Light-Induced Metastable Magnetization

• Light-induced metastable magnetization is the process where ultrafast optical pulses create long-lived magnetic states via nonthermal, coherent mechanisms.
• It leverages diverse methods including inverse Faraday effects, phonon-coupled spin dynamics, and defect-mediated processes for controlled magnetic switching.
• The phenomenon is enhanced near critical temperatures, with techniques like pump–probe and 2D THz spectroscopy revealing intricate spin–lattice interactions.

Light-induced metastable magnetization refers to the nonequilibrium generation of a long-lived net magnetic moment or magnetic state in a material via interaction with light (from near-IR to THz), such that the induced magnetic order persists well beyond the duration of the excitation and is stabilized via specific microscopic mechanisms. This phenomenon, which is of central interest in condensed matter physics and ultrafast spintronics, encompasses a variety of pathways—ranging from electronic (inverse Faraday effect), defect-mediated, and topological to lattice-coupled routes—facilitating tunable, switchable, and nonvolatile control of magnetization on timescales spanning femtoseconds to milliseconds.

1. Fundamental Mechanisms of Light-Induced Metastable Magnetization

The diverse mechanisms producing light-induced metastable magnetization can be grouped based on physical origins:

• Raman-type nonlinear optical processes and coherent excitation: In antiferromagnetic NiO, femtosecond laser pulses exploit impulsive stimulated Raman scattering to excite two degenerate magnetic oscillation modes, whose relative amplitude and phase determine the vectorial trajectory of magnetization. By using pairs of polarization-twisted pulses, full control over the magnetization vector can be achieved, leading to designed multidimensional magnetic motion (Kanda et al., 2011).
• Nonthermal spin transfer and femtosecond tilt: In ferromagnetic semiconductors such as (Ga,Mn)As, ultrafast (100 fs) optical pulses trigger coherent charge-spin transfer, non-thermal in nature, tilting magnetization and driving reversal between distinct metastable directions defined by magnetic anisotropy. This process is orchestrated by density matrix equations of motion coupled with tight-binding band calculations to capture carrier-spin dynamics (Kapetanakis et al., 2011).
• Inverse Faraday and related optomagnetic effects: In metals and metallic nanoparticles, circularly polarized light creates a second-order nonlinear response (IFE), coherently transferring angular momentum to electronic spins and/or orbits, resulting in sub-picosecond, non-thermal, and often helicity-dependent magnetization. The magnitude and spectral nature of the effect are dictated by the interplay of SOC, crystal symmetry, and light frequency (Berritta et al., 2016, Adamantopoulos et al., 27 Nov 2024, Cheng et al., 2019).
• Phonon-mediated and spin–lattice dynamics: In antiferromagnets and related compounds, driving specific optical phonon modes (IR or Raman active) nonlinearly modifies exchange couplings and reshapes the free-energy landscape. For example, in DyFeO₃ (Afanasiev et al., 2019) and FePS₃ (Ilyas et al., 8 Jul 2025, Ilyas et al., 19 Oct 2025), intense resonant mid-infrared or THz pulses displace lattice atoms, lowering the barrier between competing magnetic phases, and imprinting a new (metastable) magnetic configuration.
• Ultrafast quench and order parameter selection: Above-gap photoexcitation in low-symmetry antiferromagnetic semiconductors (e.g., CaMn₂Bi₂) rapidly melts equilibrium order, and nonequilibrium TDGL dynamics involving strong magnetoelastic coupling select a new metastable orientation of the AFM vector, which persists until thermal relaxation (Fichera et al., 2 Feb 2025).
• Defect-mediated and orbital phenomena: In SrTiO₃₋δ, oxygen vacancy–related defect complexes can be spin-polarized by circularly polarized sub-gap light, yielding long-lived magnetic moments at cryogenic temperatures (Rice et al., 2014).

2. Coherent Phononic Control and Nonlinear Lattice–Spin Interactions

In van der Waals antiferromagnets (e.g., FePS₃), intense THz pulses are used to resonantly and coherently control metastable magnetization by driving vibrational modes:

• IR–Raman phonon coupling: A sequence of resonant THz pulses excites a coherent IR-active phonon (4–5 THz) whose amplitude and phase can be modulated by two-pulse interference. Anharmonic (ionic Raman) coupling Q_R ∝ Q_IR² then nonlinearly drives a specific Raman mode (Ω_R ≈ 3.27 THz), whose displacement leads to asymmetric exchange modulation and hence a finite net magnetization (M_z).
• Modeling: Coupled equations for the IR phonon (Q_IR), Raman phonon (Q_R), and magnetization (M_z) under external driving highlight the necessity of anharmonic coupling for metastable state formation. First-principles DFT identifies the phonon eigenfrequencies and quantifies the spin–phonon coupling coefficients α.
• 2D THz spectroscopy: Experimentally, two-dimensional mapping of polarization-resolved THz response confirms the role of rectification (f_t ≈ 0) as evidence of strong nonlinear coupling and quasi-static lattice displacement between modes (Ilyas et al., 19 Oct 2025).
• Critical slowing down and stabilization: Proximity to the Néel temperature enhances critical fluctuations of the AFM order, amplifying both the magnitude and the lifetime of the light-induced magnetic state (see Section 3).

