Session Q54: Van der Waals Magnets

Generation and manipulation of spin current in topological semimetal heterostructures

Emerging topological and two-dimensional (2D) material systems synthesized by molecular beam epitaxy (MBE) have greatly expanded the materials toolkit used for fundamental studies of spin current and current-induced spin-orbit torques (SOTs). These MBE-grown heterostructures provide a potential pathway toward wafer-scale, energy-efficient, topological 2D spintronic devices for memory applications. In this talk, we discuss the generation and manipulation of spin current in MBE-grown topological semimetal/2D heterostructures. We first briefly discuss the measurement of charge-to-spin interconversion in 3D topological semimetals (TSs) of contemporary interest, including Dirac and Weyl semimetals [Phys. Rev. Appl. 16, 054031 (2021), ibid (in press)/arxiv: 2202.10656]. In their pristine state, the spin-Hall conductivity (SHC) of MBE-grown TS films quantitatively agrees with the intrinsic values predicted by first-principles calculations. We also observe that an oxidized interface enhances the SOT in these TSs. Next, we focus on the synthesis and characterization of a full van der Waals epitaxial heterostructure consisting of a Dirac semimetal, ZrTe₂, and a 2D ferromagnet, CrTe₂ [Nat. Comm. 13, 2972 (2022)].  We observe robust ferromagnetism with perpendicular magnetic anisotropy in quasi-2D CrTe₂ thin films down to one-unit-cell thickness. Anomalous Hall effect measurements in thicker CrTe₂ films suggest the existence of chiral magnetic structures. Finally, we demonstrate SOT-assisted magnetization switching in quasi-2D CrTe₂ via the current-induced SOT from ZrTe₂. Work done with W. Yanez, R. Xiao, M. Stanley, S. Ghosh, B. Zheng, Y.-S. Huang, T. Pillsbury, A. Richardella, T. Low, C.-X. Liu, V. H. Crespi, B.-H. Yan. K. A. Mkhoyan, and N. Samarth.   

Magnetic Phases of Semiconducting CrI2 monolayer

The recently discovered van der Waals magnetic semiconductor CrI₂ shows promise for spintronic applications. Using density functional theory (DFT) calculations, we show that the CrI₂ appears at the low Iodine concentration in the Cr-I phase diagram and possesses excellent magnetic properties, including large magnetic moment (~ 4.0 μB/Cr) and large magneto-crystalline anisotropy ~0.7 meV/Cr and good semiconducting properties including sizeable band gap ~ 2.0 eV with completely spin-polarized valence, and conduction bands with large exchange splitting of 0.44 eV and 1.66 eV respectively. Further, we show that the compound is easily exfoliable with cleavage energy, ~ 0.25 J/m², similar to CrI₃. Using the first and second nearest exchange interactions obtained by mapping DFT and Heisenberg Hamiltonian, we construct a magnetic phases diagram including all phases competing with minimum energy ferromagnetic phase. Our work contributes to understanding the CrI₂ monolayer and its applications in spintronic devices.

    

The generation, transmission and detection of magnons in graphene/2D magnet heterostructures

Spin waves, also known as magnons, are low-energy collective excitations of a magnetic system. The generation and control of magnons offer an alternative and potentially low-energy pathway of transmitting quantum information. Magnons are charge neutral and therefore more difficult to detect. Following previous studies [1][2], here we present an electric emission/detection scheme that allows the studies of magnons in a van der Waals magnet through the use of graphene quantum Hall edge states. We make graphene/2D magnet heterostructures and launch and detect magnons through non-local transport measurements in different regions of the device connected by the 2D magnet sheet only. Magnon transmission through the 2D magnet is observed. We analyze the dc bias, temperature, power and magnetic field dependence of the non-local signal to understand this process, which involves the emission of magnons in graphene, transmission through the graphene/magnet interface, and the detection in graphene again. This method can potentially be used to study the ground state magnetism and excitations of a large number of 2D magnets.

References:

[1] Wei et al, Electrical generation and detection of spin waves in a quantum Hall ferromagnet. Science, 2018, 362(6411), 229-233.

