Excitation-Induced Spin Reorientation Transition

• Excitation-induced spin reorientation transition is a process where external stimuli, such as ultrafast optical pulses or magnetic fields, abruptly alter a material's magnetic order.
• Experimental studies using techniques like XMLD and TMOKE reveal critical timescales (e.g., ~20 ps) and abrupt magnetization jumps indicative of first-order transitions.
• The phenomenon guides the design of ultrafast, energy-efficient spintronic devices by leveraging competitive anisotropy energies and tunable magnetic properties.

Excitation-induced spin reorientation transitions are processes in which the direction of magnetic order in a material undergoes an abrupt and well-defined change in response to external excitation. This excitation can take the form of ultrafast optical pulses, magnetic fields, or temperature variations. The phenomenon has been studied in a variety of systems, including frustrated antiferromagnets, itinerant alloys, rare-earth transition-metal compounds, and topological magnets, revealing a spectrum of microscopic mechanisms and dynamical responses spanning from picoseconds to quasi-static limits.

1. Spin Reorientation: Phenomenology and Definitions

A spin reorientation transition (SRT) occurs when the equilibrium direction of a system’s magnetic order parameter—whether the magnetization in ferro(magnetic) or the Néel vector in antiferromagnetic or ferrimagnetic materials—changes discontinuously between distinct crystallographic axes or planes. Excitation-induced SRT refers to these phenomena being driven not by quasi-static variation of thermodynamic control parameters (temperature, field) alone, but by external excitation such as ultrafast laser heating, pulsed fields, or current injection. The microscopic origin is typically a competition between anisotropy energies with different temperature or excitation dependencies, or a sudden field-induced instability of a particular stacking configuration. Prototypes of such behavior include:

• Field-induced 180° “umbrella” reorientation in kagome antiferromagnets, driven when the Zeeman energy overcomes interlayer couplings (Matan et al., 2011).
• Ultrafast optical switching of the Néel vector in orthoferrites, with characteristic timescales ~20 ps (&&&1&&&).
• Thermally driven collinear reorientation transitions in ferrimagnets or antiferromagnetic alloys, where competing anisotropies cross at a critical temperature or concentration (Ryan et al., 2024, Chang et al., 2018).

2. Experimental Signatures and Timescales

Excitation-induced SRTs manifest in abrupt anomalies in time-resolved or field-dependent magnetic observables. The timescale is determined by the mechanism of anisotropy change and subsequent angular momentum transfer:

• Field-driven SRT in jarosite: Magnetization M(H) shows a sharp jump at critical field $H_c$, with hysteresis indicative of a first-order transition. The jump corresponds to a 180° rotation of the canted moments on alternate kagome planes, as verified by magnetic Bragg peaks switching from (1,1,3/2) to (1,1,0) in neutron scattering. The transition is abrupt on experimental timescales, with $H_c(T)$ following a power law vanishing at $T_N$ (Matan et al., 2011).
• Ultrafast optical control in TmFeO$_3$: Pump-probe X-ray magnetic linear dichroism (XMLD) measurements directly track the time evolution of the Néel vector, revealing a single-exponential, 90° rotation in the a–c plane completed within $τ_\mathrm{rot} \simeq 20$ ps, with no measurable oscillations or out-of-plane component. The timescale is dictated by electron-phonon and spin-lattice coupling (Jana et al., 28 May 2025).
• Photoinduced reorientation in TbMn$_6$Sn$_6$: TMOKE at the Mn M-edge evidences a 12–24 ps rise time for in-plane magnetization following sub-100 fs pump pulses, with the timescale and amplitude fluence-dependent. Landau–Lifshitz–Gilbert (LLG) modeling, incorporating large meV-scale anisotropies, quantitatively tracks the experimental data. For sufficient excitation, complete 180° reversals of the net moment are achieved in $<$30 ps (Ryan et al., 2024).
• MnPt-based alloys: Temperature- and composition-driven SRTs reflect a delicate near-cancellation of magnetocrystalline anisotropy energy (MAE) contributions from distinct Brillouin zone regions. The SRT is quasi-static but its criticality and underlying mechanism can be mapped onto a dynamical framework using first-principles CPA+DLM simulations (Chang et al., 2018).

3. Microscopic Mechanisms and Theoretical Modeling

The microscopic origin of excitation-induced SRT varies with material system, yet generally emerges from interplay and competition among anisotropy terms, exchange interactions, and symmetry-allowed Dzyaloshinskii–Moriya (DM) couplings:

• Kagome antiferromagnets (jarosite): The spin Hamiltonian includes nearest-neighbor exchange $J_1$, DM vector $\mathbf{D}_{ij}$ with both in-plane ($D_p$) and out-of-plane ($D_z$) components, Zeeman coupling to field $\mathbf{H} \parallel c$, and weak ferromagnetic interplanar coupling $J_\perp$:

$$\hat{H} = \sum_{\langle ij \rangle} [ J_1\, \mathbf{S}_i \cdot \mathbf{S}_j + \mathbf{D}_{ij} \cdot (\mathbf{S}_i \times \mathbf{S}_j) ] - g\mu_B \sum_i \mathbf{H} \cdot \mathbf{S}_i + J_\perp \sum_{\langle \ell, \ell' \rangle} \sum_{i\in\ell, j\in\ell'} \mathbf{S}_{i,\ell} \cdot \mathbf{S}_{j,\ell'}$$

The SRT occurs at $g\mu_B H_c \Delta M \simeq 2|J_\perp| S^2$, with the resulting bulk magnetization jump and accompanying change in the magnetic unit cell period observed via neutron scattering (Matan et al., 2011).

