Amorphous CoFeB Research Articles

Enhanced Oxygen Evolution Reaction Performance in Co–Fe Hydroxides through Boron Doping

Among hydrogen production methods, water electrolysis stands out, but its efficiency is hampered by the substantial energy barrier of the oxygen evolution reaction (OER). To address this, incorporating electron‐deficient boron (B) into Co–Fe hydroxide (CoFeOxHy) promotes higher oxidation states of involved metals, greatly enhancing OER activity and charge transfer capabilities. Herein, the synthesis of a range of amorphous CoFeB nanoparticles with varying Fe to (Co+Fe) atomic ratios achieved through a simple chemical reduction method using CoFe‐Prussian blue analogs as precursors and employing Mössbauer spectroscopy to observe structural characteristics before and after transformation is reported. Among these nanoparticles, the CoFe0.25B variant, exhibiting favorable electrochemical properties, is chosen and subsequently subjected to hydrolysis to yield CoFe0.25BOH nanoparticles, serving as an active catalyst for OER. At a current density of 10 mA cm−2, the overpotentials for CoFe0.25OxHy and CoFe0.25BOH are 362 and 310 mV, respectively, with Tafel slopes decreasing from 393 to 93 mV dec−1. Furthermore, the i–t test reveals no significant loss of electrochemical performance within 24 h, substantiating the efficacy of enhancing the electrocatalytic performance of CoFeOxHy through the introduction of electron‐deficient elements. This research offers novel insights into the development of efficient and stable water electrolysis catalysts.

Theoretical limits of magnetic detection of structural surface defects at the nanometre scale

ABSTRACT We present a theoretical study on the magnetic signals of structural surface defects like cracks or indents combined with inhomogeneities on the surface or subsurface inclusions of soft ferromagnetic metals like body-centred cubic Fe or amorphous CoFeB. We discuss limits of early detection of small surface defects on the basis of calculated magnetic stray fields few tens of nm above the surface. The considered surface imperfections have extensions of a few nm which correspond to low multiples of the magnetic exchange lengths of Fe or CoFeB. The detection of such small inhomogeneities requires that the sensor is about as close to the surface as the size of the inhomogeneity is. Furthermore, the step width of a scanning sensor must be of the same size as well. Both these requirements may be fulfilled for instance by scanning microscopy with diamond nitrogen-vacancy-centre quantum sensors.

Fourfold anisotropic magnetic damping induced by anisotropic spin absorption in amorphous CoFeB/epitaxial IrMn3 bilayers

Magnetic damping rooted in various relaxation processes of spin angular momentum plays a crucial role in determining the energy consumption and the operating speed of emerging spintronic devices. Here, we reported a fourfold anisotropic magnetic damping extrinsically caused by spin current absorption in amorphous ferromagnetic/antiferromagnetic bilayers. The angular dependent broadband ferromagnetic resonance measurements were employed to investigate the effect of interfacial exchange coupling on magnetization dynamics of the CoFeB/IrMn3 bilayers. In addition to the conventional exchange bias, the amorphous CoFeB layer exhibits an induced fourfold magnetic anisotropy along the IrMn3⟨100⟩ axes because of the exchange coupling with the uncompensated IrMn3 moments along IrMn3⟨100⟩ caused by the 3Q antiferromagnetic structure. The magnetic damping of the CoFeB/IrMn3 bilayer also exhibits an obvious fourfold symmetry, which is ascribed to the anisotropic spin absorption caused by the uncompensated IrMn3 moments. The ratio of the fourfold anisotropic magnetic damping decreases dramatically with reducing the interfacial exchange coupling. When the interfacial exchange coupling is isolated by a Cu spacer layer, both the induced magnetic anisotropy and the magnetic damping exhibit a uniaxial symmetry.

Tuning surface spin polarization of CoFeB by boron diffusion detected by spin resolved photoemission

Abstract Investigation on the spin polarization of the magnetic CoFeB thin film is of practical importance in spintronic applications. Here, using a direct characterization technique of spinresolved photoemission spectroscopy, the surface spin polarization of amorphous Co40Fe40B20 thin films with different annealing temperatures from 100 ℃ to 500 ℃ prepared by magnetron sputtering is obtained. After high annealing temperature, a quasi-semiconductor state is gradually formed at the CoFeB surface due to the boron diffusion. While the global magnetization remains almost constant, the secondary electrons’ spin polarization, average valence band spin polarization and the spin polarization at Fermi level from spin-resolved photoemission spectroscopy show a general trend of decreasing with the increasing of the annealing temperature above 100 ℃. These distinct surface properties are attributed to the enhanced Fe-B bonding due to the boron segregation upon surface after annealing as confirmed by x-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Our findings provide insight into the surface spin-resolved electronic structure of the CoFeB thin films, which should be important for the development of high-performance magnetic random-access memory.

