Magnetic Superlattices Research Articles

Magnetostriction related to skyrmion-lattice formation in chiral magnet FeGe

The B20 type chiral magnet FeGe exhibits the formation of skyrmion-lattice (SkL) phases in the vicinity of the magnetic ordering temperature. The SkL is a magnetic superlattice composed of vortex-type topological spin objects, and it has experimentally been known that its formation requires the existence of an intermediate (IM) phase between SkL and the paramagnetic (PM) phases. We take interest in how the crystal lattice experiences the formation of these topological spin texture. In this study, we observed the so-called spin–orbit coupling induced magnetostriction related to these topological spin texture formation, in addition to the ac magnetization anomalies. The temperature and magnetic field dependences of the lattice parameter reflected the transformation of phases, such as helimagnetic (HM), SkL, IM, conical (CM), and PM phases. In the PM region, a phase characterized as gaseous skyrmions was detected similarly to the case of the same B20 type MnSi. Furthermore, the HM, CM, and IM phases were also divided into two regions. Thus, the precise phase diagram near Tc was reconstructed from the prospect of the magnetostriction such that we demonstrated that the stabilization of skyrmions needs a finite magnetic field.

Vortex superlattice induced second-order topological mid-gap states in first-order topological insulators and superconductors

Topological defects such as vortex and dislocations, support zero-energy localized states as a reflection of the bulk topology, in first-order topological insulators and superconductors. Furthermore, emergent first-order topological mid-gap states have been discovered driven by the magnetic vortex superlattice. However, whether the higher-order topological mid-gap states would emerge from the first-order topological insulators and superconductors with the vortex superlattice remains elusive. In this work, we propose vortex superlattice could induce second-order topological mid-gap states with staggered lattice spacings for vortices in first-order topological insulators and superconductors. These higher-order topological mid-gap states originate from the staggered tunneling between vortex-induced bound states and the emergent π flux on vortex superlattices, as an intrinsic exhibition of the interplay between vortices and bulk topology for the first-order topological states. Our work uncovers higher-topological characteristics of topological-defect superlattice in first-order topological states, and develops a controllable environment for the creation and exploration of higher-order topological states.

Bilayer graphene in periodic and quasiperiodic magnetic superlattices

Starting from the effective Hamiltonian arising from the tight-binding model, we study the behaviour of low-lying excitations for bilayer graphene placed in periodic external magnetic fields by using irreducible second-order supersymmetry transformations. The coupled system of equations describing these excitations is reduced to a pair of periodic Schrödinger Hamiltonians intertwined by a second-order differential operator. The direct implementation of more general second-order supersymmetry transformations allows to create non-singular Schrödinger potentials with periodicity defects and bound states embedded in the forbidden bands, which turn out to be associated with quasiperiodic magnetic superlattices. Applications in quantum metamaterials stem from the ability to engineer and control such bound states which could lead to a fast development of the subject in the near future.

Spin-valley polarization and tunneling magnetoresistance in monomer, dimer, and trimer magnetic silicene superlattices

Monomer, dimer and trimer semiconductor superlattices are an alternative for bandgap engineering due to the possibility of duplicate, triplicate, and in general multiply the number of minibands and minigaps in a specific energy region. Here, we show that monomer, dimer, and trimer magnetic silicene superlattices (MSSLs) can be the basis for tunable magnetoresistive devices due to the multiplication of the peaks of the tunneling magnetoresistance (TMR). In addition, these structures can serve as spin-valleytronic devices due to the formation of two well-defined spin-valley polarization states by appropriately adjusting the superlattice structural parameters. We obtain these conclusions by studying the spin-valley polarization and TMR of monomer, dimer, and trimer MSSLs. The magnetic unit cell is structured with one seed A with positive magnetization, and one, two, or three seeds B with variable magnetization. The number of B seeds defines the monomer, dimer, and trimer superlattice, while its magnetic orientation positive or negative the parallel (PM) or antiparallel magnetization (AM) superlattice configuration. The transfer matrix method and the Landauer–Büttiker formalism are employed to obtain the transmission and transport properties, respectively. We find multiplication of TMR peaks in staircase fashion according to the number of B seeds in the superlattice unit cell. This multiplication is related to the multiplication of the minibands which reflects as multiplication of the descending envelopes of the conductance. We also find well-defined polarization states for both PM and AM by adjusting asymmetrically the width and height of the barrier-well in seeds A and B.

