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A set of ∼9 nm CoFe2O4 nanoparticles substituted with Zn2+ and Ni2+ was prepared by thermal decomposition of metallic acetylacetonate precursors to correlate the effects of replacement of Co2+ with the resulting magnetic properties. Due to the distinct selectivity of these cations for the spinel ferrite crystal sites, we show that it is possible to tailor the magnetic anisotropy, saturation magnetization, and interparticle interactions of the nanoparticles during the synthesis stage. This approach unlocks new possibilities for enhancing the performance of spinel ferrite nanoparticles in specific applications. Particularly, our study shows that the replacement of Co2+ by 48% of Zn2+ ions led to an increase in saturation magnetization of approximately 40% from ∼103 A m2 kg−1 to ∼143 A m2 kg−1, whereas the addition of Ni2+ at a similar percentage led to an ∼30% decrease in saturation magnetization to 68–72 A m2 kg−1. The results of calculations based on the two-sublattice Néel model of magnetization match the experimental findings, demonstrating the model's effectiveness in the strategic design of spinel ferrite nanoparticles with targeted magnetic properties through doping/inversion degree engineering. [ABSTRACT FROM AUTHOR]

The characterization of (Mg–Ni–Co) nanoferrites doped with small different concentrations of gadolinium rare earth metal (Gd3+), prepared by the chemical co-precipitation method, is reported in this study. The XRD results reveal that the crystallite sizes, which are close to the particle size detected from TEM analysis, decrease upon adding Gd3+. Furthermore, the lattice constant a increases from 8.351 to 8.367 Å as the Gd3+ concentration (x) increases from 0.00 to 0.08. HRTEM images reveal the presence of different grains with different lattice plane orientations. Furthermore, the appearance of concentric circles in the SAED patterns verified the samples' polycrystalline nature. The presence of the active modes of the spinel structure was confirmed by the Raman analysis with a slight shift upon increasing the Gd3+ concentrations. Moreover, the photoluminescence (PL) analysis revealed the presence of a sharp peak in the UV region and structural defects in the prepared samples. Mössbauer spectroscopy and vibrating sample magnetometer (VSM) were used to examine the magnetic properties of the prepared samples. From the Mössbauer analysis, it was suggested that Gd3+ ions occupy the octahedral site. In addition, the saturation magnetization (Ms) and magnetocrystalline anisotropy (K) decreased from 31.87 to 15.76 emu/g and from 0.0057 to 0.0014 J/m3 with the increase of Gd3+ concentration from 0.00 to 0.08, respectively. Finally, different forms of the law of approach to saturation (LAS) for ferromagnetic materials were used for the fitting of M-H hysteresis loops at high applied fields. The obtained results show that the LAS model, mainly M = M s 1 - b H 2 + χ H , can effectively describe the doping of Gd3+ for all concentrations. This model exhibits well-fitting with high R2 values and provides relevant values of the magnetic parameters. [ABSTRACT FROM AUTHOR]

A set of non-stoichiometric Zn–Co-ferrite nanoparticles (NPs) was prepared by thermal decomposition of metallic complexes, in the presence of oleic acid, and, after a ligand-exchange process, was coated by a hydrophilic surfactant: these NPs were used as seeds in a sol–gel self-combustion synthesis to prepare nanocomposites (NCs) with a fixed weight ratio. Our focus here is the development of an efficient synthetic approach to control the magnetic coupling between a hard-magnetic matrix (Sr-ferrite) and NPs. The physico-chemical synthetic conditions (temperature, pH, colloidal stability) were optimized in order to tune their effect on the final particles' agglomeration in the matrix. We demonstrate that our synthetic approach is a novel way to produce strongly magnetically coupled NCs, where the final extrinsic properties could be tuned by controlling (i) the agglomeration of seeds in the matrix and (ii) their elemental doping. [ABSTRACT FROM AUTHOR]

