulse shown in gray. Spin dynamics calculations are performed both for full nuclear dynamics (i.e., including both preexcitation of the phonon and forces generated on the nuclei by momentum transfer from the excited electron system) and in the absence of nuclear dynamics. Displacement of atoms during the phonon modes is shown with black dashed lines. Results are shown for the two most strongly coupled phonon modes at the X-point: (A) the in-plane Pt mode (see Fig. 1D) and (B) the in-plane Fe mode. Credit: Science Advances (2022). DOI: 10.1126/sciadv.abq2021

Storing information via femto-phono-magnetism

Information can be stored and processed at a speed determined by fundamental time scales at which external fields can influence matter. Researchers have determined the fastest such route by interacting matter with the electromagnetic field of light, wherein ultrafast lasers can regulate magnetic order as a key route to determine microscopic order. This process can occur via a range of methods either via spin transfer between magnetic sub-lattices or by regulating . Of all processes, the lattice acts as an energy and momentum reservoir that retains the demagnetized angular momentum.

In this work, the team addressed the role that lattice phonon excitation play in the dynamics of magnetic order on ultrafast timescales. They used iron platinum (FePt) during the study to show how selective lattice excitation prior to applying the laser pulse resulted in significantly improved demagnetization. However, they did not observe a discernible change in the magnetic moment in the absence of the laser pulse.

Making a case for Femto-Phono-Magnetism in ultrafast times
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 in (A) to (E). The corresponding magnetic moment (in bohr magneton) as a function of time (in femtoseconds) for the Fe atoms in FePt is shown in (F) to (J). Credit: Science Advances (2022). DOI: 10.1126/sciadv.abq2021

Electron phonon coupling and femto-phono-magnetism

Sharma and colleagues regulated the spin dynamics via selected phonon modes, and observed the phonon spectra and electron-phonon coupling for the iron platinum samples. They used a double-pump setup where the phonons were pre-excited, then they included an optical laser pump to regulate spins in the presence of the excited phonon modes. The team observed the spin dynamics of iron platinum 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. Furthermore, while large electron phonon coupling is useful as a guide, it did not result in large spin-phonon coupling. The researchers explored the rationale behind enhanced demagnetization resulting from nonadiabatic coupled spin-nuclear dynamics; i.e., in which the nuclear motion was affected by more than one electronic state.

They noted a flow of spin current from iron to platinum atoms for terahertz generation—a radiative effect of crucial importance, which they intend to explore in the future. The researchers also expect to study resulting from phonomagnetism. The key outcomes of this work showed how the minority spin current between sub-lattices regulated 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, the nuclear degrees of freedom did not only function as an energy sink for pumped spins, but also facilitated improved spin-dynamics at femtosecond time-scales. The team explored the nature of spin- coupling and the microscopic mechanisms underlying the effect for enhanced demagnetization.

The outcomes will facilitate a route toward spin regulation via small amplitude coherent phonons in multicomponent metallic magnets. The scientists expect future studies to explore the effect of such currents on light emission, where the outcomes will provide a new mechanism to regulate magnetic order at femtosecond time-scales, for widespread applications in condensed matter physics.

More information: Sharma et al, Making a case for femto-phono-magnetism with FePt, Science Advances (2022). DOI: 10.1126/sciadv.abq2021

D. N. Basov et al, Towards properties on demand in quantum materials, Nature Materials (2017). DOI: 10.1038/nmat5017

Journal information: Nature Materials , Science Advances

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