Large Nonhysteretic Volume Magnetostriction in a Strong and Ductile High-Entropy Alloy.

Rapid development of smart technologies poses a big challenge for magnetostrictive materials, which should not only permit isotropic and hysteresis-free actuation (i.e., nonhysteretic volume change) in magnetic fields, but also have high strength and high ductility. Unfortunately, the magnetostriction from self-assembly of ferromagnetic domains is volume-conserving; the volume magnetostriction from field-induced first-order phase transition has large intrinsic hysteresis; and most prototype magnetostrictive materials are intrinsically brittle. Here, a magnetic high-entropy alloy (HEA) Fe35Co35Al10Cr10Ni10 is reported that can rectify these challenges, exhibiting an unprecedented combination of large nonhysteretic volume magnetostriction, high tensile strength and large elongation strain, over a wide working temperature range from room temperature down to 100 K. Its exceptional properties stem from a dual-phase microstructure, where the face-centered cubic (FCC) matrix phase with nanoscale compositional and structural fluctuations can enable a magnetic-field-induced transition from low-spin small-volume state to high-spin large-volume state, and the ordered body-centered cubic (BCC) B2 phase contributes to mechanical strengthening. The present findings may provide insights into designing unconventional and technologically important magnetostrictive materials.

An in-situ magnetising holder achieving 1.5 T in-plane field in 200 kV transmission electron microscope.

A strong in-plane magnetic field is required for Lorentz transmission electron microscopy (LTEM) to observe the evolution of the magnetic domain structure of materials with high coercivity, particularly for research on rare-earth permanent magnets. However, the maximum field of the present in-situ magnetising holder applied in 200-kV or 300-kV TEM does not exceed 0.1 T. In this study, the reason for the low field was analysed, and the field strength was significantly elevated by reducing the field application area of the field generator. From finite element method calculations and experimental measurements, a 1.5 T in-plane field was achieved by our new holder in a 200-kV TEM, and images with good quality could still be obtained. Using the newly developed holder, the magnetisation process of hot-pressed NdFeB magnets was observed. The in-situ magnetising holder can be used in research on a wide variety of magnetic materials.

Thermal Stability of Skyrmion Tubes in Nanostructured Cuboids.

Magnetic skyrmions in bulk materials are typically regarded as two-dimensional structures. However, they also exhibit three-dimensional configurations, known as skyrmion tubes, that elongate and extend in-depth. Understanding the configurations and stabilization mechanism of skyrmion tubes is crucial for the development of advanced spintronic devices. However, the generation and annihilation of skyrmion tubes in confined geometries are still rarely reported. Here, we present direct imaging of skyrmion tubes in nanostructured cuboids of a chiral magnet FeGe using Lorentz transmission electron microscopy (TEM), while applying an in-plane magnetic field. It is observed that skyrmion tubes stabilize in a narrow field-temperature region near the Curie temperature (Tc). Through a field cooling process, metastable skyrmion tubes can exist in a larger region of the field-temperature diagram. Combining these experimental findings with micromagnetic simulations, we attribute these phenomena to energy differences and thermal fluctuations. Our results could promote topological spintronic devices based on skyrmion tubes.

Hierarchically Engineered Manganite Thin Films with a Wide-Temperature-Range Colossal Magnetoresistance Response.

Colossal magnetoresistance is of great fundamental and technological significance in condensed-matter physics, magnetic memory, and sensing technologies. However, its relatively narrow working temperature window is still a severe obstacle for potential applications due to the nature of the material-inherent phase transition. Here, we realized hierarchical La0.7Sr0.3MnO3 thin films with well-defined (001) and (221) crystallographic orientations by combining substrate modification with conventional thin-film deposition. Microscopic investigations into its magnetic transition through electron holography reveal that the hierarchical microstructure significantly broadens the temperature range of the ferromagnetic-paramagnetic transition, which further widens the response temperature range of the macroscopic colossal magnetoresistance under the scheme of the double-exchange mechanism. Therefore, this work puts forward a method to alter the magnetic transition and thus to extend the magnetoresistance working window by nanoengineering, which might be a promising approach also for other phase-transition-related effects in functional oxides.

Directional Magnetization Reversal Enables Ultrahigh Energy Density in Gradient Nanostructures.

High-performance ferromagnetic materials are essential for energy conversion and electronic devices. However, the random and nonuniform magnetization reversal in ferromagnetics limits their performance that can be achieved. Here, through both micromagnetism simulations and experiments, a directional magnetization reversal that initiates first from large grains toward smaller ones is discovered by engineering Nd2 Fe14 B/α-Fe gradient nanostructures. Such directional magnetization reversal enables a rare combination of high magnetization and large coercivity, thus leading to a record-high energy density (26 MG Oe) for isotropic permanent magnetic materials, which is ≈50% higher than that of its gradient-free counterpart. The unusual magnetization reversal originates from an ordered arrangement of grain sizes in the gradient material, where the large grains have a lower reversal field than that of the smaller ones. These findings open up new opportunities for developing high-performance magnetic materials.

