Single laser pulse driven thermal limit of the quasi-two dimensional magnetic ordering in Sr$_2$IrO$_4$

TL;DR

This study shows that a single femtosecond laser pulse can induce a non-thermal magnetic state in Sr$_2$IrO$_4$, revealing a threshold behavior linked to the material's anisotropic exchange interactions and affecting its quasi-two-dimensional antiferromagnetic order.

Contribution

It uncovers a non-thermal magnetic transition triggered by ultrafast laser pulses in Sr$_2$IrO$_4$, highlighting the role of anisotropic exchange interactions in ultrafast magnetic dynamics.

Findings

Laser pulse induces glassy magnetic state in Sr$_2$IrO$_4$

Critical fluence threshold (~12 mJ/cm$^2$) for magnetic order disruption

Anisotropic exchange interactions influence ultrafast magnetic recovery

Abstract

Upon femtosecond-laser stimulation, generally materials are expected to recover back to their thermal-equilibrium conditions, with only a few exceptions reported. Here we demonstrate that deviation from the thermal-equilibrium pathway can be induced in canonical 3D antiferromagnetically (AFM) ordered Sr$_2$IrO$_4$ by a single 100-fs-laser pulse, appearing as losing long-range magnetic correlation along one direction into a glassy condition. We further discover a `critical-threshold ordering' behavior for fluence above approximately 12 mJ/cm$^2$ which we show corresponds to the smallest thermodynamically stable $c$-axis correlation length needed to maintain long-range quasi-two-dimensional AFM order. We suggest that this behavior arises from the crystalline anisotropy of the magnetic-exchange parameters in Sr$_2$IrO$_4$, whose strengths are associated with distinctly different timescales. As a result, they play out very differently in the ultrafast recovery processes, compared with the thermal equilibrium evolution. Thus, our observations are expected to be relevant to a wide range of problems in the nonequilibrium behavior of low-dimensional magnets and other related ordering phenomena.