Tunable magnetic confinement effect in a magnetic superlattice of graphene

Two-dimensional van der Waals materials such as graphene present an opportunity for band structure engineering using custom superlattice potentials. In this study, we demonstrate how self-assemblies of magnetic iron-oxide (Fe3O4) nanospheres stacked on monolayer graphene generate a proximity-induced magnetic superlattice in graphene and modify its band structure. Interactions between the nanospheres and the graphene layer generate superlattice Dirac points in addition to a gapped energy spectrum near the K and K′ valleys, resulting in magnetic confinement of quasiparticles around the nanospheres. This is evidenced by gate-dependent resistance oscillations, observed in our low temperature transport measurements, and confirmed by self-consistent tight binding calculations. Furthermore, we show that an external magnetic field can tune the magnetic superlattice potential created by the nanospheres, and thus the transport characteristics of the system. This technique for magnetic-field-tuned band structure engineering using magnetic nanostructures can be extended to a broader class of 2D van der Waals and topological materials.

First-principles study of structural, electronic and magnetic properties of diluted magnetic semiconductor and superlattices based‏ on Cr-doped ZnS and ZnSe compounds in wurtzite-type crystal

We performed first-principle calculations to investigate the structural, electronic, and magnetic properties of ZnS and ZnSe binary compounds, Zn0.5Cr0.5S and Zn0.5Cr0.5Se DMS alloys and (ZnS)2/Zn0.5Cr0.5Se and (ZnSe)2/Zn0.5Cr0.5S superlattices in the wurtzite structure using the full potential linear muffin–tin orbital (FP-LMTO) method. Features such as lattice constant, modulus of compressibility and its first derivative, spin-polarized band structures, total and local or partial electronic densities of states and magnetic properties were calculated. The electronic structure shows that Zn0.5Cr0.5S and Zn0.5Cr0.5Se DMS alloys and (ZnS)2/Zn0.5Cr0.5Se and (ZnSe)2/Zn0.5Cr0.5S superlattices are half-metallic ferromagnetic with 100% complete spin polarization. The total magnetic moments calculated show the same integer value of 4 µB, which confirms the ferromagnetic half-metallic behavior of these compounds. We found that the ferromagnetic state is stabilized by the p-d exchange associated with the double-exchange mechanism. Zn0.5Cr0.5S and Zn0.5Cr0.5Se DMS alloys and (ZnS)2/Zn0.5Cr0.5Se and (ZnSe)2/Zn0.5Cr0.5S superlattices are shown to be promising new candidates for applications in the fields of spintronics.

Enhancing and switching spin-valley filtered polarizations in a magnetic WSe2 superlattice induced by Fermi velocity modulation

The spin–valley transport is studied in a Fermi velocity-modulated monolayer WSe2 magnetic superlattice. Based on these results, a scheme that can achieve an enhancing and switching spin–valley filter is proposed in the superlattice system. We find the Fermi velocity enhanced and switched spin-valley dependent conductance transport gaps by increasing the number of Fermi velocity barriers, which are verified by the transmission probabilities and Bloch theorem. When compared to a mono-periodic superlattice, the maximum spin ↓ and valley K′ polarized conductance in a well-constructed superlattice structure are greatly improved to a detectable degree. More importantly, the crucial bias voltage or exchange field can dynamically flip and control the valley polarized conductance, something that is not possible in a mono-periodic superlattice. Our findings indicate that a Fermi velocity modulated superlattice structure can be a great way to improve the spin-valley filtered polarizations of devices based on transition metal dichalcogenides.