Antiferromagnetic fourfold anisotropy induced exchange bias and magnetization reversal behaviors in CoFeB/IrMn bilayers

Bilayers of an amorphous CoFeB ferromagnet coupled to an epitaxially grown IrMn antiferromagnet, whose anisotropy is fourfold symmetric, are modeled to study the angular (φ) dependence of magnetization reversal mechanism and exchange bias behavior, based on a modified Monte Carlo method. It is found that positive and negative exchange-bias field (HE) maximum values appear in the antiferromagnetic easy-axis directions, and the HE zero-value transition/return points exist in the antiferromagnetic hard-axis directions. Furthermore, the value of HE(φ) is well fitted by considering cos3φ/sin3φ terms with larger or comparable coefficients as compared to the first-order terms in the Fourier expansion. Finally, the microscopic spin configuration results evidence these phenomena originating from the tailoring of antiferromagnetic spins by simply rotating the field-cooling direction between antiferromagnetic easy- and hard-axis directions. We open an exchange-bias mode governed by antiferromagnetic fourfold anisotropy and propose a method to precisely detect antiferromagnetic higher-order anisotropies on an exchange-bias support.

Amorphous CoFeB nanosheets with plasmon-regulated dynamic active sites for electrocatalytic water oxidation

Exploring efficient and low-cost oxygen evolution reaction (OER) electrocatalysts is of crucial importance. Here, we report a surface plasmon resonance (SPR) engineering strategy to regulate surface reconstruction of CoFeB nanosheets by decorating plasmonic MoO2 nanospheres (MoO2/CoFeB), in which the SPR effect of MoO2 offers an additional acceleration for the conversion of inactive Co species to active cobalt oxyhydroxide on the CoFeB surface under visible light. Our results also indicate the real reactive surface for CoFeB is in the form of CoFeOOH with adsorbed BO2− that has positive effect. The MoO2/CoFeB shows superior OER performance with a low overpotential (209 mV at J=10 mA cm−2). However, such an accelerated reconstruction behavior would be self-terminated once the anodic voltage increases to thoroughly oxidize the MoO2 to high valence state (+6). This work inspires us to develop a rational strategy to improve the catalytic performance by properly regulating the surface reconstruction.

Role of Nb content in tailoring the microstructure and magnetic anisotropy of soft magnetic W/CoFeB alloy thin films prepared with varying the substrate temperature

The work describes the role of Nb addition on the magneto-structural properties of magnetron sputtered Si/SiO2/W/(CoFeB)1−xNbx (x = 0, 3, 5 and 10) metal metalloid alloy thin films. Films were grown at substrate temperature (Ts) of RT, 300 °C and 500 °C to realise the influence of Nb content in the amorphous and nano-crystalline states of (CoFeB)1−xNbx thin films. RT deposited films are soft magnetic (coercivity (Hc) ≤ 10 Oe), exhibits amorphous microstructure and display a well-defined uniaxial magnetic anisotropy (UMA) in the film plane. At 300 °C, devitrification occurs, and the film (x = 0) becomes partially crystalline with precipitation of FeCo nano-crystals, and UMA nearly disappears. With increasing Nb content, the mean grain size and amount of FeCo nano-crystals decreases, and partially crystalline CoFeB films transform to a completely amorphous microstructure with the simultaneous appearance of in-plane UMA and systematic improvement in soft-magnetic nature (Hc decreases from~22–5 Oe). Nb addition on at Ts= 500 °C results in grain growth inhibition and magnetically isotropic (CoFeB)1−xNbx films (except x = 3). The present study demonstrates that thermally stable amorphous CoFeB films with desired soft magnetic properties can be prepared by appropriate selection of Nb content and suitable growth temperature.

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO3(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh〈 110〉 to FeRh〈 100 〉 directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.

