Session Q52: Magnetization Dynamics and Spin-Orbitronics in 2D, Topological, and Semiconductor Materials

Damping, Anisotropy and Spin Wave Modes of Laser Patterned Low-Loss Ferrimagnet Vanadium Tetracyanoethylene

Vanadium tetracyanoethylene (V[TCNE]_x; x≈2) is a low magnetic damping organic-based ferrimagnet that is compatible with a wide variety of substrates. These properties make it promising for magnonics applications requiring integration of low-loss magnetic materials with microwave circuits. Here we present the integration of patterned V[TCNE]_x with a microwave waveguide. A V[TCNE]_x film is grown on an Al₂O₃ encapsulated coplanar waveguide patterned on a sapphire substrate. The film is then encapsulated with a glass coverslip and patterned with high intensity laser illumination to selectively remove magnetism. We study the damping, anisotropy, and spin wave modes of laser patterned structures. This approach enables in situ patterning and fine tuning of the resonance modes, compensating growth to growth variation in material properties like saturation magnetization and anisotropy.   

First Principles Calculations of the Electronic Structure of V(TCNE)2

Vanadium tetracyanoethylene, V(TCNE)₂, is a room temperature ferrimagnetic semiconductor with a T_c ~ 600 K, which has very low loss ferromagnetic resonance and spin-wave propagation. [1,2,3] We explore the electronic structure, optical properties, magnetic dynamics, magnetic anisotropy, and magnetoelastic properties using a plane-wave code VASP and XC functional Heyd-Scuseria-Ernzerhof (HSE06).[4,5,6,7] We also present features of Nb(TCNE)₂ and Cr(TCNE)₂, which are analogs with different transition metal ions, as well as features of doped V(TCNE)₂.

[1] H. Yu, M. Harberts, R. Adur, Y. Lu, P. C. Hammel, E. Johnston-Halperin, and A. J. Epstein, Appl. Phys. Lett.105, 012407 (2014).

[2] N. Zhu, X. Zhang, I. H. Froning, M. E. Flatté, E. Johnston-Halperin, and H. X. Tang, Appl. Phys. Lett. 109,082402 (2016).

[3] J. M. Manriquez, G. T. Yee, R. S. McLean, A. J. Epstein, and J. S. Miller, Science 252, 1415 (1991).

[4] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); ibid. 49, 14 251 (1994).

[5] G. Kresse and J. Furthmüller, Comput. Mat. Sci. 6, 15 (1996).

[6] G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11 169 (1996).

[7]  J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem. Phys. 118, 18 8207 (2003).   

Detection of nonlinear magnetic resonance phenomena using spin-dependent charge carrier recombination currents through deuterated poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV)

Experimental data have recently confirmed theoretical predictions of nonlinear magnetic resonant strong-drive phenomena using spin-dependent electronic transitions in organic semiconductor thin-films as observable [1]. Strong magnetic resonant drive occurs when the magnitude of the driving field is on the order or in excess of static Zeeman fields. We have extended this current-based spin measurement technique by a lock-in detection scheme that filters radiation-induced, spin-independent current signals from spin-dependent current signals, and we have used this to study resonant electron magnetic dipole transitions within a deuterated layer of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), similar to previous studies that were conducted without the lock-in scheme [1, 2]. The data obtained with this increased measurement accuracy confirms excellent agreement of the experimentally observed Bloch-Siegert shift for magnetic dipole transitions with theory. [1] Jamali, S., Mkhitaryan, V.V. et al.  Nat Commun 12, 465 (2021). [2] Waters, D., Joshi et al. Nature Phys 11, 910–914 (2015).   

Probing the Goldstone spin wave excitations of an easy-plane canted-antiferromagnet

Spin wave, also known as magnon, is one kind of low energy collective excitations in quantum magnets, which has the potential of transferring information in applications. Spin wave dispersions carry information about both magnon properties and the nature of the magnetic ground states. In experiments, specialized techniques, such as Inelastic neutron scattering and Brillouin light scattering, are usually needed in probing spin wave dispersions in different magnetic systems [1]. Here I will talk about a new all-electrical technique for quantum Hall magnets based on bilayer graphene van der Waals heterostructures.  With a Fabry-Pérot cavity structure, we electrically probe magnons with selective momenta and energies, which allow us to extract the dispersion. We obtained the gapless linear dispersion of spin waves in the predicted easy-plane canted antiferromagnetic order in the E=0 Landau level of bilayer graphene [2-4]. I will discuss the temperature and magnetic field dependence of the high-speed spin wave propagation based on the hydrodynamic theory. I will also discuss the potential of this technique in the exploration of collective excitations of spin and isospin-ordered ground states in van der Waals heterostructures.

