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Magnetic nanoparticles with an optimal size seek high inductive heating performance, which plays an important role in biomedical applications. This work reports the critical size of MnFe 2 O 4 particles at which the specific absorption rate (SAR) reaches its maximum value. MnFe 2 O 4 nanoparticles with different sizes from ~11 nm to ~70 nm were synthesized using the hydrothermal method. Under an applied field amplitude of 80 Oe and frequency of 236 kHz, the 18-nm MnFe 2 O 4 nanoparticles exhibited the highest SAR of 65.52 W/g. The effective magnetic anisotropy, as a function of particle size, was used to calculate the theoretical value of SAR in the framework of the linear response theory. Experimental results agreed well with the theoretical calculations in the superparamagnetic regime. This study may serve as a basis for the accurate prediction of the optimal size of magnetic nanoparticles in inductive heating.

In this study, the improved hyperthermia properties of a magnetic, biocompatible core/shell nanostructure containing 12 -nm hard magnetic CoFe2O4 nanoparticles as the core and soft magnetic Fe3O4 nanoparticles as the shell were reported. By simply using thermal decomposition synthesis method, CoFe2O4 nanoparticles were coated layer-by-layer by Fe3O4 nanoparticles to result the core/shell structure with a variable shell thickness. XRD analysis showed that the as-synthesized nanoparticles have face-centered cubic structure with ferrite spinel single phase. TEM images showed that particle size of the core–shell structure increased by 0.7 − 2.1 nm and monotonously with the Fe3O4 coating. The coercivity (Hc) of the core/shell nanoparticles decreased remarkably compared to that of CoFe2O4 core nanoparticles while saturation magnetization Ms significantly increased, indicating the successful formation of the soft magnetic Fe3O4 shell layer. Dynamic light scattering (DLS) analysis revealed that the uniform size and good stability of the magnetic fluids based on the as-synthesized CoFe2O4/Fe3O4 core/shell nanoparticles using poly(maleic anhydride-alt-1-octadecene) (PMAO) as the phase transfer ligand were achieved. Comparing to the core CoFe2O4 nanoparticle, the specific absorption rate (SAR) values of the core/shell nanoparticles were significantly higher and, especially, were more than doubled with shell thickness of 2 nm. Additionally, the cytotoxicity test indicated low toxicity of the magnetic fluid on HEK-293 normal cells. These results showed that the studied CoFe2O4/Fe3O4 core/shell nanoparticles with their high biocompatibility and improved hyperthermia properties are expected to be usable in biomedicine, especially in hyperthermia cancer treatment.

Zinc-substituted cobalt ferrites Co1−xZnxFe2O4 (x = 0.0 to 0.7) nanoparticles have been synthesized using the hydrothermal method. The pure cubic spinel powder samples prepared were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. It is found that the lattice parameter increases with Zn substitution. The average crystallite size of the particles decreases gradually from 20 to 10 nm with the increase in Zn-content, which is confirmed by transmission electron spectroscopy micrographs. The direct and indirect band gap of Co1−xZnxFe2O4 determined from UV–Vis measurements decreases with the increase of Zn concentration. The magnetic properties have been investigated by physical property measurement system and vibrating sample magnetometer. The saturation magnetization increases slightly from 71.38 emu g−1 (x = 0) to 77.59 emu g−1 (x = 0.1), then decrease with the increase in Zn substitution. Nevertheless, the coercivity significantly decreases with Zn concentrations, which can be explained using Yafet–Kittel model and the distribution of Fe3+ ions among octahedral and tetrahedral sites in samples. This result is further confirmed by photoluminescence emission spectra.

Plasmonic Au- and Ag-decorated ZnO films on a glass substrate were obtained by a plasma electrolyte oxidation method with high-voltage discharge, followed by the decoration of Au nanoparticles (NPs) on their surface. With the increasing concentration of the Ag precursor, the thickness of the Ag-ZnO films decreased and the width of the bandgap increased, causing an enhancement in the near band-edge ultraviolet (UV) emission. With the decoration of the Au NPs, the photoluminescence of the Ag-ZnO films exhibited quenching of the band-edge UV emission, accompanied by the suppression of the visible emission in the photoluminescence spectra, and a significant enhancement in the photocatalytic efficiency for the photodegradation of Rhodamine B under solar simulator irradiation.

A manganite composite series of (1 −  x )La 0.7 Sr 0.3 MnO 3 / x BaTiO 3 ( x  = 0, 0.06, 0.12, and 0.18) has been fabricated by solid-state reaction combined with a high-energy mechanical milling method. Experimental results revealed that the insulator–metal transition temperature was shifted towards lower temperatures, and resistivity increases with increasing BaTiO 3 content in (1 −  x )La 0.7 Sr 0.3 MnO 3 / x BaTiO 3 . Meanwhile, the ferromagnetic–paramagnetic transition temperature was almost unchanged. The increase in magnetoresistance was observed in the all composites at whole measurement temperatures under an applied magnetic field of 3 kOe. Here, temperature dependences of magnetoresistance display a Curie–Weiss law-like behavior. The nature of this phenomenon is explained in detail.