[2] Fu, et al, Gapless spin wave transport through a quantum canted antiferromagnet. Physical Review X, 2021, 11(2), 021012.   

Magnon spin transport in the van der Waals layered antiferromagnet CrPS4

The magnetic ordering in insulating two-dimensional (2D) van der Waals materials opens new possibilities for magnetoelectronic devices based on 2D systems. Yet, their instability in air and non-trivial device fabrication hindered the development of controllable information transport purely by spin waves -magnons- so far, allowing only the study of spin caloritronic effects driven by thermal gradients [1,2].  For the first time, we demonstrate direct magnon spin transport in air-stable insulating layered antiferromagnet Chromium thiophosphate (CrPS₄). We experimentally show magnon excitation and detection in the linear response and transport through exfoliated CrPS₄ flakes over distances up to 1 µm below the Neel temperature of CrPS4 (T_N = 38 Kelvin) and address the transport by the different magnon modes [3]. These results pave a way for the study of low dimensional antiferromagnetic spin waves and are an important step in the creation of magnonics devices for controllable spin information transport.

 

[1] T. Liu, et al., Physical Review B, 20, 205407 (2020)

[2] W. Xing et al. Physical Review X, 9.1, 011026 (2019)

[3] D.K. de Wal et al. Submitted to Physical Review Letters   

The spin-flop transition in the quasi-two-dimensional antiferromagnet MnPS3 detected via thermally generated magnon transport

Antiferromagnetic materials (AFM) have gained great interest for information storage and as a medium for spin currents in spintronic devices because they do not possess stray fields, are robust against magnetic perturbations, and have ultra-fast magnetic dynamics. Characterizing and probing magnetic transitions in (quasi) 2-dimensional magnetic van der Waals materials is crucial to understand magnetism at a low dimensional limit; for example by characterizing the spin-flop (SF) transition in uniaxial AFM. We present the detection of the spin-flop transition in the AFM van der Waals material MnPS₃ via thermally generated nonlocal magnon transport using permalloy detector strips [1]. The inverse anomalous spin Hall effect has the unique power to detect an out-of-plane (OOP) spin accumulation which enables us to detect magnons with an OOP polarization; in contrast to Pt which only possesses the spin Hall effect and is only sensitive to an in-plane spin polarization [2]. Our measurements show the detection of magnons generated by the spin Seebeck effect before and after the SF transition where the signal reversal of the magnon spin accumulation agrees with the OOP spin polarization carried by magnon modes before and after the SF transition [1].

[1] F.Feringa, J.M. Vink and B.J. van Wees, https://arxiv.org/abs/2210.01418 (2022), submitted to PRL

[2] K. S. Das, J. Liu, B. J. van Wees, and I. J. Vera-Marun, Nano Letters 18, 5633 (2018).

    

Generalized anisotropic spin Hamiltonian for NiPS3: spin-flop transition and dipole interaction

Two-dimensional (2D) van der Waals (vdW) magnets have attracted considerable attention owing to their fascinating property and promising applicability. NiPS₃ has become a focus of recent research interest by displaying several unusual phenomena such as coherent many-body exciton condensation and highly anisotropic magneto-optical behavior. However, the generalized spin Hamiltonian relevant to this material is yet to be established, while NiPS₃ was considered as a realizing 2D XY- or XXZ- model, which is incapable of entirely elucidating the experimental anisotropic natures of such materials. Furthermore, recent experiments have seen the characteristic behaviors related to a spin-flop transition which is crucially determined by magnetic anisotropy. In this study, we use torque magnetometer and density functional theory to construct a general spin Hamiltonian of NiPS₃. Our investigations clearly show a spin-flop transition which is described by our spin Hamiltonian. Our results indicate that the magnetic anisotropic energy order principally originates from magnetic dipole-dipole interactions. Our results demonstrate the merits of magnetic vdW materials and related systems for the pursuit of spintronic or magnetic device functionalities.   

