Ferromagnetic Surface Research Articles

Surface weak ferromagnet coupling induced giant room-temperature spontaneous exchange bias in antiferromagnet Fe3BO6 polycrystals.

The spontaneous exchange bias effect (SEB) has wide application prospects in information storage technologies. In this study, nanoscale raw materials were used to fabricate antiferromagnetic Fe3BO6 polycrystals. The obtained Fe3BO6 exhibited a large SEB effect, where the value of the spontaneous exchange bias field at room temperature was as large as ∼4234 Oe. The room-temperature training effect, temperature-dependence, and maximum field-dependence of the HSEB were investigated. We propose that this giant SEB originates from the exchange-coupling interactions between the weak ferromagnetic surface state and the bulk antiferromagnetic state. The theoretical analysis results were further verified by comparing the magnetic properties of the Fe3BO6 with relatively low crystallinity. The results of this investigation will help find promising candidate materials for devices based on the SEB effect.

Distinguishing surface and bulk electromagnetism via their dynamics in an intrinsic magnetic topological insulator.

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here, we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but overestimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.

Quantum-Confined Tunable Ferromagnetism on the Surface of a Van der Waals Antiferromagnet NaCrTe2.

The surface of three-dimensional materials provides an ideal and versatile platform to explore quantum-confined physics. Here, we systematically investigate the electronic structure of Na-intercalated CrTe2, a van der Waals antiferromagnet, using angle-resolved photoemission spectroscopy and ab initio calculations. The measured band structure deviates from the calculation of bulk NaCrTe2 but agrees with that of ferromagnetic monolayer CrTe2. Consistently, we observe unexpected exchange splitting of the band dispersions, persisting well above the Néel temperature of bulk NaCrTe2. We argue that NaCrTe2 features a quantum-confined 2D ferromagnetic state in the topmost surface layer due to strong ferromagnetic correlation in the CrTe2 layer. Moreover, the exchange splitting and the critical temperature can be controlled by surface doping of alkali-metal atoms, suggesting the feasibility of tuning the surface ferromagnetism. Our work not only presents a simple platform for exploring tunable 2D ferromagnetism but also provides important insights into the quantum-confined low-dimensional magnetic states.

Spontaneous splitting of d -wave surface states: Competition between circulating currents and edge magnetization

Pair-breaking edges of d-wave superconductors feature Andreev bound states at the Fermi energy. Since these states are energetically highly unfavorable they are susceptible to effects that shift them to finite energy. We investigate the free energy of two different mechanisms: spontaneous phase gradients in the superconducting order parameter and surface ferromagnetism caused by Fermi liquid interaction effects. We find that the surface magnetization appears at lower temperatures than the spontaneous current flow of the phase-crystal state. The magnetic state can, however, be energetically favorable at lower temperatures for sufficiently strong Fermi liquid effects. Thus, first-order transitions between the two states as function of system temperature are possible, suggesting a rich low-temperature phase diagram in d-wave superconductors. Published by the American Physical Society 2024

Ferroelectricity‐Induced Surface Ferromagnetism in Core–Shell Magnetoelectric Nanoparticles

Magnetoelectric nanoparticles (NPs) present an important class of nanomaterials with a wide interest in piezocatalytic and biomedical applications. Herein, the results of magnetoelectric and magnetization measurements performed on core–shell NPs having magnetic core (MnFe2O4, MFO) and ferroelectric shell (Ba0.85Ca0.15Ti0.5Zr0.5O3, BCZT) synthesized by the microwave hydrothermal method are reported. Magnetic results are compared with the measurements on reference MFO NPs prepared under identical conditions. Detailed SQUID magnetometer measurements of the magnetization hysteresis loops M(H) down to 2 K reveal the existence of a clear exchange bias effect in pure MFO NPs attributed to the coexistence of ferromagnetic and antiferromagnetic short‐range interactions. When the magnetic core is covered by the thin ferroelectric BCZT shell, it is observed that 1) the shell suppresses the apparent bias effect and 2) induces an “extra” ferromagnetic magnetization at T < 20 K. The results indicate that this “extra” ferromagnetism has a 2D character and it is most likely related to the interface interactions between the MFO core and BCZT shell. Ferroelectric properties and strong magnetoelectric effect in core–shell NPs are revealed via piezoresponse force microscopy under magnetic field. The mechanisms of the observed effects are discussed.