[1] V. V. Kruglyak et al., J. Phys. D: Appl. Phys. 43, 264001 (2010)

[2] Kharitonov, PRL 109, 046803 (2012)

[3] Takei et al., PRL 116, 216801 (2016)

[4] Pientka et al., PRL 119, 027601 (2017)   

Chirality-Tuned Spin-to-Charge Conversion in Organic Semiconductor/Ferromagnetic Bilayer

Organic semiconductors attract tremendous interest due to their flexible, large-area, low-cost optoelectronics and spintronics applications. The weak spin-orbit coupling from their light-element composition enables long-distance spin current transport but hampers spin-to-charge (StC) interconversion. Here we report a giant spin pumping effect and highly efficient StC conversion in a chiral polymer/ ferromagnetic bilayer structure. We find that the StC conversion efficiency (coherent length : λ_IEE∽0.2nm ) is orders of magnitude higher than that in conventional organic semiconductors. Furthermore, we found chiral polymers with a longer pitch exhibit a higher StC conversion efficiency, and the effects such as chirality-induced inversion symmetry and chemical bonding angles are investigated. Our work sheds novel physical insights on the spin-orbit interaction in organic spintronic devices.   

Low-symmetry quantum materials for magnetization manipulation

Spin-orbit torque (SOT) driven deterministic control of the magnetization state of a magnet with perpendicular magnetic anisotropy (PMA) is key to next generation spintronic applications including non-volatile, ultrafast, and energy efficient data storage devices. But field-free deterministic switching of perpendicular magnetization remains a challenge because it requires an out-of-plane anti-damping torque, which is not allowed in conventional spin source materials such as heavy metals (HM) and topological insulators due to the system's symmetry. The exploitation of low-crystal symmetries in emergent quantum materials offers a unique approach to achieve SOTs with unconventional forms. We will discuss our experiments demonstrating field-free deterministic magnetic switching of perpendicularly polarized magnets employing an out-of-plane anti-damping SOT generated in layered WTe₂ system.   

Spintronic devices based on 2D van der Waals magnets

Over the past few years, 2D van der Waals (vdW) magnets have emerged as an exciting platform for spintronic devices with novel functionalities. For example, 2D magnet-based Magnetic Tunnel Junctions (MTJs) with magnetoresistances (MR) much larger than conventional MTJs have been demonstrated. Interestingly, judicious combination of materials from the vast and rapidly increasing 2D material library opens promising avenues to simultaneously achieve large MR and low resistance-area products critical for device integration. Here we demonstrate improved vdW spin-filter MTJs with high MR while maintaining low resistance-area products in the ON state. We also leverage the atomically flat interfaces and excellent magnetic properties of high-resistivity 2D magnets to perform fundamental studies on emerging phenomenon and efficient mechanisms for manipulation and control of magnetization by spin-orbit torques (SOTs). We report initial results on measurements of SOTs in vdW heterostructures of insulating 2D van der Waals magnets with 3D topological insulator materials. These systems provide a unique opportunity to systematically investigate and quantify effects of topological states on SOTs in high quality vdW heterostructures.   

Interfacial Spin-Orbit Torques and Magnetic Anisotropy in WSe2/Permalloy Bilayers

In recent years, there has been a growing interest in spin-orbit torques (SOTs) for magnetization manipulation in non-volatile magnetic memory devices. SOTs rely on the spin-orbit coupling of a nonmagnetic material, such as transition metal dichalcogenides (TMDs), coupled to a ferromagnet (FM) to convert a charge current into a torque on the FM’s magnetization. However, there is still no consensus on the microscopic origin of SOTs in TMD/FM bilayers.

To address this issue, we perform thickness-dependent SOT measurements in WSe₂/permalloy heterostructures, down to the monolayer limit. We observe large out-of-plane field-like torques, and for some devices, smaller in-plane antidamping-like torques, both with no clear thickness dependence. Our results strongly point to a Rashba spin-orbit coupling at the TMD/FM interface as the key ingredient for SOTs in these systems. Unexpectedly, we also observe a strong in-plane magnetic anisotropy induced in permalloy along the armchair crystal direction of the WSe₂ flake. Our results unveil the nature of the SOTs in TMD-based devices and their interaction with FM layers, highlighting the potential of TMDs for non-volatile devices for data processing and storage.   

Spin-Orbit Torque in Bilayers of Kagome Ferromagnet Fe3Sn2and Pt

Spin-orbit torque phenomena enable efficient manipulation of the magnetization in ferromagnet/heavy metal bilayer systems for prospective magnetic memory and logic applications. Kagome magnets are of particular interest for spin-orbit torque due to the interplay of magnetic order and the nontrivial band topology (e.g., flat bands and Dirac and Weyl points). Here we demonstrate spin-orbit torque and quantify its efficiency in a bilayer system of topological kagome ferromagnet Fe₃Sn₂ and platinum. We use two different techniques, one based on the quasistatic magneto-optic Kerr effect (MOKE) and another based on time-resolved MOKE, to quantify spin-orbit torque. Both techniques give a consistent value of the effective spin Hall angle of the Fe₃Sn₂/Pt system. Our work may lead to further advances in spintronics based on topological kagome magnets.   

