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| Thermal variation of the saturation and remanent magnetizations (a) and that of the coercive and exchange magnetic fields (b) of CFO-CO ceramic, as inferred from its FC hysteresis loops (cooling field of 7 T) recorded at different temperatures. |
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| The relation between structural atomic lattice and the degree of crystallinity of NP is nicely demonstrated here. Moreover, properties (mechanical, growth processes) of supracrystals also change with the nanocrystallinity of the nanoparticles used as building blocks. |
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| The phenomenon of heat dissipation by magnetic materials interacting with an alternating magnetic field, known as magnetic hyperthermia, is an emergent and promising therapy for many diseases, mainly cancer. Here, a magnetic hyperthermia model for core–shell nanoparticles is developed. The theoretical calculation, different from previous models, highlights the importance of heterogeneity by identifying the role of surface and core spins on nanoparticle heat generation. We found that the most efficient nanoparticles should be obtained by selecting materials to reduce the surface to core damping factor ratio, increasing the interface exchange parameter and tuning the surface to core anisotropy ratio for each material combination. From our results we propose a novel heat-based hyperthermia strategy with the focus on improving the heating efficiency of small sized nanoparticles instead of larger ones. This approach might have important implications for cancer treatment and could help improving clinical efficacy. |
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| Bright-field TEM images of the 12 nm Fe/γ-Fe2O3 (a) core/shell, (b) core/void/shell, and (c) hollow nanoparticles |
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| In this work, we studied the dynamic and static magnetic properties of ZnO-core/CoFe2O4-shell and CoO-core/CoFe2O4-shell nanoparticles. Both systems are formed by a core of ∼4 nm of diameter encapsulated in a shell of ∼2 nm of thickness. The mean blocking temperature changes from 106(7) to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar interparticle interactions, where Hint is approximately −0.1 kOe for ZnO/CoFe2O4 and −0.9 kOe for CoO/CoFe2O4, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe2O4 is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ∼46(6) and ∼69(5) nm3 were found for CoO/CoFe2O4 and ZnO/CoFe2O4 nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe2O4 nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe2O4 interface rules the magnetization reversal and the nanoparticles’ thermal stability by inducing a larger energy barrier and promoting smaller switching volume. |
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(left) Schematic representation of the
inverse AFM/FiM core/shell structure and (right) hysteresis loops of the
doubly inverted nanoparticles, for different AFM core sizes (Dcore) and constant FiM shell thickness of four lattice spacings.
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| (a) Size dependence of experimentally measured (black solid (T = 5 K) and dashed (300 K) lines) and calculated saturation magnetization MS of CoO octahedra. A magnetic moment μ of (4.8 ± 0.4)μB per Co2+ ion and a shell thickness of 3 nm Co3O4 is assumed. MS arises only from the Co cations at the surfaces or interfaces of the octahedral nanocrystals, as indicated in the schematic in (b). The blue lines are calculated values assuming only effectively one layer of Co2+ cations at the Co3O4 surface, while the red lines correspond to a double layer of Co cations at the interface. |
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Cobalt
oxide octahedra were synthesized by thermal decomposition. Each
octahedron-shaped nanoparticle consists of an antiferromagnetic CoO core
enclosed by eight {111} facets interfaced to a thin (∼4 nm) surface
layer of strained Co3O4. The nearly perfectly
octahedral shaped particles with 20, 40, and 85 nm edge length show a
weak room-temperature ferromagnetism that can be attributed to
ferromagnetic correlations appearing due to strained lattice
configurations at the CoO/Co3O4 interface.