Effects of particle surface modification on magnetic behavior of soft magnetic Fe@SiO2 composites and Fe compacts

The study aims to evaluate the influence of surface modification of Fe powder on the magnetic behavior of soft magnetic compacts and composites that can possibly enhance their properties. The smoothing of ferromagnetic particle surfaces led to a decrease in the total energy loss as the most evident positive impact in all investigated classes (max. by 11% for small, 63–125 μm particle-based annealed Fe compacts, at max. induction 0.5 T and frequency 100 Hz) and to a partial increase in specific electrical resistivity (max. by 47% for small particle-based Fe@SiO2 composites) and resonant frequency (max. by 48% for large, 200–400 μm particle-based Fe@SiO2 composites) as well as partial decrease in coercivity (max. by 14% for small particle-based annealed Fe compacts). Removing surface irregularities negatively affected the maximum total permeability (max. drop by 28% for large particle-based Fe@SiO2 composites) due to increased inner demagnetizing fields. Applying the Bertotti theory for loss separation, we obtained parameters of loss components and assumed the domain structure using simultaneously active magnetic objects as predictors. The total loss decrease observed after the smoothing process originates from the significantly increased numbers of active magnetic objects, facilitating AC magnetization reversal so that domain wall displacements are accompanied by lower energy loss, manifested as a decrease in the excess loss component (max. by 61% for small particle-based Fe@SiO2 composites).

Insights into the Mechanism of Chiral-Induced Spin Selectivity: The Effect of Magnetic Field Direction and Temperature.

Chiral oligopeptide monolayers are adsorbed on a ferromagnetic surface and their magnetoresistance is measured as a function of the angle between the magnetization of the ferromagnet and the surface normal. These measurements are conducted as a function of temperature for both enantiomers. The angle dependence is found to follow a changing trend with a period of 360°. Quantum simulations reveal that the angular distribution can be obtained only if the monolayer has significant effective spin orbit coupling (SOC), that includes contribution from the vibrations. The model shows that SOC only in the leads cannot reproduce the observed angular dependence. The simulation can reproduce the experiments if it included electron-phonon interactions and dissipation.

Study of surface structure and interfacial effects on optical and magnetic properties of un-doped and Si-doped hematite films

Thermal evaporation technique has been used to synthesize the α-Fe2O3 (hematite) and Si-doped hematite films on three different substrates (SiO2/Si, Al2O3 and quartz). The post-deposition heat treatment at 800 °C has further improved the crystalline nature of the films. Rutherford backscattering spectroscopy has provided information for surface elemental composition and depth profile. Raman spectra have supported the formation of rhombohedral phase and inter-layer diffusion between substrate and films. The x-ray photoelectron spectroscopy has confirmed the mixed-valence oxidation states for Fe ions and +4 state for the Si ions in Si-doped hematite films. The optical reflectance spectra in the UV–Visible region indicated contributions from the films and film-substrate interface. The Si-doping has enhanced soft ferromagnetic properties in the hematite film with noticeable decrease of the magnetic coercivity and increase of the magnetic moment. The films showed blocking of magnetization at lower temperatures, typically below 125 K, and a wide thermomagnetic irreversibility between zero-field cooled and field cooled magnetization curves. The modified surface chemical structure, ferromagnetism and optical properties in the Si doped hematite films promise their potential applications in spintronics.

Enhanced room-temperature magnetic field effect on spin-polarized excitons of cesium lead bromine-magnetite nanocomposites

The generation and manipulation of spin-polarized electrons are the basis of spintronic applications. Spin injection from a ferromagnetic surface or heterojunction to a semiconductor adjacent layer is often used to create spin-polarized carriers; however, for halide perovskites, due to the mismatch of lattice parameters with most ferromagnets at room temperature, this spin injection is challenging. In this study, we synthesize all-inorganic perovskite CsPbBr3 surrounded by Fe3O4 magnetic nanoparticles. It is found that this nanocomposite shows both magnetization and magnetic field-induced circularly polarized photoluminescence at room temperature. Specifically, with the attachment of Fe3O4 nanoparticles closely on the surface of CsPbBr3 nanocrystals, both the degree of circular polarization and g-factor enhanced by 3.5 times compared with that of pure CsPbBr3. The phenomenon should be due to the formation of exciton magnetic polaron through the coupling of the magnetic aligned nanoparticles with the excitonic state of the host semiconductor in the external magnetic field. The effective spin injection provides a method of controlling the excitonic spin polarization of all-inorganic halide perovskites for spintronic applications.

Enantioselective Adsorption on Magnetic Surfaces.

From the beginning of molecular theory, the interplay of chirality and magnetism has intrigued scientists. There is still the question if enantiospecific adsorption of chiral molecules occurs on magnetic surfaces. Enantiomer discrimination wasconjectured to arise from chirality-induced spin separation within the molecules and exchange interaction with the substrate's magnetization. Here, it is shown that single helical aromatic hydrocarbons undergo enantioselective adsorption on ferromagnetic cobalt surfaces. Spin and chirality sensitive scanning tunneling microscopy reveals that molecules of opposite handedness prefer adsorption onto cobalt islands with opposite out-of-plane magnetization. As mobility ceases in the final chemisorbed state, it is concluded that enantioselection must occur in a physisorbed transient precursor state. State-of-the-art spin-resolved ab initio simulations support this scenario by refuting enantio-dependent chemisorption energies. These findings demonstrate that van der Waals interaction should also include spin-fluctuations which are crucial for molecular magnetochiralprocesses.

