Ferromagnetic Insulator Research Articles

Novel dopant-free ferromagnetic Mott-like insulator and high-energy correlated-plasmons in unconventional strongly correlated s band of low-dimensional gold

Ferromagnetic insulators and plasmons have attracted a lot of interest due to their rich fundamental science and applications. Recent research efforts have been made to find dopant-free ferromagnetic insulators and unconventional plasmons independently both in strongly correlated electron systems. However, our understanding of them is still lacking. Existing dopant-free ferromagnetic insulator materials are mostly limited to complex d- or f-systems with extremely low Curie temperature, low-symmetry structure, and strict growth conditions on specific substrates, limiting their compatibility with industrial applications. Unconventional plasmon is, on the other hand, a quasiparticle that originates from the collective excitation of correlated-charges, yet they are rarely explored, particularly in ferromagnetic insulator materials. Herewith, we present a novel, room temperature dopant-free ferromagnetic Mott-like insulator with a high-symmetry structure in unconventional strongly correlated s band of low-dimensional highly oriented single-crystal gold quantum dots (HOSG-QDs) on MgO(001). Interestingly, HOSG-QDs show new high-energy correlated-plasmons with low-plasmonics-loss. With a series of state-of-the-art experimental techniques, we find that the Mott-insulating state is tunable with surprisingly strong spin-splitting and spin polarization accompanied by strong s–s transitions, disappearance of Drude response, and generating new Mott-like gap. Supported with a series of theoretical calculations, the interplay of quantum confinement, many-body electronic correlations, and hybridizations tunes electron–electron correlations in s band and determines the ferromagnetism, Mott-like insulator, and high-energy correlated-plasmons. Our result shows a new class of room temperature dopant-free ferromagnetic Mott-like insulator and high-energy correlated-plasmons with low-loss in strongly correlated s band and opens unexplored applications of low-dimensional gold in spin field-effect transistors and plasmonics.

Thermally Driven Spin Transport of Epitaxial FeRh Films with a Non-magnetic Pt Layer via the Longitudinal Spin Seebeck Effect.

FeRh has been demonstrated to be an important material for the observation of magnetic phase transitions, such as the first-order transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state, in response to changes in the temperature. This is because of the magnetic moment induced in Rh atoms above the magnetic phase transition temperature. In the present study, we focus on the longitudinal spin Seebeck effect (LSSE), which involves the generation of spin voltage as a result of a temperature gradient in FM materials or FM insulators, and experimentally assess the effect of the crystalline quality of FeRh films and the properties of the substrate on the LSSE thermopower during the FM-AFM phase transition. The measured LSSE thermopower of an epitaxial (110)-oriented FeRh film grown on an Al2O3 substrate is approximately 60 times higher than that of a polycrystalline FeRh film on a SiO2/Si substrate. This can be explained by the high magnetic sensitivity and superior FM properties of (110)-oriented epitaxial FeRh films. Furthermore, by comparing the transverse thermoelectric voltage for in-plane magnetized (IM) and perpendicularly magnetized (PM) configurations, we quantitively evaluate the contribution of the exclusive anomalous Nernst effect (ANE) to the LSSE signals in the FeRh/Al2O3 structure, finding it to be approximately 15-30% over a temperature range of 75-300 K. LSSE measurements in Pt/FeRh films are thus demonstrated to provide a versatile pathway for the development of thermoelectric power generation applications and other practical spintronics and neuromorphic computing devices.

One-dimensional magnetic excitonic insulators

Abstract Dimensionality significantly affects exciton production and condensation. Despite the report of excitonic instability in one-dimensional materials, it remains unclear whether these spontaneously produced excitons can form Bose–Einstein condensates. In this work, we first prove statistically that one-dimensional condensation exists when the spontaneously generated excitons are thought of as an ideal neutral Bose gas, which is quite different from the inability of free bosons to condense. We then derive a general expression for the critical temperature in different dimensions and find that the critical temperature increases with decreasing dimension. We finally predict by first-principles GW-Bethe–Salpeter equation calculations that experimentally accessible single-chain staircase Scandocene and Chromocene wires are an antiferromagnetic spin-triplet excitonic insulator and a ferromagnetic half-excitonic insulator, respectively.

