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Coherent optical excitations in van der Waals materials (2D polaritons) span a large range of optical phenomena which arise from their extraordinary dispersion relations [1]. Among the different polaritons, it is particularly interesting to examine polaritons in hyperbolic materials (hyperbolic phonon-polaritons, hPhP) which show rich physical behavior, ranging from negative refraction and subdiffraction imaging. The phononic resonance creates a dispersion relation that contains multiple branches, which were shown to be tunable by the 2D material geometry, thickness, and surrounding environment [2], reaching relatively low losses in room temperature [3].
Among the various experimental techniques that are used in the field of 2D polaritons, scanning nearfield optical microscopy (SNOM) and its variants dominates the direct nearfield imaging of 2D polaritons. Specifically, recent advances in time-resolved SNOM allowed adressing the polariton’s dynamical properties as group velocity from the changes of interference of scattered polaritons with different time delays [4]. However, this interferometric technique cannot image the wavepacket dynamics, as it excludes the phase difference between different wavelengths. Different imaging techniques such as electron energy loss spectroscopy (EELS), where the 2D polariton dispersion was recently extracted [5], obtains only time-independent information on the polaritonic modes. 
Here we present how an ultrafast transmission electron microscope (UTEM), driven by femtosecond mid-infrared pulses, can probe the spatiotemporal dynamics of 2D wavepackets.
We record how the mid-infrared pulses excites hPhP wavepacket and measure the wavepacket&#39;s trajectory inside the sample, revealing unforseen physical behaviors: multi-branch wavepacket splitting and wavepacket acceleration and deceleration. These phenomena are especially surprising since conventional wavepackets propagate in a fixed group velocity. We show that it is the unique dispersion relations of the hyperbolic PhPs that facilitates these phenomena, while recording group velocities between c/45 to c/850. These results examplify the wide physical spatiotemporal dynamical phenomena that can be probed through a non-destructive measurement using the unique combination of femtosecond temporal and nanometer spatial resolution of the UTEM.

Refs:
[1] D. N. Basov, A. Asenjo-Garcia, P. J. Schuck, X. Zhu, A. Rubio, Polariton panorama. Nanophotonics 10, 549–577 (2020).
[2] S. Dai et al. Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride. Science. 343, 1125–1129 (2014).
[3] A. J. Giles et al. Ultralow-loss polaritons in isotopically pure boron nitride. Nat. Mater. 17, 134–139 (2018).
[4] E. Yoxall et al. Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity. Nat. Photonics. 9, 674–678 (2015).
[5] N. Li et al. Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride. Nat. Mater. 20, 43–48 (2021).

MATERIALS FOR HUMANITY (MH 21)

Imaging the dynamics of 2D polariton wavepackets

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EMC 2021

Coherent optical excitations in van der Waals materials (2D polaritons) span a large range of optical phenomena which arise from their extraordinary dispersion relations [1]. Among the different polaritons, it is particularly interesting to examine polaritons in hyperbolic materials (hyperbolic phonon-polaritons, hPhP) which show rich physical behavior, ranging from negative refraction and subdiffraction imaging. The phononic resonance creates a dispersion relation that contains multiple branches, which were shown to be tunable by the 2D material geometry, thickness, and surrounding environment [2], reaching relatively low losses in room temperature [3].
Among the various experimental techniques that are used in the field of 2D polaritons, scanning nearfield optical microscopy (SNOM) and its variants dominates the direct nearfield imaging of 2D polaritons. Specifically, recent advances in time-resolved SNOM allowed adressing the polariton’s dynamical properties as group velocity from the changes of interference of scattered polaritons with different time delays [4]. However, this interferometric technique cannot image the wavepacket dynamics, as it excludes the phase difference between different wavelengths. Different imaging techniques such as electron energy loss spectroscopy (EELS), where the 2D polariton dispersion was recently extracted [5], obtains only time-independent information on the polaritonic modes. 
Here we present how an ultrafast transmission electron microscope (UTEM), driven by femtosecond mid-infrared pulses, can probe the spatiotemporal dynamics of 2D wavepackets.
We record how the mid-infrared pulses excites hPhP wavepacket and measure the wavepacket's trajectory inside the sample, revealing unforseen physical behaviors: multi-branch wavepacket splitting and wavepacket acceleration and deceleration. These phenomena are especially surprising since conventional wavepackets propagate in a fixed group velocity. We show that it is the unique dispersion relations of the hyperbolic PhPs that facilitates these phenomena, while recording group velocities between c/45 to c/850. These results examplify the wide physical spatiotemporal dynamical phenomena that can be probed through a non-destructive measurement using the unique combination of femtosecond temporal and nanometer spatial resolution of the UTEM.