• Ultrafast SRT in TmFeO$_3$ and TbMn$_6$Sn$_6$: The dynamics of the order parameter (Néel vector or net moment) are governed by the LLG equation in the presence of temperature-dependent anisotropy fields, with the free energy expanded as $$E_\mathrm{anis}(\theta) = K_1(T) \sin^2\theta + K_2(T) \sin^4\theta + ...$$, where $K_1$ and $K_2$ can be rapidly modulated by optical heating and subsequent thermalization. The torque generated by abrupt changes in $\partial E_\mathrm{anis}/\partial \theta$ drives fast, damped rotation of the order parameter without long-lived oscillations, with thermal relaxation setting the ultimate timescale (Jana et al., 28 May 2025, Ryan et al., 2024).
• Band-structure origins in MnPt alloys: The MAE arises from reciprocal-space-resolved contributions, with large, nearly cancelling positive (A-point) and negative (Γ–Z line) terms split by spin-orbit coupling. Temperature or excitation broadens these features asymmetrically, allowing the net anisotropy constant $K(T)$ to change sign, driving easy-axis (out-of-plane) to easy-plane (in-plane) SRT (Chang et al., 2018).
• Rare-earth orbital physics in TbMn$_6$Sn$_6$: The SR transition is driven by the thermal population of an isotropic excited state of the Tb$^{3+}$ ion separated by $\Delta E \simeq 15$ meV from the strongly uniaxial ground state. The two-state "orbital alloy" model predicts that when the fraction of Tb ions in the isotropic state exceeds a critical value ($n_\mathrm{iso}(T_{SR}) \simeq 0.3$), the average anisotropy vanishes and the Mn moments collectively reorient (Riberolles et al., 2023).

4. Characteristic Equations and Quantitative Relationships

The SRT is marked by abrupt changes in key observables, with theory and experiment aligned through analytic and numerical relations:

Observable	System/Method	Relevant Quantities & Equations
Magnetization jump at $H_c$	Jarosite (field-driven) (Matan et al., 2011)	$g\mu_B H_c \Delta M = 2|J_\perp|S^2$, hysteresis ~2 T, $\Delta M(0) \approx 0.054 \mu_B$
Ultrafast SRT time	TmFeO$_3$ (XMLD) (Jana et al., 28 May 2025)	$$\theta(t) \simeq 90^\circ [1 - \exp(-t/\tau_{rot})]$$, $\tau_{rot} \simeq 20$ ps
Photoinduced switching time	TbMn$_6$Sn$_6$ (TMOKE) (Ryan et al., 2024)	$A(t) = C[1 - \exp(-(t - t_0)/\tau)]$, with $\tau$ 12–24 ps (low fluence), LLG model matches quantitatively
SRT condition (anisotropy)	MnPt or TbMn$_6$Sn$_6$ (Chang et al., 2018, Riberolles et al., 2023)	$E_\mathrm{anis}(T) = K(T) \sin^2\theta + ...$, SRT at $K(T_s) = 0$ or when $n_\mathrm{iso}(T_{SR}) = n_c$

Theoretical and numerical simulation (CPA+DLM for alloys; LLG for dynamics) enable direct comparison with experiments.

5. Broader Context, Generality, and Applications

Excitation-induced SRTs represent an accessible route to ultrafast, energy-efficient switching of strongly anisotropic magnetic states. Notable implications include:

• Device Concepts: Potential for THz-speed antiferromagnetic spintronics, “write”/“read” cycles exceeding the speed of ferromagnetic analogues (Jana et al., 28 May 2025, Ryan et al., 2024).
• Topological Switching: Switching between easy-axis and easy-plane magnetic configurations tunes the Chern gap in kagome magnets; changes in moment orientation couple directly to topological charge transport (Riberolles et al., 2023).
• Ultrafast, All-Optical Switching: In ferrimagnets such as TbMn$_6$Sn$_6$, 180° switching of the net magnetization between stable states can be achieved on sub-30 ps timescales by modest laser heating, offering a path for robust, low-fluence data storage (Ryan et al., 2024).
• Criticality and Tunability: Dynamical and static SRTs are sensitive to material composition, disorder, dimensionality, and external stimuli, pointing to tunable magnetic properties controlled by excitation conditions (Chang et al., 2018, Matan et al., 2011).

6. Unresolved Issues and Future Directions

Key open questions and research frontiers involve:

• Control of Damping and Domain States: Realization of deterministic switching in device geometries requires control over domain size, spatial uniformity of excitation, and intrinsic damping, particularly in materials with strong inhomogeneity or slow relaxation (Ryan et al., 2024).
• Topological and Magnetoelectric Coupling: The link between SRT, net scalar chirality (as in jarosite), and emergent topological phenomena remains a fertile area for both experiment and theory (Matan et al., 2011, Riberolles et al., 2023).
• Extension to Multiferroic and Chiral Magnets: SRTs involving noncollinear orders or magnetoelectric coupling open the possibility for electric-field, strain, or current-driven ultrafast reorientation transitions.
• Timescale Engineering: Potential to engineer reorientation dynamics over a wide window—fs to ms—through choice of excitation, composition, and device structure.

Excitation-induced spin reorientation transitions, as revealed by these model systems and techniques, stand as prototypical examples of externally tunable, ultrafast switching phenomena in correlated electron systems, uniting band-structure physics, orbital-level anisotropy, and magnetic criticality in a common framework (Matan et al., 2011, Jana et al., 28 May 2025, Ryan et al., 2024, Chang et al., 2018, Riberolles et al., 2023).