Huge Coercivity Research Articles

NIR-II light-encoded 4D-printed magnetic shape memory composite for real-time reprogrammable soft actuator

For the intelligent 4D-printed actuators, the excellent performance, including quickly reversible spatial-shape transformation and locking, digital and precise shape manipulation in real-time, and remote actuation in special spaces or harsh environments, is significantly desirable but still challenging. Here, using a UV-curable system containing the shape memory polymer (SMP) and NdFeB particles, namely the magSMP composite, we fabricate a real-time reprogrammable soft actuator via high-resolution Digital Light Processing (DLP)-based 4D printing. The printed structure is composed of an array of physical binary magSMP composite elements (m-bits), analogous to digital bits. Owing to the NdFeB's photothermal effect, each m-bit can be independently and reversibly switched between unlocking or locking states (allowing or prohibiting responsive shape-morphing) in response to the on/off state of NIR-II light. Through projecting NIR-II light patterns for encoding a set of binary instructions onto 4D-printed actuators, the real-time light-programmed deformations are induced precisely under an actuation magnetic field due to the NdFeB's huge coercivity. Thus, the synergistic magnetic and light field-manipulated multimodal deformations of actuators, including mimosa shape changing, grasping, and wire guiding, are achieved. This study shines lights on the fabrication of soft structures of arbitrary sizes and provides their future perspectives in soft robot design.

Probing Griffiths phase like behavior and exchange bias phenomena in 6H Ba3Co0.5Mn0.5Ru2O9 by B-site doping

Compositional tuning of magnetic properties to provide technologically useful results is in the forefront of research in complex oxides. Taking advantage of the highly sensitive nature of magnetism in 6H barium ruthenates (Ba3MRu2O9, M = transition metals or rare earth ions) to composition, we have carried out M-site dual doping by 50% each of Co and Mn. The high pressure and high temperature synthesized sample of 6H Ba3Co0.5Mn0.5Ru2O9 shows giant exchange bias (5.71 kOe at 5 K and 50 kOe cooling field) and huge coercivity (9.42 kOe at 5 K) at low temperature along with Griffiths phase like behavior in the 136 K < T < 260 K temperature range. The presence of ferromagnetic (FM) clusters in a non-collinear antiferromagnetic (AFM) background for temperature below 136 K is assumed. These FM clusters extend above 136 K to persist until 260 K, and the system finally reaches the paramagnetic (PM) state closer to room temperature. The exchange bias phenomena diminish above 20 K and this could be correlated with the peculiar nature of magnetic isotherms above and below this temperature. The transition at 136 K is regarded as AFM short-range ordering from ac and dc magnetization studies and heat capacity measurement. The presence of short-range interactions at temperatures until the PM region forms the basis of Griffiths phase like behavior in this sample.

Significant coercivity enhancement of Ga-doped sintered Nd-Fe-B magnet by grain boundary diffusion perpendicular to the c-axis

Grain boundary diffusion (GBD) process is the most effective approach for enhancing coercivity of Nd-Fe-B magnets with minimal consumption of heavy rare earth (HRE). However, understanding of GBD mechanism is still limited, especially on the anisotropic diffusion behavior within sintered magnets. Grain boundary (GB) phase is the key factor in determining the diffusion process, which acts as diffusion channel for HRE atoms. The effects of doping Ga on the grain boundary microstructure and diffusion behavior of Tb in the sintered Nd-Fe-B magnet were investigated. Sintered magnets with different Ga content (0.15 wt%, 0.45 wt%) were prepared. It should be noted that Tb was diffused perpendicular to the easy axis (c-axis) of the magnets. The coercivity increased more significantly to 2.802 T (1.006 T higher than that of the post-sinter annealed state) in magnets with high Ga content than the low Ga content magnets (2.291 T, 0.669 T higher than the post-sintered annealed state). Such a huge coercivity increment almost reached the same value by diffusing Tb along the easy axis reported by other researchers. Chemical compositions and microstructure of the GB phase were found to be key factors in influencing the HRE diffusion process. A considerable amount of Nd6Fe13Ga phase was observed near triple junctions after post-sinter annealing in the high Ga content magnets, which markedly decreased Fe concentration in the intergranular phase. In addition, amorphous intergranular metallic Cu-rich phases provided smooth diffusion channels for HRE atoms. (Nd1-x, Tbx)2Fe14B shell was much more obvious in the high Ga content magnets. Ga dopping induced novel modifications in diffusion behavior of HRE atoms and strongly influenced the magnetic properties of permanent magnets. The present approach provide new path for the manufacture of high performance magnets, which is interesting both for technical applications and fundamental studies.

