Showing papers on "Exchange interaction published in 2021"

Ultrafast control of magnetic interactions via light-driven phonons.

Abstract: Resonant ultrafast excitation of infrared-active phonons is a powerful technique with which to control the electronic properties of materials that leads to remarkable phenomena such as the light-induced enhancement of superconductivity1,2, switching of ferroelectric polarization3,4 and ultrafast insulator-to-metal transitions5. Here, we show that light-driven phonons can be utilized to coherently manipulate macroscopic magnetic states. Intense mid-infrared electric field pulses tuned to resonance with a phonon mode of the archetypical antiferromagnet DyFeO3 induce ultrafast and long-living changes of the fundamental exchange interaction between rare-earth orbitals and transition metal spins. Non-thermal lattice control of the magnetic exchange, which defines the stability of the macroscopic magnetic state, allows us to perform picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic spin orders. Our discovery emphasizes the potential of resonant phonon excitation for the manipulation of ferroic order on ultrafast timescales6. Non-thermal lattice control of exchange interactions allows for picosecond coherent switching between competing antiferromagnetic and weakly ferromagnetic order.

Artificial heavy fermions in a van der Waals heterostructure

Abstract: Heavy-fermion systems represent one of the paradigmatic strongly correlated states of matter1–5. They have been used as a platform for investigating exotic behaviour ranging from quantum criticality and non-Fermi liquid behaviour to unconventional topological superconductivity4–12. The heavy-fermion phenomenon arises from the exchange interaction between localized magnetic moments and conduction electrons leading to Kondo lattice physics, and represents one of the long-standing open problems in quantum materials3. In a Kondo lattice, the exchange interaction gives rise to a band with heavy effective mass. This intriguing phenomenology has so far been realized only in compounds containing rare-earth elements with 4f or 5f electrons1,4,13,14. Here we realize a designer van der Waals heterostructure where artificial heavy fermions emerge from the Kondo coupling between a lattice of localized magnetic moments and itinerant electrons in a 1T/1H-TaS2 heterostructure. We study the heterostructure using scanning tunnelling microscopy and spectroscopy and show that depending on the stacking order of the monolayers, we can reveal either the localized magnetic moments and the associated Kondo effect, or the conduction electrons with a heavy-fermion hybridization gap. Our experiments realize an ultimately tunable platform for future experiments probing enhanced many-body correlations, dimensional tuning of quantum criticality and unconventional superconductivity in two-dimensional artificial heavy-fermion systems15–17. A study demonstrates the synthesis and characterization of a two-dimensional van der Waals heterostructure hosting artificial heavy fermions, providing a tunable platform for investigations of heavy-fermion physics.

Exchange Coupling in a Linear Chain of Three Quantum-Dot Spin Qubits in Silicon.

Abstract: Quantum gates between spin qubits can be implemented leveraging the natural Heisenberg exchange interaction between two electrons in contact with each other. This interaction is controllable by electrically tailoring the overlap between electronic wave functions in quantum dot systems, as long as they occupy neighboring dots. An alternative route is the exploration of superexchange-the coupling between remote spins mediated by a third idle electron that bridges the distance between quantum dots. We experimentally demonstrate direct exchange coupling and provide evidence for second neighbor mediated superexchange in a linear array of three single-electron spin qubits in silicon, inferred from the electron spin resonance frequency spectra. We confirm theoretically, through atomistic modeling, that the device geometry only allows for sizable direct exchange coupling for neighboring dots, while next-nearest neighbor coupling cannot stem from the vanishingly small tail of the electronic wave function of the remote dots, and is only possible if mediated.

