Effects Of Interfacial Coupling Research Articles

Nonvolatile magnetoelectric coupling in two-dimensional van der Waals sandwich heterostructure CuInP2S6/MnCl3/CuInP2S6.

Electrical control of magnetism is of great interest for low-energy-consumption spintronic applications. Due to the recent experimental breakthrough in two-dimensional materials, with the absence of hanging bonds on the surface and strong tolerance for lattice mismatch, heterogeneous integration of different two-dimensional materials provides a new opportunity for coupling between different physical properties. Here, we report the realization of nonvolatile magnetoelectric coupling in vdW sandwich heterostructure CuInP2S6/MnCl3/CuInP2S6. Using first-principles calculations, we reveal that when interfacing with ferroelectric CuInP2S6, the Dirac half-metallic state of monolayer MnCl3 will be destroyed. Moreover, depending on the electrically polarized direction of CuInP2S6, MnCl3 can be a half-metal or a ferromagnetic semiconductor. We unveil that the obtained ferromagnetic semiconductor in MnCl3 can be attributed to the different gain and loss of electrons on the two adjacent Mn atoms due to the sublattice symmetry broken by interlayer coupling. The effects of interfacial magnetoelectric coupling on magnetic anisotropy and ferromagnetic Curie temperature of MnCl3 are also investigated, and a multiferroic memory based on this model is designed. Our work not only provides a promising way to design nonvolatile electrical control of magnetism but also renders monolayer MnCl3 an appealing platform for developing low-dimensional memory devices.

Evidence for Interfacial Octahedral Coupling as a Route to Enhance Magnetoresistance in Perovskite Oxide Superlattices

AbstractEngineering octahedral rotations in oxide heterostructures is a promising route for controlling magnetic properties in perovskites, with recent work focusing on magnetic‐ordering temperatures and magnetic anisotropies. Here the effects of interfacial octahedral coupling on magnetoresistance are demonstrated in a series of (La0.7Sr0.3MnO3)n/(LaFeO3)10 superlattices grown on (001)‐ and (111)‐oriented SrTiO3 substrates. The different crystallographic orientations allow for the interfacial octahedral connectivity to be tuned, with weaker interfacial coupling present at the (001)‐oriented than the (111)‐oriented structures as revealed by density functional theory calculations. In n = 14 superlattices, the effect of orientation on the physical properties is minimal with both (001)‐ and (111)‐oriented samples exhibiting similar magnetoresistance. As the fraction of interfacial volume within the LSMO layers is increased by decreasing n, the magnetoresistive behavior of the samples diverges with significantly larger magnetoresistance magnitudes present in the (111)‐oriented superlattices. The results are consistent with octahedral coupling playing a greater role in the functional properties at (111)‐heterointerfaces and demonstrate a structure‐driven approach to tuning interfacial magnetoresistance in complex‐oxide heterostructures.

Interfacial coupling, oxygen deficiency, and orbital reconstruction in oriented La0.7Ca0.3MnO3/DyMnO3 bilayers

The orthorhombic La0.7Ca0.3MnO3/DyMnO3 (LCM/DMO) bilayers and referenced LCM single-layer films were epitaxially deposited by polymer-assisted deposition on (001)- and (011)-oriented SrTiO3 (STO) substrates, respectively, and the effects of interfacial coupling, oxygen deficiency, and orbital reconstruction on the physical properties of the LCM/DMO bilayers have been investigated. It is found that the magnetic and transport properties of LCM/DMO bilayers are weaker than those of LCM referenced films, due to the orientation-adjusted interfacial coupling and oxygen deficiency. More importantly, the introduced larger strain from DMO layer promotes the eg orbital splitting with consequences for orbital reconstruction and the enhancement of antiferromagnetic phase in LCM layer, which strongly affects the physical properties of LCM/DMO bilayers cooperated with interfacial coupling and oxygen deficiency.

Electrically Driven Reversible Magnetic Rotation in Nanoscale Multiferroic Heterostructures.

