Antiferromagnet materials have recently gained renewed interest due to their possible use in spintronics technologies, where spin transport is the foundation of their functionalities. In that respect metallic antiferromagnets are of particular interest, since they enable complex interplays between electronic charge transport, spin, optical, and magnetization dynamics. Here we review phenomena where the metallic conductivity provides unique perspectives for the practical use and fundamental properties of antiferromagnetic materials.

Perspective on Metallic Antiferromagnets

Weak hyperfine interactions and spin-orbit coupling (SOC) in organic materials result in long spin lifetimes, which is very promising for spintronics. On the other hand, they also make it challenging to achieve spin polarization, which is of crucial importance for spintronics devices. To overcome this obstacle, we have proposed a physical model for spin-polarized electron transport through a chiral molecule based on the chiral-induced spin selectivity. Because the transport in the chiral molecule is not an isolated one, but rather an electron coupled to its surrounding lattice distortions, namely, a spatial localized polaron, an indispensable polaron effect is incorporated in our model. We show that the polaron transport through the chiral molecule exhibits a spin-momentum-locked feature. Interestingly, no matter what their initial spin state is, all of the polarons could transmit through the molecule with their spins being aligned to the same orientation due to the effective ``inverse Faraday effect.'' The coexistence of the electron-lattice coupling and SOC results in the spin and lattice being coupled, which leads to a strongly enhanced spin coherence and then a very high spin polarization of $70%$. In addition, the effects of the helix pitch, polaron size, and drift velocity on spin polarization are also discussed. Our results open the possibility of using chiral molecules in spintronics applications and offer a paradigm for information processing and transmission.

Chiral-induced spin selectivity: A polaron transport model

Using the time-resolved magneto-optical Kerr effect method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after a single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales, and a defined magnetization reversal time of 460 fs is shown—the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed which reproduce HD-AOS demonstrating a linear path for magnetization reversal.

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Roles of heating and helicity in ultrafast all-optical magnetization switching in TbFeCo

Information technology depends on how one can control and manipulate signals accurately and quickly. Transistors are at the core of modern technology and are based on electron charges. But as the d...

Understanding all-optical spin switching: Comparison between experiment and theory

Laser-induced femtosecond demagnetization, femtomagnetism, offers a potential route to develop faster magnetic storage devices. It is generally believed that the traditional spin-wave theory, which is developed for thermally driven slow demagnetization, cannot explain this rapid demagnetization by design. Here, we show that this traditional spin-wave theory, once augmented by laser-induced spin–orbit torque, provides a highly efficient paradigm for demagnetization, by capturing low-energy spin-wave excitation that is absent in existing mechanisms. Our paradigm is different from existing ones but does not exclude them. Microscopically, we find that optical spin–orbit torque generates massive spin waves across several hundred lattice sites, collapsing the long-range spin-spin correlation within 20 fs. Our finding does not only explain new experiments but also establishes an alternative paradigm for femtomagnetism. It is expected to have far-reaching impacts on future research.

Spin-orbit torque-mediated spin-wave excitation as an alternative paradigm for femtomagnetism

All-optical spin switching is a potential trailblazer for information storage and communication at an unprecedented fast rate free of magnetic fields. However, the current wisdom is largely based on semiempirical models of effective magnetic fields and heat pulses, so it is difficult to provide high-speed design protocols for actual devices. Here, we carry out a massively parallel first-principles and model calculation for 13 spin systems and magnetic layers, free of any effective field, to establish a simpler and alternative paradigm of laser-induced ultrafast spin reversal and to point out a path to a full-integrated photospintronic device. It is the interplay of the optical selection rule and sublattice spin orderings that underlines seemingly irreconcilable helicity-dependent and -independent switchings. Using realistic experimental parameters, we predict that strong ferrimagnets, in particular, Laves phase C15 rare-earth alloys, meet the telecommunication energy requirement of 10 fJ, thus allowing a cost-effective subpicosecond laser to switch spin in the gigahertz region.

Y. Xue

Papers

First-principles and model simulation of all-optical spin reversal