Role of 3d–4f exchange interaction and local anti-site defects in the magnetic and magnetocaloric properties of double perovskite Ho2CoMnO6 compound

Abstract: The strong coupling between 3d and 4f based magnetic sublattices in double perovskite (DP) compounds results in various exotic complex magnetic interactions, and the ground state contains multiple fascinating and remarkable magnetic states. In this article, we have performed a detailed investigation of the crystal structure, magnetic, and magnetocaloric properties of the ordered monoclinic polycrystalline double perovskite Ho 2 CoMnO 6 (HCMO) compound. A study of the magnetization dynamics employing temperature and magnetic field shows a powerful correlation between Ho and Co/Mn sublattices. Due to the presence of the ferromagnetic superexchange interaction in between Co 2 + − O − Mn 4 + networks, the system undergoes an ordered state at the transition temperature, T C ≈ 77 K. Below T C, a clear compensation point continued by negative magnetization is noticed in the virgin state of the compound. The reduction of the saturation magnetization ( M S) in the hysteresis curves (M-H) can be explained by the existence of local anti-site defects or disorders and anti-phase boundaries in the system. Temperature dependence of magnetic entropy change ( − Δ S) curves shows a maximum value of 13.4 J/kg K for Δ H = 70 kOe at a low temperature along with a noticeable inverse magnetocaloric effect. Moreover, the material holds reasonable values of magnetocaloric parameters. The absence of thermal hysteresis along with a large value of | Δ S | makes the system a potential candidate for low temperature as well as liquid nitrogen temperature-based magnetic refrigeration. Additionally, our experimental findings should encourage further detailed studies on the complex 3d–4f exchange interaction in the double perovskite system.

Insight into dynamic magnetic properties of YMnO3/FM bilayer in a time-dependent magnetic field

Abstract: Based on Monte Carlo simulation, a mixed-spin (5/2, 2, 3/2) Ising model is constructed to investigate the dynamic magnetic properties of antiferromagnetic/ferromagnetic YMnO3/FM bilayer under a time-dependent magnetic field. The effects of exchange interaction, magnetic field and temperature are involved in this work. Masses of numerical results of the dynamic order parameter, susceptibility, internal energy, and critical temperature are obtained under the influence of the diverse physical parameters. Moreover, the phase diagrams and the hysteresis loops of the system are discussed.

Shell Filling and Trigonal Warping in Graphene Quantum Dots.

Abstract: Transport measurements through a few-electron circular quantum dot in bilayer graphene display bunching of the conductance resonances in groups of four, eight, and twelve. This is in accordance with the spin and valley degeneracies in bilayer graphene and an additional threefold ``minivalley degeneracy'' caused by trigonal warping. For small electron numbers, implying a small dot size and a small displacement field, a two-dimensional $s$ shell and then a $p$ shell are successively filled with four and eight electrons, respectively. For electron numbers larger than 12, as the dot size and the displacement field increase, the single-particle ground state evolves into a threefold degenerate minivalley ground state. A transition between these regimes is observed in our measurements and can be described by band-structure calculations. Measurements in the magnetic field confirm Hund's second rule for spin filling of the quantum dot levels, emphasizing the importance of exchange interaction effects.

Curvature-driven homogeneous Dzyaloshinskii-Moriya interaction and emergent weak ferromagnetism in anisotropic antiferromagnetic spin chains

Abstract: Chiral antiferromagnets are currently considered for a broad range of applications in spintronics, spin-orbitronics, and magnonics. In contrast to the established approach relying on materials screening, the anisotropic and chiral responses of low-dimensional antiferromagnets can be tailored relying on the geometrical curvature. Here, we consider an achiral, anisotropic antiferromagnetic spin chain and demonstrate that these systems possess geometry-driven effects stemming not only from the exchange interaction but also from the anisotropy. Peculiarly, the anisotropy-driven effects are complementary to the curvature effects stemming from the exchange interaction and rather strong as they are linear in curvature. These effects are responsible for the tilt of the equilibrium direction of vector order parameters and the appearance of the homogeneous Dzyaloshinskii–Moriya interaction. The latter is a source of the geometry-driven weak ferromagnetism emerging in curvilinear antiferromagnetic spin chains. Our findings provide a deeper fundamental insight into the physics of curvilinear antiferromagnets beyond the σ-model and offer an additional degree of freedom in the design of spintronic and magnonic devices.

