A groundbreaking study from Hebrew University of Jerusalem challenges long-held assumptions in magneto-optics by revealing that light’s magnetic field plays a much larger role than previously thought. Traditionally, the Faraday effect, which causes polarization rotation of light in a magnetic field, was attributed mostly to the electric field of light. However, the new research shows that the magnetic field of light contributes up to 75% of the effect in certain wavelengths. Using Terbium Gallium Garnet (TGG), a material known for its high magnetic susceptibility, the researchers discovered that light’s magnetic component exerts significant torque on material spins, reshaping our understanding of light-matter interactions. This finding opens up new possibilities for applications in spintronics, quantum computing, and advanced optical technologies, offering a new path for all-optical switching and data storage.
Long Version
Revolutionizing Magneto-Optics: The Surprising Influence of Light’s Magnetic Field on Material Interactions
In a landmark study published on November 19, 2025, researchers from the Hebrew University of Jerusalem unveiled a discovery that reshapes our understanding of light-matter interactions. For nearly 180 years since Michael Faraday’s initial observation in 1845, scientists have attributed the Faraday effect—where a static magnetic field induces circular birefringence and causes polarization rotation in light passing through a material—primarily to the electric field of light. However, this new research demonstrates that the magnetic field of light plays a far more substantial role than previously assumed, contributing up to 75% of the effect in certain wavelengths. Using Terbium Gallium Garnet (TGG), a paramagnetic crystal known for its high magnetic susceptibility and commonly employed in magneto-optical devices, the team quantified this magnetic torque, challenging foundational assumptions in optics and magnetism.
Historical Context and the Faraday Effect Fundamentals
The Faraday effect represents a cornerstone of light-matter interactions, manifesting as the rotation of the polarization plane of linearly polarized (LP) light under an applied static magnetic field. This phenomenon arises from circular dichroism, where right circularly polarized (RCP) and left circularly polarized (LCP) components of light experience differential absorption and phase shifts due to Zeeman energy splitting in atomic spins. Quantified by the Verdet constant, which measures the rotation angle per unit magnetic field and path length, the effect has been pivotal in developing optical isolators, sensors, and modulators. Traditionally, explanations centered on the electric field of light, with the optical magnetic field dismissed as negligible due to its relative weakness—typically orders of magnitude smaller than the electric component in electromagnetic radiation.
Yet, this oversight persisted despite hints from related phenomena like the inverse Faraday effect (IFE), where intense light induces magnetization in materials without an external field. In IFE, mechanisms such as ultrafast demagnetization, spin-orbit torque, and optical spin transfer torque have been explored, often invoking the Landau-Lifshitz-Gilbert (LLG) equation to model spin dynamics. The LLG equation describes magnetization evolution as (\frac{d\mathbf{m}}{dt} = -\gamma \mathbf{m} \times \mathbf{H} + \alpha \mathbf{m} \times \left( \frac{d\mathbf{m}}{dt} \right)), incorporating gyromagnetic ratio (\gamma), Gilbert damping (\alpha), and effective fields including anisotropy field and exchange energy. Recent advancements in all-optical helicity-dependent switching (AO-HDS) further highlighted non-thermal optomagnetic mechanisms, but the magnetic component’s role remained underexplored until now.
To enhance this understanding, consider how these dynamics extend to broader contexts: in spintronics, the interplay of spin angular momentum and angular momentum transfer enables precise control over electron heating and phonon relaxation, often modeled through the two-temperature model. This model separates electron and lattice temperatures, revealing how ultrafast timescales—from femtosecond pulses to picosecond and nanosecond exposures—govern thermal relaxation times and non-adiabatic transitions.
Challenging Assumptions: The Role of Light’s Magnetic Component
The Hebrew University study directly addresses this gap by demonstrating that the optical magnetic field exerts a magnetic torque on spins, akin to a static field, through Zeeman energy interactions. In experiments and simulations, the researchers applied the macrospin approximation to model spin behavior in TGG, revealing that this torque scales with optical intensity and fluence, particularly under continuous wave (CW) illumination or Gaussian pulse excitation. For instance, in the visible range around 633 nm or 800 nm, the magnetic contribution accounts for approximately 17% of the observed polarization rotation, rising to 70-75% in the infrared range at 1.3 µm. This wavelength independence in the off-resonant condition contrasts with the traditional 1/λ dependence of the Verdet constant, underscoring a photomagnetic mechanism distinct from thermal or electric-field-dominated processes.