Co-CoO composite powders were prepared by high-energy ball milling and subsequent annealing with the aim to probe an exchange bias (EB) effect. A microstructure consisting of flakes with a thickness of about 100 nm was revealed from scanning electron microscopy images. X-ray diffraction phase identification indicated that the optimal annealing temperature of as-milled Co for the formation of Co-CoO composite structure is 300°C. Magnetic measurements showed that saturation magnetization, Ms, of annealed Co-CoO decreased as compared to that of as-milled Co. This implies that a fraction of the oxide phase was formed after heat treatment. Furthermore, the hysteresis loop measured at 5 K after cooling in a magnetic field of 50 kOe from 350 K showed a presence of the EB effect, which reached the value of 120 Oe. It is closely related to the formation of an antiferromagnetic (AFM) CoO phase, which interacts with the adjacent Co ferromagnetic (FM) phase. A Monte-Carlo simulation was also performed to demonstrate the EB effect in FM–AFM structured materials. A better agreement between simulated and experimental hysteresis loops was obtained when averaged characteristics of the randomly-oriented individual powders were taken into account. Simulation results also showed that EB is largest when easy axes of FM and AFM phases are parallel to the magnetic field, and a critical fraction of AFM phase was suggested to be necessary for appearance of the EB effect.

Iron oxide nanoparticles (NPs) are currently a very active research field. To date, a comprehensive study of iron oxide NPs is still lacking not only on the size dependence of structural phases but also in the use of an appropriate model. Herein, we report on a systematic study of the structural and magnetic properties of iron oxide NPs prepared by a co-precipitation method followed by hydrothermal treatment. X-ray diffraction and transmission electron microscopy reveal that the NPs have an inverse spinel structure of iron oxide phase (Fe 3 O 4 ) with average crystallite sizes ( D XRD ) of 6–19 nm, while grain sizes ( D TEM ) are of 7–23 nm. In addition, the larger the particle size, the closer the experimental lattice constant value is to that of the magnetite structure. Magnetic field-dependent magnetization data and analysis show that the effective anisotropy constants of the Fe 3 O 4 NPs are about five times larger than that of their bulk counterpart. Particle size ( D ) dependence of the magnetization and the non-saturating behavior observed in applied fields up to 50 kOe are discussed using the core–shell structure model. We find that with decreasing D , while the calculated thickness of the shell of disordered spins ( t  ∼ 0.3 nm) remains almost unchanged, the specific surface areas S a increases significantly, thus reducing the magnetization of the NPs. We also probe the coercivity of the NPs by using the mixed coercive Kneller and Luborsky model. The calculated results indicate that the coercivity rises monotonously with the particle size, and are well matched with the experimental ones.

Iron nanoparticles (FeNPs) have been successfully prepared by high-energy ball milling in air for various milling times from 1 h to 32 h. Their structure, particle size, elemental composition, magnetic, and inductive heating properties were investigated by means of x-ray diffraction (XRD) analysis, field-emission scanning electron microscopy, energy-dispersive x-ray (EDX) spectroscopy, vibrating-sample magnetometry, and magnetic induction heating, respectively. XRD analysis showed that the average crystallite size decreased to 11 nm after 10 h of milling, then remained almost unchanged for longer milling times. Coexistence of iron (Fe) and iron oxide (FeO) phases was detected after 12 h of milling. EDX analysis also confirmed the occurrence of oxidation, which can be reconciled with the corresponding decrease and increase in saturation magnetization ( M s ) with milling time when exposed to oxygen and when annealed under H 2 ambient due to oxygen reduction. The time-dependent magnetic and inductive heating responses of the FeNPs were investigated for prospective application in magnetic hyperthermia. The effect of varying the alternating-current (AC) magnetic field strength on the saturation heating temperature and specific loss power of FeNP-containing ferrofluid with concentration of 4 mg/mL was also studied and is discussed.

In this paper, we report the effect of Pb doping on the magnetic and magnetocaloric properties of La 0.7 Ca 0.3- x Pb x MnO 3 (0.15 ≤  x  ≤ 0.3) polycrystalline samples. The samples were prepared by solid state synthesis method. X-ray diffraction patterns indicate that these samples have monoclinic structure with the Pc/2 space group. The magnetization measurements indicated that all samples exhibited a paramagnetic–ferromagnetic transition and the transition temperature increased with increasing of Pb concentration. Using phenomenological model, the magnetocaloric behavior of all the samples has been calculated based on the experimental magnetization data. Our results on the magnetocaloric properties suggest that the compound La 0.7 Ca 0.15 Pb 0.15 MnO 3 is attractive as a possible material for magnetic refrigeration.

We have prepared Zn 1− x Co x Fe 2 O 4 nanoparticles (NPs) by using a hydrothermal method, and then studied their structural and magnetic properties. The analyses of x-ray diffraction (XRD) patterns and Raman scattering spectra reveal that the samples crystallized mainly in a cubic-spinel structure with the lattice parameter a ≈ 8.4 Å. Averaged crystallite sizes determined from the XRD linewidth are about 16–22 nm, close to the particle sizes of 19–28 nm determined from scanning electron microscopy images. Magnetization measurements versus temperature, M ( T ), in the field H  = 100 Oe indicate that the ferromagnetic–paramagnetic (FM-PM) phase transition temperature ( T C ) of Zn 1− x Co x Fe 2 O 4 NPs increases from 606 K for x  = 0 to ~823 K for x  = 1. The features of the M ( T ) curves also indicate magnetic inhomogeneity in the samples, and their magnetic property is unstable versus temperature. This is ascribed to the changes in the structural characterization and/or concentration of magnetic ions situated at the A and B sites in the spinel lattice. At room temperature, we found that both the saturation magnetization ( M s ) and coercivity ( H c ) increase with increasing Co content, with M s  = 59–70 emu/g and H c  = 100–1100 Oe. These results reflect that the Co doping into ZnFe 2 O 4 NPs greatly improves their magnetic property, making them more useful for practical applications. Additionally, we also assess magnetic interactions and the magnetocaloric effect in the samples based on analyzing initial magnetization data, M ( H ), recorded at temperatures around T C .