Brown's theorem on coercivity of ferromagnetic materials has predicted that the coercivity level is substantially higher than in practice for all the materials studied in experiments in the past ...seven decades, which is known as the Brown's paradox. In this paper, a system with a coercivity close to the one predicted by Brown's theorem is investigated. Cobalt nanowires are obtained by chemical synthesis that give rise to coercive forces significantly higher than the magnetocrystalline anisotropy field, verifying the Brown's theorem. It is found that the coercivity is strongly dependent on the nanowire diameter, the alignment of the wires in an assembly, and the packing density of the assembly. An analysis based on the current experimental results and related literature reveals a coercivity ceiling in consideration of geometrical dimensions and the effective magnetic anisotropy. Quantitative information is obtained about the proximity effect on the coercivity and the magnetization which shows the correlation between the energy product and the packing density. Furthermore, it is found that by coating the nanowires with Fe, the energy density can be enhanced. These findings provide a guideline for materials design of future high‐performance permanent magnets that take advantage of shape anisotropy at the nanoscale.
Coercivity is the key property for hard magnetic materials. The coercivity level predicted by the Brown's theorem has been substantially higher than the experimental practice for all ferromagnetic materials studied, which is recognized as the Brown's paradox. In this study, the theorem is proven, and the paradox is solved for the first time in ferromagnetic nanowires.
•Morphology and magnetic properties of single-crystalline (hcp) Co nanowire assemblies were investigated.•Angular dependence of coercivity was used to describe the magnetization reversal process.•The ...magnetization reversal in single domain nanowires is preceded via a coherent rotation.•Novel low-dimensional permanent magnets with high energy density for advanced applications.
Nanowires (NWs) of single crystalline hcp Co with length from 200 to 530nm and diameter from 8 to 20nm (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 200nm and average diameter 15nm (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 300K. 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.
•The giant exchange-bias field in Co/CoO core/shell nanoparticles is related to the interplay of the spin-glass and antiferromagnetic interactions.•Controlling the crystallinity and thickness of CoO ...shell it is possible to tune exchange bias field.•Formation of polycrystalline CoO sell with multiple easy axes and lower uncompensated moment, resulting in low HEB and significant training effects.
In this report, the exchange-bias effect of Co/CoO core/shell nanoparticles with the amorphous CoO shell has been investigated. At low temperatures, the developed spin-glass state with net magnetic moments in the CoO shell provides a unidirectional anisotropy in the Co core via exchange coupling, leading to a giant exchange bias field up to 3.6 kOe. By improving the crystallinity of CoO shell by performing surface oxidation at an elevated temperature, the spin-glass ordering is suppressed by the antiferromagnetic ordering, resulting in a significantly lower exchange-bias field (2.2 kOe) and a prominent training effect due to the existence of multiple easy axes in the multi-crystalline CoO shell. Theoretical studies confirm that the emerging spin-glass state in the CoO shell has a significant impact on the exchange bias of the system. These new findings provide a novel approach toward manipulating and fine-tuning the exchange bias effects in nanoscale systems for spintronics applications.
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Polyacrylic acid functionalized Fe3O4 nanoparticles (PAA-MNPs) of average size of 10 nm are prepared by a simple soft chemical approach. These PAA-MNPs are conjugated with folic acid through peptide ...bonding between the carboxylic group on the surface of PAA-MNPs and the amine group of folic acid. The good colloidal stability of FA conjugated MNPs makes it a promising candidate for targeted drug delivery, hyperthermia and as a MRI contrast agent with a transverse relaxivity R2 value of 105 mM(-1) s(-1). Folic acid conjugated magnetic nanoparticles (FA-MNPs) achieved ∼ 95% loading efficiency of doxorubicin (DOX) which could be due to strong electrostatic interaction of highly negatively charged FA-MNPs and the positively charged DOX. The drug release study shows a pH-dependent behavior and is higher in acidic pH (4.3 and 5.6) as compared to the physiological pH (7.3). Flow cytometry and confocal microscopic image analysis reveal that around 75-80% of HeLa cells undergo apoptosis due to DNA disintegration upon incubation with DOX-MNPs for 24 h. DOX-MNPs exhibit the synergistic effect due to the combination of DOX induced apoptosis and magnetic hyperthermia treatment (MHT) which enhance the cell death ∼ 95.0%. Thus, this system may serve as a potential pH sensitive nanocarrier for synergistic chemo-thermal therapy as well as a possible MRI contrast agent.
