# Coherent Oscillations in a SrRuO3/BiFeO3 Superlattice

## TL;DR

Researchers observed temperature-dependent oscillations in a SrRuO3/BiFeO3 superlattice, linked to acoustic phonons and magnetostriction effects.

## Contribution

The study reveals how magnetostriction in SrRuO3 suppresses lattice oscillations in a superlattice, as shown by temperature-dependent pump–probe measurements.

## Key findings

- Periodic oscillations in reflectivity are explained by zone-folded acoustic phonon modes in the superlattice.
- Oscillation amplitude suppression correlates with the square of magnetization in the ferromagnetic state.
- Pump-induced expansion of SrRuO3 excites oscillations via the A1 phonon mode.

## Abstract

We investigated the ultrafast dynamics of a SrRuO3/BiFeO3 superlattice grown on a SrTiO3 substrate using a near infrared pump–probe technique at various temperatures. The superlattice exhibits a ferromagnetic order inherited from the SrRuO3 layer. The pump-induced changes in the reflectivity reveal periodic oscillations. We found that the oscillation frequency can be well explained by zone-folded acoustic phonon oscillations, whose dispersion depends on the sound velocity, density, and thickness within the supercell of each constituent layer. It is found that the observed oscillation frequency corresponds to the A1 mode, which suggests that oscillations are excited due to pump-induced expansion of the SrRuO3 layer that absorbs the pump photon. Temperature-dependent measurements reveal significant suppression of the oscillation amplitude in the ferromagnetic state. The suppressed amplitude is proportional to the square of the magnetization, M(T)2. This phenomenon can be attributed to a strong magnetostriction effect of SrRuO3 that suppresses lattice expansion upon pumping.