Ferrimagnetic second-order topological insulator with valley polarization in two-dimensional magnet

Two-dimensional (2D) ferromagnetic and antiferromagnetic second-order topological insulators (SOTIs) coexisting with valley polarization have received increasing attention recently, while 2D valley-polarized ferrimagnetic (ferri-valley) SOTI has not been reported yet. In this work, we propose an effective six-band tight-binding model based on structural symmetry to confirm the possibility of coexistence of ferrimagnetism, second-order topological corner states, and valley polarization in 2D systems, and predict Mo2CSCl monolayer as the robust 2D ferri-valley SOTI with good structural stability, considerable Curie temperature estimated to be 100 K, and distinct valley polarization up to 109 meV under out-of-plane exchange field based on our model and first-principles calculations. Also, we find that the spin polarization direction of corner states combined with valley polarization can be controlled by switching the direction of the magnetization direction using an external magnetic field. These findings of the combination of intrinsic ferrimagnetism, second-order topological properties, and valley polarization in single 2D materials provide an ideal platform for practical applications in multifield-control spintronic devices.

Bridging the small and large in twisted transition metal dichalcogenide homobilayers: A tight binding model capturing orbital interference and topology across a wide range of twist angles

Many of the important phases observed in twisted transition metal dichalcogenide homobilayers are driven by short-range interactions, which should be captured by a local tight binding description since no Wannier obstruction exists for these systems. Yet, published theoretical descriptions have been mutually inconsistent, with honeycomb lattice tight binding models adopted for some twist angles, triangular lattice models adopted for others, and with tight binding models forsaken in favor of band projected continuum models in many numerical simulations. Here, we derive and study a minimal model containing both honeycomb orbitals and a triangular site that represents the band physics across a wide range of twist angles. The model provides a natural basis to study the interplay of interaction and topology in these heterostructures. It elucidates from generic features of the bilayer the sequence of Chern numbers occurring as twist angle is varied, and the microscopic origin of the magic angle at which flat-band physics occurs. At integer filling, the model successfully captures the Chern ferromagnetic and van Hove-driven antiferromagnetic insulators experimentally observed for small and large angles, respectively, and allows a straightforward calculation of the magnetoelectric properties of the system. Published by the American Physical Society 2024

Unraveling the dynamics of magnetization in topological insulator-ferromagnet heterostructures via spin-orbit torque

Spin–orbit coupling is a relativistic effect coupling the orbital angular momentum with the spin, which determines the physical properties of condensed matter. For instance, the spin–orbit coupling strongly influences spin dynamics, opening the possibility for promising applications. The topological insulator–ferromagnet heterostructure is a typical example exhibiting spin dynamics driven by current-induced spin–orbit torque. Recent observations of the sign flip of Hall conductivity imply that the spin–orbit torque is strong enough to flip magnetization within this heterostructure. Motivated by this, our study elucidates the conditions governing spin flips by studying the magnetization dynamics. We establish that the interplay between spin-anisotropy and spin–orbit torque plays a crucial role in the magnetization dynamics. Furthermore, we categorize various modes of magnetization dynamics, constructing a comprehensive phase diagram across distinct energy scales, damping constants, and applied frequencies. We also consider the effect of a magnetic field on the magnetization dynamics. This research not only offers insights into controlling spin direction but also charts a new pathway to the practical application of spin–orbit coupled systems.

Shunt-free cryogenic memory using ferromagnetic insulator-based Josephson junctions

Magnetic Josephson junctions are the preferred candidate devices for designing fast and scalable cryogenic memory elements. This is especially the case for rapid single-flux quantum-based superconducting electronics, where the speed mismatch between logic and memory elements have remained a long-standing challenge. In this Letter, we demonstrate a simple tri-layer Josephson memory device using ferromagnetic insulating (FI) GdN-based S/FI/S vertical mesa-type junctions, with reliable nonvolatile memory operation without the need of a shunt resistor at 4.2 K. The characteristic frequency of our devices is approximately 90 GHz, corresponding to an IcRn product of 177 μV. We demonstrate a thorough study of the parameter spaces required for designing these devices and identify the scope for future improvements that can lead to further miniaturization and higher operating speed of these devices.

Application of Cr2Si2Te6 saturable absorber in Er-doped fiber laser for generating dual-wavelength mode-locked pulse

As a typical two-dimensional (2D) ferromagnetic insulator (FI), the Cr2Si2Te6 (CST) has performance ferromagnetic properties. The previous investigation has shown that quantum mechanical simulation can get structure and electronic properties of CST, and the indirect gap value of CST is 0.6 eV by establishing its layered calculation. It implies that the CST is an excellent optical modulator due to larger infrared radiation absorption interval. Based on that, some groups conducted the research of fiber laser based on CST saturable absorber (SA). However, the exploration and application of 2D CST in optics is still in the early stage. In this investigation, the CST was utilized as a SA in an Er-doped fiber laser. The dual-wavelength mode-locked pulse could be observed when the pump power was adjusted from 25 to 140 mW. The CST was applied in Er-doped fiber as SA for generating dual-wavelength mode-locked pulse for the first time. It exhibits performance optical properties that provide a significant reference for exploring the application of 2D materials in ultrafast laser.