Origin of magnetic field-induced magnetic anisotropy in amorphous CoFeB thin films

Magnetic anisotropy (MA) is an important property of magnetic materials, which not only determines the orientation of the magnetic moment in the magnetic material but also influences the working frequency of magnetoelectric devices. Unrevealing the origin of MA has become an important topic and attracts lasting interest. Here, we report a quite significant magnetic field-induced uniaxial MA in amorphous CoFeB thin films containing double ferromagnetic atoms. The thickness independence of MA was obtained by observing a series of hysteresis loops and magnetic domains. The MA is proved subtly to be related to the variation of orbital magnetic moment acquired by ferromagnetic resonance. Furthermore, we found that atoms combine into clusters and incline to an order in amorphous CoFeB thin films with field-induced MA. Based on these experimental results, we proposed a direction-like order model to interpret the origin of magnetic field-induced MA in amorphous CoFeB thin films well.

Magnetization reversal processes in amorphous CoFeB thin films

Magnetization reversal processes occurring in amorphous CoFeB thin films of thicknesses 100 nm, 200 nm and 300 nm were experimentally investigated. Analysing hysteresis loops traced with a vectorial VSM and the angular behaviour of coercivity, we concluded that domain wall displacement is the leading reversal mechanism for films with 200 and 300 nm. For the thinnest sample, the same is observed for external fields applied away from the hard axis, while coherent rotation becomes import close to this axis.

Amorphous CoFeB on nickel foam as a high efficient electrocatalyst for hydrogen evolution reaction

The development of high-performance, low cost and earth abundant catalysts for hydrogen evolution reaction (HER) is desired. This work presents amorphous CoFeB supported on nickel foam (NF), prepared by a facial chemical reduction method, as an active catalyst for HER in alkaline solution. Structure characterization indicated that with the incorporation of Fe atom, CoFeB catalysts exhibit similar petal-like granular morphology as CoB. The optimal CoFeB/NF-0.15 catalyst exhibits Brunauer-Emmett-Teller (BET) surface area of 27.4 m2 g−1, nearly two times larger than 13.2 m2 g−1 for CoB, suggesting higher specific surface area. CoFeB/NF-0.15 catalyst shows excellent HER performance and reaches −10 mA cm−2 at overpotential of 35 mV in alkaline solution, and Tafel slope of 84.7 mV dec−1, indicative of Volmer-Heyrovsky reaction mechanism. The synergistic effect among Fe, Co and B atoms and the more exposed active sites as well as faster electron transfer kinetics collectively contributed to the improved intrinsic activity of CoFeB for HER. Moreover, CoFeB/NF-0.15 exhibits good stability for over 16 h.

Asymmetric magnetization reversal processes in amorphous composites (Fe40Co40B20)x(AL2O3)100-x

The amorphous composite films (Co40Fe40B20)x(Al2O3)100-x with different content of the metal (Co40Fe40B20)x “(CoFeB)x” and the dielectric (Al2O3)100-x components were obtained in the original ion-beam deposition unit. Sputtering of a composite target from an amorphous Co40Fe40B20 alloy plate with dielectric Al2O3 inserts was carried out in the argon atmosphere at an operating pressure of ∼5.10–4 Torr on glass-ceramic Sitall substrates. The presence of two regions in the IR spectra of composites with different values of х confirms the existence of the metal and dielectric components during the ion-plasma deposition of composites on a sitall substrate as a result of self-arrangement. The magnetic hysteresis was measured at room temperature for (CoFeB)x(Al2O3)100-x thin film samples for x = 1, 75 and from 9 to 69 with the step x = 10 at different angles (φ) when magnetic field applied out of a plane (φ = 90°) and in-plane (φ = 0°) of the films. The obtained results show that at low alloy (CoFeB)x contents the film composites with nominal x = 1, 9, and 19 behave as paramagnetic from the first glance. With an increase of the alloy (CoFeB)x content to nominal x = 29–75 amorphous composites films seems as superparamagnetic and ferromagnetic materials that are characterized as reversal magnetization with no coercivity and remanence. The existence of a magnetic vortex in the samples with x = 59–75 opens different remarkable applications like data storage.

Atomic arrangements in an amorphous CoFeB ribbon extracted via an analysis of radial distribution functions

We discuss the atomic structure of amorphous ferromagnetic FeCoB alloys, which are used widely in spintronics applications. Specifically, we obtain the pair-distribution functions for various atomic pairs based on high-energy x-ray diffraction data taken from an amorphous Co20Fe61B19 specimen. We start our reverse Monte Carlo cycles to determine the disordered structure with a two-phase model in which a small amount of cobalt is mixed with Fe23B6 as a second phase. The structure of the alloy is found to be heterogeneous, where the boron atoms drive disorder through the random occupation of the atomic network. Our analysis also indicates the presence of small cobalt clusters that are embedded in the iron matrix and percolating the latter throughout the structure. This morphology can explain the enhanced spin polarization observed in amorphous magnetic materials.

