Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te

arXiv:2608.26364 · cond-mat.mtrl-sci · Submitted 2026-08-26 · Read on arXiv

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Introduction to the show: ident: Condensed Matter. The day in condensed matter: materials, phases and the measurements under them.

Kai: We're looking at the paper: "Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te".

Mira: Here's the gist of the paper. Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te investigates the ultrafast magnetization dynamics of Ge0.85Mn0.15Te using time-resolved magneto-optical spectroscopy to identify two distinct laser-induced magnetic phenomena: coherent magnetization precession and transient modifications of magnetic ordering Core Investigation The study investigates the ultrafast magnetization dynamics of Ge0.85Mn0.15Te using time-resolved magneto-optical spectroscopy to identify two distinct laser-induced magnetic phenomena: coherent magnetization precession and transient modifications of magnetic ordering By separating magnetic and nonmagnetic contributions in the transient response, the researchers identified two distinct laser-induced magnetic phenomena, both resulting from photoinduced effective spin-orbit torque Coherent magnetization precession arises from a laser-induced increase in hole concentration, which modifies the occupation of Rashba-split spin-locked valence band states and alters the magnetic easy axis The transient change in magnetic ordering, observed as variations in the coercive field, is attributed to laser-induced modifications of the ferroelectric sublattice displacement, which affect the ferroelectric polarization and the associated Rashba spin-orbit interaction Experimental Setup and Characterization The experiment utilized an epitaxial Ge1-xMnxTe film with nominal xMn of 0.15 and a thickness of 50 nm grown on a BaF2(111) substrate Static magneto-optical characterization revealed that GeMnTe is a correlated spin glass at low temperatures, where coupling between individual ferrimagnetic clusters provides the long-range order The static MO characterization showed that the remanent magnetization's out-of-plane magnitude strongly depends on the magnetization history, even at zero applied field Mechanisms of Torque Generation The paper identifies two complementary pathways by which femtosecond optical excitation couples to the magnetic state of GeMnTe through its Rashba-coupled electronic structure 1.

Kai: The first mechanism is coherent magnetization precession, which is triggered by a photoinduced increase in hole concentration, altering the population of Rashba-split, spin-locked valence band states and consequently changing the magnetic easy axis This mechanism is fully analogous to the optical spin-orbit torque reported for the diluted magnetic semiconductor GaMnAs 2.

Mira: The second mechanism involves laser-induced modification of the sublattice shift, which leads to a change in the Rashba parameter and consequently modifies the band structure, acting as an effective spin-orbit torque Key Findings on Torque Effects The results demonstrate that two complementary pathways exist by which femtosecond optical excitation couples to the magnetic state of GeMnTe through its Rashba-coupled electronic structure 1.

Kai: The intensity-independent initial phase observed for 500 mT shows that the hole-concentration-related excitation mechanism triggers the magnetization precession for all intensities in the experiment 2.

Title and authors: Kai: To get into this paper, "Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te," we need to understand what that title actually means for how we control things with light.

Mira: It basically says they are using a laser to generate spin-orbit torques directly in a multiferroic semiconductor, meaning the material has both magnetic and electric properties that interact strongly.

Kai: They are showing that this isn't just static magnetism; you can use a femtosecond pulse to make the magnetization precess coherently or transiently change how it orders itself.

Lev: That implies we could potentially use light, not currents, to switch magnetic states very quickly, which is what we need for low-power memory or spintronics.

Mira: The key point they are making is that this laser excitation creates an effective spin-orbit torque through two separate mechanisms linked to the material's electronic structure.

Kai: They separate these mechanisms by looking at how they affect the hole concentration versus how they modify the ferroelectric sublattice displacement.

Lev: That separation is crucial because it tells us which physical parameter—the charge carriers or the structural displacement—is driving which magnetic effect.

The paper's summary: Kai: So, summarizing what they found in "Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te," they found that the laser triggers two different magnetic phenomena.

Mira: One is coherent magnetization precession, which comes from a photoinduced increase in hole concentration. This change modifies how the electrons occupy those Rashba-split spin-locked valence band states and it alters the magnetic easy axis of the material.

Lev: So, that's one pathway: more holes mean different spin textures because the Rashba splitting changes, and that directly shifts where things want to point magnetically.

Kai: The second phenomenon is a transient change in magnetic ordering seen as variations in the coercive field during the excitation. They attribute this to laser-induced modifications of the ferroelectric sublattice displacement, which then affects the ferroelectric polarization and its associated Rashba spin-orbit interaction.

Mira: So, that means they're seeing two effects: one is purely electronic, related to carriers affecting band states and easy axis direction, and the other is structural, related to how light shakes the electric dipoles.