Control of the exciton valley dynamics in atomically thin semiconductors by tailoring the environment

Abstract: The exciton valley dynamics in van der Waals heterostructures with transition metal dichalcogenide monolayers is driven by the long-range exchange interaction between the electron and the hole in the exciton It couples the states active in the opposite circular polarizations resulting in the longitudinal-transverse splitting of excitons propagating in the monolayer plane Here we study theoretically the effect of the dielectric environment on the long-range exchange interaction and demonstrate how the encapsulation in hexagonal boron nitride modifies the exciton longitudinal-transverse splitting We calculate the exciton spin-valley polarization relaxation due to the long-range exchange interaction and demonstrate that the variation of the monolayer environment results in significant, up to fivefold, enhancement of the exciton valley polarization lifetime

Bloch ferromagnetism of composite fermions

Abstract: In 1929, Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontaneous transition to a fully magnetized state at very low densities, because the exchange energy gained by aligning the spins exceeds the enhancement in the kinetic energy1. However, experimental realizations of this effect have been hard to implement. Here, we report the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism. Our platform utilizes the two-dimensional Fermi sea of composite fermions near half-filling of the lowest Landau level. We measure the Fermi wavevector—which directly provides the spin polarization—and observe a sudden transition from a partially spin-polarized to a fully spin-polarized ground state as we lower the density of the composite fermions. Our theoretical calculations that take Landau level mixing into account provide a semi-quantitative account of this phenomenon. Composite fermions can be tuned to very low effective density in a clean two-dimensional electron gas, which allows the formation of a Bloch ferromagnet.

Spin Fluctuations in Quantized Transport of Magnetic Topological Insulators.

Abstract: In magnetic topological insulators, quantized electronic transport is intertwined with spontaneous magnetic ordering, as magnetization controls band gaps, hence band topology, through the exchange interaction. We show that considering the exchange gaps at the mean-field level is inadequate to predict phase transitions between electronic states of distinct topology. Thermal spin fluctuations disturbing the magnetization can act as frozen disorders that strongly scatter electrons, reducing the onset temperature of quantized transport appreciably even in the absence of structural impurities. This effect, which has hitherto been overlooked, provides an alternative explanation of recent experiments on magnetic topological insulators.

Pressure-Enhanced Ferromagnetism in Layered CrSiTe3 Flakes.

Abstract: Despite recent advances in layered ferromagnets, ferromagnetic interactions in these materials are rather weak. Here, we report pressure-enhanced ferromagnetism in layered CrSiTe3 flakes revealed by high-pressure magnetic circular dichroism measurements. Below ∼3 GPa, CrSiTe3 undergoes a paramagnetic-to-ferromagnetic phase transition at ∼32 K, and the field-induced spin-flip in the ferromagnetic phase produces nearly zero hysteresis loops, demonstrating soft ferromagnetism. Above ∼4 GPa, a soft-to-hard ferromagnetic transition occurs, signaled by rectangular-shaped hysteresis loops with finite coercivity and remanent magnetization. Interestingly, as pressure increases, the Curie temperature and coercivity dramatically increase up to ∼138 K and 0.17 T at 7.8 GPa, respectively, in contrast to ∼36 K and 0.02 T at 4.6 GPa. It indicates a remarkable influence of pressure on exchange interactions, which is consistent with DFT calculations. The effective interaction between magnetic couplings and external pressure offers new opportunities in pursuit of high-temperature layered ferromagnets.

The role of 5d electrons spin in quantum ferromagnetism and transport properties of double perovskites Cs2ZCl/Br6 (Z = Ta, W) for spintronic applications

Abstract: The role of 5d electrons of transition metals in double perovskites Cs2ZCl/Br6 (Z = Ta, W) has been probed for advanced spintronic technology. The magnetic and transport properties are computed by using Wein2K code and BoltzTraP code. The exchange mechanism is explained by the measurement of crystal field energy, exchange energy and hybridization process. The half metallic ferromagnetism is illustrated by double-exchange model and exchange constants while spin polarization is illustrated by integer value of total magnetic moments and polarization factor computations. The issue of stability is clarified by formation energy and tolerance factor conditions. In the end, thermoelectric properties are analyzed by thermal to electrical conductivity ratio, Seebeck coefficient, and power factor.

