Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$

Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator ${Sr}_{3} {Ir}_{2} O_{7}$

Ernest Pastor, David Moreno-Mencía, Maurizio Monti, Allan S. Johnson, Nina Fleischmann, Cuixiang Wang, Youguo Shi, Xuerong Liu, Daniel G. Mazzone, Mark P. M. Dean, and Simon Wall

Phys. Rev. B 105, 064409 – Published 7 February 2022

Abstract

In many strongly correlated insulators, antiferromagnetic order competes with exotic and technologically relevant phases, like superconductivity. While control of spin order is critical to stabilize different functional states, elucidating the mechanism of laser-induced demagnetization in complex oxides remains a challenge. It is unknown if the optical pulse can quench magnetization nonthermally or if it instead only acts as a heat source. Here, we use ultrafast, broadband, optical spectroscopy to track the responses of the electronic, lattice, and spin degrees of freedom and their relation to antiferromagnetism in the strongly spin-orbit coupled insulator ${Sr}_{3} {Ir}_{2} O_{7}$ . We find that magnetization can be rapidly and strongly suppressed on a sub-150 fs timescale. At low excitation fluences, the magnetic recovery is fast; however, the recovery time increases dramatically with the magnitude of demagnetization. At the same time, we show that the lattice, evidenced through the $A_{g}$ phonon frequencies, appears to remain below $T_{N}$ , suggesting that the system remains nonthermal during the optical modulation of spin order. We suggest that photogenerated spin defects are responsible for the long-lived demagnetized state and discuss its implications for optical control of solids.

Received 2 November 2021
Revised 22 December 2021
Accepted 25 January 2022

DOI:https://doi.org/10.1103/PhysRevB.105.064409

Physics Subject Headings (PhySH)

Demagnetization Magnetic order Magnetic phase transitions Magnetization dynamics Order parameters Phonons Ultrafast magnetic effects

Antiferromagnets Mott insulators Strongly correlated systems

Liquid nitrogen cooling Magnetic techniques Optical techniques Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ernest Pastor ^1,*,†, David Moreno-Mencía ^1,*, Maurizio Monti ², Allan S. Johnson ¹, Nina Fleischmann^1,2, Cuixiang Wang³, Youguo Shi³, Xuerong Liu⁴, Daniel G. Mazzone⁵, Mark P. M. Dean ⁶, and Simon Wall ^1,2,‡

¹ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain
²Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
⁵Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
⁶Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA

^*These authors contributed equally to this work.
^†ernest.pastor@icfo.eu
^‡simon.wall@phys.au.dk

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Vol. 105, Iss. 6 — 1 February 2022

