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Register a new userProbing optical spin-currents using THz spin-waves in noncollinear magnetic bilayers
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Optically induced spin currents have proven to be useful in spintronics applications, allowing for sub-ps all-optical control of magnetization. However, the mechanism responsible for their generation is still heavily debated. Here we use the excitation of spin-current induced THz spin-waves in noncollinear bilayer structures to directly study optical spin-currents in the time domain. We measure a significant laser-fluence dependence of the spin-wave phase, which can quantitatively be explained assuming the spin current is proportional to the time derivative of the magnetization. Measurements of the absolute spin-wave phase, supported by theoretical calculations and micromagnetic simulations, suggest that a simple ballistic transport picture is sufficient to properly explain spin transport in our experiments and that the damping-like optical STT dominates THz spin-wave generation. Our findings suggest laser-induced demagnetization and spin-current generation share the same microscopic origin.
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Spin waves in antiferromagnetic materials have great potential for next-generation magnonic technologies. However, their properties and their dependence on the type of ground-state antiferromagnetic structure are still open questions. Here, we investigate theoretically spin waves in one- and two-dimensional model systems with a focus on noncollinear antiferromagnetic textures such as spin spirals and skyrmions of opposite topological charges. We address in particular the nonreciprocal spin excitations recently measured in bulk antiferromagnet $alpha$--$text{Cu}_2text{V}_2text{O}_7$ utilizing inelastic neutron scattering experiments [Phys. Rev. Lett. textbf{119}, 047201 (2017)], where we help to characterize the nature of the detected spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya interaction can lift the degeneracy of the spin-wave modes in antiferromagnets, resembling the electronic Rashba splitting. We consider the spin-wave excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss the features of their inelastic scattering spectra. We demonstrate that antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya interaction in certain antiferromagnets.
The emergence of two-dimensional (2D) layered magnetic materials has opened an exciting playground for both fundamental studies of magnetism in 2D and explorations of spinbased applications. Remarkable properties, including spin filtering in magnetic tunnel junctions and gate control of magnetic states, have recently been demonstrated in 2D magnetic materials. While these studies focus on the static properties, dynamic magnetic properties such as excitation and control of spin waves have remained elusive. Here we excite spin waves and probe their dynamics in antiferromagnetic CrI3 bilayers by employing an ultrafast optical pump/magneto-optical Kerr probe technique. We identify sub-terahertz magnetic resonances under an in-plane magnetic field, from which we determine the anisotropy and interlayer exchange fields and the spin damping rates. We further show tuning of antiferromagnetic resonances by tens of gigahertz through electrostatic gating. Our results shed light on magnetic excitations and spin dynamics in 2D magnetic materials, and demonstrate their unique potential for applications in ultrafast data storage and processing.
The interplay between spin and heat currents at magnetic insulator|nonmagnetic metal interfaces has been a subject of much scrutiny because of both fundamental physics and the promise for technological applications. While ferrimagnetic and, more recently, antiferromagnetic systems have been extensively investigated, a theory generalizing the heat-to-spin interconversion in noncollinear magnets is still lacking. Here, we establish a general framework for thermally-driven spin transport at the interface between a noncollinear magnet and a normal metal. Modeling the interfacial coupling between localized and itinerant magnetic moments via an exchange Hamiltonian, we derive an expression for the spin current, driven by a temperature difference, for an arbitrary noncollinear magnetic order. Our theory reproduces previously obtained results for ferromagnetic and antiferromagnet systems.
In cavity quantum electrodynamics, the multiple reflections of a photon between two mirrors defining a cavity is exploited to enhance the light-coupling of an intra-cavity atom. We show that this paradigm for enhancing the interaction of a flying particle with a localized object can be generalized to spintronics based on van der Waals 2D magnets. Upon tunneling through a magnetic bilayer, we find the spin transfer torques per electron incidence can become orders of magnitude larger than $hbar/2$, made possible by electrons multi-reflection path through the ferromagnetic monolayers as an intermediate of their angular momentum transfer. Over a broad energy range around the tunneling resonances, the damping-like spin transfer torque per electron tunneling features a universal value of $frac{hbar}{2} tan{frac{theta}{2}}$, depending only on the angle $theta$ between the magnetizations. These findings expand the scope of magnetization manipulations for high-performance and high-density storage based on van der Waals magnets.
Propagation of backward magnetostatic surface spin waves (SWs) in exchange coupled Co/FeNi bilayers are studied by using Brillouin light scattering (BLS) technique. Two types of SWs modes were identified in our BLS measurements. They are magnetostatic surface waves (MSSWs) mode and perpendicular standing spin waves (PSSWs) mode. The dispersion relations of MSSWs obtained from the Stokes and Anti-Stokes measurements display respectively positive and negative group velocities. The Anti-Stokes branch with positive phase velocities and negative group velocities, known as backward magnetostatic surface mode originates from the magnetostatic interaction of the bilayer. The experimental data are in good agreement with the theoretical calculations. Our results are useful for understanding the SWs propagation and miniaturizing SWs storage devices.
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