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This report presents new findings of exchange bias and related structural and magnetic properties in iron carbide/magnetite (Fe5C2/Fe3O4) core/shell nanoparticles. The exchange bias emerges from an energetic landscape, namely a first-order phase transition–the Verwey transition at 125 K, during which the Fe3O4 shell changes from the cubic to monoclinic structure. The phase transition leads to the exchange bias because it results in abrupt changes in magnetocrystalline anisotropy and exchange coupling. Another unique phenomenon identified in this composite system is enhanced magnetic coercivity due to the uniaxial anisotropy of the monoclinic phase. An analysis of the correlations between the observed phenomena is given based on the temperature dependence of the coercivity, the exchange bias field values, and the Verwey transition temperature.

Self-assembly of nanoparticles into ordered patterns is a novel approach to build up new consolidated materials with desired collective physical properties. Herein, nanoparticle assemblies of composition-modulated bimagnetic nanoparticles have been produced via slow evaporation of their colloidal suspension in the absence or presence of magnetic fields. The assemblies obtained in the presence of the magnetic fields exhibit oriented nanoparticle chains in face-centered cubic superlattice structures, compared with the hexagonal closed-packed superlattice obtained without the magnetic field. The oriented structure has an alignment of the easy magnetization axis along the chains. This alignment leads to enhanced intra-superlattice interactions. As a result, the field-induced assembly displays collective magnetic properties with significantly enhanced magnetic anisotropy, remanent magnetization and coercivity. It is also found that the bimagnetic FeCo/CoFe2O4 core/shell nanostructure enhances the intra-particle interaction and thus is beneficial for the growth of oriented assembly of nanoparticles. Furthermore, the collective magnetic behavior is evidenced by the observation of a superferromagnetic-like magnetization relaxation in the ac-susceptibility curves.

Herein, for the first time, we report copper-cysteamine (Cu-Cy) nanoparticles having Cu1+ instead of Cu2+ as an efficient heterogeneous Fenton-like catalyst for highly selective cancer treatment. I...

The Dzyaloshinskii–Moriya interaction (DMI) is well known to favor a chiral rotation of the magnetic moments, which accounts for the emergence of the skyrmions. The DMI is a combined effect of spin–orbit coupling with broken inversion symmetry in magnets. Most of the noncentrosymmetric magnetic materials that bear skyrmions involve nonmagnetic elements. This work shows that strong DMIs exist in elemental cobalt with a β-Mn-type metastable structure. The variation of DMI among different cobalt pairs largely follows the variation of hopping magnitude in which p electrons play an important role. Although the DMIs between different atomic pairs partly cancels with each other, the net interaction is sufficient to result in a left-handed Bloch-type spiral. Spin dynamics simulation shows that a critical magnetic field of 2.9 T stabilizes skyrmions at 0 K.

Anisotropy is an important and widely present characteristic of materials that provides desired direction-dependent properties. In particular, the introduction of anisotropy into magnetic nanoparticles (MNPs) has become an effective method to obtain new characteristics and functions that are critical for many applications. In this review, we first discuss anisotropy-dependent ferromagnetic properties, ranging from intrinsic magnetocrystalline anisotropy to extrinsic shape and surface anisotropy, and their effects on the magnetic properties. We further summarize the syntheses of monodisperse MNPs with the desired control over the NP dimensions, shapes, compositions, and structures. These controlled syntheses of MNPs allow their magnetism to be finely tuned for many applications. We discuss the potential applications of these MNPs in biomedicine, magnetic recording, magnetotransport, permanent magnets, and catalysis.

