Nonmagnetic Surface Research Articles

Study of the magnetic properties of a core/shell cylindrical structure with RKKY interaction using the Monte Carlo method

In this paper, we use Monte Carlo simulations to study the influence of physical parameters on the magnetic properties of a cylindrical core/shell structure separated by nonmagnetic cylindrical surfaces. The coupling between the two cylinders includes the spins σ=3/2 and S = 2 belonging to the core and shell, respectively. This study is based on the RKKY (Ruderman-Kittel-Kasuya-Yoshida) type of interaction. In order to complete this study, we carry out the hysteresis cycles of the system under study for a different number of non-magnetic surfaces.

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.

Effect of Surface Oxidation and Crystal Thickness on the Magnetic Properties and Magnetic Domain Structures of Cr2Ge2Te6.

van der Waals (vdW) magnetic materials, such as Cr2Ge2Te6 (CGT), show promise for memory and logic applications. This is due to their broadly tunable magnetic properties and the presence of topological magnetic features such as skyrmionic bubbles. A systematic study of thickness and oxidation effects on magnetic domain structures is important for designing devices and vdW heterostructures for practical applications. Here, we investigate thickness effects on magnetic properties, magnetic domains, and bubbles in oxidation-controlled CGT crystals. We find that CGT exposed to ambient conditions for 5 days forms an oxide layer approximately 5 nm thick. This oxidation leads to a significant increase in the oxidation state of the Cr ions, indicating a change in local magnetic properties. This is supported by real-space magnetic texture imaging through Lorentz transmission electron microscopy. By comparing the thickness-dependent saturation field of oxidized and pristine crystals, we find that oxidation leads to a nonmagnetic surface layer that is thicker than the oxide layer alone. We also find that the stripe domain width and skyrmionic bubble size are strongly affected by the crystal thickness in pristine crystals. These findings underscore the impact of thickness and surface oxidation on the properties of CGT, such as saturation field and domain/skyrmionic bubble size, and suggest a pathway for manipulating magnetic properties through a controlled oxidation process.

The Effect of Oxidation of Vicinal Au Surface on the Magnetic Properties of Surface‐Supported Mn Nanowires

It is well‐established that oxidation of a stepped surface can lead to the formation of 1D metal oxide nanowires along the steps. However, it is still uncertain how the extent of surface oxidation impacts the characteristics of these nanowires. Ab initio study of the effect of the oxidation state of a nonmagnetic vicinal Au surface on the magnetic properties of 1D Mn nanowires formed on the step edge is presented. Herein, the magnetic properties and electronic structure of two different types of nanowires are investigated: 1D manganese oxides (MnO) and 1D dioxides (MnO2), which correspond to different levels of oxidation of the substrate surface.

Effect of a magnetic field on droplet freezing and frost formation on cold surfaces

The effect of a magnetic field on condensed droplet freezing and frost formation was investigated using visualized experimental devices in this study. The size, shape, freezing time of droplets, and frost shape on the magnetic field considerably differ from those on the nonmagnetic surface. Moreover, the magnetic field could suppress droplet freezing and frost formation. The magnetic field suppression effect on droplet freezing and frost formation was analyzed according to the polarity characteristics of water molecules.

Experimental investigation of the effect of stable magnetic field on droplet freezing

In this study, the effect of magnetic field on droplet freezing was investigated based on a visualization system. Possible influencing factors on droplet freezing, such as magnetic field intensity, cold surface temperature, and droplet volume, were considered in this study. During the two stages of droplet supercooling and freezing process, the changes of the droplet internal temperature, the formation of initial crystal nucleus, and the freezing time were observed. Experimental results show that in comparison with the non-magnetic surface, the supercooling degree of droplets was increased in the range of 150–300 mT magnetic field intensity, thereby prolonging the freezing time of droplets. Particularly, the droplet was best suppressed on a cold surface with 250 mT magnetic field intensity. The freezing time of droplets was prolonged by 132.0 %. However, the droplets were accelerated to freeze on the cold surface with magnetic field intensities of 350 mT and 400 mT. Droplet freezing on cold surfaces was also affected by surface temperature and droplet volume. As the surface temperature of magnetic cooling dropped from −10 °C to −20 °C, the ability of the magnetic field to inhibit droplet freezing would be gradually weakened. With the increase of droplet volume from 20 μL to 50 μL, the initial freezing time of droplets was shortened. These results are related to the influence of magnetic field on the nucleation process and hydrogen bond in the water molecular cluster.