Recent Research Trend in Powder Process Technology for High-Performance Rare-Earth Permanent Magnets

Higher performance is constantly required in rare earth permanent magnets, which are an indispensable component of the motors of electric vehicles. When producing sintered magnets, advanced structural control is necessary in the powder metallurgy process in order to achieve high performance. Especially in recent years, it has become important to develop processes for Sm-Fe-N magnets and metastable phase magnets as next-generation magnets to replace the Nd-Fe-B magnets. Because the crystal grain refinement of sintered magnets is most effective for improving coercivity, production methods for raw powders have evolved from the traditional pulverization to chemical synthesis approaches, and as a result, a submicron-sized Sm-Fe-N powder with huge coercivity has been developed. State-of-the-art physical synthesis methods have also been applied successfully to the synthesis of nanopowders. Since control of the grain boundary is very effective in Nd-Fe-B magnets, this approach has also been evolved to Sm-Fe-N magnets by nano coating. On the other hand, since technologies for crystalline orientation control and high-density sintering are indispensable for improvement of remanence, new low-thermal load consolidation techniques such as spark plasma sintering are being developed for Sm-Fe-N magnets and metastable phase magnets in order to overcome the inherent low thermal stability of these materials.

In situ annealing achieves an ultrafast synthesis of high coercive strontium ferrite foams and beyond.

Strontium ferrite nanostructures have attracted intensive interest recently due to the increasing demand for cost-effective features and good chemical corrosion resistance of magnetic materials, yet the ultrafast synthesis of strontium ferrite with desired coercivity is still experiencing a severe challenge. Herein, porous strontium ferrite foams with a coercivity up to 23.35 kOe were prepared by ultrafast in situ annealing for 1 min based on an auto-combustion strategy. The high coercivity of strontium ferrite benefits from the increasing magnetocrystalline anisotropy caused by the ion substitution and the appropriate grain size close to the critical single-domain size of strontium ferrite. In addition, this ultrafast synthesis can be extended to prepare a series of porous spinel, lanthanide-based perovskites, and their high-entropy counterpart foams. We also demonstrate that this strategy is feasible for preparing biphasic composite oxide foams. Furthermore, this work provides important guidance for the design of porous permanent magnet materials and the efficient preparation of porous oxide foam materials.

A comprehensive research on new DyCo2Mnx (x = 0.0–1.0) Laves phases

DyCo2Mnx (x = 0.0, 0.2, 0.4, 0.6, and 1.0) compounds were synthesised and found to all be single-phase, isostructural to DyCo2 (MgCu2 Laves phase cubic structure (Fd-3 m) at room temperature). DyCo2Mn exhibits a huge coercivity field HC = 21,170 Oe at 5 K. The magnetic transition at Curie temperature TC, was observed as first order for DyCo2 but second order for the Mn doped compounds. Using Kouvel-Fisher plot, critical isotherm analysis and Arrott-Noaks plot techniques, a detailed critical exponents analysis was carried out on these transitions: critical exponents β ≈ 0.5, γ ≈ 1.0, and δ ≈ 3.0 were derived indicating that all the second-order transitions belong exhibit long-range magnetic interactions during the magnetic transitions. High-resolution synchrotron X-ray measurements confirm that there are crystal structure transformations sychronised with the magnetic transitions from paramagnetism to ferrimagnetism (PM-FM) at TC in the x = 0.0 and 0.2 cases with crystal structure changing from cubic (Fd-3 m) to tetragonal (I41/amd), However structures remain cubic (Fd-3 m) for x = 0.4, 0.6, and 1.0.

