Strong Magnetic Interaction Research Articles

Chemical ordering and magnetism in face-centered cubic CrCoNi alloy

The impact of magnetism on chemical ordering in face-centered cubic CrCoNi medium entropy alloy is studied by a combination of ab initio simulations, machine learning potentials, and Monte Carlo simulations. Large magnetic energies are revealed for some mixed L12/L10 type ordered configurations, which are rooted in strong nearest-neighbor magnetic exchange interactions and chemical bonding among the constituent elements. There is a delicate interplay between magnetism and stability of MoPt2 and L12/L10 type of order, which may explain opposing experimental and theoretical findings.

Nonaqueous Synthesis of Low-Vacancy Chromium Hexacyanochromate.

Prussian blue analogues (PBAs) are a highly tunable family of materials with properties suitable for a wide variety of applications. Although their straightforward aqueous synthesis allows for the facile preparation of a diverse set of compositions, the use of water as the solvent has hindered the preparation of specific compositions with highly sought-after properties. A typical example is Cr[Cr(CN)6]: its predicted strong magnetic interactions have motivated many attempts at its synthesis but with limited success. The lack of control over vacancies, crystallinity, and the oxidation state has prevented the experimental validation of its theoretical magnetic properties. Here, we report the nonaqueous synthesis of vacancy-suppressed, nanocrystalline chromium hexacyanochromate. The control over vacancies and the oxidation state leads to stronger magnetic interactions with a markedly increased absolute Weiss temperature (Θ = -836(6) K) and magnetic ordering temperature of (240 ± 10) K. Our results challenge the notion of the solvent as merely reaction medium and introduce a pathway for exploring moisture- and air-sensitive PBA compositions.

Strong Magnetic Exchange Interactions and Delocalized Mn-O States Enable High-Voltage Capacity in the Na-Ion Cathode P2-Na0.67[Mg0.28Mn0.72]O2.

The increased capacity offered by oxygen-redox active cathode materials for rechargeable lithium- and sodium-ion batteries (LIBs and NIBs, respectively) offers a pathway to the next generation of high-gravimetric-capacity cathodes for use in devices, transportation and on the grid. Many of these materials, however, are plagued with voltage fade, voltage hysteresis and O2 loss, the origins of which can be traced back to changes in their electronic and chemical structures on cycling. Developing a detailed understanding of these changes is critical to mitigating these cathodes' poor performance. In this work, we present an analysis of the redox mechanism of P2-Na0.67[Mg0.28Mn0.72]O2, a layered NIB cathode whose high capacity has previously been attributed to trapped O2 molecules. We examine a variety of charge compensation scenarios, calculate their corresponding densities of states and spectroscopic properties, and systematically compare the results to experimental data: 25Mg and 17O nuclear magnetic resonance (NMR) spectroscopy, operando X-band and ex situ high-frequency electron paramagnetic resonance (EPR), ex situ magnetometry, and O and Mn K-edge X-ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Spectroscopy (XANES). Via a process of elimination, we suggest that the mechanism for O redox in this material is dominated by a process that involves the formation of strongly antiferromagnetic, delocalized Mn-O states which form after Mg2+ migration at high voltages. Our results primarily rely on noninvasive techniques that are vital to understanding the electronic structure of metastable cycled cathode samples.

Tailored Magnetic Spatial Confinement with Enhanced Polarization and Magnetic Response for Electromagnetic Wave Absorption.

