Exchange Field Research Articles

Modulation of antichiral edge states in zigzag honeycomb nanoribbons by side potentials

The antichiral edge states induced by the modified Haldane model have been predicted in the previous studies. In this study, other types of antichiral edge states are proposed by applying the side potentials composed of a potential field, staggered electric field, and exchange field along the boundaries of zigzag honeycomb nanoribbons (zHNRs). Their corresponding transport properties are investigated. The results show that the side potentials can lift the spin degeneracy of the edge modes, inducing five types of antichiral edge states. By calculating the spin-dependent energies in K’ and K valleys of the edge modes, an interpretation for generating antichiral edge states is proposed. In addition, the spin/charge switcher in the three-terminal device consisting of zHNRs is developed based on the induced edge states. We believe that these results can be used in the design of future spintronic devices.

Rashba spin–orbit coupling induced rectified currents in monolayer graphene with exchange field and sublattice potential

We investigate the effect of Rashba spin–orbit coupling (RSOC) on photoconductivities of rectified currents in monolayer graphene with exchange field and sublattice potential. The system shows that the photoconductivities of resonant shift and injection current contributions are nonzero, while the photoconductivities of non-resonant shift current contribution are zero. We find that the RSOC induces a warping term, which leads to the nonzero rectified currents. Moreover, the photoconductivities of resonant injection (shift) current contribution are (not) related to the relaxation rate. The similar behavior can be found in other Dirac materials, and our findings provide a way to tune the nonlinear transport properties of Dirac materials.

Electrical control of antiferromagnetic domain walls in Weyl semimetals

The electric-field-induced angular force on the N\'eel vector in antiferromagnetic (AFM) Weyl semimetals (WSMs) is theoretically investigated. Unlike in the ferromagnetic (FM) counterparts, the magnetic textures in the AFM WSMs, such as the domain walls (DWs) appear to lack the torsion in the magnetization and, thus, unable to benefit from this highly efficient mechanism that originates from the axial magnetic effect. Contrarily, our calculations illustrate that the addition of the Dzyaloshinskii-Morya interaction can introduce a twist in the magnetization around the DW location, giving rise to a nonzero axial magnetic field. This axial magnetic field, when combined with an external electric field, can lead to an imbalance in the fermion density of Weyl cones of opposite chirality and, thus, a spatially localized net electron spin polarization. The resulting effective exchange field can exert an angular force on the AFM textures for spatial movement, which can be significant in certain AFM WSMs even under a moderate external electric field. The dynamics of the DW motion under this emergent angular force is analyzed by considering the balance of energy absorption and dissipation. Our investigation reveals the need to account for the contribution of the exchange dissipation mechanism beyond the typical Gilbert-like (relativistic) term to compensate the unusual superlinear rate of energy absorption by the AFM textures. The obtained DW velocity vs electric-field characteristics show a significant speedup for the N\'eel DWs in the AFM WSMs over the counterparts in the FM WSMs as well as those in the nontopological magnets. The analysis also elucidates the dependence of the DW motion on the DW chirality in these materials. Our results clearly indicate the significance of the energy-efficient axial magnetic effect in the dynamics of spin textures in AFM WSMs with broken inversion symmetry.

Magnetic circular dichroism in the dd excitation in the van der Waals magnet CrI3 probed by resonant inelastic x-ray scattering

We report on a combined experimental and theoretical study on ${\mathrm{CrI}}_{3}$ single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr $3d$ orbital splitting ($dd$ excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The $dd$ excitation energies are similar on the two sides of the ferromagnetic transition temperature, ${T}_{C}\ensuremath{\sim}61$ K, although MD in RIXS is predominant at 0.4 T magnetic field below ${T}_{C}$. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr $3d$ orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr $3d$ intraorbital $dd$ excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr $3d$ hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in ${\mathrm{O}}_{h}$ and ${\mathrm{D}}_{3d}$ symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.

Electrical Manipulation of Spin-Dependent Anisotropy of a Dirac Cone in a Graphene Superlattice with Alternating Periodic Electrostatic and Exchange Fields

We studied the spin-dependent behavior of the electronic properties of alternating periodic potentials applied to finite and infinite graphene superlattices coupled with tunable electrostatic and exchange fields. The band structures were evaluated using the transfer matrix approach. The results of tuning the coupled electrostatic potential and exchange field showed that the spin-dependent anisotropy of a Dirac cone depends on the difference between the amplitude of periodically modulated coupling. Spin-dependent collimation occurs when the modulations become zero-average potentials with the ratio of both periodically modulated strengths equals one, in which one spin can be moved freely, but the other one is highly collimated. In addition, we find that the number of extra Dirac points in the infinite superlattice is spin-dependent. In terms of spin-ups, their number increases with an increase in the strength of both modulated fields. To ensure this calculation, we also compute the conductance of finite periodic modulation at zero energy. It is shown that the peaks of the conductance occur when the extra Dirac point emerges. This result may be utilized to design graphene-based devices with highly spin-polarized collimators.