This report presents new findings of exchange bias and related structural and magnetic properties in iron carbide/magnetite (Fe

Dislocation activity is critical to ductility and the mechanical strength of metals. Dislocations are the primary drivers of plastic deformation, and their interactions with each other and with other microstructural features such as grain boundaries (GBs) lead to strengthening of metals. In general, suppressing dislocation activity leads to brittleness of polycrystalline materials. Here, we find an intermetallic that can accommodate large plastic strain without the help of dislocations. For small grain sizes, the primary deformation mechanism is GB sliding, whereas for larger grain sizes the material deforms by direct amorphization along shear planes. The unusual deformation mechanisms lead to the absence of traditional Hall-Petch (HP) relation commonly observed in metals and to an extended regime of strength weakening with grain refinement, referred to as the inverse HP relation. The results are first predicted in simulations and then confirmed experimentally., The deformation of materials depends on dislocation activity, and suppressing dislocations should lead to brittleness. Here, the authors combine simulations and experiments to show a samarium-cobalt intermetallic can exhibit plasticity without dislocations.

The interlayer plays a crucial role in improving the properties of many extremely important magnetic materials and devices by regulating the magnetic exchange coupling, while it is still not fully understood in a quantitative way. The main obstacle is that there is still no effective way to characterize quantitatively the ferromagnetic exchange coupling (FEC) strength after inserting the interlayer. In this paper, a rationally designed hard/interlayer/soft layered film is used to study the interlayer effects. It is demonstrated that the magnetic properties of nanocomposite magnets can be effectively enhanced through the modification of interface. Here a new concept, using the nucleation field Hns of soft phase as a “detector” to directly and quantitatively characterize/measure the FEC strength across interface and interlayer, is put forward. It is applied to address the dependence of FEC strength on thickness of nonmagnetic Cr and rare earth-rich phase interlayers. The detailed, distinguishing and typical decay behavior and length of FEC strength for these interlayers are successfully resolved for the first time. The new conception and experimental results get also strongly supported by the theoretical calculation. The nature or mechanism of exchange coupling between ferromagnetic grains or layers after adding the nonmagnetic interlayer is identified by using the hard/interlayer/soft layered film as a model system. Our results demonstrate the validity for characterizing/measuring quantitatively the FEC strength across different interlayer materials via strategy established in this work. It opens up new perspectives to gain insight into the interlayer property, and mechanism comprehension and high-performance design for magnetic materials and devices, such as permanent magnets, high-density magnetic recording media, advanced spin-orbit torque devices, etc.

Grain boundary modification induced giant coercivity enhancement from 945 kA/m to 1904 kA/m was achieved in Sm25Co48Fe20Cu5Zr2 magnets by doping 0.4 wt% CuO. Different magnetization reversal behaviors were clearly observed between un-doped and doped magnets. Compared with un-doped magnets, Cu enrichment along grain boundary promoting regular cellular structure was obviously found in doped magnet. The in-situ Kerr observation indicated that complete cellular structure could strengthen the pinning field of grain boundary regions and the propagations of magnetization reversals into interiors of grains were restrained by the modified grain boundaries, which resulted in the remarkable coercivity increment.

Nanograin magnets are potential candidates for ultrastrong magnets with high coercivity and high energy product. However, existing nanograin bulk magnets exhibit modest energy products because desired structures with nanoscale grain size and strong orientation (texture) are often difficult to obtain. This study describes a synchrony of the nanoscale grain size and strong texture in bulk Nd-Fe-B magnets by plastic deformation with slow strain rates at temperatures well below the melting point of the Nd-rich phase. High grain orientation (>90%) has been achieved for nanoscale grain sizes of 80-110 nm, giving the nanograin bulk magnets record high energy products (≥45 MGOe). Loosely-packed thick grain boundaries and creep-like stress-strain curves have been observed. Grain boundary mediated creep-like deformation is proposed as the mechanism for the high-degree nanoscale grain alignment under the deformation conditions without liquid phase. The effect of the strain rate on the texture and magnetic properties has been investigated systematically.