Micromagnetic Configuration of Variable Nanostructured Cobalt Ferrite: Modulating and Simulations toward Memory Devices.

Magnetic nanostructures with flux-closure state or single-domain state have widespread application in diverse memory devices. However, an insight into the modulation of these variable states within one specific magnetic material is rarely reported but still needed. Herein, these micromagnetic configurations within prototypical cobalt ferrite (CoFe2O4) nanostructures in different size and dimension were studied by modulating the assembly of CoFe2O4 building blocks. We find that the CoFe2O4 nanowire (NW) has a multidomain structure when the diameter is about 90 nm, in which the domain walls (DWs) locate preferentially at the grain boundary and can convert to single-domain state when the diameter is reduced. Alternatively, a flux-closure domain state is obtained when the CoFe2O4 nanostructure changes from NW to nanosheet (NS), where the DWs location depends on the overall shape of NS. In addition, we further confirm that the magnetic anisotropy and magnetostatic energy are two main factors affecting the micromagnetic configuration in CoFe2O4 nanostructures by crystallographic analysis and micromagnetic simulations. Our experimental and simulation results demonstrate that the modulation of morphology and dimension are efficient to tailor the micromagnetic configuration in magnetic nanostructures.

Magnetic hardening of Nd-Ce-Fe-B films with high Ce concentration.

Partial substitution of Ce in Nd-Fe-B magnets is a feasible way to cope with the crisis of Nd and Dy in Nd-Fe-B production and reduce the cost of Nd-Fe-B magnets. In the present paper, the Nd-Ce-Fe-B films with high performance have been successfully fabricated by using an ultra-high vacuum (UHV) magnetron sputtering system. High magnetic performance with a ceorcivity of 13.3 kOe, a remanence of 11.4 kGs and a maximum energy product of 29.4 GMOe is obtained with the Ce substitution for more than 50 wt.% Nd without Dy addition. The high coercivity and (BH)max achieved in this work are much larger than those of previously reported Nd-Ce-Fe-B magnets with the same Ce concentration. The phase structure, microstructure and coercivity mechanism are analyzed. The coercivity mechanism is determined to be mainly dominated by nucleation. Based on the microstructure observation and coercivity mechanism analysis, the fine and well separated grains, smooth grain surface, small and less inhomogeneities should be responsible for the high coercivity. Our results encourage the further improvement of magnetic properties in Ce magnets including the bulk material with high Ce concentration.

Direct observation of dynamical magnetization reversal process governed by shape anisotropy in single NiFe2O4 nanowire.

Discovering how the magnetization reversal process is governed by the magnetic anisotropy in magnetic nanomaterials is essential and significant to understand the magnetic behaviour of micro-magnetics and to facilitate the design of magnetic nanostructures for diverse technological applications. In this study, we present a direct observation of a dynamical magnetization reversal process in single NiFe2O4 nanowire, thus clearly revealing the domination of shape anisotropy on its magnetic behaviour. Individual nanoparticles on the NiFe2O4 nanowire appear as single domain states in the remanence state, which is maintained until the magnetic field reaches 200 Oe. The magnetization reversal mechanism of the nanowire is observed to be a curling rotation mode. These observations are further verified by micromagnetic computational simulations. Our findings show that the modulation of shape anisotropy is an efficient way to tune the magnetic behaviours of cubic spinel nano-ferrites.

Direct Observation of Magnetocrystalline Anisotropy Tuning Magnetization Configurations in Uniaxial Magnetic Nanomaterials.

Discovering the effect of magnetic anisotropy on the magnetization configurations of magnetic nanomaterials is essential and significant for not only enriching the fundamental knowledge of magnetics but also facilitating the designs of desired magnetic nanostructures for diverse technological applications, such as data storage devices, spintronic devices, and magnetic nanosensors. Herein, we present a direct observation of magnetocrystalline anisotropy tuning magnetization configurations in uniaxial magnetic nanomaterials with hexagonal structure by means of three modeled samples. The magnetic configuration in polycrystalline BaFe12O19 nanoslice is a curling structure, revealing that the effect of magnetocrystalline anisotropy in uniaxial magnetic nanomaterials can be broken by forming an amorphous structure or polycrystalline structure with tiny grains. Both single crystalline BaFe12O19 nanoslice and individual particles of single-particle-chain BaFe12O19 nanowire appear in a single domain state, revealing a dominant role of magnetocrystalline anisotropy in the magnetization configuration of uniaxial magnetic nanomaterials. These observations are further verified by micromagnetic computational simulations.

