We conduct a comprehensive study of three different magnetic semiconductors, CrI3, CrBr3, and CrCl3, by incorporating both few-and bi-layer samples in van der Waals tunnel junctions. We find that the interlayer magnetic ordering, exchange gap, magnetic anisotropy, as well as magnon excitations evolve systematically with changing halogen atom. By fitting to a spin wave theory that accounts for nearest neighbor exchange interactions, we are able to further determine a simple spin Hamiltonian describing all three systems. These results extend the 2D magnetism platform to Ising, Heisenberg, and XY spin classes in a single material family. Using magneto-optical measurements, we additionally demonstrate that ferromagnetism can be stabilized down to monolayer in more isotropic CrBr3, with transition temperature still close to that of the bulk.

Abstract:We use a combination of Raman spectroscopy and transport measurements to study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below ~250 K, we find that in moderately thin samples below ~12 nm, a single orthorhombic phase exists up to and beyond room temperature. This could be due to the effect of c-axis confinement, which lowers the energy of an out-of-plane hole band and stabilizes the orthorhombic structure. Our results suggest that Weyl nodes, predicated upon inversion symmetry breaking, may be observed in thin MoTe2 at room temperature. Main text:

While the anomalous Hall effect can manifest even without an external magnetic field, time reversal symmetry is nonetheless still broken by the internal magnetization of the sample. Recently, it has been shown that certain materials without an inversion center allow for a nonlinear type of anomalous Hall effect whilst retaining time reversal symmetry. The effect may arise from either Berry curvature or through various asymmetric scattering mechanisms. Here, we report the observation of an extremely large c-axis nonlinear anomalous Hall effect in the non-centrosymmetric Td phase of MoTe2 and WTe2 without intrinsic magnetic order. We find that the effect is dominated by skew-scattering at higher temperatures combined with another scattering process active at low temperatures. Application of higher bias yields an extremely large Hall ratio of E⊥/E|| = 2.47 and corresponding anomalous Hall conductivity of order 8 × 107 S/m.

We use both classical magnetotransport and quantum oscillation measurements to study the thickness evolution of the extremely large magnetoresistance (XMR) material and type-II Weyl semimetal candidate, -MoTe2, protected from oxidation. We find that the magnetoresistance is systematically suppressed with reduced thickness. This occurs concomitantly with both a decrease in carrier mobility and increase in electron-hole imbalance. We model the two effects separately and conclude that the XMR effect is more sensitive to the former. Main text:Among the layered transition metal dichalcogenides (TMDCs), MoTe2 is a unique member that crystallizes in both semiconducting 2H and semimetallic 1T-type structures, making it an appealing candidate for novel phase-changing electronics [1]. It has already been demonstrated, for example, that transitions between the two can be controlled by strain, alloying, and electrostatic gating [2][3][4][5][6]. The semimetal polytype itself is interesting and exhibits different phases. First, true 1T coordination is unstable as in-plane bond distortions dimerize the Mo atoms along the b-axis. Two stacking configurations of these distorted layers along the c-axis give rise to distinct three-dimensional (3D) structures: the centrosymmetric (or 1T') phase at high temperature (above ~250K) and the noncentrosymmetric (or Td) phase at low temperature, with the difference being only a ~4 ∘ tilt in the unit cell. The latter structure notably hosts type-II Weyl nodes [7-13] and exhibits extremely large magnetoresistance (XMR) below ~20K [14]. XMR materials may be useful for spintronics and sensing applications. While both -MoTe2 and -WTe2, a structurally similar compound, have been shown to demonstrate XMR [14,15], its origin in the former is under debate. Transport studies have attributed the cause to a close compensation of electron and hole concentrations at low temperature for both materials [16-19]; however, angle-resolved photoemission experiments report that MoTe2 remains uncompensated at all temperatures [20], in contrast to WTe2 [21]. One can directly test the effect of charge (un)compensation on XMR in MoTe2 by changing the relative carrier concentrations, but this is generally difficult to do in bulk systems without introducing unwanted disorder.Recently, several of the authors have shown that the phase is realized in thin MoTe2

Memristive devices whose resistance can be hysteretically switched by electric field or current are intensely pursued both for fundamental interest as well as potential applications in neuromorphic computing and phase-change memory. When the underlying material exhibits additional charge or spin order, the resistive states can be directly coupled, further allowing for electrical control of the collective phases. Here, we report the observation of abrupt, memristive switching of tunneling current in nanoscale junctions of ultrathin CrI 3 , a natural layer antiferromagnet. The coupling to spin order enables both tuning of the resistance hysteresis by magnetic field, and electric-field switching of magnetization even in multilayer samples.