The direct ordering of amorphous or disordered nanoparticles without high-temperature post-annealing is of great significance for their controllable synthesis and improvement of their properties; however, it remains challenging. In this work, the effects of substrate bias voltage (BV) on the direct crystallization ordering and magnetic properties of Sm–Co–X nanoparticles fabricated by cluster beam deposition (CBD) were explored. It was found that applying BV within ±4 kV did not affect the shape and size of the final deposited nanoparticles. As BV increased, the coercivity and remanence ratio of the Sm–Co–Ni nanoparticle films were significantly improved, which originated from the better crystallinity of the nanoparticles upon landing after applying BV. Further investigations were carried out on thick Sm–Co–Ni nanoparticle films with various thicknesses at a fixed BV of +1 kV. Appreciable coercivity of 6.1 kOe at room temperature was achieved for appropriate thickness without thermal annealing. The composition and structure of the Sm–Co–Ni nanoparticles were characterized by transmission electron microscopy (TEM). It was found that all three elements were homogeneously distributed in the nanoparticles, and no surface segregation was observed. The better crystallinity of Sm–Co–Ni after applying BV was also confirmed by TEM observations. Our results demonstrate that applying high BV may serve as a novel way for ordering amorphous or disordered nanoparticles without shape and size changes without the necessity of high-temperature annealing.

Herein, the Nd–Ce–Fe–B film was used as a model system to evaluate the full potential of Ce magnets with high Ce concentration and characterize the high-coercivity mechanism of magnets. A significant coercivity enhancement and the high-coercivity of 24.8 kOe (much higher than previously reported values) were achieved in very high Ce-substituted (67.3 at%) film magnets by the design of Ce/Cu diffusion layers via a new multifaceted approach. The mechanism of enhanced coercivity was comprehensively analyzed based on the microstructure and domain-structure observations as well as micro-magnetic theory. The magnetically well-separated grains with fine size and unexpected superior microstructure are responsible for the extremely high coercivity in these high Ce-containing film magnets. The microstructure parameters α and Neff in the present high-coercivity Ce film magnet are superior than those in the Nd–Fe–B magnets due to the specific character of Ce-metal boundary phases. Our study indicates that although the intrinsic anisotropy HA decreases distinctly in high Ce-substituted magnets, a high coercivity can still be obtained due to the accessible superior microstructure as compared to the case of the Nd–Fe–B magnets. Our study opens up the door for more effective utilization of highly abundant and cheap Ce resource. It is also significant for diffusion layer designs in high-performance diffusion-processed magnets.

Esophageal cancer (EC) is the sixth leading cause of cancer deaths worldwide with a low 5-year survival rate. More effective chemotherapeutic drugs, either new or repurposing ones, are urgently needed. Disulfiram (DSF) is a safe and public domain drug for alcohol addiction treatment and later shown to have anti-cancer capability, especially when administrated together with copper. The present study is to test the hypothesis that a newly developed copper-cysteamine (Cu-Cy) nanoparticles (NPs) can enhance the anti-tumor effect of DSF on esophageal cancer with reduced risk of copper poisoning. Our results showed that Cu-Cy NPs could greatly facilitate DSF to inhibit cell proliferation in cultured human esophageal cancer cells. Interestingly, the combined inhibitory function could be further enhanced when DSF and Cu-Cy NPs were present at an optimal molar ratio of 1:4. The results of the change in physical color, UV-vis absorption and fluorescence spectra, X-ray diffraction patterns, and FTIR spectra from a mixture of DSF and Cu-Cy NPs suggest a possible reaction between DSF and Cu-Cy NPs and the formation of new materials. Furthermore, cellular mechanistic studies revealed that the combination of DSF and Cu-Cy NPs resulted in reactive oxygen species (ROS) accumulation, and blocked nuclear translocation of NF-ƙB (p65) in esophageal cancer cells. Moreover, in xenograft nude mice, combined administration of DSF and Cu-Cy NPs greatly inhibited tumor growth without noticeable histological toxicity, while any single agent at the same doses presented no inhibitory function. Together, this study demonstrates an effective anti-cancer function of combined treatment of DSF and Cu-Cy NPs in vitro and in vivo, which could be a promising new chemotherapy for esophageal cancer patients.