For materials with coexisting phases, the transition from a random to an ordered distribution of materials often generates new mechanisms. Although the magnetic confinement effect has improved the electromagnetic (EM) performance, the transition from random to ordered magnetic confinement positions remains a synthetic challenge, and the underlying mechanisms are still unclear. Herein, precise control of magnetic nanoparticles is achieved through a spatial confinement growth strategy, preparing five different modalities of magnetic confined carbon fiber materials, effectively inhibiting magnetic agglomeration. Systematic studies have shown that the magnetic confinement network can refine CoNi NPs size and enhance strong magnetic coupling interactions. Compared to CoNi@HCNFs on the hollow carbon fibers (HCNFs) outer surface, HCNFs@CoNi constructed on the inner surface induce stronger spatial charge polarization relaxation at the interface and exhibit stronger magnetic coupling interactions at the inner surface due to the high-density magnetic coupling units at the micro/nanoscale, thereby respectively enhancing dielectric and magnetic losses. Remarkably, they achieve a minimum reflection loss (RLmin) of -64.54dB and an absorption bandwidth of 5.60GHz at a thickness of 1.77mm. This work reveals the microscale mechanism of magnetic confinement-induced different polarization relaxation and magnetic response, providing a new strategy for designing magnetic materials.

Green synthesis of CoFe2O4 nanoparticles assisted with bio-waste (Gomutram): Sintering effect on the properties of structural, microstructural and vibrational bands

Synthesis of CoFe2O4 nanoparticles has been effectively accomplished by the cow-urine (Gomutram) as a chelating agent. The liberation of hazardous gases during the sol-gel synthesis process are minimized using cow-urine (Gomutram) as a chelating agent. The Energy Dispersive X-ray Spectrum offers insights on the current composition of ferrite by providing information on the stoichiometric ratio of its constituent elements. The X-ray diffraction patterns provide substantiation for the formation of the spinel phase of present ferrite system. The ferrite systems that underwent sintering at lower temperatures exhibited a more conspicuous amorphous morphology. The experimental lattice parameter (a) and average crystallite size (D) were found to be within the range of 8.0688–8.3779 Å and 3.7–9.4 nm respectively. Both a and D demonstrate an augmentation in magnitude due to the heat treatment procedure. The function of activation energy in the ripening of crystalline size during the sintering process is significant particularly in relation to grain boundary energy. The aggregation of nanoparticles into large clusters can be attributed to the strong magnetic interactions present among them. The inherent characteristics of scanning electron micrographs unambiguously demonstrate. The vibrational frequencies (υ1 and υ2) observed from Fourier Transform Infrared spectra confirm the spinel phase of the present ferrite system. The changes seen in vibrational frequencies do not offer empirical support for the association between annealing and the expansion of the lattice constant. But, offers an empirical support for the hypothetical redistribution of cations in the spinel structure.

Elucidation of the trapping characteristics of magnetic disk-like particles in a Hagen-Poiseuille flow via Brownian dynamics simulations (usage of multi-magnetic poles)

In the present study, we elucidate the behaviour of a suspension composed of magnetic disk-like particles that flow in a Hagen-Poiseuille flow in a gradient magnetic field and the trapping characteristics of the magnetic particles. To this end, we utilise multiple pairs of magnetic poles located outside the cylindrical system. Brownian dynamics simulations were performed in order to clarify the dependence of these characteristics on a variety of factors. It is seen that an increase in the magnetic field strength improves the trapping characteristics of disk-like particles if the influence of an applied magnetic field is sufficiently more dominant than that of a flow field and that of the magnetic particle-particle interaction. In the case of a strong magnetic particle-particle interaction, thin chain-like clusters are formed from an anchored particle trapped around a magnetic pole. If the two poles are sufficiently close to each other, an arch-type cluster is formed between them. We expect that this arch-type cluster may give rise to good trapping performance even in the presence of a strong flow field if the formation arises at a sufficiently large separation distance between the pair of magnetic poles. Highlights We have performed Brownian dynamics simulations for a magnetic disk-like particle suspension in a Hagen-Poiseuille flow. If the magnetic particle-particle interaction is sufficiently more dominant than the flow field, thin chain-like clusters are formed from an anchored particle. As the pole separation distance decreases, an arch-type configuration tends to be formed along the surface of the wall. This arch-type cluster that is formed between the pair of magnetic poles may be expected to improve the trapping characteristics depending on the pole separation distance.