Ferromagnetic insulating substrate for magnetic proximity studies: LaCoO3thin film.

Ferromagnetic insulators (FMIs) are intriguing not only due to their rare nature, but also due to their potential applications in spintronics and various electronic devices. One of its key promising applications is based on an FMI-induced magnetic proximity effect, which can impose an effective time-reversal symmetry breaking on the target ultrathin layer to realize novel emergent phenomena. Here, we conduct systematic studies on thin film LaCoO3, an insulator known to be ferromagnet under tensile strain, with varying thicknesses, to establish it as an FMI platform to be integrated in heterostructures. The optimal thickness of the LaCoO3layer, providing a smooth surface and robust ferromagnetism with large remanence, is determined. A heterostructure consisting of an ultrathin target layer (2 uc SrRuO3), the LaCoO3FMI layer, and the La0.5Sr0.5CoO3conducting layer has been fabricated and the angle-resolved photoemission spectroscopy measurement on the multi-layer system demonstrates a sharp Fermi edge and a well-defined Fermi surface without the charging effect. This demonstrates the feasibility of the proposed heterostructure using LaCoO3thin film as the FMI layer, and further lays a groundwork to investigate the magnetic proximity induced phases in quantum materials.

Investigation of the conditions of movement of magnetic transport devices on ferromagnetic surfaces of various orientations

Every year, more and more enterprises are automated, robotic facilities are produced in tens of thousands. A special niche is occupied by transport devices moving along curved surfaces. Sheathing of ships’ hulls, tanks, towers, gas storage facilities, and similar metal structures is convenient to maintain during operation and repair with magnetic transport devices. The computational schemes and mathematical models of transport devices of magnetic type with wheel propulsors on ferromagnetic surfaces are presented in this paper. Geometric and force parameters affecting movements on horizontal, inclined, and vertical surfaces were investigated. The gravity of the transport device, the reaction of the processing equipment, the traction between the drive wheels of the bogie and the ferromagnetic surface, the normal reaction of the ferromagnetic surface to the drive wheels, the attraction developed by the electromagnet were taken into account. The results obtained will contribute to the introduction of mathematical modeling methods into the practice of designing magnetic transport devices moving on ferromagnetic surfaces.

Variation of color of magnetic noise in Er3+ molecular magnets contacting metal surface in ferromagnetic composite

Interaction between single molecule magnets (SMMs) and metal surface is a hot topic promising controllable deposition of SMMs, programmable frequency of magnetic relaxation and addressed reading of information. In this paper, we develop strategy of the Er3+ based SMM functionalization by its penetration inside the arrays of Ferromagnetic Micro Particles (FMPs). We show that predictable residual magnetic field of the FMPs arrays is an effective tool for regulation of the frequency maximum of imaginary magnetic susceptibility. We analyzed color of magnetic noise in respect to physical origin of magnetic relaxation. An additional channel for control over spin noise is formation of chemical bonds between SMM and the FMPs surface. Oxidation of an Er complex transforms 20 ± 5 % of the initial 10-coordinated SMM complex to the 9-coordinated complex with reduced relaxation time. Memorized residual magnetization of the composite material allows one to set desirable relaxation frequency in the individual micronized cells to create a sound frequency register. The proposed compaction of SMMs of different types in ferromagnetic composites provides engineering of the chemically designed spin interface on ferromagnetic surface for spintronics and quantum computing.

Electronic and spin states at edges of finite p-orbital helical atomic chain.

In connection to the chiral-induced spin selectivity effect, we theoretically analyze the electronic and spin states of edges of a finite p-orbital helical atomic chain with the intra-atomic spin-orbit interaction. This model can host the spin-filtering state in which two up-spins propagate in one direction and two down-spins propagate in the opposite direction without breaking the time-reversal symmetry (TRS). The enhancement of charge modulations concentrated at the edges due to the evanescent states is induced, although the spin density is absent because of the TRS. A Zeeman field at an edge of the atomic chain, which breaks the TRS, yields a finite spin polarization, whose direction depends on the chirality of the molecule. The chirality change induces a reasonable amount of the energy difference, which may provide an insight into the enantioselective adsorption of chiral molecules on the ferromagnetic surface.

Spin-Selective Electron Transport Through Single Chiral Molecules.

