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Register a new userUnderstanding all-optical spin switching: Comparison between experiment and theory
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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.
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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.
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.
Transmission and reflectivity of La_x Ca_14-x Cu_24 O_41 two-leg spin-1/2 ladders were measured in the mid-infrared regime between 500 and 12000 1/cm. This allows us to determine the optical conductivity sigma_1 directly and with high sensitivity. Here we show data for x=4 and 5 with the electrical field polarized parallel to the rungs (E||a) and to the legs (E||c). Three characteristic peaks are identified as magnetic excitations by comparison with two different theoretical calculations.
We report all-optical switching due to state-filling in quantum dots (QDs) within a Mach-Zehnder Interferometric (MZI) switch. The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530-1570 nm probe beam is switched by optical excitation of one MZI-arm from the top. By exciting below the InGaAsP bandgap, we prove that the refractive index nonlinearity is only due to the QDs. The switching efficiency is 2 rad/(microW absorbed power). Probe wavelength insensitivity was obtained using a broad distribution of QDs.
The interaction of interfacial water with graphitic carbon at the atomic scale is studied as a function of the hydrophobicity of epitaxial graphene. High resolution X-ray reflectivity shows that the graphene-water contact angle is controlled by the average graphene thickness, due to the fraction of the film surface expressed as the epitaxial buffer layer whose contact angle (contact angle theta_c = 73{deg}) is substantially smaller than that of multilayer graphene (theta_c = 93{deg}). Classical and ab initio molecular dynamics simulations show that the reduced contact angle of the buffer layer is due to both its epitaxy with the SiC substrate and the presence of interfacial defects. This insight clarifies the relationship between interfacial water structure and hydrophobicity, in general, and suggests new routes to control interface properties of epitaxial graphene.
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