Trends and prospects of 2-D tungsten disulphide (WS2) hybrid nanosystems for environmental and biomedical applications

Recently, 2D layered transition metal dichalcogenides (TMDCs) with their ultrathin sheet nanostructure and diversified electronic structure have drawn attention for various advanced applications to achieve high-performance parameters. Unique 2D TMDCs mainly comprise transition metal and chalcogen element where chalcogen element layers sandwich the transition metal element layer. In such a case, various properties can be enhanced and controlled depending on the targeted application. Among manipulative 2D TMDCs, tungsten disulphide (WS2) is one of the emerging nano-system due to its fascinating properties in terms of direct band gap, higher mobility, strong photoluminescence, good thermal stability, and strong magnetic field interaction. The advancement in characterization techniques, especially scattering techniques, can help in study of opto-electronic properties of 2D TMDCs along with determination of layer variations and investigation of defect. In this review, the fabrication and applications are well summarized to optimize an appropriate WS2-TMDCs assembly according to focused field of research. Here, the scientific investigations on 2D WS2 are studied in terms of its structure, role of scattering techniques to study its properties, and synthesis routes followed by its potential applications for environmental remediation (e.g., photocatalytic degradation of pollutants, gas sensing, and wastewater treatment) and biomedical domain (e.g., drug delivery, photothermal therapy, biomedical imaging, and biosensing). Further, a special emphasis is given to the significance of 2D WS2 as a substrate for surface-enhanced Raman scattering (SERS). The discussion is further extended to commercial and industrial aspects, keeping in view major research gaps in existing research studies.

BaSn0.96-XZr0.04MXO3 (M = Mn, Fe or Co, x = 0, 0.02) for spin-based devices: Ferromagnetic properties

In this study, Zr and/or Mn, Fe or Co ions were used to induce a robust room temperature ferromagnetic nature in perovskite BaSnO3 for high performance information applications. Dilute magnetic semiconductor compositions with formula of BaSn0.96-XZr0.04MXO3 (M = Mn, Fe, Co, x = 0, 0.02) have been synthesized at 1150 °C. For all compositions, mono-phase of cubic BaSnO3 structure with very small impurities of SnO2 were recorded based on X-ray diffraction analysis. The relation between ionic size of dopants and crystal dimension of BaSnO3 suggests the of Sn4+-site by Zr4+, Mn2+, Fe2+ or Co2+ ions. Extended absorption tails to visible light region were found for BaSn0.94Zr0.04Mn0.02O3 and BaSn0.94Zr0.04Co0.02O3 samples, owing to strong defect band transition absorption. Due to the addition of Zr and/or Mn, Fe or Co ions high refine of BaSnO3 morphology from inhomogeneous large grains to well distribute small one were seen. The XPS results of BaSn0.96Zr0.04O3 and BaSn0.94Zr0.04Co0.02O3 samples confirm that Ba, Sn, O and Zr are in the oxidation states of +II, +IV, −II and +IV/+III, respectively. Magnetically, incorporation of 4 wt% Zr4+ ions (empty 4d-orbital) induced a full-saturated spin ferromagnetic order with saturation magnetization of 1.16 emu/g, retentivity of 0.11 emu/g and coercivity of 147 Oe. Also, codoping by Zr/Co ions stimulates a full-saturated spin ferromagnetic order with saturation magnetization of 1.36 emu/g, retentivity of 0.17 emu/g and coercivity of 149 Oe. The empty 4d-Zr4+ ions have high ability in reversing the diamagnetic nature of BaSnO3 to ferromagnetic one through interaction with oxygen vacancies. The outer 4d-shell of Zr and 3d-orbital of Co with the oxygen vacancies induce the strong magnetic exchange interaction. Through looking into the previous studies on the magnetic properties of BaSnO3, the recorded saturation magnetization of 4 wt% Zr and 4 wt% Zr + 2 wt% Co ions are remarkable values.