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Figure 1
${Sr}_{3} {Ir}_{2} O_{7}$ and its optical response. (a) Temperature dependence of the static reflectivity above and below the Néel temperature $T_{N} \sim 280 K$ . The signal is shown relative to the value at 77 K [antiferromagnetic (AFM)]. Top: Lineout at ∼610 nm showing a change in reflectivity between low and high temperatures. For clarity, the signal is smoothed; raw data are displayed in Fig. S1 in the Supplemental Material [23]. (b) Schematic summary of the electronic structure of ${Sr}_{3} {Ir}_{2} O_{7}$ [25]. The strong spin-orbit coupling generates a $J_{eff} = \frac{1}{2}$ Mott-like insulating state characterized by relatively small bandgap ( $E_{g}$ ), a filled lower Hubbard band (LHB) and an empty upper Hubbard band (UHB). The spectral window of our broadband experiments is primarily sensitive to changes in the amplitude and energy of the charge transfer resonance induced by the magnetic, structural, and electronic degrees of freedom.
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Figure 2
Transient reflectivity response. Transient reflectivity change upon 800 nm excitation ( $F = 5 mJ {cm}^{- 2}$ ) at (a) 295 K [paramagnetic (PM)] and (b) 77 K [antiferromagnetic (AFM)]. For each temperature, the top panel shows a lineout at 0.14 ps time delay. Also displayed is an example of the coherent oscillations that modulate the transient response (obtained within 650–700 nm range). The amplitude of the oscillations is notably larger at 77 K when the sample is in the AFM state (for full kinetic traces see the Supplemental Material [23]).
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Figure 3
Changes in the magnetic degree of freedom. (a) Temperature dependence of the transient reflectivity at different probe wavelengths. For clarity, the traces are offset. The 610 nm region is both sensitive to magnetic order while showing a temperature-independent response above $T_{N}$ . (b) Temperature dependence of the magnetic degree of freedom ( $Δ R_{M}$ ) at different excitation fluences measured at 360 fs time delay obtained as Δ $R (T) - Δ R (T > T_{N}$ ) in the 610 nm region. The signal saturates at high fluences and at temperatures close the Néel temperature, as expected for light-induced demagnetization. We normalize the maximum $Δ R_{M}$ value at $10 mJ {cm}^{- 2}$ to 100% demagnetization. (c) Fluence dependence of the magnetic signal at different temperatures showing that full demagnetization is more rapidly attained at higher temperatures. (d) Time evolution of the magnetic signal measured at 77 K at different fluences showing fast and large (>40%) demagnetization as well a change from rapid recovery to permanent demagnetization at $F > 4 mJ {cm}^{- 2}$ (see also Fig. S5 in the Supplemental Material [23]). For comparison, the time evolution of the magnetic Bragg peak obtained from Ref. [20] is also shown (solid translucent lines, scaled).
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Figure 4
Changes in the lattice degree of freedom. (a) Fourier transform of the oscillations in the transient reflectivity at 77 K [antiferromagnetic (AFM)] and 295 K [paramagnetic (PM)] at $F = 2.5 mJ {cm}^{- 2}$ . The characteristic high and low energy $A_{1}$ _g modes can be captured. The amplitude decreases and the central frequency shifts with increasing temperature. (b) Change in the mode frequency as a function of excitation fluence at 77 K (AFM). For comparison, the frequency measured at 295 K (PM) at low fluences is also displayed (red square). Despite large demagnetization achieved at all fluences, the modes remain mostly unchanged and exhibit only an inflection at the highest fluences. Spectral response of the 146 ${cm}^{- 1}$ (c) and 182 ${cm}^{- 1}$ (d) modes as a function of fluence. The $182 {cm}^{- 1}$ mode couples to low wavelengths, while the $146 {cm}^{- 1}$ mode couples to longer wavelengths and exhibits a redshift with increasing fluence. (e) Maximum amplitude of the phonon modes as a function of fluence (obtained by fitting the spectral response, see the Supplemental Material for details [23]).
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Figure 5
Disorder-mediated recovery of magnetic order. (a) Ground state characterized by a large exchange coupling $(J)$ that leads to antiferromagnetic (AFM) order. (b) Photoexcitation at low fluences results in the formation of a small density of spin vacancies and doubly occupied states with zero net moment. Effective destruction of the moment is enabled by the small charge gap. Rapid recombination events regenerate AFM order, but recombination paths that produce parallel spin alignments are prevented by the dominant AFM exchange coupling. (c) Photoexcitation at high fluences. The formation of a large density of spin vacancies relaxes the recombination constraint, leading to a long-lived disordering of the spin system. (d) Monte Carlo magnetization dynamics in a three-dimensional spin- $\frac{1}{2}$ Ising model resulting from instantaneous and random spin flips at different degrees of excitation. If a small fraction of spins is flipped, the system recovers fast. In contrast, when many spins are flipped, inducing large disorder, long-lived change in magnetization occurs.
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Physical Review B

covering condensed matter and materials physics

Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator ${Sr}_{3} {Ir}_{2} O_{7}$

Ernest Pastor, David Moreno-Mencía, Maurizio Monti, Allan S. Johnson, Nina Fleischmann, Cuixiang Wang, Youguo Shi, Xuerong Liu, Daniel G. Mazzone, Mark P. M. Dean, and Simon Wall

Phys. Rev. B 105, 064409 – Published 7 February 2022

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Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator Sr3Ir2O7

Ernest Pastor, David Moreno-Mencía, Maurizio Monti, Allan S. Johnson, Nina Fleischmann, Cuixiang Wang, Youguo Shi, Xuerong Liu, Daniel G. Mazzone, Mark P. M. Dean, and Simon Wall

Phys. Rev. B 105, 064409 – Published 7 February 2022

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Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator ${Sr}_{3} {Ir}_{2} O_{7}$