2D Heterostructure of Amorphous CoFeB Coating Black Phosphorus Nanosheets with Optimal Oxygen Intermediate Absorption for Improved Electrocatalytic Water Oxidation.

The oxygen evolution reaction (OER) plays a paramount role in a variety of electrochemical energy conversion devices, and the exploration of highly active, stable, and low-cost electrocatalysts is one of the most important topics in this field. The exfoliated black phosphorus (EBP) nanosheet with a two-dimensional (2D) layered structure has high carrier mobility but is limited by excessive oxygen-containing intermediate absorption and fast deterioration in air. We here report the fabrication of nanohybrids of amorphous CoFeB nanosheets on EBP nanosheets (EBP/CoFeB). The 2D/2D heterostructure, thanks to the electronic interactions and oxygen affinity difference between EBP and CoFeB nanosheets, is capable of balancing the oxygen-containing intermediate absorption to an optimal status for facilitating the OER process. While the crystalline EBP contributes to the improved conductivity, the amorphous coating protects EBP and thus ensures the catalytic stability. The EBP/CoFeB electrocatalyst shows excellent OER performance with an ultralow overpotential of 227 mV at 10 mA cm-2 with an ultrasmall Tafel slope of 36.7 mV dec-1 with excellent stability. This study may inspire more researches to develop heterostructured nanohybrid electrocatalysts for a diversity of electrochemical reactions.

Effect of substrate roughness and film thickness on the magnetic properties of CoFeB films on polymer substrate

Correlation between the magnetic properties and surface morphology of CoFeB film has been investigated. The CoFeB films of various thicknesses are deposited on Ar plasma etched polymethyl methacrylate (PMMA) substrate. As the etching time increases, the substrate and surface roughness of the film increase, suggesting a conformal deposition that has a strong correlation between the substrate roughness and film surface. Furthermore, as the etching time increases, coercivity (Hc) increases, however, the saturation magnetization (Ms) decreases. The opposite characteristics of Hc and Ms with increasing etching time are related to domain-wall pinning and local anisotropy owing to the rough surface. In particular, the change in Hc is significantly different according to the film thickness with the increase in etching time. The different behaviors of Hc depending on the film thickness and etching time is due to the continuity or discontinuity of CoFeB film on the substrate. In addition, the magnetic properties and surface characteristics are different before and after the etching time of 2 min, indicating that the surface roughness formed by the Ar plasma etching and magnetic properties are closely correlated. These results can be effectively applied to the study of modulation of amorphous CoFeB films on polymer substrates.

Anisotropic coercivity and the effects of interlayer exchange coupling in CoFeB/FeRh bilayers

In an amorphous CoFeB layer, coercivity becomes anisotropic with fourfold symmetry when the CoFeB layer exchange couples to an FeRh layer. The angular dependence of coercivity of the CoFeB layer coincides with the in-plane easy-axis direction of the FeRh layer and experiences a ${45}^{\ensuremath{\circ}}$ shift with the occurrence of a metamagnetic phase transition of the FeRh layer from antiferromagnetism at room temperature to ferromagnetism at 400 K. The intriguing phenomena are well reproduced by our unbiased Monte Carlo simulation. The interfacial exchange and anisotropy energies, as well as the interfacial magnetization in the CoFeB/FeRh bilayer, are disentangled to demonstrate the strong dependence of the imprinting of anisotropy in the CoFeB layer on the interfacial exchange coupling. The evolution of the easy-axis direction of the induced anisotropy arises from the reconstruction of the interfacial exchange energy profile accompanied with the change of the magnetic state of FeRh, which governs the magnetization reversal of the CoFeB layer at both branches. Moreover, the imprinting is further applicable for the uniaxial magnetocrystalline anisotropy. This work not only presents the possibility of directly duplicating anisotropy between dissimilar materials, but it also provides a powerful tool to probe the hidden magnetic structures and/or the properties of materials that have weak magnetism, such as antiferromagnetic materials.

