Article Cite This: ACS Photonics 2019, 6, 1524−1532
pubs.acs.org/journal/apchd5
Shaping and Storing Magnetic Data Using Pulsed Plasmonic Nanoheating and Spin-Transfer Torque Frank Bello,*,† Stefano Sanvito,† Ortwin Hess,‡ and John F. Donegan† †
School of Physics, CRANN, and AMBER, Trinity College Dublin, Dublin 2, Ireland Blackett Laboratory, Department of Physics, Imperial College London, London, SW7 2AZ, United Kingdom
‡
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S Supporting Information *
ABSTRACT: Spin transfer techniques have in recent years become attractive as an alternative to electronic-based devices for use in a wide range of magnetic devices such as spin valves for magnetic sensing, tunnel junctions and random access memories (MRAM; see the following refs: Chappert, C.; Fert, A.; Nguyen Van Dau, F. The emergence of spin electronics in data storage. Nat. Mater. 2007, 6, 813−823; Prejbeanu, I. L.; Kerekes, M.; Sousa, R. C.; Sibuet, H; Redon, O.; Dieny, B.; Nozières, J. P. Thermally assisted MRAM. J. Phys. Condens. Matter 2007, 19, 165218; Jain, S., Ranjan, A.; Roy, K.; Raghunathan, A. Computing in memory with spin-transfer torque magnetic RAM. IEEE Transactions on Very Large Scale Integration (VLSI) Systems 2018, 26, 470−483). Much effort has been placed on pursuing ultrathin media, which are not only ferromagnetic, but demonstrate a large spin-dependent Seebeck effect (SDSE) capable of generating the required spin currents for efficient nanoscale operation of these devices (see the following refs: Xiao, J. In Spintronics for Next Generation Innovative Devices; Katsuaki, S., Saitoh E., Eds.; Wiley: New York, 2015; pp 125−140; Ghiasi, T. S.; Ingla-Aynés, J.; Kaverzin, A. A; van Wees, B. J. Large proximity-induced spin lifetime anisotropy in transition-metal dichalcogenide/graphene heterostructures. Nano Lett. 2017, 17, 7528−7532; Comtesse, D.; Geisler, B.; Entel, P.; Kratzer, P.; Szunyogh, L. First-principles study of spin-dependent thermoelectric properties of half-metallic Heusler thin films between platinum leads. Phys. Rev. B 2014, 89, 094410). By generating very large temperature gradients on the order of 10 K/nm, the SDSE can produce a spin current that transfers the spin polarization of a magnetic domain (MD) along the gradient (∝ heat current). As spin accumulates in an adjacent domain, it is in turn able to induce a torque on its magnetic moments (see Choi, G-M.; Min, B-C.; Lee, K-J.; Cahill, D.G. Spin current generated by thermally driven ultrafast demagnetization. Nat. Commun. 2014, 5, 4334). As improvements in magnetic materials continue, we show by assuming reasonable enhancements of SDSE that a plasmonic near-field transducer (NFT) is able to create the very high temperature gradients in order to manipulate spin states in the heated region. This advancement allows the spin polarization to be adjusted without going above the material’s Curie limit. It also allows the spin-transfer torque (STT) to be applied over areas as small as a few tens of nm2 within the film, thus being able to write magnetic data more precisely without affecting the alignment of other nearby domains. This is crucial in order to have a high-fidelity and high-density bit writing process on the nanoscale. Herein we demonstrate via numerical analysis the ability to control the time dynamics of STT, which range from the ultrafast (pico) to the nanosecond regime, using only the heat currents produced by the NFT. This enables direct control over the alignment of the magnetization within the domains using plasmonic nanoheating. KEYWORDS: spin-transfer torque, spin caloritronics, ultrathin films, heat-assisted magnetic recording, plasmonics, spin-dependent Seebeck effect, quantum information processing
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version.2−4 Heat-assisted magnetic recording, or HAMR, is a model example that merges these disciplines. HAMR is widely regarded as the signature technology to be used in the next generation of magnetic storage devices (due to market in 2019) as it takes advantage of plasmonic near-field transducers (NFTs) that are capable of subdiffraction focusing of light near or at room temperature.5−7 The NFT couples a photonic
pin caloritronics, in which the effects of introducing heat currents on magnetic moments is investigated, has lately attracted increasing attention as the fabrication of magnetic devices on the nanoscale progresses.1 At the same time, recent advancements in plasmonics and spintronics have created opportunities for the transformative manipulation of fields and processes within nanostructures. By combining caloritronics, plasmonics, and spintronics, one has a multidisciplinary field able to create scalable networks that involve ultrafast information processing, along with light−heat energy con© 2019 American Chemical Society
Received: February 21, 2019 Published: May 15, 2019 1524
DOI: 10.1021/acsphotonics.9b00295 ACS Photonics 2019, 6, 1524−1532
ACS Photonics
Article
Figure 1. Minimal model of a near-field transducer (NFT) with tapered semiconductor (Si), insulator (SiO2), and metal (Au) NFT used to generate heat along with sample dimensions. The input mode into the Si waveguide has a wavelength of 830 nm. A write pole and additional heatsink may be added for use with HAMR (see Supporting Information). The NFT sits above a magnetic layer, suitable for magnetic recording, in this case a ferromagnetic film (FePt) with two separate magnetic domains and a high Seebeck coefficient. The FePt film lies above underlayers (see Supporting Information) used to optimize the grain structure of the magnetic film along with a heatsink to improve the heat flow and temperature gradients that maximize the spin-dependent Seebeck effect (SDSE). A very large temperature gradient (∇T ≈ 10 K/nm) is able to produce a spin current via the spin-dependent Seebeck effect while the buildup of different spin currents also leads to changes in spin chemical potential (∇μS). This allows spin to diffuse between domains and contribute to the applied torque in MD2 (see main text).
spin current. The combination of the two avoids the need to include an external spin injector as in many previous applications of STT. Half-metals and Heusler alloys such as CoFeAl, CoMnSi, and NiMnGa are possible candidates with reported spin-dependent Seebeck coefficients approaching 102 μV/K and Curie temperatures ranging from 600 to 1000 K.10 FePt, in particular, is highly desirable in magnetic devices for its magnetic anisotropy, optimal grain sizes (5−25 nm), and Curie temperatures within the same temperature range. Though reported thermopower generation is low in FePt, as interest in heat-assisted STT has grown, FePt nanoparticle arrays or alloys have shown potential to increase electrical and spin conductance by a full order of magnitude.11−13 For device realization, MD1 and MD2 may either be part of two separate films or a single ferromagnetic material, for example, a FePt layer with magnetic domains set at two different spin polarizations. An appropriately tuned nonmagnetic spacer layer can also be used, which would allow the relative orientation of the magnetization in the two layers/ domains to be set as one wants. If the spacer layer is thin (
