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Using an ultrafast laser pulse to manipulate the spin degree of freedom has broad technological appeal. It allows one to control the spin dynamics on a femtosecond time scale. The discipline, commonly called femtomagnetism, started with the pioneering experiment by Beaurepaire and coworkers in 1996, who showed subpicosecond demagnetization occurs in magnetic Ni thin films. This finding has motivated extensive research worldwide. All-optical helicity-dependent spin switching (AOS) represents a new frontier in femtomagnetism, where a single ultrafast laser pulse or multiple pulses can permanently switch spin without any assistance from a magnetic field. This review summarizes some of the crucial aspects of this new discipline: key experimental findings, leading mechanisms, controversial issues, and possible future directions. The emphasis is on our latest investigation. We first develop the all-optical spin switching rule that determines how the switchability depends on the light helicity. This rule allows one to understand microscopically how the spin is reversed and why the circularly polarized light appears more powerful than the linearly polarized light. Then we invoke our latest spin-orbit coupled harmonic oscillator model to simulate single spin reversal. We consider both cw excitation and pulsed laser excitation. The results are in a good agreement with the experimental result. a We then extend the code to include the exchange interaction among different spin sites. We show where the "inverse Faraday field" comes from and how the laser affects the spin reversal nonlinearly. Our hope is that this review will motivate new experimental and theoretical investigations and discussions.

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-Faster magnetic recording technology is indispensable to massive data storage and big data sciences. All-optical spin switching offers a possible solution, but at present it is limited to a handful of expensive and complex rare-earth ferrimagnets. The spin switching in more abundant ferromagnets may significantly expand the scope of all-optical spin switching. Here by studying 40,000 ferromagnetic spins, we show that it is the optical spin-orbit torque that determines the course of spin switching in both ferromagnets and ferrimagnets. Spin switching occurs only if the effective spin angular momentum of each constituent in an alloy exceeds a critical value. Because of the strong exchange coupling, the spin switches much faster in ferromagnets than weakly-coupled ferrimagnets. This establishes a paradigm for all-optical spin switching. The resultant magnetic field (65 T) is so big that it will significantly reduce high current in spintronics, thus representing the beginning of photospintronics.

In magneto-optics the spin angular momentum Sz of a sample is indirectly probed by the rotation angle and ellipticity, which are mainly determined by the off-diagonal susceptibility χ (1) xy . A direct and analytic relation between Sz and χ (1) xy is necessary and of paramount importance to the success of magneto-optics, but is often difficult to acquire since quantum mechanically the relation is hidden in the sum-over-states. Here we propose a new and simple model to establish such a much needed relation. Our model is based on the Hookean model, but includes spin-orbit coupling. Under cw excitation, we show that χ (1) xy (ω) is indeed directly proportional to Sz for a fixed photon frequency ω. Such an elegant relation is encouraging, and we wonder whether our model can describe spin dynamics as well. By allowing the spin to change dynamically, to our surprise, our model predicts that an ultrafast laser pulse can induce a spin precession; with appropriate parameters, the laser can even reverse spin from one direction to another. This works for both the circularly and linearly polarized light. The spin reversal window is narrow. These unexpected results closely resemble all-optical helicity-dependent magnetic switching found in much more complicated ferrimagnetic rare earth compounds. Therefore, we believe that our spin-orbit coupled model may find some important applications in spin switching processes, a hot topic in femtomagnetism.Introduction. -Magneto-optical Faraday and Kerr techniques are indispensable to modern magnetism investigations [1,2]. Such a technique is an ideal tool to investigate both static and dynamic evolutions of spin [3][4][5][6], since it is a photon-in and photon-out technique and leaves the sample intact before and after the experimental measurement. The origin of the magneto-optics is rooted in the spin-orbit coupling and exchange interaction [7,8], where the rotational angle and ellipticity carry the information of the spin moment change. The classical understanding is developed through the harmonic oscillator model (Hookean model), augmented with the Lorentz force term [7,8],