Correction: Well protected SmCo nanoclusters: fabrication and transformation to single crystals.

[This corrects the article DOI: 10.1039/C7RA10158A.].

Oxygen vacancies controlled multiple magnetic phases in epitaxial single crystal Co0.5(Mg0.55Zn0.45)0.5O(1-v) thin films.

High quality single-crystal fcc-Co(x)(Mg(y)Zn(1-y))(1-x)O(1-v) epitaxial thin films with high Co concentration up to x = 0.5 have been fabricated by molecular beam epitaxy. Systematic magnetic property characterization and soft X-ray absorption spectroscopy analysis indicate that the coexistence of ferromagnetic regions, superparamagnetic clusters, and non-magnetic boundaries in the as-prepared Co(x)(Mg(y)Zn(1-y))(1-x)O(1-v) films is a consequence of the intrinsic inhomogeneous distribution of oxygen vacancies. Furthermore, the relative strength of multiple phases could be modulated by controlling the oxygen partial pressure during sample preparation. Armed with both controllable magnetic properties and tunable band-gap, Co(x)(Mg(y)Zn(1-y))(1-x)O(1-v) films may have promising applications in future spintronics.

Growth mechanism and magnetic properties of monodisperse L1(0)-Co(Fe)Pt@C core-shell nanoparticles by one-step solid-phase synthesis.

In this report, we present a novel one-step solid-phase reaction method for the synthesis of L10-CoPt@C core-shell nanoparticles (NPs) using organic metal precursors without surfactants. The obtained CoPt@C NPs have a good face-centered tetragonal single crystal structure and regular shape. The mean size of CoPt is 14 nm with a uniform carbon shell. The evolution of the core-shell structure during the synthesizing process is investigated in detail. Firstly organic metal precursors are decomposed, followed by the formation of grains/clusters in a metal-carbon intermediate state. Then the metal-carbon small grains/clusters agglomerate and recrystallize into single crystal metal alloy NPs covered with a carbon layer. The carbon shell is effective in preventing the coalescence of L10-CoPt NPs during high temperature sintering. The prepared L10-FePt nanoparticles have a high coercivity of up to 12.2 kOe at room temperature. This one-step solid-state synthesizing method could also be employed for the preparation of other types of nanostructures with high crystallinity, monodispersity and chemically ordered phase.

Dependency of magnetic microwave absorption on surface architecture of Co20Ni80 hierarchical structures studied by electron holography.

To design and fabricate rational surface architecture of individual particles is one of the key factors that affect their magnetic properties and microwave absorption capability, which is still a great challenge. Herein, a series of Co20Ni80 hierarchical structures with different surface morphologies, including flower-, urchin-, ball-, and chain-like morphologies, were obtained using structure-directing templates via a facile one-step solvothermal treatment. The microwave reflection loss (RL) of urchin-like Co20Ni80 hierarchical structures reaches as high as -33.5 dB at 3 GHz, with almost twice the RL intensity of the ball- and chain-like structures, and the absorption bandwidth (<-10 dB) is about 5.5 GHz for the flower-like morphology, indicating that the surface nanospikes and nanoflakes on the Co20Ni80 microsphere surfaces have great influences on their magnetic microwave absorption properties. Electron holography analysis reveals that the surface nanospikes and nanoflakes could generate a high density of stray magnetic flux lines and contribute a large saturation magnetization (105.62 emu g(-1) for urchin-like and 96.41 emu g(-1) for flower-like morphology), leading the urchin-like and flower-like Co20Ni80 to possess stronger microwave RL compared with the ball-like and chain-like Co20Ni80 alloys. The eddy-current absorption mechanism µ''(µ')(-2)(f)(-1) is dominant in the frequency region above 8 GHz, implying that eddy-current loss is a vital factor for microwave RL in the high frequency range. It can be supposed from our findings that different surface morphologies of magnetic hierarchical structures might become an effective path to achieve high-performance microwave absorption for electromagnetic shielding and stealth camouflage applications.

Growth mechanisms and size control of FePt nanoparticles synthesized using Fe(CO)x (x < 5)-oleylamine and platinum(ii) acetylacetonate.

