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Register a new userAll-optical spin switching under different spin configurations
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We employ an atomic spin model and present a systematic investigation from a single spin to a large system of over a million spins. To have an efficient spin switching, the electron initial momentum direction must closely follow the spins orientation, so the orbital angular momentum is transverse to the spin and consequently the spin-orbit torque lies in the same direction as the spin. The module of the spin-orbit torque is $lambda |{bf S}||{bf r}||{bf P}| sqrt{cos^2alpha+cos^2beta-2cosalpha cosbeta cosgamma} $, where $alpha(beta)$ is the angle between spin {bf S} and position {bf r}(momentum { bf P}) and $gamma$ is the angle between {bf r} and {bf P}. These findings are manifested in a much larger system. The spin response depends on underlying spin structures. A linearly polarized laser pulse creates a dip in a uniform inplane-magnetized thin film, but has little effects on eel and Bloch walls. Both right- and left- circularly polarized light ($sigma^+$ and $sigma^-$) have stronger but different effects in both uniform spin domains and Neel walls. While $sigma^+$ light creates a basin of spins pointing down, $sigma^-$ light creates a mound of spins pointing up. In the vicinity of the structure spins are reversed, similar to the experimental observation. $sigma^+$ light has a dramatic effect, disrupting spins in Bloch walls. By contrast, $sigma^-$ light has a small effect on Bloch walls because $sigma^-$ only switches down spins up and once the spins already point up, there is no major effect.
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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 device dimension shrinks, heating becomes a major problem. The spintronics explores the spin degree of electrons and thus bypasses the heat, at least in principle. For this reason, spin-based technology offers a possible solution. In this review, we survey some of latest developments in all-optical switching (AOS), where ultrafast laser pulses are able to reverse spins from one direction to the other deterministically. But AOS only occurs in a special group of magnetic samples and within a narrow window of laser parameters. Some samples need multiple pulses to switch spins, while others need a single-shot pulse. To this end, there are several models available, but the underlying mechanism is still under debate. This review is different from other prior reviews in two aspects. First, we sacrifice the completeness of reviewing existing studies, while focusing on a limited set of experimental results that are highly reproducible in different labs and provide actual switched magnetic domain images. Second, we extract the common features from existing experiments that are critical to AOS, without favoring a particular switching mechanism. We emphasize that given the limited experimental data, it is really premature to identify a unified mechanism. We compare these features with our own model prediction, without resorting to a phenomenological scheme. We hope that this review serves the broad readership well.
Interest in all-optical spin switching (AOS) is growing rapidly. The recent discovery of AOS in Mn$_2$RuGa provides a much needed clean case of crystalline ferrimagnets for theoretical simulations. Here, we attempt to simulate it using the state-of-the-art first-principles method combined with the Heisenberg exchange model. We first compute the spin moments at two inequivalent manganese sites and then feed them into our model Hamiltonian. We employ an ultrafast laser pulse to switch the spins. We find that there is a similar optimal laser field amplitude to switch spins. However, we find that the exchange interaction has a significant effect on the system switchability. Weakening the exchange interaction could make the system unswitchable. This provides a crucial insight into the switching mechanism in ferrimagnets.
Since the first experimental observation of all-optical switching phenomena, intensive research has been focused on finding suitable magnetic systems that can be integrated as storage elements within spintronic devices and whose magnetization can be controlled through ultra-short single laser pulses. We report here atomistic spin simulations of all-optical switching in multilayered structures alternating n monolayers of Tb and m monolayers of Co. By using a two temperature model, we numerically calculate the thermal variation of the magnetization of each sublattice as well as the magnetization dynamics of [Tbn/Com] multilayers upon incidence of a single laser pulse. In particular, the condition to observe thermally-induced magnetization switching is investigated upon varying systematically both the composition of the sample (n,m) and the laser fluence. The samples with one monolayer of Tb as [Tb1/Co2] and [Tb1/Co3] are showing thermally induced magnetization switching above a fluence threshold. The reversal mechanism is mediated by the residual magnetization of the Tb lattice while the Co is fully demagnetized in agreement with the models developed for ferrimagnetic alloys. The switching is however not fully deterministic but the error rate can be tuned by the damping parameter. Increasing the number of monolayers the switching becomes completely stochastic. The intermixing at the Tb/Co interfaces appears to be a promising way to reduce the stochasticity. These results predict for the first time the possibility of TIMS in [Tb/Co] multilayers and suggest the occurrence of sub-picosecond magnetization reversal using single laser pulses.
All-optical spin switching is a potential trailblazer for information storage and communication at an unprecedented fast rate and 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 thirteen 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/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 GHz region.
Terahertz spin waves could be generated on-demand via all-optical manipulation of magnetization by femtosecond laser pulse. Here, we present an energy balance model, which explains the energy transfer rates from laser pulse to electron bath coupled with phonon, spin, and magnetization of five different magnetic metallic thin films: Iron, Cobalt, Nickel, Gadolinium and Ni$_{2}$MnSn Heusler alloy. Two types of transient magnetization dynamics emerge in metallic magnetic thin films based on their Curie temperatures (T$_{C}$): type I (Fe, Co, and Ni with T$_{C}$ > room temperature, RT) and type II films (Gd and Ni$_{2}$MnSn with T$_{C}$ $approx$ RT). We study the effect of laser fluence and pulse width for single Gaussian laser pulses and the effect of metal film thickness on magnetization dynamics. Spectral dynamics show that broadband spin waves up to 24 THz could be generated by all-optical manipulation of magnetization in these nanofilms.
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