Quantum Spin Torque Driven Transmutation of an Antiferromagnetic Mott Insulator.

Abstract: The standard model of spin-transfer torque (STT) in antiferromagnetic spintronics considers the exchange of angular momentum between quantum spins of flowing electrons and noncollinear-to-them localized spins treated as classical vectors. These vectors are assumed to realize N\'eel order in equilibrium, $\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\cdots}\ensuremath{\uparrow}\ensuremath{\downarrow}$, and their STT-driven dynamics is described by the Landau-Lifshitz-Gilbert (LLG) equation. However, many experimentally employed materials (such as archetypal NiO) are strongly electron-correlated antiferromagnetic Mott insulators (AFMIs) whose localized spins form a ground state quite different from the unentangled N\'eel state $|\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\cdots}\ensuremath{\uparrow}\ensuremath{\downarrow}⟩$. The true ground state is entangled by quantum spin fluctuations, leading to the expectation value of all localized spins being zero, so that LLG dynamics of classical vectors of fixed length rotating due to STT cannot even be initiated. Instead, a fully quantum treatment of both conduction electrons and localized spins is necessary to capture the exchange of spin angular momentum between them, denoted as quantum STT. We use a recently developed time-dependent density matrix renormalization group approach to quantum STT to predict how injection of a spin-polarized current pulse into a normal metal layer coupled to an AFMI overlayer via exchange interaction and possibly small interlayer hopping---mimicking, e.g., topological-insulator/NiO bilayer employed experimentally---will induce a nonzero expectation value of AFMI localized spins. This new nonequilibrium phase is a spatially inhomogeneous ferromagnet with a zigzag profile of localized spins. The total spin absorbed by AFMI increases with electron-electron repulsion in AFMIs, as well as when the two layers do not exchange any charge.

Gate-tunable heavy fermion quantum criticality in a moir\'e Kondo lattice.

Abstract: We propose a realization of Kondo-lattice physics in moir\'e superlattices at the interface between a WX$_2$ homobilayer and MoX$_2$ monolayer (where X=S,Se). Under appropriate gating conditions, the interface-WX$_2$-layer forms a triangular lattice of local moments that couple to itinerant electrons in the other WX$_2$-layer via a gate-tunable Kondo exchange interaction. Using a parton mean-field approach we identify a range of twist-angles which support a gate-tuned quantum phase transition between a heavy-fermion liquid with large anomalous Hall conductance and a fractionalized chiral spin-liquid coexisting with a light Fermi liquid, and describe experimental signatures to distinguish among competing theoretical scenarios.

Valley-polarized quantum anomalous Hall phase in bilayer graphene with layer-dependent proximity effects

Abstract: Realizations of some topological phases in two-dimensional systems rely on the challenge of jointly incorporating spin-orbit and magnetic exchange interactions. Here, we predict the formation and control of a fully valley-polarized quantum anomalous Hall effect in bilayer graphene, by separately imprinting spin-orbit and magnetic proximity effects in different layers. This results in varying spin splittings for the conduction and valence bands, which gives rise to a topological gap at a single Dirac cone. The topological phase can be controlled by a gate voltage and switched between valleys by reversing the sign of the exchange interaction. By performing quantum transport calculations in disordered systems, the chirality and resilience of the valley-polarized edge state are demonstrated. Our findings provide a promising route to engineer a topological phase that could enable low-power electronic devices and valleytronic applications as well as putting forward layer-dependent proximity effects in bilayer graphene as a way to create versatile topological states of matter.