Magnetocrystalline anisotropy imprinting of an antiferromagnet on an amorphous ferromagnet in FeRh/CoFeB heterostructures

Magnetic anisotropy is a fundamental key parameter of magnetic materials that determines their applications. For ferromagnetic materials, the magnetic anisotropy can be easily detected by using conventional magnetic characterization techniques. However, due to the magnetic compensated structure in antiferromagnetic materials, synchrotron measurements, such as X-ray magnetic linear dichroism, are often needed to probe their magnetic properties. In this work, we observed an imprinted fourfold magnetic anisotropy in the amorphous ferromagnetic layer of FeRh/CoFeB heterostructures. The MOKE and ferromagnetic resonance measurements show that the easy magnetization axes of the CoFeB layer are along the FeRh〈110〉 and FeRh〈100〉 directions for the epitaxially grown FeRh layer in the antiferromagnetic and ferromagnetic states, respectively. The combined Monte Carlo simulation and first-principles calculation indicate that the fourfold magnetic anisotropy of the amorphous CoFeB layer is imprinted due to the interfacial exchange coupling between the CoFeB and FeRh moments from the magnetocrystalline anisotropy of the epitaxial FeRh layer. This observation of imprinting the magnetocrystalline anisotropy of antiferromagnetic materials on easily detected ferromagnetic materials may be applied to probe the magnetic structures of antiferromagnetic materials without using synchrotron methods.

Microstructure and phase dependence of magnetic properties of CoFeB nanowires

Arrays of amorphous CoFeB nanowires with the diameter of 250 nm and length of 20 μm were fabricated by electrodepositing in pores of anodic aluminum oxide (AAO) templates. The corresponding changes of coercivity (Hc), remanence squareness (Mr/Ms), and the shape of hysteresis loops of NWs before and after annealing with the microstructure evolution have been investigated. The XRD and TEM results showed the precipitation of partial Co nanocrystallites and complete crystallization for the annealing treatment at 573 K and 773 K, respectively. The VSM results indicated a strong magnetic anisotropy in the amorphous nanowires and the easy magnetization axis was in perpendicular to the long wire axis. Both Hc and Mr/Ms increased with increasing annealing temperature. The magnetization reversal process occurred by the curling model for the amorphous nanowires and exhibited a transition for magnetization reversal after thermal annealing. In all, the magnetic properties of CoFeB nanowires can be controlled by tuning their microstructure with thermal annealing.

Giant anisotropy of magnetic damping and significant in-plane uniaxial magnetic anisotropy in amorphous Co40Fe40B20 films on GaAs(001)**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the National Natural Science Foundation of China (Grant Nos. 51971109, 51771053, and 51471085), and Scientific Research Foundation of Nanjing Institute of Technology (Grant Nos. ZKJ201708 and CKJB201708).

Tuning magnetic damping constant in dedicated spintronic devices has important scientific and technological implications. Here we report on anisotropic damping in various compositional amorphous CoFeB films grown on GaAs(001) substrates. Measured by a vector network analyzer-ferromagnetic resonance (VNA-FMR) equipment, a giant magnetic damping anisotropy of 385%, i.e., the damping constant increases by about four times, is observed in a 10-nm-thick Co40Fe40B20 film when its magnetization rotates from easy axis to hard axis, accompanied by a large and pure in-plane uniaxial magnetic anisotropy (UMA) with its anisotropic field of about 450 Oe. The distinct damping anisotropy is mainly resulted from anisotropic two-magnon-scattering induced by the interface between the ferromagnetic layer and the substrate, which also generates a significant UMA in the film plane.

XPS and XAS investigations of multilayer nanostructures based on the amorphous CoFeB alloy

We analyzed two amorphous multilayer nanostructures (MLNS) [(CoFeB)60C40/SiO2]200and [(CoFeB)34(SiO2)66/C]46with the same element composition, but with the inverse location of the non-metallic phases C and SiO2in the metal-composite layers or interlayers. Based on the analysis of XPS and X-ray absorption spectra, it was found that in the metal-composite layers the chemical bonds prevailing at the interphase boundaries between the CoFeB metal clusters and the elements of the surrounding matrix, forming predominantly carbide or oxide phases depending on the C or SiO2 matrix composition.Quantitative fitting of the Fourier transforms for the Fourier transformed EXAFS signal from two MLNSs shows, that in [(CoFeB)60C40/SiO2]200with carbon in metal-composite the Fe and Co atoms of metal clusters have coordination numbers comparable to each other (6.73 and 7.68).In another MLNS [(CoFeB)34(SiO2)66/C]46, in which carbon forms interlayers, and its place in metal-composite layers is taken by SiO2, the number of atoms in the nearest environment of Co (6.71) close to the previous one, and the number of atoms in the environment of iron atoms (3.37) most two times less and, rather, is typical for compounds of iron with oxygen from the composite matrix in this MLNS. This lead to noticeable violations of the planarity of the interface and the anisotropy of electromagnetic properties in this MLNS.