Uncovering the path to low-field colossal magnetoresistance: A microscopic view of field driven percolative insulator-to-metal transition in manganites

For perovskite manganites, their colossal magnetoresistance (CMR) requires a large field, which limits their potential applications. In order to uncover the path to achieve low-field CMR, it is crucial to understand the microscopic process of the field driven insulator-to-metal transition (IMT) in manganites. This is particularly true considering the fact that the IMT is of a percolative type, in which the interplay between nucleation and growth of the electronic phase separation domains under magnetic field is not well investigated. In this work, we investigate the magnetic field driven percolative IMT in a model system of La1−x−yPrxCayMnO3 in real space via magnetic force microscopy (MFM). Our experimental observations show unambiguously three stages of the IMT phase transition where domain nucleation and domain growth exhibit distinctly different features in the global initial magnetization measurements. Moreover, MFM reveals that domain growth requires a much lower field than domain nucleation, which provides critical information on how to achieve low-field CMR. It is believed that the exchange field provided by ferromagnetic metallic domains at the boundary with antiferromagnetic insulating domains plays a critical role in assisting the domain growth process. Optimizing such internal exchange fields in manganites is a potential route to achieve CMR without the need of a large external field.

Anisotropic differential conductance of a mixed-parity superconductor/ferromagnet structure

We study the electronic transport properties of a superconductor (S) with a mixed s+p-wave pairing attached to a ferromagnetic metal (F) and a normal electrode (N) in an SFN configuration. Using the quasiclassical Green's function method, we compute the differential conductance $\sigma$ of the junction and demonstrate its dependence on the direction of the exchange field relative to the direction of the d-vector of the pair potential. If the p-wave triplet dominates the pairing, the zero bias conductance depends on the relative direction between the triplet d-vector and the exchange field. In contrast, if the s-wave singlet dominates the pairing, the zero bias conductance is isotropic with respect to the field direction. Furthermore, at zero temperature, the zero bias conductance height can only take two values as a function of $r$, the parameter quantifying the relative amount of s- and p-wave pairing, with an abrupt change at $r=1$ when the superconductor goes from a singlet to triplet dominated ground state. Moreover, we show that the relative amount of s- and p-wave pairing, can be estimated from the dependence of the finite bias conductance on the exchange field direction. Our results provide a way to characterize parity-mixed superconductors performing electrical measurements.

Machine learning nonequilibrium electron forces for spin dynamics of itinerant magnets

We present a generalized potential theory for conservative as well as nonconservative forces for the Landau-Lifshitz magnetization dynamics. Importantly, this formulation makes possible an elegant generalization of the Behler-Parrinello machine learning (ML) approach, which is a cornerstone of ML-based quantum molecular dynamics methods, to the modeling of force fields in adiabatic spin dynamics of out-of-equilibrium itinerant magnetic systems. We demonstrate our approach by developing a deep-learning neural network that successfully learns the electron-mediated exchange fields in a driven s-d model computed from the nonequilibrium Green’s function method. We show that dynamical simulations with forces predicted from the neural network accurately reproduce the voltage-driven domain-wall propagation. Our work also lays the foundation for ML modeling of spin transfer torques and opens a avenue for ML-based multi-scale modeling of nonequilibrium dynamical phenomena in itinerant magnets and spintronics.

Study of the Antiferromagnetic State Nematics in EuFe2As2 by Using Spin-Resonance and Magnetic Measurements

Using electron spin resonance spectroscopy and SQUID-magnetometry we obtained direct evidence of the occurrence of magnetic domains in the antiferromagnetically ordered state of a {text{EuF}}{{{text{e}}}_{{text{2}}}}{text{A}}{{{text{s}}}_{{text{2}}}} single crystal. The resonance spectra of europium ions were measured in the temperature range from 4 to 200 K. Using an equation for the resonance field in an antiferromagnet that takes into account the exchange and anisotropy fields, we have performed an analysis of the angular dependence of the spectrum at a temperature of 4.8 K, measured upon the crystal rotation around the c axis. Data analysis showed that {text{EuF}}{{{text{e}}}_{{text{2}}}}{text{A}}{{{text{s}}}_{{text{2}}}} is the antiferromagnet with easy anisotropy plane. Besides, we found in the ab-plane the second order axes of easy magnetization for each of the two types of magnetic domains, related to the structural transition and the formation of twins. Magnetic anisotropy caused by the exchange interaction of europium ions with iron ions indicates the occurrence of nematic magnetic ordering in the basal ab plane. An estimate of the magnitude of the exchange field and the anisotropy field is obtained from the angular dependence of the resonance fields.