Interlayer engineering of Fe3GeTe2: From 3D superlattice to 2D monolayer

The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546, 265-269 (2017) and B. Huang et al., Nature 546, 270-273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14, 408-419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe3GeTe2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature (TC), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

Magnetohydrodynamic Redeposition of Cations Onto the Anode

Dissolution of metallic and metal-based catalysts occurs at the anode surfaces. The oxygen evolution reaction (OER) has been reported to cause the catalyst to be dissolved into cationic species during the electrolysis.[1–5] This phenomenon is not limited to non-noble metals (e.g. Fe, Co, Cu), but also occurs with noble metals (e.g. Ru, Rh, Pd, Os, Ir, Pt, Au).[1–4] Even with thermodynamically stable stoichiometric oxide catalysts, the dissolution cannot be completely avoided during the OER.[5] Upon dissolution, the as-formed cationic species will be attracted to the negatively charged cathode surface, resulting in catalyst loss from the anode over time.Mass transport in diffusion layers during electrochemical reactions can be tuned by means of magnetohydrodynamics (MHD). This magnetic convection is a result of the Lorentz force that arises when there is an angular mismatch between the magnetic field and the local current density. Therefore, in the presence of a magnetic field, vortices can be generated around microscale protuberances on the electrode surface, and this phenomenon is often referred to as micro-MHD.[6] Conventional applications of MHD include cathodic electrodeposition and anodic corrosion or electropolishing studies. The difference in application according to the redox tendency of the target reaction is electrochemically rational on the basis of E H–pH diagrams.Despite a rather elusive combination, we presume that the magnetically-induced vortex can contribute to the electrochemical redeposition of as-dissolved cationic species onto the anode surface. Provided that a vortex can enhance the retention of the cations remaining in the vicinity of the surface, the chance of oxidative electrodeposition will increase. If this is the case, the protuberances on the anode are expected to grow under magnetic fields over the course of the OER. This will be realized as a result of facilitated O2 bubble detachment or increased local ionic concentration, or both.In this work, we present that the morphology of the anode surface can change in the presence of a magnetic field. For the demonstration, we prepare a nanometrically-flat electrode consisting of a magnetic superlattice and an electrocatalyst layer. We perform the OER on the flat catalyst surface using magnetic and nonmagnetic samples and compare micrographs of pre-experimental and post-experimental sample surfaces. The findings show that the protuberances build up along their axes on the magnetic sample surface whereas the nonmagnetic sample involves the flattening of the surface.

Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers.

The advent of twist engineering in two-dimensional crystals enables the design of van der Waals heterostructures with emergent properties. In the case of magnets, this approach can afford artificial antiferromagnets with tailored spin arrangements. Here we fabricate an orthogonally twisted bilayer by twisting two CrSBr ferromagnetic monolayers with an easy-axis in-plane spin anisotropy by 90°. The magnetotransport properties reveal multistep magnetization switching with a magnetic hysteresis opening, which is absent in the pristine case. By tuning the magnetic field, we modulate the remanent state and coercivity and select between hysteretic and non-hysteretic magnetoresistance scenarios. This complexity pinpoints spin anisotropy as a key aspect in twisted magnetic superlattices. Our results highlight control over the magnetic properties in van der Waals heterostructures, leading to a variety of field-induced phenomena and opening a fruitful playground for creating desired magnetic symmetries and manipulating non-collinear magnetic configurations.

Giant, Linearly Increasing Spin-Orbit Torque Efficiency in Symmetry-Broken Spin-Orbit Torque Superlattices.

