Session B14: 2D Materials (Metals, Superconductors, and Correlated Materials) -- Magnetism

Magnetism in two dimensions

Selected by the organization chairs Kin Fai Mak and Cory Dean.   

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe3GeTe2

The advent of two-dimensional van der Waals crystals creates new possibilities in developing novel spintronic devices. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr₂Ge₂Te₆ and CrI₃ at low temperatures. Here, we developed a new device fabrication technique, and successfully isolated monolayers from layered metallic magnet Fe₃GeTe₂. We found that the itinerant ferromagnetism persists in Fe₃GeTe₂ down to monolayer. The ferromagnetic transition temperature, T_c, is suppressed in pristine Fe₃GeTe₂ thin flakes. An ionic gate, however, dramatically raises the T_c up to room temperature. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe₃GeTe₂ opens up opportunities for potential voltage-controlled magnetoelectronics.   

Magnetic properties of vanadium selenide epitaxial thin films

The discoveries of ferromagnetism in atomically-thin Cr₂Ge₂Te₆ and CrI₃ crystals have opened up an opportunity for integration of magnetic 2D materials into van der Waals heterostructures. Moreover, the recent discovery of emergent room-temperature ferromagnetism in monolayer VSe₂ [1], which has been known as a paramagnetic metal in its bulk form, should provide an important step toward spintronics applications based on 2D materials, although the existence of ferromagnetism in monolayer VSe₂ is still controversial [2]. We have recently found that vanadium selenide epitaxial thin films fabricated by molecular-beam epitaxy (MBE) with our growth recipe [3] exhibit peculiar magnetic properties with clear anomalous Hall effect down to 2D limit. In this presentation, we will show transport properties of our MBE-grown vanadium selenide epitaxial thin films, and discuss possible origins of this emergent ferromagnetism in our samples. [1] M. Bonilla, et. al., Nat. Nanotechnol. 13, 289 (2018). [2] J. Feng, et. al., Nano Lett. 18, 4493 (2018). [3] M. Nakano, et. al., Nano Lett. 17, 5595 (2017).   

Gate-controlled charge-doping of a Mott insulator in Graphene/α-RuCl3 Heterostructures

The layered antiferromagnetic Mott insulator α-RuCl3 exhibits many phenomena consistent with the existence of a Kitaev quantum spin liquid. While most works on α-RuCl3 so far have focused on pristine bulk samples, this material can be readily exfoliated down to monolayer thicknesses. Here we study the electronic transport of van der Waals heterostructure devices containing thin a-RuCl3 flakes in contact with monolayer graphene Hall bars. We find an anomalously large conductivity implying the RuCl3 has become charge-doped and is now conducting. The Hall coefficient data show a sharp increase in the density of the second conducting band (in the a-RuCl3) as graphene is gated from hole- to electron-doped. Additionally, the resistivity at low temperature shows clear signals associated with magnetic phase transitions at temperatures 2-3 times higher than the native antiferromagnetic transition in a-RuCl3.   

Ab initio mismatched interface theory of graphene on α-RuCl3: doping and magnetism

The possibilities for creating van der Waals heterostructures are limitless with the rich palette including Mott insulating α-RuCl₃ and semi-metallic graphene. However, the study of such "mismatched interfaces'' calls for an innovative way of capturing the underlying physics of these complex systems, that goes beyond standard ab initio methods. We propose a general strategy for studying such interfaces, "mismatched interface theory," and apply it to a α-RuCl₃-graphene heterostructure in which there is significant lattice mismatch between the two crystals. Our results indicate charge transfer from graphene to the Ru layer in α-RuCl₃ directly proportional to the number of carbon atoms present in the system, corresponding to uniform doping of 4.7% in the α-RuCl₃-graphene heterostructure. We further demonstrate that this doping may be tuned by applying uniaxial pressure to the heterostructure, and investigate the effects of the doping on the zigzag- and ferromagnetic-ordered low-energy stable states of α-RuCl₃ which lie close to the antiferromagnetic Kitaev spin liquid.   

Crystal Structure Reconstruction on Surface of Quantum Spin Liquid Candidate: α-RuCl3

α-RuCl_3, has emerged as a novel 2D material that potentially hosts quantum spin liquid state (QSL). Recent experimental reports of neutron scattering and thermal quantum Hall effect have provided indirect but promising evidence for the existence of QSL in α-RuCl₃. However, numerous controversies still remain in the current literatures about α-RuCl₃, such as its crystal structure and electronic structure. We used a unique surface sensitive technique, low energy electron microscopy (LEEM), combined with dynamical selected-area low energy electron diffraction (μLEED-IV) to study the 2D crystal structure of α-RuCl_3. We found an unexpected diffraction pattern on the surface of α-RuCl₃, which indicates intrinsic surface reconstruction that forms a superlattice. The superlattice most likely originates from the slight shift of atomic positions which results in unit cell distortion. The existence of a surface superlattice may significantly impacts the electronic and magnetic properties which in turn would potentially influence the existence of QSL in α-RuCl₃.   