Refs:
[1] D. N. Basov, A. Asenjo-Garcia, P. J. Schuck, X. Zhu, A. Rubio, Polariton panorama. Nanophotonics 10, 549–577 (2020).
[2] S. Dai et al. Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride. Science. 343, 1125–1129 (2014).
[3] A. J. Giles et al. Ultralow-loss polaritons in isotopically pure boron nitride. Nat. Mater. 17, 134–139 (2018).
[4] E. Yoxall et al. Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity. Nat. Photonics. 9, 674–678 (2015).
[5] N. Li et al. Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride. Nat. Mater. 20, 43–48 (2021).

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

We will discuss research on generating a wide range of colors by nanostructuring materials that are colorless in bulk. Unlike pigments and dyes that have specific colors due to their chemical composition, we generate colors by precise nanopatterning structures using two-photon polymerization (TPL) and electron beam lithography (EBL), shaping them into a variety on geometry out of metals, semiconductors, stacks of metal/dielectrics, and most recently low refractive-index polymers. For instance, we introduce a heat-shrinking method to produce 3D-printed photonic crystals with a 5x reduction in lattice constants, achieving sub-100-nm features exhibiting a full range of colors. We recently demonstrate four-dimensional (4D) printing of shape memory polymer (SMP) that imparts time responsive properties to 3D structures. A new SMP photoresist based on Vero Clear was used achieving print features at a resolution of~ 300 nm half pitch using TPL. Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. These structural colors could have an impact on humanity by providing potentially alternative solutions to spectral control in sensors to decorative colors in consumer products.

Colorless Materials For Humanity’s Color Needs

Unidirectional magnetoresistance (UMR) is a new type of MR effect, which is strongly related to the spin-orbit coupling (SOC) in various types of magnetic heterostructures, consisting of either conventional transition heavy metals or emergent materials systems such as topological insulators (TIs). UMR differs from other well-reported spin-dependent MR effects, such as the spin Hall magnetoresistance (SMR), that the resistance is current direction dependent. Although some seminal works have proposed its origin, especially for the UMR in normal metal/ferromagnetic metal (NM/FM) and TI/FM bilayer systems, the physical mechanisms that can lead to UMR still remain elusive. In this presentation, I will first introduce some of the common features and measurement protocols for UMR. Then I will discuss the possible scenarios that can lead to a sizable spin-dependent UMR in magnetic heterostructures: the spin Hall effect (SHE), Rashba-Edelstein effect (REE), and topological surface states (TSSs). Employing UMR for fast-track spin-orbit torque (SOT) characterization in a pump-probe fashion will also be demonstrated.

Unidirectional Spin-Dependent Magnetoresistance: Physics and Applications

Spin-orbit interaction (SOI) has been playing one of the most pivotal roles in condensed matter physics. Especially, the role of the SOI in spintronics and spin-orbitronics is quite significant and much effort has been paid to explore novel physics related with SOI in these research fields. 
Although a magnitude of the SOI of solids has been thought to be inherently determined and unchanged, our finding of gate-tunable SOI, resulting in control of the inverse spin Hall effect, opened a new field of gate-tuneable SOI [1,2]. Furthermore, we have recently discovered that a strong electric field application to a solid with inherently small SOI enables creation of synthetic Rashba fields and gate-tunability of them [3]. 
In this presentation, the recently-found gate modulation of SOI in solids will be discussed. 

References:
[1] S. Dushenko, M. Shiraishi et al., Nature Communications 9, 3118 (2018). 
[2] S. Yoshitake, M. Shiraishi et al., Appl. Phys. Lett. 117, 092406 (2020). 
[3] S. Lee, M. Shiraishi et al., to appear. 


Gate modulation of spin-orbit interaction in solids

Two-dimensional magnets based on van der Waals materials are currently fostering great expectations for the advancement of spin-orbitronics, which aims to exploit the spin-orbit coupling in non-centrosymmetric magnetic heterostructures to enable current-driven magnetic torques and stabilize homochiral magnetic textures. Van der Waals magnets are particularly appealing for this purpose as their properties can be tuned by surface engineering at the atomic level. In this talk, I will present our investigation of the spin-orbitronics properties of selected highly promising van der Waals candidates, both in pristine and Janus configurations, from first principles and effective tight-binding models. I will first discuss the spin-orbit torque that emerges from inversion symmetry breaking in magnetic transition metal dichalcogenide monolayers [1] and demonstrate that sizable torques can be obtained, whose magnitude is controlled by the electric dipole due to the chalcogen elements. Most importantly, I will show the existence of a unique spin-orbit torque component that allows for field-free current-driven switching, of highest interest for applications. Then, I will discuss the nature of Dzyaloshinskii-Moriya interaction in these systems, and how they can stabilize magnetic skyrmions, but also magnetic bimerons depending on the magnetic anisotropy properties [2]. Finally, I will discuss the spin-orbitronics properties of Fe3Ge2Te2 and show that in spite of its high symmetry, its inherent mirror symmetry breaking enables the onset of an in-plane Dzyaloshinskii-Moriya interaction that can stabilize planar chiral textures [3].