By using Fe(CO)x-OAm (oleylamine, x < 5) as the Fe precursor to slow down the formation rate of FePt nanoparticles (NPs), a time dependence of the NPs' nucleation and growth process was observed by transmission electron microscopy (TEM). The complexing temperature of OAm and Fe(CO)5 at which Fe(CO)x-OAm was formed has a strong influence on the nucleation rate and growth process of the NPs. TEM analyses indicated that the NPs with isotropic shape were single crystalline throughout the synthesis and were formed by a diffusion-controlled Ostwald-ripening (OR) growth mechanism. The nanorod particles were first formed via joining of arbitrarily oriented single crystals and the two crystals formed a uniform particle afterwards, as described by the oriented-attachment (OA) mechanism. The ratio of OAm to Fe(CO)5 used in the preparation of Fe(CO)x-OAm has a significant influence on the growth process, and subsequently the shape, size and size distribution of the FePt NPs. By adjusting the ratio and its complexing temperature, single-crystal FePt NPs with controllable size and isotropic shape were obtained. The insight into the exploration of the specific roles of the reaction conditions and the formation mechanisms provided important information for controlling the morphology of the nanoparticles.

Electron holography of magnetic field generated by a magnetic recording head.

The magnetic field generated by a magnetic recording head is evaluated using electron holography. A magnetic recording head, which is connected to an electric current source, is set on the specimen holder of a transmission electron microscope. Reconstructed phase images of the region around the magnetic pole show the change in the magnetic field distribution corresponding to the electric current applied to the coil of the head. A simulation of the magnetic field, which is conducted using the finite element method, reveals good agreement with the experimental observations.

Magnetization distribution of magnetic vortex of amorphous FeSiB investigated by electron holography and computer simulation.

The three-dimensional spin structure of the magnetic vortex of FeSiB, an amorphous soft magnetic material, was investigated by holography observation and computer simulation. Magnetization distribution in the neighborhood of the vortex center was estimated from the phase distribution obtained by holography observation. To confirm this magnetization distribution, sample-tilting experiments were performed: when the sample was tilted with respect to the electron beam direction, the phase-image center was found to shift along the tilting axis. Finite-element computer simulation was carried out to estimate the amount of shifts of the phase-image center in the sample tilting from the experimental magnetization distributions in the no sample-tilting conditions. We found that the simulated shifts of the phase-image center were in good agreement with those in the sample-tilting experiment, thus confirming the magnetization distribution near the vortex center obtained by holography observation.

Quantitative evaluation of magnetic flux density in a magnetic recording head and pseudo soft underlayer by electron holography.

The magnetic interaction between the pole tip of a single-pole head and a pseudo soft underlayer in perpendicular magnetic recording was observed by electron holography. The magnetic flux density inside the soft underlayer was quantitatively evaluated. The distribution of magnetic flux density was calculated using the finite element method, and the influences of the modulation of the reference wave and stray fields were investigated by comparison with experimental results. The flux density observed was found to be underestimated due to the modulation of the phase shift in reference wave. The magnetic flux measured experimentally was larger than that inside the specimen because of the relatively large stray fields above and below the specimen in the direction of the electron beam.

Observations of a magnetic microstructure in a Co-CoO obliquely evaporated tape using electron holography.

The magnetic microstructure in a Co-CoO obliquely evaporated tape that was subjected to a recording bit length of 250 nm was studied using electron holography. The reconstructed phase image demonstrated a periodic pattern of magnetic flux loops that were inclined to the film normal due to a well-developed columnar structure. When a magnetic field was applied to the tape for observing the remanent state by holography, the periodic pattern of the flux loops gradually disappeared. Interestingly, on applying a large magnetic field, the contour lines in the reconstructed phase image became approximately parallel to the longitudinal axis of the sliced tape, i.e. the contour lines were made to virtually deviate from the easy magnetization axis. The observations were supported by a computer simulation in which the effect of the stray magnetic field was considered.

Direct observation of field emission in a single TaSi2 nanowire.

The electric potential change in a single TaSi2 nanowire during field emission was visualized by means of electron holography. During the field emission, the interference fringes of the electron hologram were blurred locally between the TaSi2 nanowire and anode. This phenomenon was interpreted as being due to a change in the electric potential of approximately 1 V in the TaSi2 nanowire after each ballistic emission. The experiments on the single TaSi2 nanowire field emission behavior provide the useful information for understanding the field emission in the nano-field-emitting device.

Electron holography on dynamic motion of secondary electrons around sciatic nerve tissues.

By means of electron holographic visualization of detailed electric potential distribution around sciatic nerve tissues coated with C and OsO(4), we show that the steady state of these specimens subjected to intense charging with electron irradiation is accompanied with a dynamic motion of collective secondary electrons; the secondary electrons emitted from the coated specimens revolve around the positively charged specimens forming stationary orbits. Further, this study clarified the possibility of the direct visualization of a part of the orbits of the collective secondary electrons without disturbing their motions.