Observation of flat band, Dirac nodal lines and topological surface states in Kagome superconductor CsTi3Bi5

Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena due to the strong correlations, spin-orbit coupling and/or magnetic interactions involved in such a lattice. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi3Bi5, which is isostructural to the AV3Sb5 (A = K, Rb or Cs) kagome superconductor family and possesses a two-dimensional kagome network of titanium. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. In agreement with calculations, we identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi3Bi5 from the measured electronic structures. In addition, around the Brillouin zone centre, {{mathbb{Z}}}_{2} nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.

Structure-Related Evolution of Magnetic Order in Anisidinium Tetrachlorocuprates(II).

Tetrachlorocuprate(II) hybrids of the three anisidine isomers (ortho-, meta-, and para-, or 2-, 3-, and 4-methoxyaniline, respectively) were prepared and studied in the solid state via X-ray diffraction and magnetization measurements. Depending on the position of the methoxy group of the organic cation, and subsequently, the overall cation geometry, a layered, defective layered, and the structure comprising discrete tetrachlorocuprate(II) units were obtained for the para-, meta-, and ortho-anisidinium hybrids, respectively. In the case of layered and defective layered structures, this affords quasi-2D-layered magnets, demonstrating a complex interplay of strong and weak magnetic interactions that lead to the long-range ferromagnetic (FM) order. In the case of the structure with discrete CuCl42- ions, a peculiar antiferromagnetic (AFM) behavior was revealed. The structural and electronic origins of magnetism are discussed in detail. To supplement it, the method for calculation of dimensionality of the inorganic framework as a function of interaction length was developed. The same was used to discriminate between n-dimensional and "almost" n-dimensional frameworks, to estimate the organic cation geometry limits for layered halometallates, and to provide additional reasoning behind the observed relation between cation geometry and framework dimensionality, as well as their relation to differences in magnetic behavior.

Complex magnetic ordering behavior in the frustrated perovskite Ba2MnMoO6

New and exotic ground states of magnetic materials are highly sought after and are extensively studied for the insights they provide into the thermodynamics of disorder and fundamental magnetic interactions. By controlling the crystal structure of an appropriate magnetic lattice, it is possible to cause the strong magnetic exchange interactions to sum to zero and so be frustrated. Due to the presence of this frustration, the lowest energy configuration that results may be crucially dependent on the tiniest of energy differences between a multitude of states that have (almost) the same energy. The keen interest in these materials arises from the fact that these finely balanced systems offer a way of probing classical or quantum mechanical interactions that are of fundamental importance but are too weak to be observed in non-frustrated systems. Here, we combine local and crystallographic probes of the cation-ordered double perovskite Ba2MnMoO6 that contains a face-centered cubic lattice of S = 5/2 Mn2+ cations. Neutron diffraction measurements below 9.27(7) K indicate that a fourfold degenerate non-collinear antiferromagnetic state exists with almost complete ordering of the Mn2+ spins. Muon spin relaxation measurements provide a local probe of the magnetic fields inside this material over the t1/2 = 2.2 µs lifetime of a muon, indicating a slightly lower Néel transition temperature of 7.9(1) K. The dc susceptibility data do not show the loss of magnetization that should accompany the onset of the antiferromagnetic order; they indicate that a strongly antiferromagnetically coupled paramagnetic state [θ = −73(3) K] persists down to 4 K, at which temperature a weak transition occurs. The behavior of this material differs considerably from the closely related compositions Ba2MnMO6 (M = W, Te), which show collinear ordering arrangements and well defined antiferromagnetic transitions in the bulk susceptibility. This suggests that the Mo6+ cation leads to a fine balance between the nearest and next-nearest neighbor superexchange in these frustrated double perovskite structures.

VdW Mediated Strong Magnetic Exchange Interactions in Chains of Hydrogen-Free Sublimable Molecular Qubits.

