Quantum Theory Explains Ultrafast Demagnetization in FePt and FePd Through Second-Order Spin Moment Generation

The rapid demagnetization of materials with laser pulses represents a long-standing challenge in magnetism, lacking a comprehensive theoretical explanation due to its complex nonlinear optical behaviour. Now, G. P. Zhang, Y. H. Bai, and Thomas F. George from the University of Missouri-St. Louis, address this gap by developing a novel nonlinear optical theory focused on the fundamental spin moment, rather than magnetic susceptibility. Their work establishes that the light-induced spin moment arises from second-order processes, specifically difference frequency generation, which dominates the observed changes, and successfully distinguishes between the closely related magnetic materials FePt and FePd. Importantly, the theory accurately predicts a stronger light-induced spin moment in FePt compared to FePd, aligning with both ultrafast simulations and experimental observations, and provides a powerful new framework for understanding and comparing femtosecond demagnetization across diverse materials without relying on computationally intensive real-time calculations.

A ferromagnet has, for a long time, lacked a complete analytic theory because it falls neither within nonlinear optics nor traditional magnetism. This research addresses this gap by developing a nonlinear optical theory centred on the spin moment, rather than the more conventional susceptibility. The team employed group theory to determine that the lowest order of a nonzero spin moment in a centrosymmetric system is second order, where the second-order density matrix contains four terms of sum frequency generation and four terms of difference frequency generation. By tracing over the product of the density matrix and the spin matrix, the researchers compute the light-induced spin moment.

Laser-Induced Demagnetization in FePt and FePd Alloys

This research investigates ultrafast demagnetization in FePt and FePd alloys using first-principles calculations, aiming to understand how laser pulses affect their magnetic moments. Scientists modelled the electronic structure of these materials, revealing the availability of electronic states for excitation and examining the second-order density of states, which relates to their nonlinear optical response. Calculations, validated by testing computational settings, revealed that difference frequency generation is the dominant mechanism for inducing the magnetic moment, while sum frequency generation plays a lesser role. The study found that FePt exhibits a stronger response to laser pulses than FePd, with shorter pulses and higher energies generally leading to stronger demagnetization.

Intraband transitions also contribute significantly to this process. Comparing different theoretical approaches, scientists found that the choice of approximation can influence the predicted strength of demagnetization, but the overall trends remain consistent. These findings provide insights into the fundamental mechanisms of ultrafast demagnetization, offering a means to predict and compare the behaviour of different materials and paving the way for advanced magnetic storage and spintronic devices.

Second-Order Nonlinearity Drives Ultrafast Demagnetization

This work presents a new theoretical framework for understanding ultrafast demagnetization, a process where laser pulses alter the magnetic properties of materials on incredibly short timescales. Researchers developed a nonlinear optical quantum theory focused on the change in spin moment within ferromagnets, specifically examining FePt and FePd, crucial for magnetic recording technologies. Symmetry analysis revealed that the lowest non-zero spin moment occurs at the second order, establishing the foundation for their nonlinear optical theory. Through their calculations, scientists demonstrated that FePt exhibits a stronger spin moment reduction than FePd under the same laser fluence, attributed to the stronger spin-orbit coupling within FePt. Real-time simulations of spin moment changes confirmed strong agreement with experimental results, consistently showing that FePt demagnetizes more rapidly than FePd, regardless of laser photon energy, pulse duration, or fluence. This consistency with experimental data, alongside the theoretical framework, represents a significant step forward, offering a means to compute and compare different materials at a fundamental level without relying on computationally intensive real-time calculations, and promising to greatly enhance accessibility and reproducibility for researchers in ultrafast spintronics and all-optical spin switching.

Optical Rectification Drives Spin Moment Change

This research presents the first nonlinear optical quantum theory of demagnetization, addressing a long-standing gap in understanding how light interacts with magnetic materials. Scientists developed a theoretical framework starting with symmetry analysis, revealing that the second-order spin moment is the lowest order response in centrosymmetric systems. A key finding is that difference frequency generation processes dominate over sum frequency generation, demonstrating optical rectification in the change of spin moment, and that competition between two difference frequency generation terms determines the overall spin change. Importantly, this theory allows for the computation and comparison of light-induced spin moment changes between different magnetic materials from first principles, bypassing the need for time-consuming real-time calculations. Applying the theory to FePt and FePd, materials crucial for magnetic recording, researchers demonstrated that FePt demagnetizes more readily than FePd, even though their crystal and electronic structures are remarkably similar, a result confirmed by both real-time simulations and experimental observations.