Self-Hybridization and Tunable Magnon-Magnon Coupling in van der Waals Synthetic Magnets

Abstract: Van der Waals magnets are uniquely positioned at the intersection between two-dimensional materials, antiferromagnetic spintronics, and magnonics. The interlayer exchange interaction in these materials enables antiferromagnetic resonances to be accessed at gigahertz frequencies. Consequently, these layered antiferromagnets are intriguing materials out of which quantum hybrid magnonic devices can be fashioned. Here, we use both a modified macrospin model and micromagnetic simulations to demonstrate a comprehensive antiferromagnetic resonance spectra in van der Waals magnets near the ultrathin (monolayer) limit. The number of optical and acoustic magnon modes, as well as the mode frequencies, are found to be exquisitely sensitive to the number of layers. We discover a self-hybridization effect where pairs of either optical or acoustic magnons are found to interact and self-hybridize through the dynamic exchange interaction. This leads to characteristic avoided energy level crossings in the energy spectra. Through simulations, we show that, by electrically controlling the damping of surface layers within heterostructures, both the strength and number of avoided energy level crossings in the magnon spectra can be controlled.

Room temperature superconductivity dome at a Fano resonance in superlattices of wires

Abstract: Recently room temperature superconductivity with degrees Celsius has been discovered in a pressurized complex ternary hydride, CSHx , which is a carbon- and hydrogen-doped H3 S alloy. The nanoscale structure of H3 S is a particular realization of the 1993 patent claim of superlattice of quantum wires for room temperature superconductors and the maximum T C occurs at the top of a superconducting dome. Here we focus on the electronic structure of materials showing nanoscale heterostructures at the atomic limit made of a superlattice of quantum wires like hole-doped cuprate perovskites, and organics focusing on A 15 intermetallics and pressurized hydrides. We provide a perspective of the theory of room temperature multigap superconductivity in heterogeneous materials tuned at a shape resonance or Fano resonance in the superconducting gaps near a Lifshitz transition focusing on H3 S where the maximum T C occurs where the multiband metal is tuned by pressure near a Lifshitz transition. Here the superconductivity dome of T C vs. pressure is driven by both electron-phonon coupling and contact exchange interaction. We show that the T C amplification up to room temperature is driven by the Fano resonance between a superconducting gap in the anti-adiabatic regime and other gaps in the adiabatic regime. In these cases the T C amplification via contact exchange interaction is the missing term in conventional multiband BCS and anisotropic Migdal-Eliashberg theories including only Cooper pairing.

Hydrogen Control of Double Exchange Interaction in La0.67Sr0.33MnO3 for Ionic–Electric–Magnetic Coupled Applications

Abstract: The dynamic tuning of ion concentrations has attracted significant attention for creating versatile functionalities of materials, which are impossible to reach using classical control knobs Despite these merits, the following fundamental questions remain: how do ions affect the electronic bandstructure, and how do ions simultaneously change the electrical and magnetic properties? Here, by annealing platinum-dotted La067 Sr033 MnO3 films in hydrogen and argon at a lower temperature of 200 °C for several minutes, a reversible change in resistivity is achieved by three orders of magnitude with tailored ferromagnetic magnetization The transition occurs through the tuning of the double exchange interaction, ascribed to an electron-doping-induced and/or a lattice-expansion-induced modulation, along with an increase in the hydrogen concentration High reproducibility, long-term stability, and multilevel linearity are appealing for ionic-electric-magnetic coupled applications

Heisenberg Exchange and Dzyaloshinskii–Moriya Interaction in Ultrathin Pt(W)/CoFeB Single and Multilayers

Abstract: We present results of the analysis of Brillouin light-scattering (BLS) measurements of spin waves performed on ultrathin single and multirepeat CoFeB layers with adjacent heavy metal layers. From a detailed study of the spin-wave dispersion relation, we independently extract the Heisenberg exchange interaction (also referred to as symmetric exchange interaction), the Dzyaloshinskii–Moriya interaction (DMI, also referred to as antisymmetric exchange interaction), and the anisotropy field. We find a large DMI in CoFeB thin films adjacent to a Pt layer and nearly vanishing DMI for CoFeB films adjacent to a W layer. Furthermore, the influence of the dipolar interaction on the dispersion relation and on the evaluation of the Heisenberg exchange parameter is demonstrated. Eventually, an experimental analysis of the impact of the DMI on the spin-wave lifetime is presented.