Electrically driven magnetic switching (EDMS) is highly demanded for next-generation advanced memories or spintronic devices. The key challenge is to achieve repeatable and reversible EDMS at sufficiently small scale. In this work, we reported an experimental realization of room-temperature, electrically driven, reversible, and robust 120° magnetic state rotation in nanoscale multiferroic heterostructures consisting of a triangular Co nanomagnet array on tetragonal BiFeO3 films, which can be directly monitored by magnetic force microscope (MFM) imaging. The observed reversible magnetic switching in an individual nanomagnet can be triggered by a small electric pulse within 10 V with an ultrashort time of ∼10 ns, which also demonstrates sufficient switching cycling and months-long retention lifetime. A mechanism based on synergic effects of interfacial strain and exchange coupling plus shape anisotropy was also proposed, which was also verified by micromagnetic simulations. Our results create an avenue to engineer the nanoscale EDMS for low-power-consumption, high-density, nonvolatile magnetoelectric memories and beyond.

Effect of local-action on electro-birefringence in bilayer composite and micro-displacement sensing

A bilayer composite with electro-birefringence effect was fabricated using ferroelectric and stress-birefringence medium. Its electro-optical response was studied. The transmission light intensity was found to change with displacement of the incident beam to the elastic-optical layer in two dimensions perpendicular to the beam. The effects of local-action and interfacial elastic coupling on the stress distribution in the elastic-optical material were analyzed. Starting from the basic equations of elasticity, a physical model of electro-stress birefringence for the bilayer composite was derived. It was found that the theoretical calculations in general accord with the experimental results without considering ferroelectric relaxation. And the micro-displacement sensing of two-dimension was realized with the bilayer composite of electro-stress birefringence.

Spin-orbit torques from interfacial spin-orbit coupling for various interfaces.

We use a perturbative approach to study the effects of interfacial spin-orbit coupling in magnetic multilayers by treating the two-dimensional Rashba model in a fully three-dimensional description of electron transport near an interface. This formalism provides a compact analytic expression for current-induced spin-orbit torques in terms of unperturbed scattering coefficients, allowing computation of spin-orbit torques for various contexts, by simply substituting scattering coefficients into the formulas. It applies to calculations of spin-orbit torques for magnetic bilayers with bulk magnetism, those with interface magnetism, a normal metal/ferromagnetic insulator junction, and a topological insulator/ferromagnet junction. It predicts a dampinglike component of spin-orbit torque that is distinct from any intrinsic contribution or those that arise from particular spin relaxation mechanisms. We discuss the effects of proximity-induced magnetism and insertion of an additional layer and provide formulas for in-plane current, which is induced by a perpendicular bias, anisotropic magnetoresistance, and spin memory loss in the same formalism.

Studying the effects of interfacial coupling in La0.5Sr0.5CoO3-δ thin films on SrTiO3 using in-situ cooling experiments

Studying the effects of interfacial coupling in La_0.5Sr_0.5CoO_3-δ thin films on SrTiO₃ using in-situ cooling experiments

The effects of interfacial exchange coupling in Fe/ErFeO3 heterostructures

Exploring exchange bias in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures is vital for both fundamental magnetism and practical application. However, in the case of conventional FM/AFM systems, the essential field cooling process above the Néel temperature of AFM materials hinders their application if the Néel temperature is far higher than room-temperature. Here, we report the effects of interfacial exchange coupling in Fe/ErFeO3 heterostructures. The magnetic-field-induced switchable exchange bias, originating from the AFM exchange coupling between Fe film and Dzyaloshinskii–Moriya-interaction-induced net moment of ErFeO3 along c axis, is successfully achieved without field cooling or in-field growth process of AFM. Different from the most previous pinning layer using a hard FM or traditional AFM, ErFeO3 pinning layer has the advantages of both the magnetic field sensitivity (~780 Oe) and ultrahigh dynamic frequency. In addition, although Fe film is polycrystalline, it exhibits a strong uniaxial magnetic anisotropy resulted from the so-called ‘spin-flop-coupling effect’, i.e. the magnetic coupling between Fe film and the compensated G-type AFM spins of EFO along a axis. Interestingly, the exchange bias field and asymmetric switching field offer entirely different information about the asymmetry of magnetization reversal near hard axis. The asymmetric switching field is further proved to be an effective measure to determine the weak unidirectional magnetic anisotropy for film with nearly 180° domain wall displacement. Our experimental results provide a practical method to establish room-temperature exchange bias in FM/G-type AFM without field cooling. Furthermore, the magnetic-field-induced switchable exchange-bias, the spin-flop coupling effect and the angular dependent asymmetry of magnetization reversal in the vicinity of hard axis in Fe/ErFeO3 heterostructures may provide new insights on the interfacial exchange coupling in FM/AFM systems.