Lev: That structural link makes it interesting because we have a direct way to couple an optical excitation to the ferroelectric state, which we usually treat separately from magnetism.

The paper's improvements: Kai: The authors suggest improvements by showing how these two mechanisms work together. They show that for the hole-concentration related excitation mechanism, there's an intensity-independent initial phase observed for a field of five hundred mT, meaning it triggers precession regardless of the laser intensity in that range.

Mira: And for the second mechanism involving sublattice shift modification, they suggest this is what causes variations in the hysteresis loops for intensities exceeding Ith. They also note that this effect saturates when the laser fluences go above two point five Ith, which is about sixteen mJ cm minus squared.

Lev: From an experimental perspective, knowing those intensity thresholds helps us know how much power we need to pump to get the structural change versus just getting a simple electronic response.

Kai: They also highlight that the pump-wavelength dependence doesn't strongly affect the magnetic dynamics within their measured range, which is good for designing a practical system.

Mira: The paper implies that by understanding these two distinct pathways, we might be able to engineer one effect over the other depending on what material parameters we tune.

Conclusion: Kai: To wrap up this talk on "Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te," the main point is that we have two complementary pathways for ultrafast optical excitation to couple to the magnetic state through the Rashba-coupled electronic structure.

Mira: More broadly, they demonstrate that the strong interplay among charge carriers, ferroelectric polarization, spin-orbit coupling, and magnetization enables ultrafast control of magnetic properties in these multiferroic semiconductors.

Lev: From a hardware perspective, it means we have two distinct control knobs: one for carrier density effects and one for structural displacement effects.

Kai: This second effect involving the sublattice shift modification was something they hadn't seen before in this material, which highlights new physics resulting from that multiferroic coupling.

Mira: It shows how the electric and magnetic degrees of freedom are tightly interwoven, which is a big deal for designing future devices.

Lev: For us building real hardware, understanding how these two mechanisms manifest under different laser fluences will be key to predicting system response overhead.

Faculty of Mathematics and Physics, Charles University · Institute of Physics ASCR, v.v.i. · Institut für Halbleiter-und FestkŤphysik, Johannes Kepler University · Institut de Physique, École Polytechnique Fພrale de Lausanne · Centre for Photon Science, Paul Scherrer Institut

cond-mat.mtrl-sci

Submitted: 2026-08-26

Updated: 2026-10-29

Comments: v2: AAM; licence: CC BY 4.0; Supplementary material is a part of this submission; Data necessary for replication of results are available in the Zenodo repository at http://doi.org/10.5281/zenodo.22116843

DOI: 10.1002/advs.78355

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 83/100

The gist: Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te investigates the ultrafast magnetization dynamics of Ge0.85Mn0.15Te using time-resolved magneto-optical spectroscopy to

Key concepts

Coherent Magnetization Precession: This effect occurs when a laser pulse increases hole concentration. This change modifies how electrons occupy Rashba-split valence band states, which in turn shifts the magnetic easy axis, leading to a coherent precession of the magnetization. It is analogous to optical spin-orbit torque in other semiconductors.
Transient Modification of Magnetic Ordering: This mechanism involves laser excitation causing modifications in the displacement of the ferroelectric sublattice. These changes affect both the ferroelectric polarization and the Rashba spin-orbit interaction, which manifests as a transient change in magnetic ordering, observable as variations in the coercive field.
Rashba Spin-Orbit Torque: This is a mechanism where spin-orbit coupling couples an electron's momentum to its spin. In this material, laser excitation modifies the electronic structure (via hole concentration or lattice shift), which acts as an effective torque that directly controls and modifies the magnetic state of GeMnTe.
Differential Signal ($\Delta\gamma_{dif}$): To isolate the true laser-induced magnetic dynamics from non-magnetic optical noise, researchers calculate $\Delta\gamma_{dif}$. This is done by taking the difference between magneto-optical signals measured with opposite field polarities. This differential signal is specifically identified as the component containing information about the laser-induced magnetization dynamics.

Terminology

Polarization — 96× in this paper · explained in 2 episodes
Hysteresis — 41× in this paper · explained in 3 episodes
Anisotropy — 7× in this paper · explained in 4 episodes
Spin-Orbit Interaction — 5× in this paper · explained in 2 episodes
Symmetry Breaking — 4× in this paper · explained in 4 episodes
Degeneracy — 2× in this paper · explained in 4 episodes
Phase Transition — 2× in this paper · explained in 3 episodes
Berry Curvature — 1× in this paper · explained in 8 episodes
Clarity — 1× in this paper · explained in 3 episodes
Core — 1× in this paper · explained in 2 episodes
Disentanglement — 1× in this paper · explained in 2 episodes
Entanglement — 1× in this paper · explained in 5 episodes
Exchange Interaction — 1× in this paper · explained in 2 episodes
Frustration — 1× in this paper · explained in 3 episodes
Linear Dichroism — 1× in this paper · explained in 2 episodes
Phase Transitions — 1× in this paper · explained in 2 episodes
Quantum Interference — 1× in this paper · explained in 3 episodes
Time-Reversal Symmetry Breaking — 1× in this paper · explained in 2 episodes

