Element-specific ultrafast lattice dynamics in FePt nanoparticles.

Area of Science:

• Nanoscale science and technology
• Materials science
• Condensed matter physics

Background:

• Investigating light-matter interactions at the nanoscale is crucial for magnetic recording technologies.
• Element-specific atomic motion in magnetic alloys remains largely unexplored, despite spin dynamics being accessible.
• FePt nanoparticles are promising for high-density magnetic recording applications.

Purpose of the Study:

• To probe the element-specific atomic motion in FePt nanoparticles following ultrafast laser excitation.
• To understand the dynamics of lattice vibrations and expansion in response to laser energy transfer.

Main Methods:

• Utilized ultrafast electron diffraction (UED) to analyze temporal evolution of lattice Bragg peaks.
• Studied FePt nanoparticles embedded in a carbon matrix.
• Excited samples with an optical femtosecond laser pulse.

Main Results:

• Demonstrated significantly larger mean square vibration amplitudes for Fe sublattices compared to Pt, consistent with mass differences.
• Observed an increase in vibration amplitudes as energy transferred from excited electrons to the lattice.
• Revealed a counterintuitive laser-induced lattice expansion where Pt atoms expanded more than Fe atoms within the first picosecond.

Conclusions:

• Element-specific atomic motion, particularly lattice expansion, can be precisely measured using UED.
• The observed preferential Pt expansion suggests strain-wave driven dynamics, with longitudinal acoustic Pt motion dominating Fe motion.
• Findings offer insights into nanoscale thermal transport and lattice dynamics in magnetic nanomaterials.