This investigation reports the synthesis and characterization of (1-x)Ni0.5Zn0.5Fe2O4/(x)Zn0.95Co0.05O nanocomposites, with 0.0 ≤ × ≤ 0.5. Fourier transform infrared (FTIR) and Raman spectroscopies confirmed the purity of the samples and the presence of bands corresponding to octahedral and tetrahedral iron occupancies for Ni0.5Zn0.5Fe2O4 nanoparticles. A shift in peak positions of these bands was detected upon the addition of Zn0.95Co0.05O nanoparticles. The magnetic properties of the nanocomposites were examined using Mössbauer spectrometry at both room temperature and 77 K. Room temperature analysis showed the existence of both ferromagnetic and superparamagnetic behaviors, while at 77 K, all nanocomposites showed ferromagnetic behavior. The adsorption performance of the nanocomposite on the removal of malachite green (MG) dye solution was investigated by varying the contact time, adsorbent concentration, and reaction temperature. The adsorption reaction followed the second-order kinetics and the sample with x = 0.3 showed the highest adsorption rate. The adsorption rate showed an increase with the increase in the reaction temperature. The adsorption isotherm was determined by applying different adsorption isotherms (Langmuir, Freundlich, and Temkin isotherms), and the results are well-fitted with the Langmuir theoretical model. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets. [ABSTRACT FROM AUTHOR]

Synthesis atmosphere (i.e., air and nitrogen) effects on the physical properties and formation mechanism of spinel iron oxide nanoparticles prepared via the co-precipitation method have been investigated using a multi-technique approach. The obtained magnetic nanoparticles (MNPs) were characterized using the X-ray diffraction, transmission electron microscopy (TEM), SQUID magnetometry, Mössbauer spectroscopy and X-ray absorption near-edge Structure spectroscopy techniques. The synthesis procedure leads to the formation of a spinel structure with an average crystallite size of 9.0(9) nm. The morphology of the particles synthetized under an inert atmosphere was quasi-spherical, while the nanoparticles prepared in air present a faceted shape. The small differences observed in morphological properties are explained by the influence of the reaction atmosphere on the formation mechanism of the MNPs. The magnetic characterization indicates that both samples exhibit superparamagnetic behavior at 300 K. The investigation by means of the Langevin approach at 300 K also leads to equal values for the mean size of the magnetic cores (Dm). Additionally, the analysis of the Mössbauer spectra revealed the lack of spin disorder for both samples, resulting in a high saturation magnetization. The fit of XANES spectrum suggests that about 2/3 of the iron ions reside in a local environment close to that of γ-Fe2O3 and about 1/3 close to that of Fe3O4 for the sample synthetized in inert atmosphere. [ABSTRACT FROM AUTHOR]

The exploitation of the exchange coupling between hard and soft magnetic materials has been proposed for enhancing the magnetic performances of rare-earth free permanent magnets, with the aim of extending their use to all applications where moderate energy product (35–100 kJ m−3) is required. Strontium hexaferrite (SFO)/spinel ferrite composites seem particularly promising to achieve this target, although the conditions to maximize the effect while using techniques easily scalable to industrial production have not yet been identified. Within this framework, the optimization of the structural, chemical, and magnetic properties of the two moieties before the coupling procedure is crucial to enhance the energy product of the final composite. Here we report the syntheses of both nanometric SFO with high coercivity (ca. 525 kA m−1) and quasi-bulk saturation magnetization (68 Am2 kg−1) and a series of nanosized zinc-doped ferrite (ZnxFe3−xO4, 0.0 ⩽ x ⩽ 0.4) through cheap, easily scalable and eco-friendly approaches. The structural and chemical stability of the two magnetic phases as a function of temperature were investigated up to 1100 °C, with the aim of finding the best compromise between preservation of the nanometric scale and magnetic properties. A very high-magnetization (106 Am2 kg−1) ferrite was obtained by annealing Zn0.3Fe2.7O4 nanopowder at the highest investigated temperature. A preliminary attempt at coupling the two phases, starting from a mixture of the nanopowders, was performed through a classic annealing process in the temperature range 500 °C–1100 °C. The adopted procedure allowed for obtaining an exchange coupled composite at 1100 °C where the two phases are intimately and homogeneously mixed, with micrometric (0.3–5 μm) and nanometric (up to 50 nm) spinel ferrite particles. Despite these promising results, no enhancement of the energy product was found, highlighting the need for further experimental efforts to improve the coupling procedure. [ABSTRACT FROM AUTHOR]