Ultra-high coercivity Sm-Co bulk magnets with remarkable thermal stability

The Sm-Co bulk magnet prepared in this work using unique Sm-Co alloy nanopowder as a precursor prepared by the lox oxygen-induction thermal plasma (LO-ITP) process displays a giant coercivity of 4.1 MA/m (52 kOe) at 300 K, which is the highest coercivity among the reported rare-earth permanent bulk magnets with high density. The Sm-Co magnet has a high coercivity of 2.4 MA/m (30 kOe) at 473 K and 1.6 MA/m (20 kOe) at 573 K, which shows the good temperature coefficient of coercivity of −0.22%/K. The well-organized microstructure with fine Sm-Co alloy single-crystal grains with an average grain size of 0.63 μm, sharp grain boundary, and better crystallinity, leads to a huge coercivity and remarkable thermal stability.

Novel Powder Processing Technologies for Production of Rare-earth Permanent Magnets

Post-neodymium magnets that possess high heat resistance, coercivity, and (BH)max are desired for future-generation motors. However, the candidate materials for post-neodymium magnets such as Sm2Fe17N3 and metastable magnetic alloys have certain process-related problems: low sinterability due to thermal decomposition at elevated temperatures, deterioration of coercivity during sintering, and the poor coercivity of the raw powder. Various developments in powder processing are underway with the aim of overcoming these problems. So far, the development of advanced powder metallurgy techniques has achieved Sm2Fe17N3 anisotropic sintered magnets without coercivity deterioration, and advances in chemical powder synthesis techniques have been successful in producing Sm2Fe17N3 fine powders with huge coercivity. The challenge of a new powder process is expected to open the way to realizing post-neodymium magnets.

Efficient Brownian oscillators and nanoheaters based on gallium-doped ε-Fe2O3.

Wireless actuation at the nanoscale is vital in many contexts, and magnetic fields acting on nanoparticles (NPs) are among the most effective tools when actuation concerns linear forces. However, effective tools to apply torques at the nanoscale are still missing, because NPs where the magnetic moment is strongly coupled to the lattice agglomerate due to their high magnetic moment. Here, we show that gallium-doped ε-iron oxide NPs have small interparticle magnetic interactions and huge lattice-coupling for efficiently applying torques at the nanoscale. In this view, they are expected to be useful tools to efficiently apply mechanical forces to induce cellular apoptosis and to discern between mechanical and thermal contributions to cellular apoptosis currently under debate.

Giant enhancement in coercivity of ferromagnetic α-Fe2O3 nanosheet grown on MoS2

To study the 2D materials, an interface interaction is very important to tune the physical properties because of the large specific surface area. When the antiferromagnetic α-Fe2O3 is grown in a single-crystalline form on a diamagnetic MoS2, enormous changes in magnetic properties are observed in the 2D composite system. Strong ferromagnetism with a giant coercivity of 1.8 T is observed in this single-crystalline α-Fe2O3/MoS2 heterostructure. It is noted that thinner sheets show better coercivity than the thicker sheets, and the coercivity decreases with the increase in temperature. This huge coercivity in larger and thinner single-crystalline α-Fe2O3 sheets grown on a MoS2 arises due to charge transfer from “S” to “Fe” and the surface pinning effect at the interface. A large negative magnetoresistance with a maximum value of 15% is achieved due to the spin-spin interaction, and a positive magnetoresistance is also observed at low field and high temperature as a result of the spin splitting effect.

Weak ferromagnetic insulator with huge coercivity in monoclinic double perovskite La2CuIrO6

Insulating ferromagnets with high TC are required for many new magnetic devices. More complexity arises when strongly correlated 3d ions coexist with strongly spin–orbit coupled 5d ones in a double perovskite. Here, we perform the structural, magnetic, and density functional theory (DFT) study of such double perovskite La2CuIrO6. A new P21/n polymorph is found according to the comprehensive analysis of x-ray, Raman scattering and phonon spectrum. The magnetization reveals a weak ferromagnetic (FM) transition at TC = 62 K and short range FM order in higher temperature range. A huge coercivity is found as high as HC ~ 11.96 kOe at 10 K, which, in combination with the negative trapped field, results in the magnetization reversal in the zero field cooling measurement. The first principle calculations confirm the observed FM state and suggest La2CuIrO6 of this polymorph is a Mott insulating ferromagnet assisted by the spin–orbit coupling.