Layer dependance of Kondo temperature and linear magnetoresistance of 2D Ferromagnets: Fe5Ge2Te2 and Fe3GeTe2

Two-dimensional (2D) metallic ferromagnets including Fe₅Ge₂Te₂ (FG2T) and Fe₃GeTe₂ (FGT) exhibit a variety of fascinating physical properties. In particular, recent experimental studies on FGT have shown evidence of strong electron correlation indicative of f-electron-like characteristics. In this work, we focus on the low-temperature transport and magneto-transport behaviors in both FGT and FG2T nano-devices over a range of thicknesses. We fabricate FG2T and FGT nanodevices by exfoliating flakes from crystals grown by solid-state reaction down to the monolayer thickness. Most FGT and FG2T devices are protected by a thin layer of sputtered Al2O3. For some thin FGT devices, the FGT flake is picked up by BN and transferred onto pre-patterned electrodes inside glovebox to avoid further lithography and air exposure.  In devices thicker than 3 layers, we find that the resistivity shows a clear minimum, followed by an upturn which leads to a saturation at lower temperatures. This overall low-temperature behavior can be very well fitted by the Kondo effect with a nearly constant Kondo temperature value of 34 K, independent of the number of layers in both FG2T and FGT. In these devices, the magnetoresistance is found to be linear up to 14 T.  The resistivity minimum in thin devices (3 layers and below) is shifted to much higher temperatures due to an apparent insulating temperature-dependent background. Discussions of the low-temperature transport and magneto-transport data will be presented.   

Surface magnetoresistance in antiferromagnetic semiconductor CrSb2

Surface transport in bulk semiconducting materials has attracted much attention recently. In this respect, CrSb₂ is an interesting material, since it is an antiferromagnetic narrow-gap semiconductor with a metallic surface state [1]. The previous report [1] has shown that the surface conduction dominates over bulk one at low temperatures below ~15 K. However, the magnetic property of the surface state remains to be investigated in contrast to the established antiferromagnetic structure of the bulk state [2,3]. In the present work, we perform the low-temperature measurement of angle–dependent magnetoresistance (ADMR) in high magnetic fields up to 24 T to explore novel magneto-transport property due to the possible surface magnetization. At 1.4 K where the surface conduction is well dominant, we show that ADMR at 24 T exhibits strong two-fold symmetry, which can be attributed to anisotropic magnetoresistance in the magnetic surface state. We will report the systematic results of temperature and magnetic-field dependence of surface ADMR.

[1] Q. Du et al., Phys. Rev. Res., 2, 043085 (2020).

[2] M. B. Stone et al., Phys. Rev. Lett., 108, 167202 (2012).

[3] M. B. Stone et al.,108, 167202 (2012).   

Ferroelectric field effect in few-layer CrCl3 tunnel junctions top-gated by PbZr0.2Ti0.8O3 membranes

We report the ferroelectric gating control of few-layer antiferromagnetic CrCl₃ tunnel junctions. High-quality CrCl₃ flakes are synthesized on mica by the physical vapor transport technique [1]. Epitaxial ferroelectric PbZr_0.2Ti_0.8O₃ (PZT) films (50 nm) are deposited on Sr₃Al₂O₆ (SAO) buffered (001) SrTiO₃ substrates and suspended via dissolving the SAO layer in water. Selected few-layer CrCl₃ flakes are fabricated into graphite/CrCl₃/graphite tunnel junctions by the all-dry stamping transfer method, which are encapsulated by either PZT membranes or h-BN flakes. Using conductive atomic force microscopy, we pole the PZT top-layer to uniformly polarized up and down states. Polarization reversal leads to nonvolatile modulation of the tunneling current, with an on/off ratio of 10⁶ obtained at room temperature. Compared with h-BN encapsulated devices, the PZT-gated CrCl₃ tunnel junctions exhibit distinct magnetotransport properties. The tunneling magnetoresistance changes sign at low temperature, suggesting a change of magnetic state. Our study provides an effectively strategy to design CrCl₃-based nonvolatile memory and spintronic applications.

