Cadmium sulfide (CdS) quantum dots (QDs) doped with different concentrations of manganese (Mn = 0, 1, 3, 5, and 7%) are synthesized by using the thermolysis of diselenocarbamato complexes of cadmium ...in the trioctylphosphine oxide (TOPO) method. The QDs show quantum size effects in the optical spectra and exhibit band-edge emission. The TEM images revealed that the QDs are spherical within the range of 3 to 5 nm. The nonlinear optical studies and optical limiting characteristics are studied. From open and closed aperture data, the nonlinear absorption coefficient (β) and refractive indices (n2) values are in the range of 4-16 x10-9 cm2/w and 8-14x10-15 cm2/w. The cross relaxations and energy transfer mechanism of quantum dots are studied using the transient absorption pump-probe experiment.
Rhombohedral nickel carbide (Ni3Cx, x=0.7, 1.2 and 1.5) nanoparticles (∼ 110 nm) with enhanced magnetic coercivity (HC up to 1.3 kOe) at temperature below the spin-glass freezing (T
f
) have been ...demonstrated. The presence of spin-glass state, which is seen at ∼17 K, is evident by the field dependence of the freezing temperature following the de Almeida–Thouless (AT) relationship and frequency dependence of T
f
. Moreover, the spin-glass state is irreversible to the sweeping applied field and results in high HC at 10 K. With increases of the carbon content, we have found a gradual increasing trend in the saturation magnetization (MS: 6.6 to 9.2 emu/g) and coercivity (350 Oe to 1.3kOe) at 10 K. This is attributed to the increase of spin-glass contribution and the weak ferromagnetic phase.
Ferromagnetic Co-doped α-Fe2O3 cubic shaped nanocrystal assemblies (NAs) with a high coercivity of 5.5 kOe have been synthesized via a magnetic field (2 kOe) assisted hydrothermal process. The X-ray ...diffraction pattern and Raman spectra of α-Fe2O3 and Co-doped α-Fe2O3 NAs confirms the formation of single-phase α-Fe2O3 with a rhombohedral crystal structure. Electron microscopy analysis depict that the Co-doped α-Fe2O3 NAs synthesized under the influence of the magnetic field are consist of aggregated nanocrystals (∼30 nm) and of average assembly size 2 μm. In contrast to the NAs synthesized with no magnetic field, the average NAs size and coercivity of the Co-doped α-Fe2O3 NAs prepared with magnetic field is increased by 1 μm and 1.4 kOe, respectively. The enhanced coercivity could be related to the well-known spin–orbit coupling strength of Co2+ cations and the redistribution of the cations. The size increment indicates that the small ferromagnetic nanocrystals assemble into cubic NAs with increased size in the magnetic field that also lead to the enhanced coercivity.
We report surface cleaning of magnetic nanoparticles (SmCo5 nanochips and CoFe2O4 nanoparticles) by using cold plasma. SmCo5 nanochips and CoFe2O4 nanoparticles, coated with surfactants (oleic acid ...and oleylamine, respectively) on their surfaces, were treated in cold plasmas generated in argon, hydrogen or oxygen atmospheres. The plasmas were generated using a capacitively coupled pulsed radio frequency discharge. Surface cleaning of nanoparticles was monitored by measurement of the reduction of surface carbon content as functions of plasma processing parameters and treatment times. EDX and XPS analyses of the nanoparticles, obtained after the plasma treatment, revealed significant reduction of carbon content was achieved via plasma treatment. The SmCo5 nanochips and CoFe2O4 nanoparticles treated in an argon plasma revealed reduction of atomic carbon content by more than 54 and 40 in atomic percentage, compared with the untreated nanoparticles while the morphology, crystal structures and magnetic properties are retained upon the treatments.
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.
Nanocrystalline RCo5 (R = Ce, La0.35Ce0.65, and misch-metal noted as MM) ribbons with hexagonal crystal structure and an average grain size of 5 nm have been prepared via a one-step melt-spinning ...technique. Coercivity as high as 13.0, 13.8, and 10.9 kOe has been obtained at 300 K for the CeCo5, La0.35Ce0.65Co5, and MMCo5 ribbons, respectively. High thermal stability is also achieved as shown by the high coercivity of 9.3 kOe, 10.2 kOe, and 8.8 kOe at 400 K for CeCo5, La0.35Ce0.65Co5, and MMCo5 ribbons, respectively. The coercivity mechanism is studied by magnetization analysis and microstructural observations. The nanocrystalline grains promote a strong exchange interaction, as indicated by the positive δM and the relatively high remanence ratio (∼0.8). In addition, the temperature dependence of coercivity of RCo5 ribbons shows the low coercivity temperature coefficient of −0.2% to −0.25%/K.
Low-dimensional hard magnetic materials Mohapatra, Jeotikanta; Joshi, Pramanand; Ping Liu, J.
Progress in materials science,
September 2023, 2023-09-00, Volume:
138
Journal Article
Peer reviewed
Open access
When a piece of ferromagnetic material reduces its size to nanometer scale, its magnetic ordering will be altered and its magnetic properties will be consequently changed, because ferromagnetism is a ...size-sensitive physical phenomenon related to electronic exchange, magnetocrystalline anisotropy, and magnetostatic interactions, which are all effected by geometric parameters of the material at nanometer scale. In this review, we systematically discuss the scientific and technological problems of low-dimensional ferromagnetic materials, from their synthesis strategies to the geometric confinement in fine nanoparticles (0D), nanowires (1D), and thin layers (2D) that give rise to new magnetic effects, including the size, surface, and shape effects on magnetic properties, particularly on magnetic anisotropy and coercivity. Various ferromagnetic materials are studied, including metals, alloys and ceramics. These low-dimensional nanoscale magnets can find broad applications in green energy, information storage, and biomedicine. On the other hand, low-dimensional objects can be used as building blocks to assemble new types of advanced 3D bulk magnets. Research in this area is significant not only for technological applications but also for a fundamental understanding of magnetic anisotropy and interactions.