Orbital selective 5f electron character, indication of Kondo effect, and subatomic features of single uranium atoms

In actinide-based materials, the multiplicity of $5f$ orbitals provides a rich setting to elucidate the large and outstanding issues of the competition and interplay among different channels of interactions. Among them, the dual character of $5f$ electrons has been studied in some $5f$ electron Kondo lattice compounds. Here, by using scanning tunneling microscopy/spectroscopy (STS), we present the indication of Kondo effect driven by $5f$ electrons and subatomic features of single U atoms on graphene/6H-SiC(0001), ${\mathrm{Cu}}_{2}\mathrm{N}/\mathrm{Cu}(100)$, and Bi(110). For all three systems, an asymmetric-dip spectral feature can be observed around the Fermi energy, which is an indication of Kondo resonance in STS. The size of the ``screening cloud'' for single U atoms on Bi(110) is much larger than that observed on graphene/6H-SiC(0001) and ${\mathrm{Cu}}_{2}\mathrm{N}/\mathrm{Cu}(100)$, indicating the $5f$ electrons of single U atoms on Bi(110) are more itinerant than that on the other two substrates. In addition to the asymmetric spectrum feature around the Fermi energy, various subatomic spectrum features are also observed from the $dI/dV$ maps of U atoms on different substrates. Further density-functional theory calculations show that these distinct subatomic features arise from the orbital selective hybridizations between U atoms and the substrates. Our results present the distinct observation of the orbital selective $5f$ electron character, indication of Kondo effect, and together with the subatomic features of individual $5f$ impurity magnetic atoms adsorbed on nonmagnetic surfaces.

Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi2Te4 and its nonmagnetic counterpart Bi2Te3 host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi2Te4 and MnBi4Te7 following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi2Te4 and Bi2Te3 surface terminations of MnBi4Te7, revealing an instantaneous occupation of states associated with the Bi2Te3 surface layer followed by carrier extraction into the adjacent MnBi2Te4 layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi2Te4 septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi2Te4-based van der Waals compounds.

Theory of spin-polarized high-resolution electron energy loss spectroscopy from nonmagnetic surfaces with a large spin-orbit coupling

The scattering theory of low-energy (slow) electrons has been developed by Evans and Mills [Phys. Rev. B 5, 4126 (1972)]. The formalism is merely based on the electrostatic Coulomb interaction of the scattering electrons with the charge-density fluctuations above the surface and can describe most of the interesting features observed in the high-resolution electron energy-loss spectroscopy experiments. Here we extend this theory by including the spin-orbit coupling in the scattering process. We discuss the impact of this interaction on the scattering cross section. In particular, we discuss cases in which a spin-polarized electron beam is scattered from nonmagnetic surfaces with a strong spin-orbit coupling. We show that under some assumptions one can derive an expression for the scattering cross section, which can be used for numerical calculations of the spin-polarized spectra recorded by spin-polarized high-resolution electron energy-loss spectroscopy experiments.

Design and Control of Turtle Locomotion-Inspired Robots Actuated By Antagonistic Shape Memory Alloy Springs

Inspired by turtle locomotion, a novel type of robot with flippers actuated by antagonistic shape memory alloy (SMA) springs is presented that can crawl based on two different variable friction mechanisms. First, unlike robots incorporating a variable friction mechanism based on weight, the turtle-inspired robot bends its flippers through “ <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on-off”</small> control of the SMA springs in an open-loop system and achieves high friction based on the electromagnet. It can crawl on a vertical surface under a load with a load ratio of 78.5% without accurate locomotion control. To achieve fast and accurate displacement for each locomotion sequence, a radial basis function (RBF) compensator based on the minimum parameter algorithm and a mechanical model of the flippers is used to control the antagonistic SMA springs. The robot can regulate the bending shape of its flippers using an RBF compensator and climb a nonmagnetic surface with an inclination angle of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$72^\circ $</tex-math></inline-formula> when it utilizes suction units in the foot for its variable friction mechanism. The robot can crawl forward along both the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> - and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> -axis directions. In conclusion, antagonistic SMA actuation with an RBF compensator provides fast and accurate performance and is a feasible alternative to the disturbance of the control of the flippers. This article can be used to develop design guidelines for crawling robots.

Origin of Enhanced Ammonia Synthesis on Ru–Co Catalysts Unraveled by Density Functional Theory

Bimetallic Ru–Co catalysts have been reported as promising NH3 production catalysts that are better than various Ru–X alloys, including RuFe and pure Ru, under mild conditions; however, a systematic understanding of their superior activity is still lacking. Here, we report a comprehensive theoretical study on NH3 synthesis of Ru–Co catalysts using density functional theory and microkinetic modeling. Indeed, the RuCo surface enables more facile N2 dissociation than the Ru surface, which results from the manifested Co-induced spin symmetry breaking of Ru. We also investigated the surface phase diagram of RuCo(0001) with partial pressures of H2 and N2 gases and found that the most stable phase of the RuCo surface consists of a fraction of both N and H atoms under experimental Haber–Bosch pressure conditions; however, NH3 can be readily produced on the surfaces without severe surface poisoning issues. Furthermore, this study shows that the spin-symmetry breaking of nonmagnetic surfaces can enhance the catalytic activity for NH3 synthesis, which provides an alternative strategy to catalyst design.