Magnetic heterostructures with high spin-orbit torque efficiency and low impedance have great promise for low-power spintronic technologies. We report a symmetry-broken spin-orbit superlattice [Pt0.75Cu0.25/Co/Ta]n, in which the dampinglike spin-orbit torque efficiency accumulates linearly with the repeat number n and achieves a giant value of >200% when n = 16, which is 100 times stronger than that of a conventional magnetic heterostructure with a clean Pt (e.g., 2% at a resistivity of 7 μΩ cm). The giant spin-orbit torque effect arises predominantly from the spin Hall effect of Pt0.75Cu0.25. The anomalous Nernst effect increases remarkably as the repeat number n increases, implying a critical need to include the thermal effect in the analysis of magnetic superlattices and multilayers. The giant spin-orbit torque, low resistivity, and strong anomalous Nernst effect suggest the great potential of the superlattice [Pt0.75Cu0.25/Co/Ta]n for low-power memory and logic technologies as well as high-performance thermoelectric battery and sensor applications.

Neutron-powder-diffraction studies of the nuclear and magnetic structure of the double-perovskite CaxSr2−xWMnO6 (x = 0.5, 1.0, and 1.5)

The nuclear and magnetic structures of CaxSr2−xWMnO6 (x = 0.5, 1.0, 1.5) have been investigated by neutron diffraction at 295, 30 or 40 K, and 4 or 5 K. The compounds crystallized in the monoclinic symmetry with a space group P21/n. The nuclear structure consists of ordered WO6 and Mn2+O6 octahedral frameworks, with lattice constant a decreasing more rapidly than b with increasing Ca content. No structural change was found around the magnetic transition temperature, although the Mn octahedron below the magnetic order temperature exhibited a slightly compressed Jahn–Teller distortion (0.1–0.3 Å) along the c axis. A standard magnetic model was used to solve the magnetic structure, with the Mn magnetic moments ferromagnetically coupled along the a axis and anti-ferromagnetically coupled along the b and c axes, resulting in a magnetic propagation vector of k = (0,1/2, 1/2) and a magnetic superlattice of 1a × 2b × 2c. In each crystallographic cell, the Mn1–Mn2 atoms are antiferromagnetically configurated, compensating the total moment to zero by symmetry. A layer-type magnetism along the (111) crystallographic plane was confirmed to couple antiferromagnetically between these planes and ferromagnetically within them. The magnetic structure refinement confirmed an average Mn moment of 4.5 μB. Also, it reveals that the collinear magnetic Mn configuration has three possible spin orientations, namely, (with their standard deviation) [μx = (3.3 ± 0.4) μB, μy = (2.5 ± 0.3) μB, μz = (−0.7 ± 0.7) μB], [μx = (3.0 ± 0.4) μB, μy = (0.5 ± 0.2) μB, μz = (−2.8 ± 0.2) μB, and [μx = (−1.8 ± 0.4) μB, μy = (2.9 ± 0.2) μB, μz = (−2.4 ± 0.3) μB. The first configuration is estimated to be the ground state, while the other magnetic configurations are an optimum local value derived from the refinements. Combined with the density functional theory calculations, these experimental results confirm a high-spin state Mn2+ (d5) electron configuration.

Design of quasiperiodic magnetic superlattices and domain walls supporting bound states

We study the simplest Lamé magnetic superlattice in graphene, finding its allowed and forbidden energy bands and band-edge states explicitly. Then, we design quasiperiodic magnetic superlattices supporting bound states using Darboux transformations. This technique enables us to add any finite number of bound states, which we exemplify with the most straightforward cases of one and two bound states in the designed spectrum. The topics of magnetic superlattices and domain walls in gapped graphene turn out to be connected by a unitary transformation in the limit of significantly large oscillation periods. We show that the generated quasiperiodic magnetic superlattices are also linked to domain walls, with the bound states keeping their nature in such a limit.