Sulfur-rich molecular complexes of dithiolene-like ligands are appealing candidates as molecular spin qubits because spin coherence properties are enhanced in hydrogen-free environments. Herein, we employ the hydrogen-free mononegative 1,3,2-dithiazole-4-thione-5-thiolate (dttt-) ligand as an alternative to common dinegative dithiolate ligands. We report the first synthesis and structural characterization of its Cu2+, Ni2+, and Pt2+ neutral complexes. The XPS analysis of thermal deposition of [Cu(dttt)2] in UHV conditions indicates that films of intact molecules can be deposited on surfaces by sublimation. Thanks to a combined approach employing DC magnetometry and DFT calculations, we highlighted AF exchange interactions of 108 cm-1 and 36 cm-1 attributed to the two different polymorph phases. These couplings are exclusively mediated by S···S VdW interactions, which are facilitated by the absence of counterions and made particularly efficient by the diffuse electron density on S atoms. Furthermore, the spin dynamics of solid-state magnetically diluted samples was investigated. The longest observed T m is 2.3 μs at 30 K, which significantly diverges from the predicted T m > 100 μs. These results point to the diluting matrix severely affecting the coherence lifetime of Cu2+ species via different factors, such as the contributions of neighboring 14N nuclei and the formation of radical impurities in a non-completely controllable way. However, the ease of processing [Cu(dttt)2] via thermal sublimation can allow dispersion in matrices better suited for coherent spin manipulation of isolated molecules and the realization of AF-coupled VdW structures on surfaces.

Investigate the effect of the synthesis method on the magnetic dynamic properties of SrAl4Fe8O19 hexaferrite

M-type SrAl4Fe8O19 hexaferrite was synthesized through three different synthesis methods, i.e., solid-state, co-precipitation, and sol–gel auto-combustion, to study the effect of synthesis on the structural and magnetic properties. X-ray diffraction (XRD), Rietveld refinement, high-resolution scanning electron microscope, and SQUID-magnetic property measurement system (MPMS) magnetometer were performed. The structural analysis demonstrated the formation of magnetoplumbite structure by these three synthesis methods. The variation in bond length and bond angle was also observed due to the synthesis methods, which affected the super-exchange interaction of the system. The solid-state method produced larger grains and more significant saturation magnetization i.e., 4.03 µm and 43.16 emu/gm, respectively, than the co-precipitate and sol–gel auto combustion methods. Magnetization–temperature (M−T) measurement revealed that the solid-state synthesized sample showed a higher blocking temperature than the rest. Magnetic susceptibility (χ) was performed with temperature dependence on different frequencies to demonstrate the magnetic dynamic properties of the SrAl4Fe8O19 hexaferrite. AC susceptibility results showed very strong magnetic interaction between particles leading to spin glass action at very low temperatures. The Neel–Arrhenius (N-A) & Vogel-Fulcher (V-F) models were used to determine the jump attempt time (τ0) and E0/KB (energy barrier or activation energy and Boltzmann constant) through AC susceptibility of magnetic particles. The τ0 and E0/KB were observed at about 2.84 × 10-11 sec and 871.31, respectively, for the solid-state synthesized sample by the V-F model.

The Curie line in protoplanetary disks and the formation of Mercury-like planets

In laboratory experiments, we heated chondritic material up to 1400 K in a hydrogen atmosphere. Mössbauer spectroscopy and magnetometry reveal that, at high temperatures, metallic iron forms from silicates. The transition temperature is about 1200 K after 1 h of tempering, likely decreasing to about 1000 K for longer tempering. This implies that in a region of high temperatures within protoplanetary disks, inward drifting solids will generally be a reservoir of metallic iron. Magnetic aggregation of iron-rich matter then occurs within the magnetic field of the disk. However, the Curie temperature of iron, 1041 K, is a rather sharp discriminator that separates the disk into a region of strong magnetic interactions of ferromagnetic particles and a region of weak paramagnetic properties. We call this position in the disk the Curie line. Magnetic aggregation will be turned on and off here. On the outer, ferromagnetic side of the Curie line, large clusters of iron-rich particles grow and might be prone to streaming instabilities. To the inside of the Curie line, these clusters dissolve, but that generates a large number density that might also be beneficial for planetesimal formation by gravitational instability. One way or the other, the Curie line may define a preferred region for the formation of iron-rich bodies.