Spin-wave resonance frequency in a multi-layer film

The spin-wave resonance (SWR) frequency in a ferromagnetic bilayer or triple-layer film (for bilayer or triple-layer films, including symmetric and asymmetric films) with single-ion anisotropy has been studied by using the linear spin-wave approximation and Green’s function technique. The effects of interfacial coupling, surface anisotropy, interface anisotropy, bulk anisotropy and external magnetic field on the SWR frequency have been investigated. For a ferromagnetic bilayer or triple-layer film, the SWR frequencies (for the same spin-wave modes) all increase with increasing external magnetic field, surface anisotropy, interface anisotropy and bulk anisotropy and with decreasing film thickness. As the interfacial coupling increases, the SWR frequencies of some modes increase, while the SWR frequencies of the other modes do not change. The SWR frequencies of a triple-layer film are different from those of a bilayer film and the SWR frequencies of an asymmetric film are also different from those of a symmetric film. The present results show the method to enhance and adjust the SWR frequency of ferromagnetic multi-layer films.

Anomalous temperature and interfacial‐coupling dependence of exchange bias in antiferromagnetic (core)/ferromagnetic (shell) nanoparticles

AbstractIn a single composite ferromagnetic/antiferromagnetic (AFM) nanoparticle with an unconventional core/shell morphology, the effects of temperature and interfacial coupling on the exchange bias (EB) are studied by performing a modified Monte Carlo method. With the decrease of temperature, three distinct behaviors of the EB are obtained as a result of the state transitions of the AFM spins from completely free via partially free and partially frozen, and, finally, to completely frozen. The behaviors of the coercivity are nonmonotonic due to the energy competitions and the thermal fluctuations. Moreover, the cooling field affects strongly the low‐temperature behaviors of the EB. The EB field obtained after field cooling is negative and almost constant, independent of the interfacial coupling. However, after zero‐field cooling, the EB field may exhibit a sign‐reversal behavior with increasing interfacial coupling at low temperature. On the basis of the theory of the surplus magnetization, it is revealed that the drastic temperature dependence and the energy competition mechanism at the interface in the cooling process are responsible for this unique property in such novel nanostructures.

Effects of Depolarization Field and Interfacial Coupling on the Polarization of Ferroelectric Bilayers

A ferroelectric bilayer model considering depolarization field and interfacial coupling is proposed and the expression of the depolarization field is derived. The spatial profiles of spontaneous polarization and hysteresis loops are calculated using the numerical method with and without considering the depolarization field. The effects of the depolarization field and interfacial coupling on the polarization of second-order ferroelectric bilayers are studied systematically. When interfacial coupling is ferroelectric coupling, the interface spontaneous polarization increases and the area of hysteresis loop becomes larger with increasing coupling. When interfacial coupling is antiferroelectric coupling, the depolarization field makes the central loop become smaller and the shape of the hysteresis loop becomes steep. Meanwhile, as interfacial coupling increases, the outer loops stretch further out horizontally and the size of the central loop widens.

Exchange bias in a columnar nanocrystallineNi80Fe20∕CoOthin film

The effects of interfacial coupling at the boundary of ferromagnetic and antiferromagnetic components in a nanoscale columnar-structured thin film of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Co}\mathrm{O}$ have been examined. Field-cooling the film results in very different temperature dependences of the enhanced coercivity and exchange-bias shift of the hysteresis loop. The exchange-bias temperature dependence is well described by thermal fluctuations of the interfacial spins while the coercivity temperature dependence indicates that single-domain-like columns are being coherently rotated by the thermal fluctuations of the interface spins. Furthermore, only a portion of the spins in the antiferromagnetic layer seem to be associated with the spin coupling that results in exchange bias. X-ray magnetic resonant scattering measurements show clearly the presence of canted Co interfacial moments that provide a local field which enables exchange bias at a significantly higher temperature than the onset of an enhanced coercivity.