Ferromagnetic nanowires (NWs) are novel materials that offer unique magnetic properties, as the geometrical dimensions become comparable to key length scales in magnetism, such as the exchange length ...or the domain wall width. In this work, compositionally modulated Co1−xNix nanowires (diameter 10–15 nm) with high coercivity have been synthesized via a thermal decomposition method. The structural analysis demonstrates that the hexagonal close-packed (hcp) crystal structure and the wire-shape morphology are maintained up to Ni content of x = 0.3. Based on the shape anisotropy and orientation, the aligned Co1−xNix nanowire assemblies show that the coercivity at room temperature decreases from 11.4 to 5.4 kOe with increasing x from 0 to 0.3. The monotonous decrease in coercivity with Ni content is related to the effective magnetic anisotropy and nanowire diameter which are found to be strongly varied with Ni addition. In addition, it is found that the increase of Ni content in the nanowires brings more resistance to oxidation than the pristine Co nanowires. The exchange bias study indicates that the Ni addition leads to lower blocking temperature of the CoNiO grains and consequently the switch-on temperature for the exchange bias field shifts to low temperatures with the increase of Ni content. Further, the exchange bias behavior that is associated with the existence of antiferromagnetic and spin-glass-like states are confirmed by temperature-dependent magnetization measurements.
•CoNi alloy composition determines the crystal nanostructure and influences the effective magnetic anisotropy.•The effects of structural and morphological parameters on magnetic coercivity are presented.•The synergistic contribution of shape and crystalline anisotropies to the coercivity is systematically discussed.•The temperature dependence of the exchange bias study reveals the existence of antiferromagnetic and spin-glass-like states.
Controlling the semiconductor nanoparticles (NPs) size can alter their optical and electronic properties, which is an important feature for many optoelectronic device applications. In this study, we ...demonstrated a simple and economical approach to synthesize size-controlled Cu2ZnSnS4 (CZTS) NPs and their application as an absorber layer in solar cells. The size of the CZTS NPs has been controlled from 2.5 to 8 ( ± 0.5) nm by variation of the amine to the precursor mole ratio. The impact of the particle size on the structural, optical, and electrical performance of the devices are studied systematically. XRD and Raman spectroscopy measurements reveal the formation of pure kesterite phase of the CZTS. Moreover, the UV–vis spectroscopy data show that the CZTS NP films have a high optical absorption coefficient (104 cm−1) in the visible region, and its optical band gap is in the range of 1.50–1.62 eV. The power conversion efficiency of a solar cell fabricated using CZTS NPs is enhanced considerably from 3.6% to 4.8% with an increase of nanoparticles size, within an active area of 1.0 ± 0.1 cm2. The maximum external quantum efficiency of 59% is obtained for the solar cell with CZTS thin film comprising 8 nm particles. The observed changes in the device performance parameters might be due to the variation of the thin film microstructure.
•A simple and cost-effective approach has been used to synthesize size-selective Cu2ZnSnS4 nanoparticles (CZTS NPs).•The size of the CZTS NPs has been controlled by a simple variation of amine to precursor mole ratio.•The influence of CZTS NPs size on the structural, optical and solar cell performance has been studied.•The fabricated devices show efficiencies ranging from 3.6% to 4.8% depending on the CZTS NPs average diameter.
It has been possible to incorporate cadmium ions in ZnS quantum dots (QDs). It is studied how the substitution of Cd
2+
ions by zinc ions affects the structural, morphological, and optical properties ...of ZnS QDs. Zn
1−
x
Cd
x
S QDs are prepared by a simple beaker chemistry approach and characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy, and UV-visible and photoluminescence (PL) spectroscopies. XRD studies confirmed that all the prepared samples are in zinc-blende phase. With the increase of cadmium content, the diffraction peaks shifted towards lower diffraction angles and the lattice constant increased linearly. Optical studies revealed that the strong absorption edge also shifted towards the higher wavelength region with the increase of Cd content. Hence, the optical bandgap of the QDs decreases with the increase of Cd content. Due to the quantum confinement of the carriers in the QDs, the bandgap energy is higher than that of the corresponding bulk material. The PL spectrum of the undoped ZnS QDs contains five peaks (centered at 365, 400, 420, 450, and 470 nm) which are attributed to the recombination of the defect states of ZnS.
