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 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 , which we show corresponds to the smallest thermodynamically stable -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 , 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.
- Received 12 April 2021
- Revised 16 August 2021
- Accepted 3 September 2021
DOI:https://doi.org/10.1103/PhysRevX.11.041023
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Optical manipulation of magnetic order in materials has great potential for information-technology applications. Ultrashort laser pulses can demagnetize a ferromagnet in tens of femtoseconds or can even switch the polarity of magnetic domains. After a short laser pulse, common wisdom suggests that spins will settle back to their thermal equilibrium condition, a picture that has so far proven applicable to many simple systems. We show that this is not necessarily the case for systems where the spin interactions are not so simple.
In our experiments, we optically stimulate the 3D long-range magnetic order in a layered material with a single femtosecond laser pulse. The resultant magnetic state is well ordered in two dimensions, while along the third dimension, between the layers it is fractured into very thin domains. Such unique quantum disorder persists indefinitely after the excitation. Interestingly, with increased laser fluence, the thickness of the ordered slabs saturates at a minimum value, which corresponds to the thermal limit for the quasi-2D magnetic order to be stable.
These observations naturally link to the dimensionality of the material and the magnetic coupling within a layer, which is far stronger than the coupling between layers. Because of this, the long-range 2D order is quickly reestablished after being briefly scrambled by photoexcitation but before it can be registered between layers, thus halting the recovery of the interlayer correlation. We refer to this new mechanism as “time-window mismatch.”
Our findings can offer guidance for a wide range of future studies on the nonequilibrium behavior of low-dimensional magnets and related ordering phenomena.