Spin-orbit torque facilitated switching of Bi2Se3/NiFe heterostructures

As worldwide demand for computing grows exponentially and processors become increasingly powerful, the need for memory technologies that are energy efficient, non-volatile and capable of ultrafast read/write operations is urgent. One promising technique is the use of materials with spin-orbit coupling to control magnetic layers using spin-orbit torque. In this talk I will discuss our group’s ongoing work to leverage spin-momentum locking in topological insulators to manipulate magnets, as measured using Kerr Rotation and using both pulsed and DC current modalities. I will focus on recent results in which we have switched the in-plane moment of a NiFe layer in a Bi₂Se₃/NiFe structure in which the Bi₂Se₃ has been specially engineered to be bulk insulating, maximizing the spin-momentum locking in the charge current. We find that we are able to switch the NiFe layer using a current density in the Bi₂Se₃ more than an order of magnitude lower than required in more commonly used heavy metals. I will contextualize these results with transport and spin-torque ferromagnetic resonance measurements before closing with some discussion of future work.   

Spin-charge Dynamics in Topological Insulator Low-dimensional Ferromagnet Heterostructures

The recent experimental realization of two-dimensional (2D) ferromagnetic (FM) crystals like CrI₃ and CrGeTe₃ has sparked great interest with possible use in applications ranging from, spintronics, valleytronics, and application in magnetic memories. The low out-of-plane anisotropy and the experimental observation of 2D magnons in Cr-compounds, makes them an interesting candidate for fast memory devices. An interesting avenue of research lies in coupling 2D FMs with topological insulators (Tis). The edge states of topological insulators can act as spin-channels with high spin-polarizability. Moreover, depending on the direction of applied bias, the spin of the edge states can be switched due to spin-momentum locking. In this work, we theoretically model a 2D TI-FM heterostructure using an a tight-binding Hamiltonian for the TI and a Heisenberg Hamiltonian for the FM. We model the spin-charge dynamics using the non-equilibrium Green’s function and the time-quantified Monte-Carlo simulations. We show that by applying an electric bias on the 2D TI, the magnetic domain of the 2D FM can be switched. We finally show that the magnetic order of 2D FM also impacts the transport in the interfacing 2D TI, which can be used to design ultrafast memory devices.  

Spin-orbit torque study on MBE-grown Bi1-xSbx thin films

Strong spin-orbit torque (SOT) generated by the surface and bulk states in topological insulator (TI)/ferromagnet heterostructures offers new opportunities in thin film magnetization manipulation for low power-consumption magnetoresistive random-access memory devices. The Bi_1-xSb_x alloy system has recently attracted interest in this context because of energy-efficient SOT switching [Nature Mater. 17, 808 (2018)]. This motivated us to study current-induced SOT in thin film Bi_1-xSb_x/ferromagnet heterostructures via spin-torque ferromagnetic resonance (ST-FMR). We briefly describe the synthesis by molecular beam epitaxy of Bi_1-xSb_x thin films and their characterization via reflection high energy diffraction, x-ray diffraction, transmission electron spectroscopy, and in vacuo angle-resolved photoemission spectroscopy. We then discuss the ST-FMR measurement of SOT at room temperature in Bi_1-xSb_x thin films as we vary the composition within and outside the topological regime.   

Origin of the unidirectional and bilinear magnetoresistance in topological insulator/ferromagnet heterostructures

Unidirectional magnetoresistance (UMR) has been observed in heavy metal/ferromagnet (HM/FM) [1] and magnetic topological insulator (TI) [2] heterostructures. In both systems, magnon scattering (MS) is shown to play a significant role. However, whether the MS-UMR in these two systems share the same origin remains unclear. On the other hand, bilinear magnetoresistance (BMR) [3] was also reported in the topological insulator Bi₂Se₃, and several mechanisms including hexagonal warping, particle-hole asymmetry, and spin-orbit inhomogeneities have been revealed or proposed. In this study, we observed both UMR and BMR in various TI/FM heterostructures. By using an insulating magnetic layer, we clearly distinguish the MS-UMR in HM/FM and TI/FM as two different mechanisms. By further depositing Bi₂Se₃ on magnetic and nonmagnetic substrates, we show that the UMR and BMR in TI/FM originate from magnetic-proximity-effect-induced band distortion and nonlinear spin-to-charge conversion under time-reversal symmetry breaking conditions.

 

[1] C. O. Avci et al, Nat. Phys. 11, 570 (2015).

[2] K. Yasuda et al, Phys. Rev. Lett. 117, 127202 (2016).

[3] P. He et al, Nat. Phys. 14, 495 (2018).