Two-dimensional Magnetic Insulators Research Articles

Small-voltage multiferroic control of two-dimensional magnetic insulators

Small-voltage multiferroic control of two-dimensional magnetic insulators

Superfluid spin transistor

We propose to use the Hall response of topological defects, such as merons and antimerons, to spin currents in two-dimensional magnetic insulator with in-plane anisotropy for identification of the Berezinskii-Kosterlitz-Thouless (BKT) transition in a transistorlike geometry. Our numerical results relying on a combination of Monte Carlo and spin dynamics simulations show transition from spin superfluidity to conventional spin transport, accompanied by the universal jump of the spin stiffness and exponential growth of the transverse vorticity current. We propose a superfluid spin transistor in which the spin and vorticity currents are modulated by changes in density of free topological defects, e.g., by injection of vorticity or by tuning the in-plane magnet across the BKT transition by changing the exchange interaction, magnetic anisotropy, or temperature.

Topological properties of multilayer magnon insulators

Two-dimensional magnetic insulators can be promising hosts for topological magnons. In this study, we show that ABC-stacked honeycomb lattice multilayers with alternating Dzyaloshinskii-Moriya interaction (DMI) reveal a rich topological magnon phase diagram. Based on our bandstructure and Berry curvature calculations, we demonstrate jumps in the thermal Hall behavior that corroborate with topological phase transitions triggered by adjusting the DMI and interlayer coupling. We connect the phase diagram of generic multilayers to a bilayer and a trilayer system. We find an even-odd effect amongst the multilayers where the even layers show no jump in thermal Hall conductivity, but the odd layers do. We also observe the presence of topological proximity effect in our trilayer. Our results offer new schemes to manipulate Chern numbers and their measurable effects in topological magnonic systems.

Quantum anomalous Hall effect in two-dimensional magnetic insulator heterojunctions

Recent years have witnessed tremendous success in the discovery of topological states of matter. Particularly, sophisticated theoretical methods in time-reversal-invariant topological phases have been developed, leading to the comprehensive search of crystal database and the prediction of thousands of topological materials. In contrast, the discovery of magnetic topological phases that break time reversal is still limited to several exemplary materials because the coexistence of magnetism and topological electronic band structure is rare in a single compound. To overcome this challenge, we propose an alternative approach to realize the quantum anomalous Hall (QAH) effect, a typical example of magnetic topological phase, via engineering two-dimensional (2D) magnetic van der Waals heterojunctions. Instead of a single magnetic topological material, we search for the combinations of two 2D (typically trivial) magnetic insulator compounds with specific band alignment so that they can together form a type-III broken-gap heterojunction with topologically non-trivial band structure. By combining the data-driven materials search, first-principles calculations, and the symmetry-based analytical models, we identify eight type-III broken-gap heterojunctions consisting of 2D ferromagnetic insulators in the MXY compound family as a set of candidates for the QAH effect. In particular, we directly calculate the topological invariant (Chern number) and chiral edge states in the MnNF/MnNCl heterojunction with ferromagnetic stacking. This work illustrates how data-driven material science can be combined with symmetry-based physical principles to guide the search for heterojunction-based quantum materials hosting the QAH effect and other exotic quantum states in general.

Probing magnetism via spin dynamics in graphene/2D-ferromagnet heterostructures

The recent discovery of two-dimensional magnetic insulators has generated a great deal of excitement over their potential for nanoscale manipulation of spin or magnetism. One intriguing use for these materials is to put them in contact with graphene, with the goal of making graphene magnetic while maintaining its unique electronic properties. Such a system could prove useful in applications such as magnetic memories, or could serve as a host for exotic states of matter. Proximity to a magnetic insulator will alter the spin transport properties of graphene, and the strength of this interaction can be probed with Hanle spin precession experiments. To aid in the analysis of such experiments, in this work we derive an explicit expression for Hanle spin precession in graphene interfaced with a ferromagnetic insulator whose magnetization points perpendicular to the graphene plane. We find that this interface results in a shifted and asymmetric Hanle response, and we discuss how this behavior can be used to interpret measurements of spin transport in these systems.

Spin tunnel field-effect transistors based on two-dimensional van der Waals heterostructures

A transistor based on spin rather than charge—a spin transistor—could potentially offer non-volatile data storage and improved performance compared with traditional transistors. Many approaches have been explored to realize spin transistors, but their development remains a considerable challenge. The recent discovery of two-dimensional magnetic insulators such as chromium triiodide (CrI3), which offer electrically switchable magnetic order and an effective spin filtering effect, can provide new operating principles for spin transistors. Here, we report spin tunnel field-effect transistors (TFETs) based on dual-gated graphene/CrI3/graphene tunnel junctions. The devices exhibit an ambipolar behaviour and tunnel conductance that is dependent on the magnetic order in the CrI3 tunnel barrier. The gate voltage switches the tunnel barrier between interlayer antiferromagnetic and ferromagnetic states under a constant magnetic bias near the spin-flip transition, thus effectively and reversibly altering the device between a low and a high conductance state, with large hysteresis. By electrically controlling the magnetization configurations instead of the spin current, our spin TFETs achieve a high–low conductance ratio approaching 400%, suggesting they could be of value in the development of non-volatile memory applications. A tunnel field-effect transistor with spin-dependent outputs that are voltage controllable and reversible can be created using a dual-gated graphene/CrI3/graphene tunnel junction.