Switching modulation of spin transport in ferromagnetic tetragonal silicene

We study the band structure and transport properties of ferromagnetic tetragonal silicene nanoribbons by using the non-equilibrium Green's function method. The band structure and spin-dependent conductance are discussed under the combined effect of the external electric field, potential energy, exchange field and the spin-orbit coupling. One can easily realize a phase transition from a semimetallic to a semiconducting state by changing the transverse width of the nanoribbon. Separation of spin-dependent conductances arises from the effect of exchange field and the spin-orbit coupling, while zero-conductance behaviors exhibit spin-dependent band gaps induced by the electric field. We propose a device configuration of four-terminal tetragonal silicene nanoribbon with two central channels. It is found that spin current can be controlled by utilizing two switches. The switch with a high potential barrier can block electrons flowing from the central scattering region into other terminals. Interestingly, applying only one switch can realize spin-dependent zero conductance and large spin polarization. Two switches can provide multiple operations for controlling spin-dependent transport properties. The two-channel ferromagnetic tetragonal silicene nanoribbon can realize an effective separation of spin current, which may be a potential candidate for spintronic devices.

Exchange field enhanced upper critical field of the superconductivity in compressed antiferromagnetic EuTe2

Understanding the interplay between superconductivity and magnetism has been a longstanding challenge in condensed matter physics. Here we report high pressure studies on the C-type antiferromagnetic semiconductor EuTe2 up to 36.0 GPa. A structural transition from the I4/mcm to the C2/m space group is identified at ~16 GPa. Superconductivity is observed above ~5 GPa in both structures. In the low-pressure phase, magnetoresistance measurements reveal strong couplings between the local moments of Eu2+ and the conduction electrons of Te 5p orbits. The upper critical field of superconductivity is well above the Pauli limit. While EuTe2 becomes nonmagnetic in the high-pressure phase and the upper critical field drops below the Pauli limit. Our results demonstrate that the high upper critical field of EuTe2 in the low-pressure phase is due to the exchange field compensation effect of Eu2+ and the superconductivity in both structures may arise in the framework of the Bardeen-Cooper-Schrieffer theory.

Triplet proximity effects in superconductor/ferromagnet hybrid structures exhibiting intrinsic spin–orbit coupling

The conventional spin–singlets with momentum zero can be transformed into spin-polarized triplet states with non-zero momentum at the superconductor/ferromagnet interface by an inhomogeneous magnetic exchange field. While the decaying spectrum of triplet pairs in ferromagnetic metals has received significant attention, less is known about the decay of these pairs in ferromagnets with intrinsic spin–orbit coupling (SOC) present at the superconductor/ferromagnet interface. We have performed simulations to study the triplet proximity effects in hybrid structures composed of a BCS superconductor and ferromagnet with intrinsic spin orbit coupling (SOC) in the diffusive limit where the quasiclassical boundary conditions have been modified to incorporate the effect of SO coupling in Usadel equations and the self-consistent equation. We laid emphasis on the modification of the density of states inside the ferromagnet near the Fermi energy. The generated short ranged triplets (SRTs) present themselves in the form of zero energy gap in the density of states (DOS) even in the presence of comparatively larger values of the exchange field oriented either in plane or out of plane of the hybrid structure and the Long ranged triplets (LRTs) present themselves in the form of zero energy peak in the DOS for feasible magnitude and orientation of the exchange field. Understanding and modeling these observations is important as it is difficult to model such hybrid structures experimentally. Where earlier research has focused on the scenario of substantial spin orbit coupling in the ballistic threshold/clean limit, in this paper we provide conclusions that are applicable to the diffusive transport/dirty limit regime. Also, the findings reveal that the SO coupling generates a distinct imprint in the DOS, which exhibits extremely nonmonotonic behavior with magnetization orientations.