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| Increased coercive and tunable exchange bias field values were registered in hybrid CoO@Fe3O4 core–shell octahedron-shaped nanoparticles of different average size. In this strained morphology, the metal cation chemical potentials at the interface between the antiferromagnetic and ferrimagnetic oxides become a very dynamic variable. This causes the effective magnetic anisotropy to increase and the type of interface to change after growing the magnetite shell epitaxially onto the cobalt oxide {111} surface facets and, consequently, to tune the exchange bias effect. |
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| Morphological and structural evolution of magnetic nanoparticle and correlated tunability of nanomagnetism. (a) Magnetic NPs with various structural motifs exhibiting differences in size, surface anisotropy, and exchange anisotropy. (b) Magnetism tuning by the systematicchanges of magnetic nanoparticles. Graphs i−iv correspond to the nanoparticles shown in part a where modulation of structural motifs is needed to control parameters such as K, Hc, Ms, or Mr. |
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| Images and magnetization behaviors of cube and sphere nanoparticles. (a) TEM images of cube (18 nm (σ ≈ 5%) in edge length) and (b) sphere nanoparticle (22 nm (σ ≈ 7%) in diameter). Nanoparticles have identical composition (Zn0.4Fe2.6O4) and magnetic volume (5.8 × 10–24 m3). (c) High resolution TEM image of cube exhibiting well-defined lattice fringes of {100} faces. (d) M-H curves of cube and sphere measured at 300 K using SQUID. Ms of cube is 165 emu/g(Fe+Zn), and that of sphere is 145 emu/g(Fe+Zn). Simulated magnetic spin states of (e) cube and (f) sphere by using OOMMF program. The color map indicates the degree of spin canting against external magnetic field (B0) where red indicates nondeviated spins and blue indicates highly canted spins. Local spin states on the surfaces of nanoparticles are depicted on the right corners of parts e and f. Cube exhibits lower spin disorder rate of 4% than sphere of 8%. |
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| Schematic Illustration of Synthesis and Modification of Fe3O4/TaOx Core/Shell NPs |
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| Temperature dependence of ZFC and FC magnetization taken in a field of 50 Oe for (a) 10 nm and (b) 15 nm hollow particles. |
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| Temperature dependence of the net moment of frozen spins ( M f ) for (a) 15 nm, (b) 10 nm, and (c) 8 nm Fe/γ-Fe 2O 3 nanoparticles. H EB and M f are plotted as a function of particle size (d). |
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| Hysteresis loops (M–H) measured in the ZFC and FC protocols for cubic, spherical, and octopod-shaped FeO/Fe3O4 nanoparticles. The inset of (a) shows an enlarged portion of the M–H curves featuring the low-field jump in the FC magnetization of the cubic nanoparticles. This feature is less pronounced in the case of the octopod-shaped nanoparticles (c) and is almost absent in the case of the spherical nanoparticles (b). |
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| (a) Temperature dependence of exchange bias field for cooling field of 1 T and (b) temperature dependence of isothermal remanence. Insets in (a) show the ZFC and FC hysteresis loops at 10 K and 100 K. Inset in (b)shows the temperature dependence of blocking function distribution. |
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| The FC M( H) loops taken at 5 K for (a) 9.2 nm and (b) 18.7 nm hollow nanoparticles. The TEM image of the 18.5 nm solid nanoparticles is shown in the inset of (b). |
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| Novel OER catalysts – monodisperse Au@Co3O4 core-shell nanocrystals – have been prepared by synthesizing Au nanocrystals, followed by deposition of Co shells and their conversion to Co3O4 shells. Owing to the synergistic effect, Au@Co3O4 nanocrystals have an OER activity 7 times as high as a Au and Co3O4 nanocrystals mixture or Co3O4 nanocrystals alone, and 55 times as high as Au nanocrystals alone |
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| Schematic illustration of the transformation of Mn3O4 NCs, showing the evolution of their morphology via the localized dissolution of Mn3O4 and the surface precipitation of g-Fe2O3. (B to E) HRTEM images of the hollow nanostructures synthesized by the reaction of Mn3O4 NCs with 1 ml of aqueous solutions of iron(II) perchlorate having different concentrations. |
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| (a) and (b) Bipolar TS scan at 20K and 130K under FC condition indicatingthe anisotropy fields and g, (c) temperature dependence of g, (d) temperature dependence of shift in the first anisotropy peaks in the bipolar scans. |
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| Snapshots of the spin configurations in the vertical section central plane through the symmetry axis of ellipsoid-shaped nanoparticle with KC=0.5 in different fields around the coercive fields of field-cooled hysteresis loop. Hl and Hr are the fields around the coercive fields at the descending and ascending branches, respectively. (a) Hl=−0.07, (b) Hl=−0.08, (c) Hl=−0.09, (d) Hl=−0.10, (e) Hl=−0.11, (f) Hr=0.03, (g) Hr=0.05, (h) Hr=0.07, (i) Hr=0.08, (j) Hr=0.09. |