Optimization of the tunneling magnetoresistance and spin-valley polarization in complex magnetic silicene structures

Aperiodic order is ubiquitous in nature and quite relevant in science and technology. There are extensive works in aperiodic structures studying fundamental characteristics in physical properties, such as fractality, self-similarity, and fragmentation. However, there are fewer reports in which aperiodicity signifies an improvement in physical quantities with practical applications. Here, we show that the aperiodicity of fractal or self-similar type optimizes the tunneling magnetoresistance and spin-valley polarization of magnetic silicene structures, raising the prospects of spin-valleytronics. We reach this conclusion by studying the spin-valley-dependent transport properties of complex (Cantor-like) magnetic silicene structures within the lines of the transfer matrix method and the Landauer–Büttiker formalism. We find that the self-similar arrangement of magnetic barriers in conjunction with structural asymmetry reduces the conductance oscillations typical of periodic magnetic silicene superlattices and more importantly makes the K′-spin-down conductance component dominant, resulting in nearly perfect positive and negative spin-valley polarization states accessible by simply reversing the magnetization direction. The tunneling magnetoresistance is not as prominent as in periodic magnetic silicene superlattices; however, it is better than in single magnetic junctions. Furthermore, the optimization of the spin-valley-dependent transport properties caused by the complex structure is superior than the corresponding one reported in typical aperiodic structures, such as Fibonacci and Thue–Morse magnetic silicene superlattices.

Tuning the spin transport properties of a magnetic MoS2 superlattice

In this study, the author theoretically investigates the effect of the external Rashba interaction on the properties of spin transport in a magnetic MoS2 superlattice. The results show that the spin conductance of the magnetic MoS2 superlattice heavily relied on the applied external magnetic field in the presence of more than one barrier. Moreover, the spin conductances with and without the spin-flip exhibited a gap, the size of which increased with the number of the barriers in the superlattice. The spin-dependent conductance in the magnetic MoS2 superlattice could be controlled by changing the number of barriers to the superlattice and the strength of the Rashba interaction. The spin polarization exhibited oscillating behavior in terms of the energy of the incident electron and the strength of the Rashba interaction. Remarkably, full-spin polarization could be achieved by using an MoS2 superlattice under an external magnetic field. The results of this study confirm that the magnetic MoS2 under the Rashba interaction can be used in spintronics and electronics devices in the future.

Tuning the magnetoresistance properties of phosphorene with periodic magnetic modulation

Periodic superlattices constitute ideal structures to modulate the transport properties of two-dimensional materials. In this paper, we show that the tunneling magnetoresistance (TMR) in phosphorene can be tuned effectively through periodic magnetic modulation. Deltaic magnetic barriers are arranged periodically along the phosphorene armchair direction in parallel (PM) and anti-parallel magnetization (AM) fashion. The theoretical treatment is based on a low-energy effective Hamiltonian, the transfer matrix method and the Landauer–Büttiker formalism. We find that the periodic modulation gives rise to oscillating transport characteristics for both PM and AM configurations. More importantly, by adjusting the electrostatic potential appropriately we find Fermi energy regions for which the AM conductance is reduced significantly while the PM conductance keeps considerable values, resulting in an effective TMR that increases with the magnetic field strength. These findings could be useful in the design of magnetoresistive devices based on magnetic phosphorene superlattices.

Atomic scale interfacial magnetism and origin of metal-insulator transition in (LaNiO_3)_n/(CaMnO_3)_m superlattices: a first principles study