Holes and magnetic polarons in a triangular lattice antiferromagnet

The intricate interplay between charge motion and magnetic order in geometrically frustrated lattices is central for the properties of many two-dimensional quantum materials. The triangular lattice antiferromagnet is a canonical example of a frustrated system, and here we analyze the dynamics of a hole in such a lattice focusing on observables that have become accessible in a new generation of experiments. Using the $t\ensuremath{-}J$ model, we solve the problem exactly within linear spin wave theory in the limit of strong magnetic interactions, showing that the ground state is described by a coherent state of spin waves. The derivation highlights the crucial role played by the interaction between a static hole and the neighboring spins, which originates in the geometric frustration and has often been omitted in earlier works. Furthermore, we show that the nonequilibrium dynamics after a hole has abruptly been inserted at a lattice site is given by a coherent state with time-dependent oscillatory coefficients. Physically, this describes a burst of magnetic frustration propagating through only two-thirds of the lattice sites, since a destructive interference of spin waves leaves spins parallel to that removed by the hole unperturbed. After the wave has propagated through the lattice, the magnetization relaxes to that of the ground state. We then use our analytical solution to benchmark the widely used self-consistent Born approximation (SCBA) in the limit of strong magnetic interactions, showing that it is very accurate also for a triangular lattice. The magnetic polaron spectrum is analyzed for general magnetic interactions using the SCBA, and we compare our results with those for a square lattice.

Multi-dimensional Ni@C-CoNi composites with strong magnetic interaction toward superior microwave absorption

• Multi-dimensional Ni@C-CoNi with 0D CoNi alloy and 1D Ni@C were synthesized. • The multi-layer heterogeneous interfaces trigger strong interfacial polarization. • Inner 1D Ni interacts with outer 0D CoNi alloy to induce magnetic coupling. • An optimal R L value of -51.4 dB and bandwidth of 4.6 GHz were achieved. Dielectric-magnetic integrated absorbers have attracted arousing attention in microwave absorption, however, it still remains a great challenge to simultaneously achieve superior dielectric polarization and strong magnetic loss. Herein, we propose a multi-scale structure optimization strategy to anchor CoNi-MOFs derived 0D CoNi alloy onto 1D core-shell Ni@C surface. By decorating with the poly-dopamine layer, the connection between 1D NiO and CoNi-MOFs precursors was greatly improved via the electrostatic interaction. Benefiting from the overlapping conductive networks, enhanced interfacial polarization among the multi-dimensional heterogeneous interfaces and strong magnetic interaction, the fabricated multi-dimensional Ni@C-CoNi composites exhibit outstanding microwave absorption. Typically, the optimal reflection loss is as high as –51.4 dB at 1.9 mm, and the effective absorption bandwidth achieves 4.6 GHz with a thickness of only 1.3 mm. This multi-scale structure optimization strategy inspires us with an efficient method to fabricate ideal microwave absorbers and the obtained multi-dimensional composites can be used as promising candidates in electromagnetic radiation protection.

Electron-spin spectral diffusion in an erbium doped crystal at millikelvin temperatures

Erbium doped crystals offer a versatile platform for hybrid quantum devices because they combine magnetically sensitive electron-spin transitions with telecom-wavelength optical transitions. At the high doping concentrations necessary for many quantum applications, however, strong magnetic interactions of the electron-spin bath lead to excess spectral diffusion and rapid decoherence. Here we lithographically fabricate a 4.4 GHz superconducting planar microresonator on a ${\text{CaWO}}_{4}$ crystal doped with Er ions at a concentration of 20 ppm relative to Ca. Using the microwave resonator, we characterize the spectral diffusion processes that limit the electron-spin coherence of Er ions at millikelvin temperatures by applying two- and three-pulse echo sequences. The coherence time shows a strong temperature dependence, reaching 1.3 ms at 23 mK for an electron-spin transition of $^{167}\mathrm{Er}$.