Probing magnetism in atomically thin van der Waals insulators using electron tunneling

Layered magnetic insulators that can be exfoliated are highly desirable, as we can potentially create topological states (e.g. quantum anomalous Hall effect) by incorporating them in van der Waals heterostructures. Such materials were finally realized in 2017 with the isolation of monolayer CrI₃ and bilayer CrGeTe₃. However, this field is in its infancy and few layer films seem to have complex properties different from bulk, such as the ferromagnetic to antiferromagnetic transition in CrI₃. In this talk, I will discuss the use of electron tunneling through ultrathin magnetic insulators as a probe of their magnetic ground state and excitations [1]. We report giant magnetoresistance effects when tunneling through CrI₃ and CrCl₃ barriers with graphite electrodes, resulting from polarization of their antiferromagnetic ground states under an applied magnetic field. In CrI₃ we also find inelastic electron tunneling suggesting the observation of Dirac magnon excitations. Finally, we note a large enhancement of the interlayer exchange in thin CrCl₃ compared to bulk, and discuss its origins.

1. D. R. Klein, D. MacNeill, J. L. Lado, D. Soriano, E. Navarro-Moratalla, K. Watanabe, T. Taniguchi, S. Manni, P. Canfield, J. Fernández-Rossier, P. Jarillo-Herrero, Science 360, 1218 (2018).   

Increased Interlayer Exchange in the Layered Magnetic Insulator CrCl3

The family of layered chromium trihalides has been studied for decades due to its rich magnetic phases coupled with insulating properties. The recent isolation of monolayer CrI₃ has renewed interest in these materials to incorporate magnetism into 2D van der Waals heterostructures. Transition metal halide magnets have revealed intriguing ground states differing from the bulk when cleaved to thin films but this phenomenon is not yet understood. Here, we use electron tunneling through few-layer crystals of the layered antiferromagnet CrCl₃ to probe its magnetic order in the ultrathin limit. By measuring the magnetoresistance in spin filter magnetic tunnel junctions, we measure the interlayer exchange in CrCl₃ barriers two to four layers in thickness and find that it is greatly increased in thin films compared to bulk.   

Microscopic understanding of magnetic interactions in bilayer CrI3

We performed a detailed microscopic analysis of the inter-layer magnetic couplings for bilayer CrI₃. As the first step toward understanding the recent experimental observations and utilizing them for device applications, we estimated magnetic force response as well as total energy. Various van der Waals functionals unequivocally point to the ferromagnetic ground state for the low-temperature structured bilayer CrI₃ which is further confirmed independently by magnetic force response calculations. The calculated orbital-dependent magnetic forces clearly show that e_g-t_2g interaction is the key to stabilize this ferromagnetic order. By suppressing this ferromagnetic interaction and enhancing antiferromagnetic orbital channels of e_g-e_g and t_2g-t_2g, one can realize the desirable antiferromagnetic order. We showed that high-temperature monoclinic stacking can be the case. Our results provide unique information and insight to understand the magnetism of multi-layer CrI₃ paving the way to utilize it for applications.   

Pressure induced ferromagnetism and magnetic anisotropy in bilayer CrI3

Two diemnsional CrI3 system has attracted extensive research interest because it is an intrinsic 2D ferromagnetic material with a band gap. It has been found that the monolayer of CrI3 possesses an intrinsic ferromagnetic ordering with a finite band gap of 1.1 eV and it has also a strong perpendicular magnetocrystalline anisotropy. Besides, it has been experimentally reported that the anti-ferromagnetic interlayer coupling in bilayer CrI3 structure could be change by external charge doping. Nonetheless, despite a few experimental works on CrI3 system, it is rare to find the theoretical studies on bilayer CrI3. We investigated the magnetic properties of bilayer CrI3 using density functional approach. We found that the pristine bilayer CrI3 had an anti-ferromagnetic ground state. Interestingly, we obtained that the external pressure could induce a transition from anti-ferromagnetic to ferromagnetic state in bilayer CrI3 and the perpendicular magnetic anisotropy was still preserved even under the external pressure.   

Transport properties of van der Waals heterostructures based on two-dimensional magnet

Emerging properties of van der Waals (vdW) materials at two-dimensional (2D) limit have been broadening their varieties, ranging from spin-orbit coupled transport properties, valley-polarized luminescence, spin-valley-locked superconductivities, to topological quantum transport. As one of big advantages of 2D materials, 2D physical phenomena could be more and more enriched by integrating different 2D materials into a heterostructure, so-called vdW heterostructures, providing a new high-quality platform for 2D physics. One recent breakthrough in 2D physics is observation of magnetism in vdW materials, which turned out to survive down to 2D limit. This type of magnet, so-called 2D magnet, has been integrated into various types of vdW heterostructures, while tuning magnetism itself at vdW heterostructures has been less investigated so far. Here we created magnetic vdW heterostructures by molecular-beam epitaxy, where a new type of 2D magnet, vanadium selenide epitaxial thin film, was incorporated. In the presentation, we will show transport properties of those heterostructures, and discuss the interface effect on 2D magnetism.