Ultrasmall Yttrium Iron Garnet Nanoparticles with High Coercivity at Low Temperature Synthesized by Laser Ablation and Fragmentation of Pressed Powders.

Pulsed laser ablation of pressed yttrium iron garnet powders in water is studied and compared to the ablation of a single-crystal target. We find that target porosity is a crucial factor, which has far-reaching implications on nanoparticle productivity. Although nanoparticle size distributions obtained by analytical disc centrifugation and transmission electron microscopy (TEM) are in agreement, X-ray diffraction and energy dispersive X-ray analysis show that only nanoparticles obtained from targets with densities close to that of a bulk target lead to comparable properties. Our findings also show why the gravimetrical measurement of nanoparticle productivity is often flawed and needs to be complemented by colloidal productivity measurements. The synthesized YIG nanoparticles are further reduced in size by laser fragmentation to obtain sizes smaller than 3 nm. Since the particle diameters are close to the YIG lattice constant, these ultrasmall nanoparticles reveal an immense change of the magnetic properties, exhibiting huge coercivity (0.11 T) and irreversibility fields (8 T) at low temperatures.

Study of structure and magnetic properties of rare earth doped BiFeO3

RExBi1−xFeO3 (RE=La, Gd and Ho) samples were synthesized by a solid state reaction method and investigated for structure and magnetic properties. XRD shows that La and Ho dopings at A-site effectively reduce the secondary phase formation. A structural phase transition is observed to orthorhombic phase with all RE elements above certain concentration limit. La doping results in the observation of huge coercivity of 10kOe and Ho doping results in high magnetic moment among all the RE elements. Gd and Ho doped samples show a pinching in the M–H loops with minimum HC.

A Comprative Study Of Sol-gel And Solid-state Prepared La3+ Doped Multiferroic BiFeO3

LaxBi1-xFeO3 (LBFO) samples were prepared by sol-gel route using citric acid as chelating agent for x = 0.0 - 0.4. The structure, dielectric and magnetic properties of the LBFO compounds were studied and compared with the corresponding properties of the materials prepared by a conventional solid state reaction. The use of the sol–gel method in preparation lowered the reaction threshold temperature by 200 °C. Effects of the preparation routes and conditions on the phase and microstructures of the materials were investigated in this study using XRD and SEM. The pure BFO without bismuth loss, which cannot be prepared by the solid state reaction, was obtained by the sol–gel method. Sol-gel synthesis could yield a pure phase material at relatively lower temperatures while the solid state method yielded powder with a small amount of the secondary Bi25FeO40 phase. Single phase LBFO prepared by sol-gel method (SG) revealed huge value of dielectric constant than same obtained by the solid state reaction method (SS). Maxwell-Wagner type dielectric dispersion is observed in sol-gel method. Dielectric constant and loss tangent are found to be higher for SG as compared to SS. Huge coercivity (HC) of the order of ~ 15 kOe is observed in both SG and SS samples due to the high anisotropy in these samples. The increase in the magnetization is observed due to the destruction of spin cycloid structure. The enhanced properties made LBFO a promising candidate for the applications in novel memory devices and spintronics.

Extraordinary magnetization of amorphous TbDyFe films

A study of the magnetic properties and their relationship with the magnetoelastic properties of amorphous (Tb0.3Dy0.7)40Fe60 films prepared by dc magnetron sputtering is presented. The magnetostrictive coefficient λ reaches values up to 300ppm in a field of 0.1T. At low temperatures the films present spin-glass behavior with Tf≅160K. At room temperature the films show low coercivity values, less than 30mT, whereas at temperatures below 40K huge coercivities are observed which reach values up to 2.5T at 5K.