Real-space calculation of thermal Hall effects in strained and disordered kagome ferromagnets

Ferromagnetic insulators may exhibit magnon Hall effects when subjected to a temperature gradient due to the Dzyaloshinsky–Moriya interaction. In this study, we investigate the magnon thermal Hall conductivity κxy of kagome ferromagnets in real space using the kernel polynomial method. We first establish the formalism in real space within the framework of linear response theory, which enables efficient numerical calculations of thermal transport properties under various imperfections. The validity and accuracy of the real-space approach are confirmed by comparing the calculations with those obtained in momentum space. This approach is particularly advantageous for computing the thermal transport coefficients of disordered lattices. We consider two types of disorder in kagome ferromagnets and observe that both types significantly influence κxy across the entire temperature range. This is in contrast to the effects of strains, where strains primarily impact the maximum values of κxy .

Induced superconducting correlations in a quantum anomalous Hall insulator

Thin films of ferromagnetic topological insulator materials can host the quantum anomalous Hall effect without the need for an external magnetic field. Inducing Cooper pairing in such a material is a promising way to realize topological superconductivity with the associated chiral Majorana edge states. However, finding evidence of the superconducting proximity effect in such a state has remained a considerable challenge due to inherent experimental difficulties. Here we demonstrate crossed Andreev reflection across a narrow superconducting Nb electrode that is in contact with the chiral edge state of a quantum anomalous Hall insulator. In the crossed Andreev reflection process, an electron injected from one terminal is reflected out as a hole at the other terminal to form a Cooper pair in the superconductor. This is a compelling signature of induced superconducting pair correlation in the chiral edge state. The characteristic length of the crossed Andreev reflection process is found to be much longer than the superconducting coherence length in Nb, which suggests that the crossed Andreev reflection is, indeed, mediated by superconductivity induced on the quantum anomalous Hall insulator surface. Our results will invite future studies of topological superconductivity and Majorana physics, as well as for the search for non-abelian zero modes.

Exchange bias in heterostructures combining magnetic topological insulator MnBi2Te4 and metallic ferromagnet Fe3GeTe2

Introducing magnetism into topological insulators enables exotic phenomena such as quantum anomalous Hall effect. By fabricating van der Waals (vdW) heterostructures using layered magnetic materials, we can not only induce a gap in the non-magnetic topological surface states through magnetic proximity but also further manipulate the magnetic properties of magnetic topological insulators. However, the scarcity of 2D ferromagnetic insulator materials limits the fabrication of such heterostructures. Here, we demonstrate the vdW heterostructure devices comprising metal ferromagnetic Fe3GeTe2 nanoflakes and few-layer antiferromagnetic topological insulator MnBi2Te4 separated by an insulating hexagonal-boron nitride spacer. These devices exhibit significant exchange bias with the exchange bias field of over 100 mT under certain conditions. Our results prove that besides magnetic insulators, metallic magnets can also effectively adjust the magnetic properties of topological insulators, thereby inspiring diverse configurations of the heterostructures between topological insulators and magnetic materials.

Probing magnetic and triplet correlations in spin-split superconductors with magnetic impurities

A superconductor (SC) in proximity to a ferromagnetic insulator (FMI) is predicted to exhibit mixed singlet and triplet correlations. The magnetic proximity effect of FMI spin splits the energy of Bogoliubov excitations and leads to a spin polarization at the surface for superconducting films that are thinner than the superconducting coherence length. In this work, we study manifestations of these phenomena in the properties of a magnetic impurity coupled via Kondo coupling to this FMI/SC system. Using the numerical renormalization group (NRG) method, we compute the properties of the ground state and low-lying excited states of a model that incorporates the Kondo interaction and a Ruderman-Kittel-Kasuya-Yosida (RKKY)-like interaction with the surface spin polarization. Our main finding is an energy splitting of the lowest even fermion-parity states caused by the proximity to the FMI. As the Kondo coupling increases, the splitting grows and saturates to a universal value equal to twice the exchange field of the FMI. We introduce a two-site model that can be solved analytically and provides a qualitative understanding of this and other NRG results. In addition, using perturbation theory, we demonstrate that the mechanism behind the splitting involves the RKKY field and the triplet correlations of the spin-split superconductor. A scaling analysis combined with NRG shows that the splitting can be written as a single-parameter scaling function of the ratio of the Kondo temperature and the superconducting gap, which is also numerically obtained. Published by the American Physical Society 2024

