Plexcitons: Dirac points and topological modes

Plexcitons are polaritonic modes that result from the strong coupling between excitons and plasmons. We consider plexcitons emerging from the interaction of excitons in an organic molecular layer with surface plasmons in a metallic film. We predict the emergence of Dirac cones in the two-dimensional bandstructure of plexcitons due to the inherent alignment of the excitonic transitions in the organic layer. These Dirac cones may open up in energy by simultaneously interfacing the metal with a magneto-optical layer and subjecting the whole system to a perpendicular magnetic field. The resulting energy gap becomes populated with topologically protected one-way modes which travel at the interface of this plexcitonic system. Our theoretical proposal suggests that plexcitons are a convenient and simple platform for the exploration of exotic phases of matter as well as of novel ways to direct energy flow at the nanoscale

Control and Local Measurement of the Spin Chemical Potential in a Magnetic Insulator

The spin chemical potential characterizes the tendency of spins to diffuse. Probing the spin chemical potential could provide insight into materials such as magnetic insulators and spin liquids and aid optimization of spintronic devices. Here, we introduce single-spin magnetometry as a generic platform for non-perturbative, nanoscale characterization of spin chemical potentials. We use this platform to investigate magnons in a magnetic insulator, surprisingly finding that the magnon chemical potential can be efficiently controlled by driving the system's ferromagnetic resonance. We introduce a symmetry-based two-fluid theory describing the underlying magnon processes, realize the first experimental determination of the local thermomagnonic torque, and illustrate the detection sensitivity using electrically controlled spin injection. Our results open the way for nanoscale control and imaging of spin transport in mesoscopic spin systems.Comment: 18 pages, 4 figure

Temporal evolution of auto-oscillations in a YIG/Pt microdisc driven by pulsed spin Hall effect-induced spin-transfer torque

The temporal evolution of pulsed Spin Hall Effect - Spin Transfer Torque (SHE-STT) driven auto-oscillations in a Yttrium Iron Garnet (YIG) / platinum (Pt) microdisc is studied experimentally using time-resolved Brillouin Light Scattering (BLS) spectroscopy. It is demonstrated that the frequency of the auto-oscillations is different in the center and at the edge of the investigated disc that is related to the simultaneous STT excitation of a bullet and a non-localized spin-wave mode. Furthermore, the magnetization precession intensity is found to saturate on a time scale of 20 ns or longer, depending on the current density. For this reason, our findings suggest that a proper ratio between the current and the pulse duration is of crucial importance for future STT-based devices.Comment: 4 pages, 3 figure

Tm3Fe5O12/Pt Heterostructures with Perpendicular Magnetic Anisotropy for Spintronic Applications

With recent developments in the field of spintronics, ferromagnetic insulator (FMI) thin films have emerged as an important component of spintronic devices. Ferrimagnetic yttrium iron garnet in particular is an excellent insulator with low Gilbert damping and a Curie temperature well above room temperature, and has been incorporated into heterostructures that exhibit a plethora of spintronic phenomena including spin pumping, spin Seebeck, and proximity effects. However, it has been a challenge to develop high quality sub-10 nm thickness FMI garnet films with perpendicular magnetic anisotropy (PMA) and PMA garnet/heavy metal heterostructures to facilitate advances in spin-current and anomalous Hall phenomena. Here, robust PMA in ultrathin thulium iron garnet (TmIG) films of high structural quality down to a thickness of 5.6 nm are demonstrated, which retain a saturation magnetization close to bulk. It is shown that TmIG/Pt bilayers exhibit a large spin Hall magnetoresistance (SMR) and SMR-driven anomalous Hall effect, which indicates efficient spin transmission across the TmIG/Pt interface. These measurements are used to quantify the interfacial spin mixing conductance in TmIG/Pt and the temperature-dependent PMA of the TmIG thin film

Evidence for ferromagnetic coupling at the doped topological insulator / ferrimagnetic insulator interface

One of the major obstacles of the magnetic topological insulators (TIs) impeding their practical use is the low Curie temperature (Tc). Very recently, we have demonstrated the enhancement of the magnetic ordering in Cr-doped Bi2Se3 by means of proximity to the high-Tc ferrimagnetic insulator (FMI) Y3Fe5O12 and found a large and rapidly decreasing penetration depth of the proximity effect, suggestive of a different carrier propagation process near the TI surface. Here we further present a study of the interfacial magnetic interaction of this TI/FMI heterostrucutre. The synchrotron-based X-ray magnetic circular dichroism (XMCD) technique was used to probe the nature of the exchange coupling of the Bi2−xCrxSe3/Y3Fe5O12 interface. We found that the Bi2−xCrxSe3 grown on Y3Fe5O12(111) predominately contains Cr3+ cations, and the spin direction of the Cr3+ is aligned parallel to that of tetrahedral Fe3+ of the YIG, revealing a ferromagnetic exchange coupling between the Bi2−xCrxSe3 and the Y3Fe5O12

Thin film rare earth iron garnets with perpendicular magnetic anisotropy for spintronic applications

Perpendicular magnetic anisotropy (PMA) in garnet thin films is important for achieving numerous spintronic applications including spin-orbit switching. In this study, we computationally investigated how to control PMA by tuning substrate strain in Holmium Iron Garnet (HoIG) films grown on five different (111) single crystal garnet substrates of Gadolinium Gallium Garnet (GGG, Gd3Ga5O12), Yttrium Aluminum Garnet (YAG, Y3Al5O12), Terbium Gallium Garnet (TGG, Tb3Ga5O12), Substituted Gadolinium Gallium Garnet (sGGG, Gd3Sc2Ga3O12), and Neodymium Gallium Garnet (NGG, Nd3Ga5O12). The negative sign of effective anisotropy energy density, Keff < 0, and anisotropy field, Ha < 0, determines the easy magnetization axis of the film to be perpendicular to the film surface. Here, we show that magnetoelastic anisotropy energy density determines the sign of the total anisotropy and it can be manipulated by altering the lattice parameter mismatch of the film and its substrate. Based on this study, HoIG is predicted to have PMA when grown on GGG, TGG and YAG among all five substrates mentioned. Moreover, the saturation field magnitude is calculated as an order of several hundreds of Oersteds, which is feasible in practical applications to saturate rare earth iron garnets with perpendicular magnetic anisotropy

Thickness-Dependent Double-Epitaxial Growth in Strained SrTi_0.7Co_0.3O_3−δ Films

Perovskite-structured SrTi_0.7Co_0.3O_3−δ (STCo) films of varying thicknesses were grown on SrTiO₃(001) substrates using pulsed laser deposition. Thin films grow with a cube-on-cube epitaxy, but for films exceeding a critical thickness of about 120 nm, a double-epitaxial microstructure was observed, in which (110)-oriented crystals nucleated within the (001)-oriented STCo matrix, both orientations being epitaxial with the substrate. The crystal structure, strain state, and magnetic properties are described as a function of film thickness. Both the magnetic moment and the coercivity show maxima at the critical thickness. The formation of a double-epitaxial microstructure provides a mechanism for strain relief in epitaxially mismatched films