Structural characterization of green synthesized magnetic mesoporous Fe3O4NPs@ME

Mesoporous magnetite nanoparticles (Fe3O4NPs) were synthesized by using the leaf extract of Mussaenda erythrophylla (ME) by a cheap and simple method – green synthesis. The synthesized Fe3O4NPs@ME were characterized by various techniques. A strong absorption spectrum without any specific peak in the UV–vis image designated the formation of iron-containing nanoparticles. XRD image confirmed the presence of only magnetite and the purity was ascertained by the absence of other forms of iron-containing nanoparticles. The average crystallite size was calculated as 18.58 nm and the lattice parameter was 8.38 Å which was close to the magnetite standards. FE-SEM image portrayed spherical aggregates and EDS showed signature peaks for Fe and O elements. XPS image showed the presence of peaks for oxygen, ferrous, and ferric ions which are consistent with magnetite along with carbon on the surface. Mesoporous structure (5.78 nm) was affirmed by BET results which showed a higher surface area (174.15 m2/g) than the commercial one. The FTIR bands obtained at 454.25 and 667.39 cm−1 corresponded to the distinctive Fe–O linkage, substantiating the formation of magnetite. The stability of the magnetite at very high temperature was validated by TGA results which showed a 15% total reduction in weight. The magnetite showed superparamagnetism with a low saturation magnetization value (5.14 emu/g) which confirmed the existence of non-magnetic surface layers arising from the phytomolecules residing in the leaf extract of M. erythrophylla. Hence, the pure, crystalline, and mesoporous Fe3O4NPs@ME with large surface area obtained by the above-mentioned facile procedure will surely make an impact in many areas such as catalysis, adsorption, and biomedical engineering.

Cooperation and competition between magnetism and chemisorption.

Chemisorption on ferromagnetic and non-magnetic surfaces is discussed within the Newns-Anderson-Grimley model along with the Stoner model of ferromagnetism. In the case of ferromagnetic surfaces, the adsorption energy is formulated in terms of the change in surface magnetic moments. Using such a formulation, we address the issue of how an adsorbate's binding strength depends on the magnetic moments of the surface and how the adsorption process reduces/enhances the magnetic moments of the surface. Our results indicate a possible scaling relationship of adsorption energy in terms of surface magnetic moments. In the case of non-magnetic surfaces, we formulate a modified Stoner criterion and discuss the condition for the appearance of magnetism due to chemisorption on an otherwise non-magnetic surface.

Local Kondo scattering in 4d-electron RuOx nanoclusters on atomically-resolved ultrathin SrRuO3 films.

Perovskite SrRuO3 is a unique 4d transition metal oxide with coexisting spin-orbit coupling (SOC) and electron-electron correlation. However, the intrinsic, non-reconstructed surface structure of SrRuO3 has not been reported so far. Here we report an atomic imaging of the non-reconstructed, SrO-terminated SrRuO3 surface by scanning tunneling microscopy/spectroscopy. Moreover, a Kondo resonant behavior is revealed in RuOx clusters located on top of the nonmagnetic SrO surface layer. The density functional theory calculations confirm that RuOx clusters possess localized 4d-electron-involved spin moments and hybridize with the conduction electrons in the metal host, resulting in the appearance of the Kondo resonance features around the Fermi level. Our work demonstrates that artificially-engineered transition metal oxides provide new opportunities to explore the Kondo physics in 4d multi-orbital systems.

Gas Evolution in Li-Ion Batteries: Modeling Ethylene Carbonate Decomposition on LiCoO2 in the Presence of Surface Magnetism