Summary

Laser-Induced Rashba Spin-Orbit Torques in Multiferroic Semiconductor (Ge,Mn)Te investigates the ultrafast magnetization dynamics of Ge0.85Mn0.15Te using time-resolved magneto-optical spectroscopy to identify two distinct laser-induced magnetic phenomena: coherent magnetization precession and transient modifications of magnetic ordering

Core Investigation

The study investigates the ultrafast magnetization dynamics of Ge0.85Mn0.15Te using time-resolved magneto-optical spectroscopy to identify two distinct laser-induced magnetic phenomena: coherent magnetization precession and transient modifications of magnetic ordering By separating magnetic and nonmagnetic contributions in the transient response, the researchers identified two distinct laser-induced magnetic phenomena, both resulting from photoinduced effective spin-orbit torque Coherent magnetization precession arises from a laser-induced increase in hole concentration, which modifies the occupation of Rashba-split spin-locked valence band states and alters the magnetic easy axis The transient change in magnetic ordering, observed as variations in the coercive field, is attributed to laser-induced modifications of the ferroelectric sublattice displacement, which affect the ferroelectric polarization and the associated Rashba spin-orbit interaction

Experimental Setup and Characterization

The experiment utilized an epitaxial Ge1-xMnxTe film with nominal xMn of 0.15 and a thickness of 50 nm grown on a BaF2(111) substrate Static magneto-optical characterization revealed that GeMnTe is a correlated spin glass at low temperatures, where coupling between individual ferrimagnetic clusters provides the long-range order The static MO characterization showed that the remanent magnetization's out-of-plane magnitude strongly depends on the magnetization history, even at zero applied field

Mechanisms of Torque Generation

The paper identifies two complementary pathways by which femtosecond optical excitation couples to the magnetic state of GeMnTe through its Rashba-coupled electronic structure 1. The first mechanism is coherent magnetization precession, which is triggered by a photoinduced increase in hole concentration, altering the population of Rashba-split, spin-locked valence band states and consequently changing the magnetic easy axis This mechanism is fully analogous to the optical spin-orbit torque reported for the diluted magnetic semiconductor GaMnAs 2. The second mechanism involves laser-induced modification of the sublattice shift, which leads to a change in the Rashba parameter and consequently modifies the band structure, acting as an effective spin-orbit torque

Key Findings on Torque Effects

The results demonstrate that two complementary pathways exist by which femtosecond optical excitation couples to the magnetic state of GeMnTe through its Rashba-coupled electronic structure 1. The intensity-independent initial phase observed for 500 mT shows that the hole-concentration-related excitation mechanism triggers the magnetization precession for all intensities in the experiment 2. The second mechanism, related to sublattice shift modification, is assumed to be responsible for the change in hysteresis loops for intensities exceeding Ith This effect saturates for laser fluences exceeding 2.5 Ith ≈ 16 mJ cm−2

Conclusion

The findings reveal two complementary pathways by which femtosecond optical excitation couples to the magnetic state of GeMnTe through its Rashba-coupled electronic structure More broadly, they demonstrate that the strong interplay among charge carriers, ferroelectric polarization, spin-orbit coupling, and magnetization enables ultrafast control of magnetic properties in multiferroic semiconductors The second effect had not been observed prior to this experiment in the multiferroic Rashba semiconductor (Ge,Mn)Te This highlights the new physics resulting from the multiferroic coupling

Methods Summary

The pump-probe experiment employed a time-resolved magneto-optical (MO) technique to detect changes in the MO response caused by a strong (pump) laser pulse To disentangle magnetic signals from nonmagnetic optical background contributions, the researchers calculated the difference between measured signals for opposite field polarities, denoted as Δγdif This signal was identified as the one from which information about the laser-induced magnetization dynamics can be deduced The results confirm that the measured MO signals are sensitive to both in-plane and out-of-plane projections of magnetization, allowing for a full reconstruction of the three-dimensional magnetization trajectory