Signatures of interfacial topological chiral modes via RKKY exchange interaction in Dirac and Weyl systems

Abstract: We theoretically investigate the features of Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between two magnetic impurities, mediated by the interfacial bound states inside a domain wall (DW). The latter separates the two regions with oppositely signed inversion symmetry broken terms in graphene and Weyl semimetal. The DW is modeled by a smooth quantum well which hosts a number of discrete bound states including a pair of gapless, metallic modes with opposite chiralities. We find clear signatures of these interfacial chiral bound states in spin response (RKKY exchange interaction) which is robust to the deformation of the quantum well.

Spin-torque switching mechanisms of perpendicular magnetic tunnel junction nanopillars

Abstract: Understanding the spin-transfer magnetization switching mechanisms of perpendicular magnetic tunnel junction nanopillars is critical to optimizing their performance in memory devices Here, we use micromagnetics to study how the free layer's exchange constant affects its switching dynamics Switching is shown to generally occur by (1) growth of the magnetization precession amplitude in the element center; (2) an instability in which the reversing region moves to the element edge, forming magnetic domain wall(s); and (3) the motion of the domain wall(s) across the element For small exchange and large element diameters, step 1 leads to a droplet with a fully reversed core that experiences a drift instability (step 2) While in the opposite case (large exchange and small diameters), the central region of the element is not fully reversed before step 2 occurs The origin of the micromagnetic structure is shown to be the free layer's non-uniform demagnetization field More coherent, energy-efficient, and faster switching is associated with larger exchange, showing that increasing the exchange interaction strength leads to improvements in device performance

Electron and Hole Spin Relaxation in CdSe Colloidal Nanoplatelets.

Abstract: Solution-processed quantum-confined nanocrystals are important building blocks for scalable implementation of quantum information science. Extensive studies on colloidal quantum dots (QDs) have revealed subpicosecond hole spin relaxation, whereas the electron spin dynamics remains difficult to probe. Here we study electron and hole spin dynamics in CdSe colloidal nanoplatelets (also called quantum wells) of varying thicknesses using circularly polarized transient absorption spectroscopy at room temperature. The clear spectroscopic features of transition bands associated with heavy, light, and spin-orbit split-off holes enabled separate probes of electron and hole dynamics. The hole spin-flip occurred within ∼200 fs, arising from strong spin-orbit coupling in the valence band. The electron spin lifetime decreased from 6.2 to 2.2 ps as the platelet thickness is reduced from 6 to 4 monolayers, reflecting an exchange interaction between the electron and the hole and/or surface dangling bond spins enhanced by quantum confinement.

Anisotropic coercivity and the effects of interlayer exchange coupling in CoFeB/FeRh bilayers

Abstract: In an amorphous CoFeB layer, coercivity becomes anisotropic with fourfold symmetry when the CoFeB layer exchange couples to an FeRh layer. The angular dependence of coercivity of the CoFeB layer coincides with the in-plane easy-axis direction of the FeRh layer and experiences a ${45}^{\ensuremath{\circ}}$ shift with the occurrence of a metamagnetic phase transition of the FeRh layer from antiferromagnetism at room temperature to ferromagnetism at 400 K. The intriguing phenomena are well reproduced by our unbiased Monte Carlo simulation. The interfacial exchange and anisotropy energies, as well as the interfacial magnetization in the CoFeB/FeRh bilayer, are disentangled to demonstrate the strong dependence of the imprinting of anisotropy in the CoFeB layer on the interfacial exchange coupling. The evolution of the easy-axis direction of the induced anisotropy arises from the reconstruction of the interfacial exchange energy profile accompanied with the change of the magnetic state of FeRh, which governs the magnetization reversal of the CoFeB layer at both branches. Moreover, the imprinting is further applicable for the uniaxial magnetocrystalline anisotropy. This work not only presents the possibility of directly duplicating anisotropy between dissimilar materials, but it also provides a powerful tool to probe the hidden magnetic structures and/or the properties of materials that have weak magnetism, such as antiferromagnetic materials.