Key to this insight is the breakdown of reciprocity between the Faraday effect and IFE at ultrafast timescales. While the Verdet constant for FE derives from steady-state equilibrium, involving DC magnetic susceptibility and relative electrical permittivity, the IFE operates in nonequilibrium conditions with non-adiabatic transitions, leading to differing constants. Simulations using the LLG equation showed longitudinal torque proportional to intensity difference (I_RCP – I_LCP), building quadratically with fluence in single-pulse regimes and linearly in multi-pulse exposures. Transverse torque, meanwhile, remains wavelength-independent, with dynamics influenced by carrier frequency, optical angular frequency, and phonon bath equilibration.
Enhancing this analysis, the research highlights how magnetic permeability and saturation magnetization modulate these torques, with Gilbert damping acting as a key dissipative factor. In materials like heavy metal (Pt, Pd) and ferromagnet (FM)/metallic bilayers, spin-orbit coupling amplifies effects, while Py nanomagnets exhibit full magnetization reversal under short exposure times, such as 0.1 seconds. Asymmetric absorption further contributes to reciprocity breakdown, and off-resonance steady states ensure minimal interference from carrier phase variations.
Experimental and Theoretical Framework
The team’s approach integrated numerical integration of the LLG equation with analytical derivations, incorporating Landau-Lifshitz-Bloch (LLB) extensions for thermal relaxation and spin-phonon equilibration. Parameters included saturation magnetization typical of Co-based films or yttrium iron garnet, with optical fluence varied via pulse duration (femtosecond to picosecond) and FWHM (full-width at half maximum). In TGG, a crystal with high Verdet constant and negligible anisotropy energy, the optical magnetic field induces spin voltage and torque, enhanced in ferromagnet (FM)/metallic bilayers like heavy metal (Pt, Pd) structures due to spin-orbit coupling.
Notably, the study reproduced IFE signatures: linear fluence dependence for fixed intensity, quadratic buildup in multi-pulse regimes, and helicity-driven switching under CW or rectangular quasi-CW pulses. Exposure times as short as 0.1 seconds enable full magnetization reversal in Py nanomagnets, with spot size and laser power modulating the effect. Asymmetric absorption and circular dichroism further amplify non-reciprocity, while off-resonance steady states minimize carrier phase impacts. Advanced considerations, such as Laguerre-Gaussian modes with orbital angular momentum and helical wavefronts, suggest potential for LP beams to mimic CP effects via stimulated magneto-Raman scattering.
Thermal mechanisms, including electron heating and phonon relaxation via the two-temperature model, interplay with optomagnetic ones, but the magnetic field’s contribution persists independently, scaling inversely with Gilbert damping and magnetic permeability. In THz excitation or ultrafast IFE scenarios, spin diffusion length and loss rates govern dynamics, with longitudinal relaxation negligible under typical conditions. The Hamiltonian incorporates free energy terms for induced magnetization, emphasizing spin texture influences like skyrmions in optical profiles.
To deepen this framework, the inclusion of nonlinear magneto-optical susceptibility allows for modeling intensity differences and helicity-dependent switching, while demagnetization processes in metallic capping layers reveal time-domain vectorial torque measurements. Optical profile variations, such as those from 800 nm or 1.3 µm wavelengths, demonstrate how wavenumber differences and pulse durations affect overall efficiency.
Broader Implications for Technology and Research
This revelation extends beyond theoretical optics, promising transformative applications in spintronics and quantum technologies. By harnessing the magnetic field of light for spin-based computing, researchers can develop low-power optical data storage and all-optical switching devices, where AO-HDS enables helicity-dependent control of nanomagnets without external fields. In quantum computing, enhanced manipulation of atomic spins via optical spin transfer torque could improve qubit stability and readout in systems reliant on nonlinear magneto-optical susceptibility.
For optical technologies, incorporating this magnetic component refines designs of isolators using TGG or similar materials, potentially boosting efficiency in visible and infrared ranges. Metallic capping layers in Co-based films amplify effects, while integration with heavy metal bilayers opens avenues for spin-orbit torque-driven devices. Future investigations may explore wavenumber differences, subpicosecond timescales, and nanosecond exposures to optimize optical intensity and fluence for practical implementations.
Researchers have noted that the static magnetic field twists the light, and the light, in turn, reveals the magnetic properties of the material. The magnetic part of light has a first-order effect—it’s surprisingly active in this process. Light interacts with matter not only through its electric field but also through its magnetic field, a component that has been largely overlooked until now. These insights position this discovery as a pivotal advancement, urging revisions to magneto-optic frameworks and inspiring innovations in spin-based devices.
In summary, this research not only rectifies a long-standing underestimation but also bridges classical optics with modern quantum applications, offering a robust foundation for future explorations in light’s multifaceted interactions with matter.