Self-assembly of nanoparticles into ordered patterns is a novel approach to build up new consolidated materials with desired collective physical properties. Herein, nanoparticle assemblies of composition-modulated bimagnetic nanoparticles have been produced via slow evaporation of their colloidal suspension in the absence or presence of magnetic fields. The assemblies obtained in the presence of the magnetic fields exhibit oriented nanoparticle chains in face-centered cubic superlattice structures, compared with the hexagonal closed-packed superlattice obtained without the magnetic field. The oriented structure has an alignment of the easy magnetization axis along the chains. This alignment leads to enhanced intra-superlattice interactions. As a result, the field-induced assembly displays collective magnetic properties with significantly enhanced magnetic anisotropy, remanent magnetization and coercivity. It is also found that the bimagnetic FeCo/CoFe2O4 core/shell nanostructure enhances the intra-particle interaction and thus is beneficial for the growth of oriented assembly of nanoparticles. Furthermore, the collective magnetic behavior is evidenced by the observation of a superferromagnetic-like magnetization relaxation in the ac-susceptibility curves.

Fe

Sm-Co multilayer films are deposited by plasma sputtering with a Cr underlayer and a Cu buffer layer, followed by annealing at different temperatures. The as-deposited films show low hard magnetic properties due to the existing amorphous Sm-Co phase. After annealing, for the films without Cu buffer layer, there is only Sm2Co17 phase in the films even at high annealing temperature and the coercivity is less than 12kOe. When inserting a Cu buffer layer, the well-crystalline SmCo5 single phase is obtained at relatively lower annealing temperature (500 °C and 550 °C) and extremely high in-plane coercivity (38kOe) is achieved. An analysis of Transmission electron microscopy (TEM) and Auger electron spectroscopy (AES) reveals that Cu atoms started to diffuse during the annealing process induced by thermal dynamics, concentration gradient and microstructure variations, and eventually uniformly distribute in SmCo layer and form Sm(Co,Cu)5 phase. The exchange interaction energy calculation indicates that small fraction of Cu atoms preferentially substitute the Co(2c) site in SmCo5 lattice, which reduces the exchange coupling effects of Sm-Co and gives rise to the coercivity. The results provide a guidance to fabrication of single-phase Sm(Co,Cu)5 multilayer films with superior performance that have applications in advanced electromagnetic and electronic devices.

An efficient heat activating mediator with an enhanced specific absorption rate (SAR) value is attained via control of the iron oxide (Fe3O4) nanoparticle size from 3 to 32 nm. Monodispersed Fe3O4 nanoparticles are synthesized via a seed-less thermolysis technique using oleylamine and oleic acid as the multifunctionalizing agents (surfactant, solvent and reducing agent). The inductive heating properties as a function of particle size reveal a strong increase in the SAR values with increasing particle size up to 28 nm. In particular, the SAR values of ferromagnetic nanoparticles (>16 nm) are strongly enhanced with the increase of ac magnetic field amplitude than that for the superparamagnetic (3–16 nm) nanoparticles. The enhanced SAR values in the ferromagnetic regime are attributed to the synergistic contribution from the hysteresis and susceptibility loss. Specifically, the 28 nm Fe3O4 nanoparticles exhibit an enhanced SAR value of 801 W g−1 which is nearly an order higher than that of the commercially available nanoparticles.

We report high-temperature magnetic properties of hybrid nanocomposite magnets composed of Sm2 Co17 and Nd2Fe14B hard magnetic phases prepared via high-energy ball milling and warm compaction. The magnetic and structural properties of the Sm2Co17/Nd2Fe 14B nanocomposite magnets were investigated for different phase fractions of the two hard phases. The effective interphase exchange coupling resulting from the small grain size (∼14 nm) of the hard phases leads to a single-phase-like demagnetization behavior with improved energy product ${(BH)}_{{{\max}}}$ . The Sm2Co17/Nd2Fe14B isotropic nanocomposite magnets exhibit a ${(BH)}_{{{\max}}}$ value up to 15.5 MG·Oe at room temperature. High-temperature magnetic characterizations show that the Sm2Co17/Nd2Fe14B hybrid nanocomposite magnets have remarkably better thermal stability than single-phase Nd2Fe14B magnets and also have enhanced ${(BH)}_{{{\max}}}$ compared to single-phase Sm 2Co17 magnets.