The interplay between chirality and magnetism is a source of fascination among scientists for over a century. In recent years, chirality-induced spin selectivity (CISS) has attracted renewed interest. It is observed that electron transport through layers of homochiral molecules leads to a significant spin polarization of several tens of percent. Despite the abundant experimental evidence gathered through mesoscopic transport measurements, the exact mechanism behind CISS remains elusive. This study reports spin-selective electron transport through single helical aromatic hydrocarbons that are sublimed in vacuo onto ferromagnetic cobalt surfaces and examined with spin-polarized scanning tunneling microscopy (SP-STM) at a temperature of 5K. Direct comparison of two enantiomers under otherwise identical conditions revealed magnetochiral conductance asymmetries of up to 50% when either the molecular handedness is exchanged or the magnetization direction of the STM tip or Co substrate is reversed. Importantly, the results rule out electron-phonon coupling and ensemble effects as primary mechanisms responsible forCISS.

Spinterface Formation of Sexithiophene (6T) on Ferromagnetic Surfaces

Interfaces between organic molecules and ferromagnetic materials play a critical role in a variety of remarkable spin-related effects. In order to employ such spinterfaces in next-generation molecular spintronic devices, it is crucial to characterise and control these spin effects. Here, we report the features of prototypical spinterfaces comprised of sexithiophene 6T (C24H14S6) deposited on ferromagnetic metal and ferromagnetic oxide surfaces. We have investigated the controlled coverage of 6T monolayers on an Fe surface using a variety of advanced characterization techniques and show that adsorption leads to the formation of monodisperse conjugated solid islands. AFM results show that a 0.5 ML 6T deposition leads to solid islands, while with an increased thickness of 1.25 ML the material is deposited in the form of aggregated islands. XPS and UPS data reveal the orientation and electronic structure of the 6T/Fe system. Comparisons are also made to 6T adsorption on the more inert Fe3O4(001) surface revealing the importance of adsorption geometry, molecular packing and surface reconstruction in determining spinterfacial electronic structure and associated device performance.

Controlling the motion of gas-lubricated adhesive disks using multiple vibration sources.

Robots capable of generating adhesion forces that can achieve free movement in application environments while overcoming their own gravity are a subject of interest for researchers. A robot with controllable adhesion could be useful in many engineered systems. Materials processing equipment, robots that climb walls, and pick-and-place machines are some examples. However, most adhesion methods either require a large energy supply system or are limited by the properties of the contact plane. For example, electromagnetic adhesion requires a ferromagnetic surface and pneumatic adhesion requires a flat surface. Furthermore, nearly all existing approaches are only used to generate adhesion forces and often require additional mechanisms to remove the adhesive component from the surface. In this study, we aimed to develop a simpler method of adhering to a surface while simultaneously moving in directions parallel to the surface, using multiple vibration sources to generate normal adhesion and propulsion. To test our approach, we constructed circular and elliptical models and conducted experiments with various inputs and model parameters. Our results show that such a gas-lubricated adhesive disk could achieve adhesive rotation and displacement in the plane without requiring any auxiliary operating system. Using only vibration sources, we were able to generate the necessary adhesion and propulsion forces to achieve the desired motion of the robot. This work represents a step towards the construction of a small-sized tetherless robot that can overcome gravity and move freely in a general environment.

Magnetic Omnidirectional Wheel for Ferromagnetic Surface Cleaning Robots

Magnetic Omnidirectional Wheel for Ferromagnetic Surface Cleaning Robots

Results of a Study of Electromagnetic Forces and Heat Release Power in Induction Systems with a Traveling Electromagnetic Field

The results of studying the influence of various factors on the distribution patterns of electromagnetic forces and heat release power in three-phase induction systems with a traveling electromagnetic field of power frequency are presented. In the analysis, the following influencing factors were considered: phase sequence of inductors, distance between the inductor magnetic circuits end parts, and air gap size between the inductor magnetic circuit poles and the flat ferromagnetic load surface. The graphs of electromagnetic forces and heat release power in the load versus the above-mentioned factors, obtained by carrying out computational experiments in the Elcut software environment, are presented.

Surface Ferromagnetism of FeO Nanoparticles Revealed by Operando Magnetoelectrochemical Measurements

In view of the long-standing controversy over the surface ferromagnetism of $\mathrm{Fe}\mathrm{O}$, we propose to prepare its nanoparticles based on the lithium-battery reaction mechanism and carry out operando magnetometry measurements. This operando magnetoelectrochemical technology allows accurate magnetic detection of electrochemical synthesis products, effectively avoiding various intractable surface reactions during sample transmission processes. Due to the large surface area of nanosized $\mathrm{Fe}\mathrm{O}$, the magnetometry measurements show strong ferromagnetism and a high Curie temperature of 754 K, which can be attributed to surface frustration and the surface-bulk exchange interaction. These critical results confirm the surface ferromagnetism of $\mathrm{Fe}\mathrm{O}$ and provide valuable insights toward the design of spintronic devices.