Iron (Fe) is the most important ferromagnetic element, not only for its high magnetic moment and high Curie temperature but for its abundance as well. Fe-based magnetic materials are therefore widely ...applied in technologies and industries, with most of the applications for soft magnetic materials, because of the low magnetocrystalline anisotropy (MCA) of Fe. However, it is possible to realize magnetic hardening in Fe-based materials as we have learned from the early carbon steel permanent magnets although their coercivity was modest. Recent efforts to search for rare-earth-free hard magnetic materials have shown more promising evidences for achieving high MCA in Fe-based materials. In this paper, we review the history and the recent developments of Fe-based hard and semi-hard magnetic materials with a focus on mechanisms of high MCA in Fe-based phases and the related crystal and electronic structures. We have tabulated and discussed the structures and the magnetic properties of the Fe-based binary or ternary systems containing p-block and d-block elements, with many of them showing considerable MCA. Furthermore, it is important to know and to understand that the MCA in Fe-based magnetic materials can be tailored/enhanced through chemical and/or structural modifications that will lead to “artificially engineered” hard and semi-hard magnetic materials for advanced permanent magnets in the future.
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•Mechanisms of high MCA in Fe-based phases, along with the corresponding crystal and electronic structures, are presented.•The magnetic properties of the Fe-based binary or ternary systems containing p-block and d-block elements are discussed.•Guidelines on how to search for the Fe-based hard and semi-hard magnetic materials that are thermodynamically stable are provided.
Cobalt (Co) is one of the most important 3d transition metal elements used in the development of magnetic materials, especially in hard magnetic materials, from the early Co-based steel magnets to ...the rare-earth permanent magnets based on the 3d-4f compounds. Like iron (Fe), Co possesses a high saturation magnetization (MS) and a high Curie temperature (TC). Unlike Fe, Co has high magnetocrystalline anisotropy due to its spin-orbit coupling. As a result, it can be used to achieve a high degree of magnetic hardening in its elemental, alloy or compound materials. For instance, Co-based magnetic recording media is based on its elemental material. On the other hand, nano-structuring of Co-based alloys gives important hard magnetic materials including Alnico magnets and Fe–Cr–Co magnets, which are probably the earliest nanostructured hard magnetic materials. Recent progress in the development of Co-containing nanoscale hard magnetic materials has been very encouraging and promising based on the outstanding magnetic properties we have seen in L10 CoPt, tetragonally distorted FeCo and Co-based intermetallics. Particularly, the morphology of the nanoparticles has also been modulated to develop high coercivity via utilizing shape anisotropy, leading to extraordinarily high coercivity in Co nanowire assemblies. This review presents an overview of the development of Co-based hard and semi-hard magnetic materials and their intrinsic and extrinsic magnetic properties in view of fundamental understanding and technological applications.
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•Research progress of Co-based hard and semi-hard magnetic materials is reviewed.•The magnetic properties are discussed in view of technological applications.•Fabrication process and magnetic properties of Co-based nanomaterials are discussed.•The effects of microstructure parameters on ferromagnetic properties are presented.
•Controlled synthesis of 3–16nm sized CoFe2O4 nanoparticles using oleylamine.•Small sized nanoparticles (9nm and below) exhibit strong interaction in dried form.•A superspin glass with enhanced MS at ...10K stems from strong interactions.•The frequency dependent blocking and memory effect confirms superspin glass freezing.
Low-temperature magnetic properties of CoFe2O4 nanoparticles (3–16nm) have been investigated by AC and DC magnetic measurements. The saturation magnetization (MS) of ultra-small CoFe2O4 nanoparticles (3–9nm) sharply increases at low temperature (10K) compared to room temperature (RT) MS value. For example, the increment in MS value for 3nm CoFe2O4 nanoparticle is 22emu/g and is null for 12nm and larger sized nanoparticles. A similar trend of increment in MS is also seen in ultra-small size Fe3O4 and MnFe2O4 nanoparticles. However, the MS enhancement in ultra-small CoFe2O4 nanoparticles is found much higher as compared to Fe3O4 and MnFe2O4 nanoparticles. The ultra-small sized nanoparticles arrange with a high packing density to induce a strong exchange as well as dipolar interactions, which renders the enhanced low temperature MS with superspin glass (SSG) state. The exchange coupling strongly depends on magnetic anisotropy energy, which increases in the order Mn2+<Fe2+<Co2+ and thus the ultra-small CoFe2O4 nanoparticles show a large enhancement of MS at low temperature due to strong exchange coupling. A noticeable enhancement of spin glass temperature (Tg) for ultra-small sized CoFe2O4 nanoparticles also confirms the presence of strong exchange coupling in this case. Fitting of the ac susceptibility χ′(T, f) data to a power-law scaling and Vogel–Fulcher model shows a satisfactory fit and the dynamic critical exponent takes value between 8.9 and 11.9 which are in a range typical for the spin-glass systems. Memory behavior in ultra-small CoFe2O4 nanoparticles suggest that the frequency dependent blocking process of ultra-small sized nanoparticles can be better described by power law model, while the interaction regime present in the 12nm and above sized nanoparticles is ascribed to a Vogel–Fulcher model.
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