Electrical conductivity enhanced dielectric and ferroelectric properties of interface-coupled ferroelectric superlattices

We have developed a model to theoretically study the interplay of the effects of interfacial coupling and electrical conductivity on the ferroelectric and dielectric properties of superlattices consisting of alternating ferroelectric and paraelectric layers based on the Landau-Ginzburg theory. The qualitative predictions of the model are compared with recent experimental results for “symmetric” and “asymmetric” superlattices. It is shown that the consideration of time-dependent space-charge-limited conductivity and “interface structure” can satisfactorily account for the enhancement of the overall ferroelectric and dielectric properties of ferroelectric-paraelectric superlattices.

A Lattice Model for Ferroelectric Superlattices

A one-dimensional lattice model based on the Landau–Ginzburg theory to study the intrinsic properties of a periodic superlattice with interfacial coupling is developed. The effects of thickness and interfacial coupling on the properties of a superlattice consisting of alternating ferroelectric and paraelectric constituent layers are investigated. Competition between the thickness effects of each constituent layer is expected and indeed can play an important role in governing the properties of an interface-coupled superlattice, depending on the strength of the interfacial coupling. This work may provide useful information on the possibility of manipulating structures to obtain the desired properties for specific applications.

Numerical simulation of interfacial coupling phenomena in two-phase flow through porous media

This paper provides details of the numerical simulation process for the interfacial coupling effect in multiphase flow through porous media. It covers the theoretical background, description of mathematical formulations, basic assumptions in the models, normalization and transformation between Lagrangian and Eulerian formulations, specification of boundary conditions, discretization and grid system, and solution methods. The main advantage of the Lagrangian formalism is that it eliminates the need for space discretization thereby reducing computation time and error. The numerical simulation process was codified into a stand-alone numerical simulator using the JavaTM Programming language. Numerical examples obtained from the numerical simulator are presented to show the applicability of the computational technique and to investigate the effects of interfacial coupling and hydrodynamic effects. Numerical results show that interfacial coupling (viscous and capillary) and hydrodynamic effects are insignificant in two-phase, cocurrent, horizontal, porous media flow. Sensitivity analyses studies carried out with the simulator show that if the viscous factor in the parameter that control the amount of viscous coupling is taken as the theoretically established value of 2 or below, then the viscous coupling effect is insignificant. Otherwise, the viscous coupling effect would have a greater effect on the physics of flow and might lead to unreasonable results.

Pyroelectric and dielectric properties of a ferroelectric bilayer with an antiferroelectric interfacial coupling

The pyroelectric coefficient and susceptibility of a ferroelectric bilayer with an antiferroelectric material coupling are investigated using the transverse Ising model within the effective field theory with a probability distribution technique. The effects of the magnitude interfacial coupling and the transverse field on the pyroelectric coefficient and susceptibility of the bilayer are discussed. It is shown that there exists a negative peak of pyroelectric coefficient which has not been found in the bilayer or the superlattice with ferroelectric interfacial coupling. We find that the pyroelectric coefficient and the susceptibility increase with the decrease of the magnitude of the interfacial coupling before the phase transition. We also find that a large transverse field and weak interfacial coupling can lead to the disappearance of some of the peaks of the pyroelectric coefficient and susceptibility of the bilayer.

Theory of Interface Structures in Double-Layer Ferroelectrics

Interface structures in double-layer ferroelectrics comprising two distinct layers of different polarization properties, with bilinear interface coupling across the interface, are examined analytically within the framework of Landau–Ginzburg phenomenological theory. The structures are investigated for coupled layers with: (i) one layer in paraelectric phase and the other in ferroelectric phase (ferroelectric–paraelectric), and (ii) both layers in ferroelectric phases (ferroelectric–ferroelectric). The effects of interfacial coupling on the total interface structure energies, interfacial coupling energies and mismatch in polarization across the interface are discussed.