Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?

All-optical spin reversal presents a new opportunity for spin manipulations, free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are found to have a perpendicular magnetic anisotropy (PMA), but it has been unknown whether PMA is necessary for the spin reversal. Here we theoretically investigate magnetic thin films with either PMA or in-plane magnetic anisotropy (IMA). Our results show that the spin reversal in IMA systems is possible, but only with a longer laser pulse and within a narrow laser parameter region. The spin reversal does not show a strong helicity dependence where the left- and right-circularly polarized light lead to the identical results. By contrast, the spin reversal in PMA systems is robust, provided both the spin angular momentum and laser field are strong enough while the magnetic anisotropy itself is not too strong. This explains why experimentally the majority of all-optical spin-reversal samples are found to have strong PMA and why spins in Fe nanoparticles only cant out of plane. It is the laser-induced spin-orbit torque that plays a key role in the spin reversal. Surprisingly, the same spin-orbit torque results in laser-induced spin rectification in spin-mixed configuration, a prediction that can be tested experimentally. Our results clearly point out that PMA is essential to the spin reversal, though there is an opportunity for in-plane spin reversal.Comment: 20 pages, 4 figures and one tabl

Switching ferromagnetic spins by an ultrafast laser pulse: Emergence of giant optical spin-orbit torque

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.Comment: 12 page2, 6 figures. Accepted to Europhysics Letters (2016). Extended version with the supplementary information. Contribution from Indiana State University,Europhysics Letters (2016

What Sets the Radial Locations of Warm Debris Disks?

The architectures of debris disks encode the history of planet formation in these systems. Studies of debris disks via their spectral energy distributions (SEDs) have found infrared excesses arising from cold dust, warm dust, or a combination of the two. The cold outer belts of many systems have been imaged, facilitating their study in great detail. Far less is known about the warm components, including the origin of the dust. The regularity of the disk temperatures indicates an underlying structure that may be linked to the water snow line. If the dust is generated from collisions in an exo-asteroid belt, the dust will likely trace the location of the water snow line in the primordial protoplanetary disk where planetesimal growth was enhanced. If instead the warm dust arises from the inward transport from a reservoir of icy material farther out in the system, the dust location is expected to be set by the current snow line. We analyze the SEDs of a large sample of debris disks with warm components. We find that warm components in single-component systems (those without detectable cold components) follow the primordial snow line rather than the current snow line, so they likely arise from exo-asteroid belts. While the locations of many warm components in two-component systems are also consistent with the primordial snow line, there is more diversity among these systems, suggesting additional effects play a role

Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: Beyond the rigid band approximation

The exchange interaction among electrons is one of the most fundamental quantum mechanical interactions in nature and underlies any magnetic phenomena from ferromagnetic ordering to magnetic storage. The current technology is built upon a thermal or magnetic field, but a frontier is emerging to directly control magnetism using ultrashort laser pulses. However, little is known about the fate of the exchange interaction. Here we report unambiguously that photoexcitation is capable of quenching the exchange interaction in all three $3d$ ferromagnetic metals. The entire process starts with a small number of photoexcited electrons which build up a new and self-destructive potential that collapses the system into a new state with a reduced exchange splitting. The spin moment reduction follows a Bloch-like law as $M_z(\Delta E)=M_z(0)(1-{\Delta E}/{\Delta E_0})^{\frac{1}{\beta}}$, where $\Delta E$ is the absorbed photon energy and $\beta$ is a scaling exponent. A good agreement is found between the experimental and our theoretical results. Our findings may have a broader implication for dynamic electron correlation effects in laser-excited iron-based superconductors, iron borate, rare-earth orthoferrites, hematites and rare-earth transition metal alloys.Comment: 16 pages, 3 figures, one supplementary material fil