Ru-Doping-Induced Spin Frustration and Enhancement of the Room-Temperature Anomalous Hall Effect in La2/3 Sr1/3 MnO3 Films.

In transition-metal-oxide heterostructures, the anomalous Hall effect (AHE) is a powerful tool for detecting the magnetic state and revealing intriguing interfacial magnetic orderings. However, achieving a larger AHE at room temperature in oxide heterostructures is still challenging due to the dilemma of mutually strong spin-orbit coupling and magnetic exchange interactions. Here, Ru-doping-enhanced AHE in La2/3 Sr1/3 Mn1-x Rux O3 epitaxial films is exploited. As the B-site Ru doping level increases up to 20%, the anomalous Hall resistivity at room temperature can be enhanced from nΩ cm to µΩcm scale. Ru doping leads to strong competition between the ferromagnetic double-exchange interaction and the antiferromagnetic superexchange interaction. The resultant spin frustration and spin-glass state facilitate a strong skew-scattering process, thus significantly enhancing the extrinsic AHE. The findings can pave a feasible approach for boosting the controllability and reliability of oxide-based spintronic devices.

Exceptionally High Saturation Magnetic Flux Density and Ultralow Coercivity via an Amorphous-Nanocrystalline Transitional Microstructure in an FeCo-Based Alloy.

High saturation magnetic flux density (Bs ) of soft magnetic materials is essential for increasing the power density of modern magnetic devices and motor machines. Yet, increasing Bs is always at the expense of high coercivity (Hc ), presenting a general trade-off in the soft magnetic material family. Here, superior comprehensive soft magnetic properties, i.e., an exceptionally high Bs of up to 1.94 T and Hc as low as 4.3 A m-1 are unprecedentedly combined in an FeCo-based alloy. This alloy is obtained through a composition design strategy to construct a transitional microstructure between amorphous and traditional nanocrystalline alloys, with nanocrystals (with < 5 nm-sized crystal-like regions around) sparsely dispersed in an amorphous matrix. Such transitional microstructure possesses extremely low magnetic anisotropy caused by the annihilation of quasi-dislocation dipoles, and a strong magnetic exchange interaction, which leads to excellent comprehensive magnetic properties. The results provide useful guidelines for the development of the next generation of soft magnetic materials, which are promising for applications of high-frequency, high-efficiency, and energy-saving devices.

Concentration dependent exchange coupling in BaFe12O19/NiFe2O4 nanocomposites

Hard Barium ferrite (BaFe12O19) and soft Nickel ferrite (NiFe2O4) nanoparticles and their nanocomposites with different concentrations, i.e. (BaFe12O19)1−x/(NiFe2O4)x where x = 0, 0.3, 0.5, 0.7 and 1 were successfully synthesized. X-ray diffraction was used to obtain the phase formation and purity of these samples and the average crystallite size was found to be in a narrow range of 27–34 nm. The merging of zero field cooled and field cooled magnetization curves for x = 0.7 was evidence of strong magnetic interactions in these nanocomposites which is due to strong exchange coupling of hard/soft magnetic phase at this particular concentration. The value of coercivity and saturation magnetization of soft ferrite (NiFe2O4) should be linearly increased by the addition of hard ferrite (BaFe12O19), however a sharp increase was found for x = 0.7 and x = 0.5 samples, which is indication for the presence of spring exchange coupling in these nanocomposites. The single peak in dM/dH vs H plot indicate the presence of strong exchange coupling for x = 0.5. Bloch’s law fit revealed the smallest value of Bloch’s constant B for x = 0.7, which is also an evidence for the presence of strong magnetic interactions in these nanocomposites. The Kneller’s law fit to the HC versus temperature data revealed the higher value of Kneller’s constant α for x = 0.7, which may be due to presence of strong magnetic interactions in these nanocomposites. In summary, hard/soft ferrites nanocomposites with spring exchange mechanism strongly depends on the concentration of hard and soft ferrite phases and may be beneficial for different technological applications.