Supercurrent reversal in ferromagnetic hybrid nanowire Josephson junctions

We report supercurrent transport measurements in hybrid Josephson junctions comprised of semiconducting InAs nanowires with epitaxial ferromagnetic insulator EuS and superconducting Al coatings. The wires display a hysteretic superconducting window close to the coercivity, away from zero external magnetic field. Using a multi-interferometer setup, we measure the current-phase relation of multiple magnetic junctions and find an abrupt switch between $\pi$ and 0 phases within the superconducting window. We attribute the 0-$\pi$ transition to the discrete flipping of the EuS domains and provide a qualitative theory showing that a sizable exchange field can polarize the junction and lead to the supercurrent reversal. Both $0$ and $\pi$ phases can be realized at zero external field by demagnetizing the wire.

Sublattice-enriched tunability of bound states in second-order topological insulators and superconductors

Bound states at sharp corners have been widely viewed as the hallmark of two-dimensional second-order topological insulators and superconductors. In this work, we show that the existence of sublattice degrees of freedom can enrich the tunability of bound states on the boundary and hence lift the constraint on their locations. We take the Kane-Mele model with honeycomb-lattice structure to illustrate the underlying physics. With the introduction of an in-plane exchange field to the model, we find that the boundary Dirac mass induced by the exchange field has a sensitive dependence on the boundary sublattice termination. We find that the sensitive sublattice dependence can lead bound states to emerge at a specific type of boundary defects named as sublattice domain walls if the exchange field is of ferromagnetic nature, even in the absence of any sharp corner on the boundary. Remarkably, this sensitive dependence of the boundary Dirac mass on the boundary sublattice termination allows the positions of bound states to be manipulated to any place on the boundary for an appropriately-designed sample. With a further introduction of conventional s-wave superconductivity to the model, we find that, no matter whether the exchange field is ferromagnetic, antiferromagnetic, or ferrimagnetic, highly controllable Majorana zero modes can be achieved at the sublattice domain walls. Our work reshapes the understanding of boundary physics in second-order topological phases, and meanwhile opens potential avenues to realize highly controllable bound states for potential applications.

Field‐Free Spin‐Orbit Torque Switching in Synthetic Ferro and Antiferromagents with Exchange Field Gradient

AbstractDeterministic switching of perpendicularly magnetized synthetic antiferromagnets using spin‐orbit torque (SOT) usually requires an in‐plane auxiliary magnetic field, which limits its practical applications. Here, an exchange field gradient is introduced into perpendicularly magnetized synthetic ferro‐ and antiferromagnets (SFs and SAFs) through the insertion of a slightly wedged Ru between two thin ferromagnetic layers, which induces field‐free switching of perpendicular SFs and SAFs with a switching ratio up to 81% regardless of the nature of the coupling. Temperature‐dependent measurement shows a robust field‐free switching even at low temperature. The experimental results show that the field‐free switching ratio and the effective SOT field are directly related to the exchange field gradient. The theoretical model and numerical simulation indicate that the dynamic noncollinear spin textures induced by the exchange field gradient lead to the field‐free switching, while the sign of the exchange field gradient determines the field‐free switching polarity. It is further revealed that the SOT efficiency is positively correlated with the antiferromagnetic exchange field for both Ru wedged and non‐wedged samples. These results provide a new avenue for simultaneously achieving field‐free switching and high SOT efficiency of perpendicularly magnetized SAFs for highly stable, high‐density, low‐stray‐field, and low‐power magnetic memory devices.

Gate-Tunable Highly Efficient Spin and Valley Quantum Heat Engine in a Magnetic Tunnel Junction Based on Transition-Metal Dichalcogenides

Recently, the nanoscale quantum heat engine (QHE) has become one of the most productive areas of research, due to its role as a conduit for excess heat. Regarding the low thermal conductance and suitable structural properties of transition-metal dichalcogenides (TMDs), here, we design a highly efficient spin and valley QHE in a normal-ferromagnetic-normal TMD-based tunnel junction. Spin splitting caused by the exchange field and the huge spin-orbit coupling (SOC) of TMDs leads to a remarkable spin-valley--dependent Seebeck coefficient with a sharp peak at $E\ensuremath{\approx}{E}_{F}$. Application of the gate voltage changes the bands from conduction to valence; thus, a remarkable spin-valley--dependent Seebeck and conductance are observed, leading to a large gate- and exchange-field-tunable spin- and valley-dependent maximum power, which is greatly enhanced compared to other two-dimensional materials such as silicene- and graphene-based QHEs. Additionally, a enormous gate-controllable figure of merit and, consequently, a high efficiency at maximum power are achieved in this device, which confirms the advantage of using TMD-based junctions for highly efficient QHEs. These findings may open the door to the design of spin-valley caloritronics devices based on TMDs.