Large-area hard magnetic L10-FePt nanopatterns by nanoimprint lithography

Large-area hard magnetic L10-FePt nanopatterns with out-of-plane texture were fabricated by using a top-down approach.For the fabrication process, ultraviolet nanoimprint lithography (UV-NIL) in combination withinductively coupled plasma reactive Ar-ion etching was used. By this technique a continuousL10-Fe51Pt49 film was nanostructured into a regular arrangement of nanodots over an area of 4 mm2. The dot dimension and distribution was specified by the stamp, resultingin a dot size of 60 nm and a periodicity of 150 nm. For the large-scaleL10-FePt nanopatterns, huge coercivities up to 4.31 T could be achieved. By means of magneticforce microscopy it could be verified that the nanodots were magnetically decoupled fromeach other and occurred in the single-domain state with perpendicular magnetization.

Synthesis of Norbergite Fe3BO6 of Single Crystallites from a Borate Glass

Iron oxyborate Fe3BO6 is one of the important ceramics of the norbergites useful for electrodes in lithium ion batteries, gas sensors and biological probes. A facile biofriendly method of rapid cooling of a borate glass-melt (starting composition 40Fe2O3-60 B2O3), after an isothermal heating for 30 min at ∼1220°C in air, is explored in view of producing Fe3BO6 single crystallites. The method is feasible to scale-up for high yield. Scanning electron microscopic images of as-cooled rectangular bars or laminates of Fe3BO6 crystallites measure 1–2 μm length, 0.51 μm width and ∼45 nm thickness. A single phase compound, Fe3BO6, as analyzed from the X-ray diffractogram, shows an orthorhombic crystal structure (Pnma space group), with lattice parameters a = 1.0048 nm, b = 0.8531 nm, c = 0.4466 nm, and density 44.76 g.cm−3. It is highly stable in air unless heating above 700°C. At room temperature these Fe3BO6 crystallites behave as the surface modified antiferromagnet, showing a nearly linearly increasing magnetization (i.e. a very small value of the order of a few 10−4 emu.g−1) against applied magnetic field and a peculiar open hysteresis loop with a huge coercivity ∼2105 Oe.

Influence of hydrogen on the growth of FePt thin films

FePt films have been prepared by pulsed laser deposition on MgO(100) in a forming gas atmosphere of 5% hydrogen in argon. At a pressure and temperature combination of 0.1 mbar and 800 °C, smooth continuous and well ordered L10 phase films can be obtained. The anisotropy field is close to the bulk value, while the coercivity is comparatively low at 0.15 T. This is in contrast to films deposited in vacuum, which are granular in nature and show huge coercivity. It is proposed that nucleation centers for island growth introduced by hydrogen on the MgO surface are responsible for this difference in growth mode. The temperature and pressure dependence of the surface morphology are explained by this assumption.

Microstructures and coercivities of SmCo x and Sm(Co,Cu) 5 films prepared by magnetron sputtering

We have investigated the influence of composition and annealing conditions on the magnetic properties and microstructural features of SmCo x films that were prepared by sputtering and subsequent annealing. A huge in-plane coercivity of 5.6 T was obtained from an optimally annealed Sm–Co film, which was attributed to the nanometer sized polycrystalline microstructure of the highly anisotropic SmCo 5 phase. Although a high density of planar defects were observed in the films that were annealed at high temperatures, they did not act as strong pinning sites for domain wall motion. The effect of Cu on [SmCo 4.5(9 nm)/Cu( xnm)] 10 multilayer thin films was also studied. An appropriate Cu content increased the coercivity.

Magnetic properties and domain structure of highly coercive Au∕FePt films

The magnetic properties of highly ordered Au∕FePt films were studied. The film morphology was controlled by varying the thickness of the FePt layer while the thickness of the Au cap layer was fixed at 60 nm. We prepared the highly coercive Au∕FePt films with surface morphology ranging from continuous films to island structure and magnetic domains structure changing from stripe multidomain to single-domain structure, respectively, with a critical FePt thickness 60 nm. The particles are slightly coalesced with nearly ellipsoidal shape as the thickness is larger than 60 nm. In thinner FePt layer such as 20 nm, single-domain particle was observed clearly in magnetic force microscopy image. The huge coercivity up to 27 kOe was measured at room temperature with thickness 10 nm. According to the temperature dependence on coercivity, the domain wall displacement plays an important role in Au∕FePt (90 nm) bilayer. However, magnetic incoherent rotation is significantly in Au∕FePt (10 nm) bilayer.