Coexistence of ferromagnetism and antiferromagnetic dimers in topological insulators

Coexistence of ferromagnetism and antiferromagnetic dimers in topological insulators

Double ferromagnetic barriers resonant tunneling diodes (RTD) based on the surface of a topological insulator

Resonant tunneling diodes (RTDs), which are based on double barrier quantum well structures, are typically achieved by combining different materials with varying band gap sizes. However, this approach often poses challenges such as material mismatching and dislocations. In this study, we present a novel resonant tunneling diode scheme utilizing the unique properties of topological insulator materials. Specifically, we exploit the gap opening in the band structure of the topological insulator by employing perpendicular magnetization. In this proposed RTD platform, the barrier regions are formed from a ferromagnetic topological insulator through the proximity effect. By adjusting the thickness and spacing of the ferromagnetic barriers, a well region with confined states emerges between the barrier regions. Theoretical analysis reveals that by tuning the back gate voltage, the I-V characteristics exhibit two significant behaviors: negative differential resistance (NDR) and step-like behavior for Fermi energy values of EF = −3 and EF = 3, respectively. Furthermore, we observe an increase in the peak-to-valley ratio (PVR) with higher magnetization values. Notably, the PVR reaches a value of 7.13 for a magnetization value of m = 9. Additionally, we investigate the influence of the well width and barrier thickness on the transport properties of the device.

Interfacial reaction behavior between ferromagnetic CoFeB and the topological insulator Sb2Te3

The successful prevention of interfacial reactions between ferromagnetic materials and topological insulators (TIs) is crucial for the realization of reliable spintronic devices using TIs. To this end, we investigated the magnetic properties and interfacial reaction behavior of a bilayer structure composed of ferromagnetic CoFeB and TI Sb2Te3. The effects of including a MgO interlayer was also investigated. Ferromagnetic resonance (FMR) studies showed a remarkably weak resonance peak for the sample annealed at 400 °C for the 2-nm-thick interlayer, and finally, no resonance peak for a film below 1 nm in thickness was observed. X-ray diffraction (XRD) results demonstrated that the Sb2Te3 peak intensities started to decrease upon annealing at 200 °C and completely disappeared for annealing temperatures >400 °C. Hard X-ray photoelectron spectroscopy (HAXPES) results also support that the core-level peaks of Sb and Te split upon annealing at temperatures >200 °C, suggesting the dissociation of Sb2Te3. These results indicate that Sb2Te3 and CoFeB react at the interface during annealing, resulting in a loss of the ferromagnetic properties of the CoFeB layer. Meanwhile, a sample containing a 3-nm-thick MgO layer retained its original Sb2Te3/MgO/CoFeB structure even after annealing at 400 °C, as evidenced by its unchanged XRD peak intensity and FMR spectrum. We expect that our findings will be highly valuable in developing TI-based spintronic devices.

Electronic, magnetic and universality properties of room-temperature antiferromagnet Fe[formula omitted]O[formula omitted] monolayer

The previously reported X2O3 (X=V, Cr, Mn, Ta) monolayers crystallize in a honeycomb–kagome (HK) structure and are found primarily to be ferromagnetic Chern insulators with remarkable quantum anomalous Hall (QAH) properties. However, in this study, it was found that the HK Fe2O3 monolayer exhibits antiferromagnetic semiconducting properties suitable for advanced spintronics applications. Through first-principle calculations based on density functional theory (DFT) calculations, we show that the Fe2O3 monolayer has good energetic, mechanical, and dynamic stability. We further evaluate the magnetic anisotropy energies (MAE) of the Fe2O3 monolayer and find that it prefers the out-of-plane easy axis magnetization direction with a sizeable MAE value of 248.75 μeV per Fe atom. Moreover, by performing Monte Carlo (MC) simulations based on the 2D classical anisotropic Heisenberg model, we predict a Néel temperature of 631.7(5) K for the Fe2O3 monolayer. We demonstrate that the Fe2O3 monolayer belongs to the 2D Ising-like universality class based on the estimated critical exponent ratios of magnetic susceptibility, magnetization, and the exponent of the correlation length. Our findings demonstrate that the Fe2O3 monolayer can be a suitable candidate for future antiferromagnetic spintronic device applications, especially in ultra-high data processing and storage, due to the coexistence of AFM and semiconducting properties.