    

Magnon Spin Nernst Effect: Current Progress, Challenges, and Outlooks

Magnons in antiferromagnetic insulators are promising information carriers for energy-efficient spintronic applications because they can transport spin angular momenta over long distances without incurring Joule heating. However, due to the lack of electrical charge and net magnetization, generating (hence manipulating) antiferromagnetic magnons remains a fundamental challenge. It has been recently proposed that the magnon spin Nernst effect (SNE), which is the thermomagnonic counterpart of the spin Hall effect, is an efficient way to generate magnonic pure spin currents without accompanying heat currents. The magnon SNE can be realized in a collinear antiferromagnet on 2D honeycomb lattice, in which the Dzyaloshinskii–Moriya (DM) interaction plays a crucial role in breaking symmetry. To reconcile with real experimental setup, we analyze detectable signals arising from the magnon SNE in the presence of spin diffusion in a finite system. We find that by proper device engineering, spin diffusion effects can substantially enhance the detectable signal and that optical measurements are generally more favorable than electrical measurements. In addition to 2D honeycomb antiferromagnets, we review a series of recent progress in the magnon SNE with different ground states such as noncollinear antiferromagnets, paramagnets, and magnets with other crystal structures. We also review alternative physical mechanisms that can lead to magnon SNE other than the DM interaction such as magnon-phonon hybridization. Finally, we briefly mention recent experimental attempts in verifying the magnon SNE.   

Dynamical mean-field theory study of a ferromagnetic CrI3 monolayer

We have employed one of the well-known many-body techniques, density functional theory plus dynamical mean-field theory (DFT + DMFT), to investigate the electronic structure of ferromagnetic monolayer CrI3 as a function of temperature and hole-doping concentration. The computed magnetic susceptibility follows the Curie's law, indicating that the ferromagnetism of monolayer CrI3 originates from localized magnetic moments of Cr atoms rather than Stoner-type itinerant ones. The DFT + DMFT calculations show a different coherent temperature for each spin component, demonstrating apparent strong spin-dependent electronic correlation effects in monolayer CrI3. Furthermore, we have explored the doping-dependent electronic structure of monolayer CrI3 and found that its electronic and magnetic properties are easily tunable by the hole-doping.   

Role of Fe intercalation on transition-metal dichalcogenide Fe1/3+δNbS2

Among 3d transition metal intercalated transition-metal dichalcogenides (TMDCs), Fe_1/3+δNbS₂ has been found to possess two competing and highly tunable long-range ordered magnetic ground states -- antiferromagnetic stripe and zigzag orders, which can lead to intriguing resistance switching and magnetic memory effects. However, how the intercalated Fe atoms interact with the van der Waals bonded host material electronically remains unclear. Here, combining angle-resolved photoemission spectroscopy (ARPES) with DFT calculations, we systematically study the electronic structure of Fe_1/3+δNbS₂. The electronic bands are significantly adjusted to the superlattice. We discuss the orbital selective charge transfer scenarios between the intercalated Fe and the NbS₂ layers. We also discuss the evolution of the electronic structure associated with the antiferromagnetic phase transition.Finally, we will discuss the effect of electronic correlation in the 3d transition metal intercalated TMDCs, especially in comparison to the weak coupling theories used to describe this material family.   

Two-dimensional ferromagnetic spin-orbital excitations in honeycomb VI3

VI₃ is a ferromagnet with planar honeycomb sheets of bonded V³⁺ ions held together by van der Waals forces. We apply neutron spectroscopy to measure the two-dimensional (J/J_c ≈ 17) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ( ≈ k_BT_c ≈ 55 K) and dispersive excitations. We apply a multilevel spin-wave formalism to describe the spectra in terms of two coexisting domains hosting differing V³⁺ orbital ground states built from contrasting distorted octahedral environments. This analysis fits a common nearest-neighbor in-plane exchange coupling (J = −4.4 ± 0.2 meV) between V³⁺ sites. The distorted local crystalline electric field combined with spin-orbit coupling provides the needed magnetic anisotropy for spatially long-ranged two-dimensional ferromagnetism in VI₃.