Conventional methods for sintering magnetic biased granular solids are generally known to induce severe grain growth and coarsening, thus making any evidence of exchange bias (EB) hard to detect in the resulting consolidates. This work explores the possibility of preparing a hetero-nanoconsolidate starting from an exchange-biased nanopowder and using Spark Plasma Sintering (SPS), in order to overcome that microstructural limitation. It also investigates the capability of the resulting solid to establish EB. The nanopowder consists of an intimate nanocrystalline mixture of the antiferromagnetic NiO and ferrimagnetic CoFe2O4 phases. It exhibits a significant broadening of the hysteresis loop at room temperature when measured under field cooling (FC) conditions. X-ray diffraction coupled to high resolution transmission electron microscopy and 57Fe Mössbauer spectrometry shows that sintering induces a drastic atomic diffusion leading to the replacement of the starting NiO and CoFe2O4 phases by their Ni1−xCoxO and Co1−xNixFe2O4 solid solutions (x value close to 0.25). As a result, exchange-bias is hindered at 300 K but always expressed at lower temperatures, far below the Néel temperature of the new antiferromagnet. The maximum EB value for the ceramics is measured at 5 K and is calculated about 48 mT under a cooling magnetic field of 7 T. This value decreases when the temperature increases, but it remains different from zero up to 200 K. [ABSTRACT FROM AUTHOR]

Abstract: Magnetically contrasted granular hetero‐nanostructures are prepared by seed‐mediated growth in polyol, properly combining two oxide phases with different magnetic order, ferrimagnetic (F) partially oxidized magnetite Fe3−xO4 and antiferromagnetic (AF) cobalt oxide. Spinel Fe3−xO4 nanoparticles are first synthesized and then used as seeds for rock salt CoO nanocrystals growth. Three different hetero‐nanostructure designs are realized, acting on the content ratio between the seeds and the deposit's precursors during the synthesis. For all of them, the spinel and the rock salt phases are confirmed by X‐ray diffraction and high‐resolution transmission electron microscopy. Both phases are obtained in high‐crystalline quality with a net epitaxial relationship between the two crystallographic lattices. Mössbauer spectrometry confirms the cobalt cation diffusion into the spinel seeds, giving favorable chemical interfacing with the rock salt deposit, thus prevailing its heterogeneous nucleation and consequently offering the best condition for exchange‐bias (EB) onset. Magnetic measurements confirm EB features. The overall magnetic properties are found to be a complex interplay between dipolar interactions, exchange anisotropy at the F/AF interface, and magnetocrystalline anisotropy enhancement in the F phase, due to Co2+ diffusion into iron oxide's crystalline lattice. These results underline the powerfulness of colloidal chemistry for functional granular hetero‐nanostructured material processing. [ABSTRACT FROM AUTHOR]

We report the synthesis of yttrium iron garnet ( YIG) combining soft chemistry route, namely the polyol process, and spark plasma sintering ( SPS) technique. The polyol process produced an intermediary amorphous phase containing both iron and yttrium cations in the desired ratio. They were annealed at 400°C in air to decompose the organic content of the reaction (polyol and acetate). To achieve the garnet phase, the polyol-obtained precursor was subjected to reactive SPS treatment at a temperature of 750°C, far below the typical temperatures (1350°C) used in the classic solid-state reaction process. In 15 min pure and high-density Y3 Fe5 O12 ceramic, with about 100 nm sized crystalline grains, was obtained. We report as well the characterization of the initial amorphous phase and the obtained YIG by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, 57 Fe Mössbauer spectrometry, and magnetization measurements. [ABSTRACT FROM AUTHOR]

The effectiveness of amine-borane as reducing agent for the synthesis of iron nanoparticles has been investigated. Large (2-4 nm) Fe nanoparticles were obtained from [Fe{N(SiMe3)2}2]. Inclusion of boron in the nanoparticles is clearly evidenced by extended X-ray absorption fine structure spectroscopy and Mössbauer spectrometry. Furthermore, the reactivity of amine-borane and amino-borane complexes in the presence of pure Fe nanoparticles has been investigated. Dihydrogen evolution was observed in both cases, which suggests the potential of Fe nanoparticles to promote the release of dihydrogen from amine-borane and amino-borane moieties. [ABSTRACT FROM AUTHOR]