Interfacial magnetism and metal-insulator transition at LaNiO_3-based oxide interfaces have triggered intense research efforts, because of the possible implications in future heterostructure device design and engineering. Experimental observation lack in some points a support from an atomistic view. In an effort to fill such gap, we hereby investigate the structural, electronic, and magnetic properties of (LaNiO_3)_n/(CaMnO_3)_m superlattices with varying LaNiO_3 thickness (n) using density functional theory including a Hubbard-type effective on-site Coulomb term. We successfully capture and explain the metal-insulator transition and interfacial magnetic properties, such as magnetic alignments and induced Ni magnetic moments which were recently observed experimentally in nickelate-based heterostructures. In the superlattices modeled in our study, an insulating state is found for n=1 and a metallic character for n=2, 4, with major contribution from Ni and Mn 3d states. The insulating character originates from the disorder effect induced by sudden environment change for the octahedra at the interface, and associated to localized electronic states; on the other hand, for larger n, less localized interfacial states and increased polarity of the LaNiO_3 layers contribute to metallicity. We discuss how the interplay between double and super-exchange interaction via complex structural and charge redistributions results in interfacial magnetism. While (LaNiO_3)_n/(CaMnO_3)_m superlattices are chosen as prototype and for their experimental feasibility, our approach is generally applicable to understand the intricate roles of interfacial states and exchange mechanism between magnetic ions towards the overall response of a magnetic interface or superlattice.

Gradient Strain-Induced Room-Temperature Ferroelectricity in Magnetic Double-Perovskite Superlattices.

Single-phase multiferroics suffer from a fundamental contradiction between polarity and magnetism in d0 electronic configuration, motivating studies of unconventional ferroelectricity in magnetic oxides. However, low critical temperature and polarization still need to be overcome. Here, it is reported that the switchable polarization behavior at room temperature in [(La2 NiMnO6 )/(La2 CoMnO6 )]n double-perovskite magnetic superlattice films is achieved by engineering a microstructure with gradient strains, and the ferromagnetic Curie temperature did not show a rapid decrease. The synergy of gradient strains and superlattice components plays a decisive role in inducing ferroelectricity via the tilting or rotation of various oxygen octahedra. Such distortion responses to gradient strains are accompanied by slight magnetic fluctuations, maximizing the preservation of the initial magnetic exchange interactions, which alleviates the contradiction of multiferroic coexistence to a certain extent. This work confirms the room-temperature ferroelectricity in double-perovskite superlattices and provides a preferred strategy for confronting the difficulty of multiferroic coexistence in single-phase materials.

Tunneling magnetoresistance and spin-valley polarization of aperiodic magnetic silicene superlattices

Magnetic silicene superlattices (MSSLs) are versatile structures with spin-valley polarization and tunneling magnetoresistance (TMR) capabilities. However, the oscillating transport properties related to the superlattice periodicity impede stable spin-valley polarization states reachable by reversing the magnetization direction. Here, we show that aperiodicity can be used to improve the spin-valley polarization and TMR by reducing the characteristic conductance oscillations of periodic MSSLs (P-MSSLs). Using the Landauer–Büttiker formalism and the transfer matrix method, we investigate the spin-valley polarization and the TMR of Fibonacci (F-) and Thue–Morse (TM-) MSSLs as typical aperiodic superlattices. Our findings indicate that aperiodic superlattices with higher disorder provide better spin-valley polarization and TMR values. In particular, TM-MSSLs reduce considerably the conductance oscillations giving rise to two well-defined spin-valley polarization states and a better TMR than F- and P-MSSLs. F-MSSLs also improve the spin-valley polarization and TMR, however they depend strongly on the parity of the superlattice generation.

Nonequilibrium chiral soliton lattice in the monoaxial chiral magnet MnNb3S6

In a magnetic superlattice composed of kinks in a ferromagnetic spin array, the change in the kink number requires the movement of the kinks to and from the crystal surface. Namely, the kinks must have a velocity, and the superlattice must be nonequilibrium. Evidence of the nonequilibrium state has never been observed in previous model compounds. In ${\mathrm{MnNb}}_{3}{\mathrm{S}}_{6}$, a long magnetization relaxation was observed, and the nature of the nonequilibrium state was more pronounced in the kink annihilation process rather than the kink creation process. The annihilation process can be phenomenologically reproduced using the unfrustrated magnetic clusters model. The nonequilibrium state in the annihilation process has a longer relaxation time than that in the nucleation process, since an energy barrier exists only in the latter.