Driving magnetic order in a manganite by ultrafast lattice excitation

TL;DR

This paper demonstrates ultrafast control of magnetic order in a manganite by exciting its lattice with mid-infrared pulses, revealing rapid magnetic and orbital order reduction within a few picoseconds, and proposing a microscopic displacement mechanism.

Contribution

It introduces a novel method of manipulating magnetism via ultrafast lattice excitation in manganites, expanding the understanding of optomagnetic control mechanisms.

Findings

Magnetic and orbital orders are reduced within a few picoseconds after lattice excitation.

The process involves a displacive exchange quench of the charge-orbital-spin structure.

A microscopic model based on nonlinear rectification explains the lattice displacement mechanism.

Abstract

Optical control of magnetism, of interest for high-speed data processing and storage, has only been demonstrated with near-infrared excitation to date. However, in absorbing materials, such high photon energies can lead to significant dissipation, making switch back times long and miniaturization challenging. In manganites, magnetism is directly coupled to the lattice, as evidenced by the response to external and chemical pressure, or to ferroelectric polarization. Here, femtosecond mid-infrared pulses are used to excite the lattice in La0.5Sr1.5MnO4 and the dynamics of electronic order are measured by femtosecond resonant soft x-ray scattering with an x-ray free electron laser. We observe that magnetic and orbital orders are reduced by excitation of the lattice. This process, which occurs within few picoseconds, is interpreted as relaxation of the complex charge-orbital-spin structure following a displacive exchange quench - a prompt shift in the equilibrium value of the magnetic and orbital order parameters after the lattice has been distorted. A microscopic picture of the underlying unidirectional lattice displacement is proposed, based on nonlinear rectification of the directly-excited vibrational field, as analyzed in the specific lattice symmetry of La0.5Sr1.5MnO4. Control of magnetism through ultrafast lattice excitation has important analogies to the multiferroic effect and may serve as a new paradigm for high-speed optomagnetism.