Geometry-Dependent Magnetoelectric and Exchange Bias Effects of the Nano L-T Mode Bar Structure Magnetoelectric Sensor.

The geometry-dependent magnetoelectric (ME) and exchange bias (EB) effects of the nano ME sensor were investigated. The sensor consisted of the Longitudinal-Transverse (L-T) mode bi-layer bar structure comprising the ferromagnetic (FM) and ferroelectric (FE) materials and the anti-ferromagnetic (AFM) material. The bi-layer ME coefficient was derived from constitutive equations and Newton's second law. The trade-off between peak ME coefficient and optimal thickness ratio was realized. At the frequency × structure length = 0.1 and 1200, minimum and maximum peak ME coefficients of the Terfenol-D/PZT bi-layer were around 1756 and 5617 mV/Oe·cm, respectively, with 0.43 and 0.19 optimal thickness ratios, respectively. Unfortunately, the bi-layer could not distinguish the opposite magnetic field directions due to their similar output voltages. PtMn and Cr2O3, the AFM, were introduced to produce the EB effect. The simulation results showed the exchange field starting at a minimum PtMn thickness of 6 nm. Nevertheless, Cr2O3 did not induce the exchange field due to its low anisotropy constant. The tri-layer ME sensor consisting of PZT (4.22 nm)/Terfenol-D (18 nm)/PtMn (6 nm) was demonstrated in sensing 2 Tbit/in2 magnetic bits. The average exchange field of 5100 Oe produced the output voltage difference of 12.96 mV, sufficient for most nanoscale magnetic sensing applications.

Magnetic ordering in Co2+-containing layered double hydroxides via the low-temperature heat capacity and magnetisation study

The low-temperature heat capacity and the magnetisation of Co2+nAl3+ layered double hydroxides (LDH) with the cobalt-to-aluminium ratio n = 2 and 3 and intercalated with different anions have been studied in a wide range of magnetic fields up to 50 kOe. The heat capacity, C(T), was found to demonstrate a Schottky-like anomaly observed as a broad local maximum in the temperature dependence below 10 K. The effect is caused by a splitting of the ground-state Kramers doublet of Co2+ in the internal exchange field and correlates with magnetic ordering in these LDH. In low applied fields, the temperature-dependent dc magnetic susceptibility demonstrates a pronounced rise, which is associated with an onset of magnetic ordering. Both the heat capacity anomaly and the magnetic susceptibility peak are more pronounced for the LDH with n = 2 than for those with n = 3. This feature is associated with an excess of the honeycomb-like CoAl coordination (which corresponds to a 2:1 CoAl ordering) over the statistical cation distribution in Co2Al LDH, while a rather random cobalt-aluminium distribution is typical for Co3Al LDH. The temperature of the Schottky-like anomaly measured in a zero field is independent of the interlayer distance. Application of the magnetic field results in a widening of the anomaly range and a shift to higher temperatures. The observed experimental data are typical for a cluster spin glass ground state.

Cr-substituted Fe[formula omitted]O[formula omitted] nanoparticles: The role of particle size in the formation of Fe[formula omitted]O sub-domains and the emergence of exchange bias

Here we study the influence of particle size on the formation of antiferromagnetic FexO subdomains and the emergence of exchange bias in novel ferrimagnetic Cr-substituted Fe3O4 nanoparticles generated using an aerosol technique based on spark ablation. Cooling in an applied magnetic field across the Néel temperature of the FexO subdomains results in significant shifts in the hysteresis loops, which decrease in magnitude with the particle diameter for the two large systems (D≈20nm&D≈40nm), whereas only minor exchange fields are observed for the smallest system (D≈10nm). The reduced exchange bias is attributed to the metastable FexO’s lower volume fraction and size due to a facile transformation into the thermodynamically stable (Fe,Cr)3O4 phase with decreasing particle diameter.

Scaling with MgO thickness of the proximity exchange field in a Fe/MgO/Si contact

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness ${t}_{\mathrm{MgO}}$ is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with ${t}_{\mathrm{MgO}}$ with a decay length of $\ensuremath{\lambda}=\ensuremath{-}0.42$ nm. Comparing this to the decay length for the tunnel conductance ($\ensuremath{\lambda}=\ensuremath{-}0.21$ nm) suggests that the exchange interaction is dominated by two-step tunneling processes.