Decomposition of liquid electrolytes used in Li-ion batteries can result in the formation of gaseous species and the release of flammable solvent vapor. These undesirable “gassing” reactions can be mediated by interactions between the electrode surfaces and the electrolyte. At present, the detailed mechanisms responsible for electrolyte gassing are not well understood. Here, first-principles calculations are used to characterize the thermodynamics and kinetics of potential gas-forming reactions that involve surface-mediated decomposition of ethylene carbonate (EC)—a commonly used electrolyte solvent—at the (101̅4) surface of a LiCoO2 (LCO) cathode. The magnetic ordering of the (101̅4) surface was explicitly taken into account. Comparisons of EC adsorption on nonmagnetic, ferromagnetic, and antiferromagnetic surface orderings reveal differences in the adsorption geometry and reaction energetics. The antiferromagnetic surface exhibited the lowest surface energy overall; EC adsorption on this surface preferentially occurs on Li sites. In contrast, the nonmagnetic surface exhibits a stronger attraction for EC—adsorption is 0.5 eV more exothermic per molecule than for the antiferromagnetic surface—and adsorption occurs on Co sites. The thermodynamic driving force for EC decomposition on the antiferromagnetic surface was predicted for several potential reaction products resulting from more than 30 bond-breaking scenarios. The most exothermic of these reactions results in the formation of CO2 and acetaldehyde (C2H4O), gaseous products that have been observed in prior experiments. Nevertheless, an evaluation of the minimum energy path for CO2/C2H4O formation on fully lithiated LCO reveals a large reaction barrier for this process, implying a kinetically limited reaction. These data suggest that the rate of solvent decomposition at the cathode may be maximized in the charged state, where LCO is partially delithiated.

Experimental investigations and optimization of machining parameters for Magneto-rheological Abrasive Honing process

ABSTRACT Being an advancement of conventional honing method, Magneto-rheological Abrasive Honing (MRAH) process is one of the unconventional nano-finishing processes which has the capability of finishing the non-magnetic freeform surfaces with good material removal rate. The magneto-rheological effect imparted by the carbonyl iron particles (magnetic particles) along with abrasives introduced through carrier medium directs the finishing action in MRAH. In MRAH process, up and down motion with rotational motion were assigned to the finishing fluid and the workpiece, respectively. Being a new process, there is a need to quantify and analyze the outcomes of machining parameters in addition to finishing time on % change in roughness value (%∆Ra). In the current study, machining parameters like magnetizing current, rotational speed of the workpiece, and finishing time were optimized to maximize %∆Ra using response surface methodology. An investigational study using center composite rotatable design followed by ‘Analysis of Variance’ was applied to evaluate the impact of influencing parameters. The regression equation obtained after elimination of insignificant terms was further optimized using response analyzer in Minitab 17. The outcomes obtained from regression equation were establishing a fine confirmation with the outcomes obtained from the experiments.

Robust Topological States in Bi2Se3 against Surface Oxidation

Nontrivial surface states in topological insulators are originated from the bulk band topology, and thus are protected against non-magnetic surface perturbations. However, previous experimental and...

Magnetic Properties of Magnetite Nanoparticles (Fe3O4-NPs) Coated with Mesoporous Silica by Surfactant Templated Sol-Gel Method

Here, we present the magnetic properties of silica-coated magnetite nanoparticles. We have coated 7 nm of Fe3O4 with cetyltrimethylammonium bromide (CTAB) for phase transformation from hydrophobic to hydrophilic. Core-shell structure of silica-coated magnetite nanoparticles have been obtained using surfactant templated sol-gel method. The obtained silica-coated magnetite nanoparticles were characterized by transmission electron microscopy (TEM), fourier transform infrared (FTIR) spectroscopy and superconducting quantum interference device (SQuID). The hysteresis loops of the coated particles were measured using SQuID and the results showed a superparamagnetic behavior at room temperature. The saturation magnetization (Ms) of the coated particles indicate the presence of non-magnetic surface layers resulting from the strong chemical attachment of the silica to the Fe3O4’s surface, also observed by FTIR spectroscopy.  Â

Single molecules sense spin

Surface Magnetism Imaging surface magnetism, and, in particular, spin excitations of adsorbed molecules or films, is challenging. Verlhac et al. demonstrate spin-sensing capability by using the magnetic exchange interaction between a surface sample and the spin-excited states of a nickelocene molecule attached to a scanning tunneling microscope tip. The spatial dependence of the exchange field at the atomic scale enabled imaging of magnetic corrugation with atomic-scale lateral resolution for iron atoms and small islands of cobalt atoms absorbed on nonmagnetic copper surfaces. Science , this issue p. [623][1] [1]: /lookup/doi/10.1126/science.aax8222

Atomic-scale spin sensing with a single molecule at the apex of a scanning tunneling microscope.

Recent advances in scanning probe techniques rely on the chemical functionalization of the probe-tip termination by a single molecule. The success of this approach opens the prospect of introducing spin sensitivity through functionalization by a magnetic molecule. We used a nickelocene-terminated tip (Nc-tip), which offered the possibility of producing spin excitations on the tip apex of a scanning tunneling microscope (STM). When the Nc-tip was 100 picometers away from point contact with a surface-supported object, magnetic effects could be probed through changes in the spin excitation spectrum of nickelocene. We used this detection scheme to simultaneously determine the exchange field and the spin polarization of iron atoms and cobalt films on a copper surface with atomic-scale resolution.