A high-performance MRI contrast agent and a drug nanocarrier have been realized in porous Fe3O4@SiO2 nanorods (NRs). The Fe3O4@SiO2 NRs of length ~520nm and diameter ~180nm are synthesized by ...annealing FeOOH@SiO2 nanorods at a temperature of 300℃ under continuous flow of forming gas. The magnetic measurement confirms that the Fe3O4@SiO2 NRs is ferrimagnetic in nature with magnetization of 20emu/g and coercivity HC ~450Oe. The aqueous suspension of the NRs is stable over a time frame of one month and exhibits a high R2 relaxivity value of 192mM−1s−1. The R2 darkening effect is also observed in HeLa cells incubated with NRs in comparison to untreated control cells. The porous Fe3O4@SiO2 NRs further work as an excellent carrier for doxorubicin (DOX) drug with loading efficiency of 65%. The drug release study shows a pH-dependent behavior and is higher in acidic pH (4.3) as compared to the physiological pH (7.4). After 72h, the cumulative DOX release is found to be ~58% at pH 4.3 and ~17% at pH 7.4. The induction heating studies of the NRs exhibit a sharp increasing trend of SAR value with the increase of magnetic field.
•Porous Fe3O4@SiO2 NRs shows enhanced MRI contrast agent and drug carrier properties.•The stable aqueous dispersion of NRs exhibits a high R2 relaxivity of 192mM−1s−1.•The porous NRs also work as a nanocarrier for DOX with a loading efficiency of 65%.•The drug release study shows a pH-dependent behavior and is higher in acidic pH (4.3).
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We present a facile green approach to synthesize monodisperse graphene quantum dots (GQDs) of sizes 2–6.5 nm using rice grains as a carbon source. As the size of the GQDs increases from 2–6.5 nm, a ...red shift (blue to cyan) in the photoluminescence emission spectra is observed due to quantum confinement effect. The colloidal solution of as synthesized GQDs is highly luminescent under 336 nm illumination. The quantum yield (QY) of the as-prepared GQDs in water is size dependent and increases from 16 to 24% with the decrease in size from 6.5 to 2 nm. The potential of these GQDs as biomarkers for cell imaging is explored further. The cytoxicity study with different concentrations of the GQDs confirms the excellent biocompatibility of the GQDs.
Abstract
Cations and anions are replaced with Fe, Mn, and Se in CZTS in order to control the formations of the secondary phase, the band gap, and the micro structure of Cu
2
ZnSnS
4
. We demonstrate ...a simplified synthesis strategy for a range of quaternary chalcogenide nanoparticles such as Cu
2
ZnSnS
4
(CZTS), Cu
2
FeSnS
4
(CFTS), Cu
2
MnSnS
4
(CMTS), Cu
2
ZnSnSe
4
(CZTSe), and Cu
2
ZnSn(S
0.5
Se
0.5
)
4
(CZTSSe) by thermolysis of metal chloride precursors using long chain amine molecules. It is observed that the crystal structure, band gap and micro structure of the CZTS thin films are affected by the substitution of anion/cations. Moreover, secondary phases are not observed and grain sizes are enhanced significantly with selenium doping (grain size ~1 μm). The earth-abundant Cu
2
MSnS
4
/Se
4
(M = Zn, Mn and Fe) nanoparticles have band gaps in the range of 1.04–1.51 eV with high optical-absorption coefficients (~10
4
cm
−1
) in the visible region. The power conversion efficiency of a CZTS solar cell is enhanced significantly, from 0.4% to 7.4% with selenium doping, within an active area of 1.1 ± 0.1 cm
2
. The observed changes in the device performance parameters might be ascribed to the variation of optical band gap and microstructure of the thin films. The performance of the device is at par with sputtered fabricated films, at similar scales.
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