Localized heat generation using nanoparticles is a promising supplementary technique to the well-established cancer treatments, such as chemotherapy and radiotherapy. Here, we demonstrate that iron carbide (Fe5C2) nanoparticles with a thin carbon shell have the collective magnetothermal and photothermal effects based on the ferromagnetic and photonic properties. When the Fe5C2 nanoparticle suspension is irradiated with a NIR laser (808 nm), it yields unprecedented heating effects. Further, owing to the observed high magnetization and coercivity, the Fe5C2 nanoparticle suspension on exposure to an alternating magnetic field (ACMF) exhibits an enhanced specific absorption rate (SAR) as compared to Fe3O4 nanoparticles of the same size. This significant improvement in the SAR arises from the cooperative contribution from the hysteresis and susceptibility losses. This work also gives quantitative information about the ACMF effects on heating ability as well as provides some guidelines for obtaining enhanced heating activity in nanoparticle suspensions of a given magnetic material.

Nanowires (NWs) of single crystalline hcp Co with length from 200 to 530 nm and diameter from 8 to 20 nm (corresponding to the aspect ratio from 10 to 66) are synthesized via a solvothermal method by controlling the Co-precursor to amine mole concentration. The increased aspect ratio leads to enhanced coercivity of randomly oriented Co NWs up to an optimum value of 6.7 kOe, for the NWs of average length 200 nm and average diameter 15 nm (aspect ratio ∼13). Alignment of the NWs in a magnetic field leads to further enhanced coercivity up to a doubled value of 12.5 kOe at 300 K. The high magnetic coercivity achieved in the random and aligned assemblies is due to both the magnetocrystalline anisotropy and the shape anisotropy. For a better understanding of the coercivity mechanism of the NWs, angular dependence of the coercivity has been experimentally investigated for the aligned NW assemblies and the corresponding magnetization reversal mode is determined to be a coherent reversal mode according to an analytical simulation based on the Stoner-Wohlfarth model.

Anisotropic SmCo5/Co nanocomposite powders have been prepared by electroless Co deposition on commercial SmCo5 powders with hydrazine as reducer. The Co particles are mainly in the range of 8–27 nm and form dense/continuous soft magnetic coatings on the surface of SmCo5 powders. Exchange coupling happened between the coated Co soft magnetic particles and the SmCo5 hard phase. As a result, SmCo5/Co nanocomposite powders with remanence of 73.58 emu/g and energy product of 13.74 MGOe were obtained in the optimum condition, as compared with those of 70.52 emu/g and 13.40 MGOe for uncoated SmCo5 powders. The effects of Co adding amount on Co particle size, coating morphology, and magnetic properties of SmCo5/Co products were investigated.

The exchange-bias field (HEB) and its angular dependence are systematically investigated in Co/CoO core-shell nanowire assemblies (∼15 nm in diameter and ∼200 nm in length) consisting of single-crystalline Co core and polycrystalline CoO shell. Giant exchange-bias field (HEB) up to 2.4 kOe is observed below a blocking temperature (TEB ∼150 K) in the aligned Co/CoO nanowire assemblies. It is also found that there is an angular dependence between the HEB and the applied magnetization direction. The HEB showed a peak at 30° between the applied field and the nanowire aligned direction, which may be attributed to the noncollinear spin orientations at the interface between the ferromagnetic core and the antiferromagnetic shell. This behavior is quantitatively supported by an analytical calculation based on Stoner–Wohlfarth model. This study underlines the importance of the competing magnetic anisotropies at the interface of Co/CoO core-shell nanowires.

Co/CoO bilayer thin films were sputter deposited on a substrate with periodically close-packed truncated 500 nm void arrays. After field cooling from room temperature to 10 K, the hysteresis loops measured at 10 K show distinguished two asymmetries: the unidirectional anisotropy resulted from exchange bias (EB) effect and a step in the second quadrant. This step is believed to be caused by unbiased free magnetic moment. The hysteresis loop representing magnetization change leading to the step show hard-axis behavior, which strongly suggests that the unbiased free moments come from the magnetization with perpendicular anisotropy.