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Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the M plots of γ-Fe2O3 nanoparticles isolated by thick SiO2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles.

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]

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]

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]

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]