3. Lifetime, Metastability, and the Role of Critical Fluctuations

Unlike typical light-induced phases (e.g., transient superconductivity or ferroelectricity), the metastable magnetization in antiferromagnetic FePS₃ displays an exceptionally long lifetime, exceeding 2.5 ms (Ilyas et al., 8 Jul 2025):

• Critical slowing down: As temperature approaches Tₙ = 118 K, critical order parameter fluctuations diverge, dramatically slowing the relaxation (τ ∼ |T – Tₙ|^–νz), stabilizing the light-induced state.
• Mechanistic pathway: The mechanism relies on nonlinear excitation of phonons—field strengths exceeding the linear regime alter exchange couplings (J(Q) ≈ J – αQ)—producing new equilibria in the Landau free energy:

$$F = \frac{1}{2}a_L(T) L^2 + \frac{1}{4}b_L L^4 + \frac{1}{2}a_M(T) M^2 + \frac{1}{4}b_M M^4 + \frac{1}{2}\Omega^2 Q_2^2 + g LMQ_2,$$

where $L$ is zigzag AFM order, $M$ is a weak FM component, $Q_2$ is the key phonon, and $g$ is the coupling constant.

• Monte Carlo and atomistic simulations show both the necessity of nonlinear phonon displacement and reproduce the observed critical timescales and temperature dependence.
• Distinct from thermal switching: Because excitation is sub-gap and preserves spin-lattice integrity, the process avoids significant thermal load and supports long-lived, easily detectable metastable magnetization.

4. Experimental Realizations and Key Observables

A variety of pump–probe and probe-specific schemes are utilized to demonstrate and characterize light-induced metastable magnetization:

Material/Platform	Excitation Scheme	Observables
NiO (AFM oxide)	fs pulses, double-pol.	THz TDS (ellipticity)
(Ga,Mn)As semicon.	100-fs pump, tight-binding	Dens. matr., Kerr/TR-MOKE
FePS₃ (vdW AFM)	Intense THz double-pulse	Probe ellipticity, 2D TS
DyFeO₃ (orthoferrite)	Mid-IR phonon resonance	Faraday rotation
SrTiO₃₋δ (defect oxide)	Circularly-pol. subgap light	SQUID, MCD
Metals (Au NP, Fe, Co)	Circularly-pol. pulses	Faraday/MOKE/IPE

• Modes and signatures: Metastable states manifest as anomalies in THz polarization, net Kerr or Faraday rotation, and, for topological textures, persistent patterns in Lorentz transmission images.
• Coherent manipulation: Pulse sequencing (time/phase delay) and polarization control are required for steering vectorial components and maximizing metastability.

5. Technological and Conceptual Implications

• Spintronics and memory: The ms-scale stability of THz-induced magnetization in FePS₃ enables readout by Hall resistivity or other transport methods, highlighting the potential for nonvolatile, fast-response magnetic memory.
• Ultrafast phase and information control: The ability to program net magnetization via coherent lattice control opens avenues for energy-efficient, sub-ps switching. Devices that leverage metastable magnetization—robust to thermal decay and rewritable via optical protocols—are prospective candidates in ultrafast computation and quantum technologies.
• Broader reach: The central finding that metastability is enhanced near critical points due to order-parameter fluctuations suggests a generic design principle: targeting materials with large order-parameter susceptibilities and strong spin–lattice coupling near phase transitions maximizes the likelihood of stabilizing hidden, non-equilibrium states.
• Contrast with conventional optical switching: In contrast to laser-induced demagnetization or heating-based reversals, these processes—especially those using high-field, sub-gap THz irradiation—achieve state transformation via non-thermal, coherent pathways. This minimizes dissipation, preserves structural order, and is compatible with fragile quantum materials and complex, layered magnets.

6. Comparison with Other Light-Induced Metastable Phases

• Distinctness: In light-induced superconductivity and ferroelectricity, induced phases are typically transient (ps–ns), often relax thermally, and usually arise from direct carrier excitation. Metastable magnetization in FePS₃ (and related compounds) is achieved by nonlinear lattice driving, which modifies magnetic exchange on symmetry-selective bonds and leverages the free energy landscape to “trap” the magnetization for orders of magnitude longer.
• Topology and disorder: In some systems (e.g., quenched Fe thin films (Eggebrecht et al., 2016)), topologically protected metastable textures (vortices, Skyrmions) form via defect-pinning during ultrafast relaxation. For defect-polarization processes (SrTiO₃₋δ), highly persistent local magnetization is observed but typically at cryogenic temperatures and without collective order.
• Conceptual integration: The underlying principle across platforms remains the non-thermal, symmetry-targeted, and fluctuation-aided stabilization of nonequilibrium magnetic states with technologically relevant lifetimes and access protocols.

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Light-induced metastable magnetization is, thus, an umbrella for a family of nonequilibrium processes in which ultrafast optical driving—electronic or lattice, coherent or impulsive—reorders magnetic degrees of freedom and, under the right material conditions, produces new, long-lived macroscopic magnetic configurations. The synergy of lattice selectivity, symmetry constraints, critical fluctuations, and sophisticated ultrafast control is central to extending magnetic phase space far beyond equilibrium possibilities—a prospect with far-reaching implications for future condensed-matter research and device engineering.