[1] Smaili et al., arXiv:2007.07579
[2] Laref et al., arXiv:2011.07813 
[3] Laref et al., Physical Review B 102, 060402(R) (2020)


Theory of spin-orbit torque and Dzyaloshinskii-Moriya interaction in van der Walls magnets

The discovery of the spin Hall effect (SHE) enabled the efficient generation and manipulation of the spin current. The magnetic spin Hall effect provides a unique opportunity to control the spin current and relevant device performance with controllable magnetization. In this talk, we report the magnetic spin Hall effect both in non-collinear antiferromagnet Mn3Pt and a collinear antiferromagnet Mn2Au. We generate tiny out-of-plane polarized spin current when the charge current is applied along the axis perpendicular to the magnetic mirror plane of Mn3Pt/permalloy bilayers, but robust when the current is parallel to the magnetic mirror plane in all of the Mn3Pt films with different orientations. In Mn2Au, the spin currents are generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. Therefore, we term the Néel vector-dependent spin Hal effect the ‘antiferromagnetic spin Hall effect’. The out-of-plane spins from the antiferromagnetic spin Hall effect are favourable for the efficient switching of perpendicular magnetized devices, which is required for high-density applications. The antiferromagnetic spin Hall effect adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin structure.

Antiferromagnetic spin Hall effect

In this presentation, microscopic structures and magnetic properties of the Fe5−xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet, are introduced. Our study demonstrate that the √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5−xGeTe2 due to its non‐centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties of the new vdW ferromagnet, Fe5−xGeTe2 for future topology‐based spin devices.

Helimagnetism of the Fe5GeTe2 crystal

The rising demand for computing power driven by real-world applications is driving significant research and development effort into discovery of new device technologies (e.g., mem-transistor, spintronics, etc.) that better meet the requirements of novel computing paradigms (e.g., bio-inspired neural networks, Ising machines, etc.). In this talk, we will present an overview of our research in exploring emerging non-volatile computing device technologies for novel computing paradigms from a devices-to-algorithms perspective. We will be highlighting some of the research gaps that need to be closed to enable architecture-level simulations that better capture the effects of stochasticity in emerging device technologies, and discussing the exploitation of physical phenomena in novel device technologies for enabling alternative computing paradigms.

A Devices-to-Algorithms Perspective on Non-volatile Computing Device Technologies

Inspired by the success in switching FM magnetization by various electrical means, AFM spintronics has received a great deal of attention in recent years. While most efforts have focused on electrical manipulations of a bulk AFM, interfacial effects between FM and AFM layers are also vitally important for spintronic devices.
Here we demonstrate that applying current pulses to Pt/Co/IrMn trilayers causes concurrent switching of FM magnetization and perpendicular exchange bias (EB)1. This implies that the FM magnetization and EB can be manipulated independently. When comparing with the conventional field-annealing approach to generate EB, SOT determines the FM magnetization direction and further provides effective disturbance to align the interfacial spins between the FM/AFM layers to the FM magnetization. We further develop a time-resolved resistance measurement technique to eliminate the possibility of EB reversal due to joule heating effects. Not only the direction of the exchange bias can be reversed, but its strength can also be controlled by electric current pulses. 
For manipulating in-plane exchange bias, we applied an anisotropic magneto-resistance (AMR) effect for studying the spin-orbit torque (SOT)-driven magnetization switching in PtMn/Co bilayer, under y-type SOT geometry2. The tailorable magneto-structural ordering of PtMn provides an additional dimension for the SOT switching. The results reveal the effective field generated by current in PtMn can be enhanced via forming L10 (antiferromagnetic) phase after annealing. On the other hand, the critical current for the y-type SOT switching is strongly associated with the PtMn/Co interfacial spin configuration. The lowest (highest) critical current is yielded when Co is antiferromagnetically (ferromagnetically) coupled to PtMn through the exchange bias. 
With further investigation and optimization along this direction, the current-pulse-induced EB shall have significant impacts on the SOT-MRAM and other spintronic devices.