Supplementary Analysis

Supplementary information provides extensive data supporting the interpretation, including polarization dependence studies which show that the field-dependent ellipticity signal is not strongly affected by the pump polarization Furthermore, measurements at temperatures above the sample Curie temperature confirm that the Δγdif signal is connected with magnetic order The dependence of the measured MO signals on external experimental geometry, such as the angle of incidence (AOI), indicates that these effects are sensitive to experimental setup The pump-wavelength dependence shows that the magnetic dynamics are not strongly affected by changing the pump wavelength within the measured range The comparison of ellipticity and rotation dynamics indicates that both magneto-optical detection channels are sensitive to the same magnetic response The long time range dynamics in the concentric geometry show that the decay of this signal does not change above Ith, resembling the dynamics of ΔT/T The oscillation period becomes longer as the magnetic field decreases, showing that the precession frequency is reduced at lower fields The oscillation amplitude A is nearly field-independent while the background amplitude B decreases for higher fields The precession damping time τ1 shows a moderate decrease at higher fields The signal Δγdif does not depend on AOI significantly The measured MO signals are not significantly affected by the wavelength of the pump pulses, indicating that the exact spectral position of the photoinjected carriers within the semiconductor band structure does not play a significant role in the torque properties The strong probe-wavelength-dependent sensitivity of the measured signals to carrier relaxation within the band structure makes probe wavelengths around 500 nm the best choice for studying early-stage magnetization dynamics The pump-induced changes in hysteresis loops reveal significantly different coercive fields compared to the static case, indicating a change in magnetic anisotropy in the excited material The most intriguing observation is the change in the ordering of the loop branches shown in Figure 6c: below-threshold intensities yield higher MO signals on the loop branch corresponding to increasing fields from negative values compared to the opposite branch, while the reverse is true above the threshold intensity The pump-induced changes in hysteresis loops reveal significantly different coercive fields (inset in Figure 5d) compared to the static case (Figure 2f), indicating a change in magnetic anisotropy in the excited material This observation raises the question of whether the laser-modified magnetic state can truly be considered a perturbed equilibrium state, or if ultrafast laser excitation brings the material to a completely different magnetic state The substantial laser-induced change of the coercive field observed in the Ge0.85Mn0.

Improvements for AI systems

Develop an all-optical spin-orbit torque generation module capable of coherent magnetization precession in multiferroic Rashba semiconductors using femtosecond laser pulses by leveraging a photoinduced increase in hole concentration to modify Rashba-split valence band states and alter the magnetic easy axis.
Create a nonvolatile memory device that utilizes ultrafast optical control over magnetic order by exploiting the second laser-induced phenomenon, where the transient change in magnetic ordering, observed as variations in the coercive field, is attributed to laser-induced modifications of the ferroelectric sublattice displacement.
Design a method for separating genuine laser-induced magnetization dynamics from non-magnetic optical background contributions by calculating the differential signal Delta gamma dif from measured data to isolate signals corresponding to laser-induced magnetization dynamics as described in Section 2.4.
Engineer a system capable of distinguishing between two distinct mechanisms for ultrafast magnetic control: one based on a photoinduced increase in hole concentration, which alters the population of Rashba-split, spin-locked valence band states, and another based on laser-induced modifications of the sublattice displacement.
Implement a device architecture where probe polarization dependence is used to fully reconstruct the three dimensional magnetization trajectory after a laser pulse impacts the studied material by analyzing signals sensitive to both in plane and out of plane projections of magnetization.

Abstract

The multiferroic semiconductor (Ge,Mn)Te exhibits ferroelectricity, strong Rashba spin-orbit coupling, and carrier-mediated magnetic order, making it a unique platform for exploring the interplay between electronic, structural, and magnetic degrees of freedom. In this study, we investigate the ultrafast magnetization dynamics of Ge0.85Mn0.15Te using time-resolved magneto-optical spectroscopy. By separating magnetic and nonmagnetic contributions in the transient response, we identify two distinct laser-induced magnetic phenomena, both arising from a photoinduced effective spin-orbit torque. Coherent magnetization precession is driven by a laser-induced increase in hole concentration, which modifies the occupation of Rashba split, spin-locked valence band states and shifts the position of the magnetic easy axis. The transient change in magnetic properties, observed as variations in the amplitude and coercive field of the pump-induced change in ellipticity hysteresis loops, is attributed to laser-induced modifications of the ferroelectric sublattice displacement, which affect the ferroelectric polarization and the associated Rashba spin-orbit interaction. While the first type of optical spin-orbit torque has already been observed in the diluted magnetic semiconductor (Ga,Mn)As, the second effect is unique to the multiferroic Rashba semiconductor (Ge,Mn)Te, opening new opportunities for all-optical manipulation of magnetic order and spin-orbit torque generation in spin-orbitronic devices.

Sources

Disentangling magnetic and optical contributions in ultrafast dynamics of antiperovskite non-collinear antiferromagnets