Spin Seebeck effect and large spin conversion in amorphous Fe2TiSb/polycrystalline Y3Fe5O12 thin films

This study investigates spin current generation in a Fe2TiSb/Y3Fe5O12 multi-layer thin film as prepared via the magnetron sputtering method. Comprehensive characterization techniques are employed to assess film properties, including X-ray diffraction, energy-dispersive X-ray spectroscopy, Scanning electron microscopy, and Vibrating sample magnetometer. The Y3Fe5O12 material exhibits a polycrystalline ferromagnetic insulator behavior, while the 20 nm-thick Fe2TiSb film displays small ferromagnetic metal properties with an amorphous structure. Spin current analysis utilizes the longitudinal spin Seebeck effect configuration, considering magnetic field and temperature dependencies and the results show that spin conversion within the Fe2TiSb/Y3Fe5O12 structure is influenced by both the spin Seebeck effect and the anomalous Nernst effect, resulting in an overall spin signal enhancement. The spin Seebeck coefficient of Fe2TiSb/Y3Fe5O12 was approximately 0.103 μV/K within a magnetic field of 300 mT.

Cooperatively tuning magnetic anisotropy and colossal magnetoresistance via atomic-scale buffer layers in highly strained La0.7Sr0.3MnO3 films

Achieving simultaneous control over multiple functional properties, such as magnetic anisotropy, magnetoresistance, and metal-insulator transition, with atomic precision remains a major challenge for realizing advanced spintronic functionalities. Here, we demonstrate a unique approach to cooperatively tune these multiple functional properties in highly strained La0.7Sr0.3MnO3 (LSMO) thin films. By inserting varying perovskite buffer layers, compressively strained LSMO films transition from a ferromagnetic insulator with out-of-plane magnetic anisotropy to a metallic state with in-plane anisotropy. Remarkably, atomic-scale control of the buffer layer thickness enables precise tuning of this magnetic and electronic phase transformation. We achieve a colossal magnetoresistance tuning of 10,000% and an exceptionally sharp transition from out-of-plane to in-plane magnetic anisotropy within just a few atomic layers. These results demonstrate an unprecedented level of control over multiple functional properties, paving the way for the rational design of multifunctional oxide spintronic devices.

Transmission of Polarized Electrons through Multilayered Spintronics

Objectives: We study transmission of polarized electrons through multilayered spintronics of the form of n-cell periodic (L/R)n and symmetric (L/R)nL multilayers. L and R are layers of ferromagnetic semiconductors or insulators. The multilayers are of arbitrary sizes with layer L of width a and layer R of width b. The number of cells n is arbitrary positive integer. For specific examples, we use ferromagnetic semiconductors Ga0.73Mn0.27N, In0.92Fe0.08As and Ga0.8Fe0.2N which have different sd-exchange energies. For the insulators, we use non-ferromagnetic insulator Al0.3Ga0.7As, and ferromagnetic insulator Al0.9Fe0.1Sb. Methods: For the purpose of the study, we will calculate transmission coefficients of polarized electrons through each multilayer by using the method of transfer matrix for finite periodic systems. Energy dependencies are analytically derived and generalize the transmission through several types of multilayered spintronics, accommodating an arbitrary number of cells and layer widths embedded between ferromagnetic semiconductors. Results will be derived for multilayers with ferromagnetic layers that are the same as or different from the ferromagnetic semiconductors embedded in the multilayer. Results: Various characteristics of transmissions are obtained from low transmissions to high transmissions with sharp peaks about certain energies, depending on the ferromagnetic semiconducting layers and the insulating layers being used for the multilayers. By varying the materials and sizes of the layers one can arrange desired transmissions such as those with high transmissions at resonances.

A theory of magnetoresistance of non-magnetic metal on magnon valves

One recent exciting development in the field of magnonics is the discovery of universal unusual anisotropic magnetoresistance (UAMR) in nanometer-thick non-magnetic (NM) metallic bars that are deposited on magnon valves of two ferromagnetic insulators (FIs) sandwiching an NM metal. This UAMR has the same angular dependencies as various bilayers consisting of at least one magnetic layer and at least one metallic layer. This suggests that the UAMR of different systems may originate from the same physics, which is yet to be fully understood. Here, we reveal the common feature shared by all these systems: two-vector dependencies of tensor quantities. Specifically, the resistivity of an NM metallic bar depends on the magnetization of its adjacent FI due to the quantum penetration of itinerant electrons of the metallic bar into the FI and on a perpendicular field at the interface of the bar and the FI. We demonstrate that the two-vector dependence of the resistivity tensor is responsible for the observed universal UAMR of metallic bars on magnon valves, independent of the details of the microscopic interactions in different materials. We also propose experiments that can test this theory.