1P.H. Lin, B. Y. Yang, M. H. Tsai, H. H. Lin, C.H. Lai, Nat. Mater. 18, 335, (2019)
2 C. Y Yang, L. C. He, Y. S. Yen, P. C. Chen, J. C. Chiu, S. D. Huang, C. H Tseng, and C. H. Lai, Appl. Phys. Lett. 118, 102403 (2021)



Manipulating exchange bias by spin–orbit torque

The precession of magnetization in a ferromagnet (FM) can transmit pure spin current into an adjacent heavy metal (HM) via spin pumping. This pure spin current gets converted to a charge current due to high spin orbit coupling (SOC) of the HM due to the inverse spin Hall effect (ISHE). I will discuss recent ISHE results on Co2Fe0.4Mn0.6Si/Pt bilayers, where Co2Fe0.4Mn0.6Si is a full Heusler alloy. Damping analysis indicates the presence of significant spin pumping at the interface of Co2Fe0.4Mn0.6Si and Pt [1]. I will also discuss ISHE experiments on some other combinations such as CoFeB/IrO2 and manganite based La0.66Sr0.34MnO3/Pt bilayers [2-3]. Recently AFM materials having high SOC have been found to be a good replacement of HM in spin current based study. We have performed the ISHE study of CoFeB (10 nm)/ AFM (d nm) where we considered various AFM such as Mn2Au, Mn3Ga, IrMn etc. The systematic angle dependent ISHE measurements have been carried out to disentangle the different spin rectification effects viz. anisotropic magnetoresistance and anomalous Hall effect [4-6]. Further I will show the ISHE study on topological insulator (TI)/ferromagnetic Bi2Se3/CoFeB films [7,8]. ISHE experiments have also been performed to demonstrate that TIs are potential candidates to replace HM as they possess high spin-orbit coupling. In the second part of my talk, I will discuss about magnetic skyrmion. Skyrmions in ultrathin films are believed to offer large performance advantages in the development of novel spintronics technologies [9]. Recent studies have shown that skyrmions can be stabilized by interfacial Dzyaloshinskii-Moriya Interaction (iDMI). The controlled nucleation, stabilization, smaller size and efficient motion of skyrmions at room temperature and low magnetic field are the major challenges for R&D purpose. In this context, I will discuss the generation of skyrmions from triangular antidot structure in a ferromagnetic nanotrack using local Oersted field [10]. The dependency of skyrmion size and shape for variable Dzyaloshinskii–Moriya interaction and uniaxial anisotropy will also be discussed [11]. Sometimes the shape of skyrmions is not perfectly circled which is explained by the presence of different local anisotropy energy [12] using micromagnetic simulations. I will discuss the effect of the interlayer exchange interaction on between skyrmion-antiskymion pair [13]. I will also discuss the current induced motion of skyrmions in Pt/CoFeB thin films. The presence of low pinning sites in the Pt/CoFeB thin films helps skyrmions to move with a low threshold current density [14].

Acknowledgement: I like to sincerely thank my group members and collaborators for their hardwork and kind cooperation. I also acknowledge the funding agencies. 

References: 
[1] B. B. Singh et al., NPG Asia Materials, 13, 9 (2021)
[2] B. Sahoo et al. arXiv:2102.03624
[3] P. Gupta et al., Nanoscale, 13, 2714-2719, (2021)
[4] B. B. Singh et al., Phy. Rev. Appl. 13, 044020 (2020)
[5] Koustuv Roy et al., arXiv:2102.03624
[6] B. B. Singh et al., Phys. Rev. B 102, 174444 (2020). 
[7] B. B. Singh et al., PSS Rapid 2018, 1800492 (2018) 
[8] B. B. Singh et al., ACS Applied Material & Interfaces 12, 53409 (2020).
[9] A. Fert et al., Nat. Rev. Mater. 2, 17031 (2017)
[10] A. K. Behera et al., J. Phys. D: Appl. Phys. 54 025001 (2021)
[11] A. K. Behera et al., J. Phys. D: Appl. Phys. 51, 285001 (2018)
[12] G. Pradhan et al., J. Magn. Magn. Mater. 528, 167805 (2021)
[13] M. M. M. Bhukta et al., arXiv:2010.07328
[14] B. Ojha et al., (unpublished).

Spintronics with heavy metals, topological insulators and antiferromagnets

We examine interfacial magnetic properties of ultra-thin 3d transition metal alloys deposited on a Pt underlayer. The interfacial Dzyaloshinskii-Moriya interaction (iDMI) and the interfacial perpendicular magnetic anisotropy (iPMA) are measured as a function of the average valence electron number per atom in 3d alloys from 25 (Mn) to 28 (Ni). Interestingly, both iDMI and iPMA exhibit the Slater-Pauling-like dependence on the electron number as the saturation magnetization does, and the three quantities are strongly correlated with each other. The iPMA and saturation magnetizations are calculated by virtual crystal approximation for FM monolayer alloys. We also calculated the iDMI, iPMA, and spin mixing conductance by employing tight binding single band model, by which the Slater-Pauling curve-like behaviors of iPMA and iDMI can be explained. Based on our finding, we claim that the strong correlation among the saturation magnetization, iPMA, and iDMI is manifested in a wide range of electron numbers.

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