Diagnosing phases of magnetic insulators via noise magnetometry with spin qubits

Two-dimensional magnetic insulators exhibit a plethora of competing ground states, such as ordered (anti)ferromagnets, exotic quantum spin liquid states with topological order and anyonic excitations, and random singlet phases emerging in highly disordered frustrated magnets. Here we show how single spin qubits, which interact directly with the low-energy excitations of magnetic insulators, can be used as a diagnostic of magnetic ground states. Experimentally tunable parameters, such as qubit level splitting, sample temperature, and qubit-sample distance, can be used to measure spin correlations with energy and wavevector resolution. Such resolution can be exploited, for instance, to distinguish between fractionalized excitations in spin liquids and spin waves in magnetically ordered states, or to detect anyonic statistics in gapped systems.

A barrier to spin filters

Electron tunnelling through a two-dimensional magnetic insulator is assisted by magnon inelastic processes that provide spin-filtering.

Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide—II

The splitting, spectroscopic g-factors, polarized relative intensities and temperature dependence of the 2 T 1, 2 T 2 and 4 T 2 states of CrBr 3 and the 4 T 2 state of CrCl 3 are examined in detail. The 2 T 1 state is split into well-resolved 2 A and 2 E sublevels by the trigonal field, with no apparent complication from vibronic coupling. A hot-magnon-assisted transition to the 2 A sublevel ( hω mag = 75cm −1) is identified and its intensity vs temperature related to the nearest-neighbor two-spin correlation function, which is also used to estimate an excited-state exchange field of 1200 kG from the shift in energy of the 2 E sublevel with temperature. A single-ion intensity mechanism is also found to be important, and the resulting strong dependence of the 2 E sublevel intensity on exchange field direction and temperature (below T c ) is analyzed in terms of 2 T 1− 4 T 2 and 2 T 1− 4 T 1 spin-orbit mixing. Closely similar results are found for the 2 T 2 state, including a strong dependence of the 2 E sublevel intensity on exchange field direction and temperature (below T c ) and a hot-magnon-assisted transition to the 2 A sublevel ( hω mag = 48cm −1). Quantitative analysis is very difficult, however, because of a complicated sideband structure, the probable existence of a Jahn-Teller effect and uncertainty as to the rigid-lattice trigonal splitting. A partial interpretation of the structure, based on Jahn-Teller interaction with one or more e g , unit-cell vibrations, is proposed. A strong tetragonal Jahn-Teller effect is found in the 4 T 2 states in CrCl 3 and CrBr 3, as evidenced by a long progression in the 250 cm −1 and the 150 cm −1 e g , unit-cell vibration, respectively. For CrCl 3, an analysis of the zero-phonon levels shows that E JT = 500 ±80 cm −1 and indicates that the excited-state exchange field is antiferromagnetic, Isotropie and slightly smaller in magnitude than the ferromagnetic ground-state exchange field. Vibrationally-excited levels of the 4 T 2 state are examined qualitatively, aided by the analysis of the zero-phonon levels. The closely-similar CrBr 3 4 T 2 vibronic structure is also analyzed through a perturbation calculation, and a good description of the splitting, relative intensities and polarizations of the zero-phonon levels is obtained, yielding a value of E JT ≈ 400 cm −1. Spurious 4 T 2 zero-phonon structure is found in CrCl 3 containing stacking faults but similar complications are not found in CrBr 3.

Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide—I

The general aspects of the crystal field spectra of the two-dimensional magnetic insulators CrCl 3 and CrBr 3 are considered in detail, with special emphasis on the broad, spin-allowed 4 T 2 and 4 T 1 bands. A fit of the point-charge model Hamiltonian to the spectra is presented, with attention to the magnitude of the trigonal field effects. The temperature dependence of the 4 T 2 and 4 T 1 dipole strengths in the range 4.2< T < 300°K is analyzed in terms of single-ion (odd-parity trigonal field and odd-parity vibronic) and exchange mechanisms. A moment analysis of the M.C.D. spectra is performed, which (together with dipole strength temperature dependence data) yields a reasonable interpretation of the spin-allowed transition mechanism. The odd-parity crystal field is found to be the most important parity-breaking mechanism in both the 4 T 2 and 4 T 1 bands. Both also receive some contribution from the vibronic mechanism. This process is more significant for the 4 T 2 state than for the 4 T 1, and may involve a slightly anharmonic low-energy vibration. Evidence for a small exchange contribution is found in the 4 T 2 band of both systems. Factors influencing the bandshapes are examined quantitatively. The antiresonance formalism of Sturge et al. is applied to the anomalous lineshapes of certain transitions in absorption and M.C.D. and one particularly complex bandshape (that for 4 A 2 → 4 T 2 - 2 E - 2 T 1 in CrCl 3) is analyzed through a computation.

Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide. II

Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide. II

Spectroscopic studies of the two-dimensional magnetic insulators chromium trichloride and chromium tribromide. I

The global number of electric vehicles is exponentially rising, due to strong marketing efforts and governmental incentives that significantly lower the price of new vehicles. This shift of consumers from internal combustion engine vehicles to electric vehicles is actually a shift from petroleum to the primary sources that generate electricity. The motivation for this paper is the holistic analysis of electric vehicles and the determination of their benefits and detriments. Starting with an exergetic assessment for all road vehicles, this paper determines the electricity needed for the propulsion of electric vehicles. When the heating and cooling requirements of the vehicle’s cabin are included, the range of the vehicles decreases significantly. The electricity requirements of the vehicles are abridged to primary energy sources using the concept of well-to-wheels efficiency. Based on the regional mix of electricity generation, the effect of the shift to electric vehicles on greenhouse gas emissions is determined. Because the charging of the batteries of electric vehicles requires significant power, it was concluded that the simultaneously charging of a number of vehicles will strain the capacity of the electricity grid. The paper also examines the effects of electric vehicles on the further utilization of renewable energy sources.