Cobalt ferrite ferrimagnetic nanoparticles (NPs) are prepared and used in this work as seeds to grow a thin antiferromagnetic poly‐ and nanocrystalline CoO shell. The major purpose is to study systematically the characteristics of the as‐produced powders, making emphasis on their internal crystallographic arrangement and their magnetic properties. 57Fe Mössbauer spectrometry evidences an evolution of the cation distribution among the spinel lattice in the cobalt ferrite core during the core–shell NPs processing. High‐resolution transmission electron microscopy shows a perfect epitaxy between the face‐centered cubic lattices of the spinel core and the rock‐salt shell. Finally, the measurements of 7T‐field‐cooled magnetic hysteresis loops at low temperature (5 K) of the composite particles exhibit a strong exchange bias coupling with an exchange field, µ0HE, of 365 mT and an enhanced coercive field, µ0HC, of 1395 mT. These values are very high compared to those of differently prepared CoO‐based oxide composite NPs and for which a giant exchange‐bias is reported. These features are attributed to the favorable material processing conditions offered by the polyol process in terms of crystalline quality, particularly at the interfaces, and for the pinning action exerted by the CoO phase on the magnetization of the CoFe2O4 phase. Oxide‐based core–shell like nanoparticles are produced by the polyol process. Their advanced structural and microstructural characterization highlighted their very exceptional crystallographic quality, making them able to exhibit giant EB feature. [ABSTRACT FROM AUTHOR]

Hard-soft nanocomposites of (1 - x ) BaFe 12 O 19 / x (Zn 0.5 Co 0.5 )Fe 2 O 4 , for x = 0.00, 0.25, 0.50, 0.75 and 1.00, were prepared via co-precipitation and high-speed ball milling techniques, respectively. The synthesized samples were characterized via x-ray diffraction, transmission electron microscope, Fourier transform infrared (FTIR), and vibrating sample magnetometer. XRD revealed the formation of hard-soft nanocomposites. TEM indicated that the two phases are well distributed and the particle size distribution is narrower for low content of soft phase, leading to better exchange coupling between the grains. Magnetic measurements were performed at 300 K and 77 K. The results showed a good single-phase magnetic behavior, verifying the good exchange coupling between hard and soft phases. For low (Zn 0.5 Co 0.5 )Fe 2 O 4 content, the dipolar interactions were dominated by the exchange-coupling interactions. Additionally, the optimum values of saturation and remanent magnetizations, coercivity, and squareness ratio were obtained for x = 0.5. This was attributed to the dominance of exchange-coupling interaction. The enhancement of magnetic properties and energy product ( BH ) max for nanocomposites at low temperature is skilled in the reduction of the thermal fluxes of magnetic moments at the surface. The maximum energy product ( BH ) max was observed in C2 at both temperatures with a smaller value than that of pure BaFe 12 O 19 ., (© 2021 IOP Publishing Ltd.)

Core-shell magnetic nanoparticles type Fe 3- x O₄ (≈10 nm)@CoO (≈3 nm) prepared by forced hydrolysis in polyol medium have been investigated through the combined use of dc magnetization measurements by SQUID and local analysis by 57 Fe Mössbauer spectrometry. A shift of the hysteresis loop along the field axis was observed, which highlights the presence of exchange bias coupling in these nanoparticles. This exchange bias coupling is accompanied by an unexpected high value of coercive field in hysteresis loop in mode ZFC. A local probe study using both zero-field and in-field Mössbauer spectrometry reveals complex hyperfine structures. Great attention is paid to the fitting procedure: it is concluded that Fe 3- x O₄@CoO nanoparticles result from a mixture of magnetite and maghemite phases in addition to the formation of an interface-like layer close to a cobalt ferrite. Such a structure explains both the high coercive field value observed by SQUID and the presence of exchange bias coupling.

The effectiveness of amine-borane as reducing agent for the synthesis of iron nanoparticles has been investigated. Large (2-4 nm) Fe nanoparticles were obtained from [Fe{N(SiMe3)2}2]. Inclusion of boron in the nanoparticles is clearly evidenced by extended X-ray absorption fine structure spectroscopy and Mössbauer spectrometry. Furthermore, the reactivity of amine-borane and amino-borane complexes in the presence of pure Fe nanoparticles has been investigated. Dihydrogen evolution was observed in both cases, which suggests the potential of Fe nanoparticles to promote the release of dihydrogen from amine-borane and amino-borane moieties., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Today the magnetic properties of multilayers and nanostructures including a metal or an insulator as a nonmagnetic spacer layer are rather well understood. But they are much more controversial for semiconductor spacers. For instance, for Co/Si multilayers short period coupling oscillations are predicted by ab initio computations but have yet to be observed. Here we show in Co/Si multilayers prepared at low temperature (90 K) strong saturation field oscillations that are consistent with the predicted coupling oscillations. However, the decay length of the oscillations is much longer than the expected one and cannot be explained within the framework of available theories.