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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 structural properties of (x)CuO/(1-x)CdFe2O4 nanocomposites for x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0 were studied. The X-ray diffraction (XRD) patterns confirmed the existence of both phases with a secondary phase of α-Fe2O3. The Fourier-transform infrared spectroscopy (FTIR) characterization showed the vibrational bands of the synthesized samples. The high resolution transmission electron microscopy (HRTEM) micrographs revealed the presence of CuO, CdFe2O4, and α-Fe2O3 phases and showed the polycrystalline nature of the prepared nanocomposites. The scanning electron microscope (SEM) study exhibited the morphology of the prepared nanostructures and indicated the agglomeration between them. The energy-dispersive X-ray spectroscopy (EDX) results revealed the presence of all chemical elements within the synthesized nanocomposites. Emission peaks of the samples were analyzed via Photoluminescence (PL) deconvolutions which approved the interface defects. The Mössbauer study showed the presence of hematite impurity and the results were coherent with XRD findings. The obtained M-H loops from the vibrating-sample magnetometer (VSM) analysis depicted a paramagnetic nature of CuO and showed the ferromagnetic behavior of the nanocomposites. [ABSTRACT FROM AUTHOR]

Exchange bias (EB) effects linked to surface spin freezing (SSF) are commonly found in iron oxide nanoparticles, while signatures of SSF in low-field temperature-dependent magnetization curves have been much less frequently reported. Here, we present magnetic properties of dense assemblies of similar-sized (∼8 nm diameter) particles synthesized by a magnetite (sample S1) and a maghemite (sample S2) method, and the influence of long-term (4 year) sample aging under ambient conditions on these properties. The size of the EB field of the different sample (fresh or aged) states is found to correlate with (a) whether a low-temperature hump feature signaling the SSF transition is detected in out-of-phase ac susceptibility or zero-field-cooled (ZFC) dc magnetization recorded at low field and with (b) the prominence of irreversibility between FC and ZFC curves recorded at high field. Sample S1 displays a lower magnetization than S2, and it is in S1 where the largest SSF effects are found. These effects are significantly weakened by aging but remain larger than the SSF effects in S2, where the influence of aging is considerably smaller. A non-saturating component due to spin disorder in S1 also weakens with aging, accompanied by, we infer, an increase in the superspin and the radius of the ordered nanoparticle cores. X-ray diffraction and Mössbauer spectroscopy provide indication of maghemite-like stoichiometry in both aged samples as well as thicker disordered particle shells in aged-S1 relative to aged-S2 (crystallographically-disordered and spin-disordered according to diffraction and Mössbauer, respectively). The pronounced diminution in SSF effects with aging in S1 is attributed to a (long-term) transition, caused by ambient oxidation, from magnetite-like to maghemite-like stoichiometry, and a concomitant softening of the spin-disordered shell anisotropy. We assess the impact of this anisotropy on the nature of the blocking of the nanoparticle superspins. [ABSTRACT FROM AUTHOR]

The nucleation and growth mechanism of exchange-biased magnetically contrasted highly crystallized and perfectly epitaxied Fe3O4–CoO nanoaggregates was investigated using local probe analysis, namely 57Fe Mössbauer and L2,3 Co XMCD spectroscopies, and advanced magnetometry. The granular nanocomposites were produced by a two-step chemical route. Spinel iron oxide nanoparticles were first synthesised by forced hydrolysis of iron acetate in a polyol and then used as seeds for rock-salt cobalt monoxide nanocrystalline growth. In one case, this crystallochemical step is carried out by separating the seeds, washing and drying them before mixing them in a fresh polyol solution of cobalt precursors. In another case, it is conducted by introducing, directly in the seed reaction solution, the cobalt precursors in the same Fe/Co nominal atomic ratio. It appears that the composite formation proceeds differently in the two cases. It is driven, in the ex situ route, by Co2+ cation diffusion into the spinel lattice leading to the formation of CoxFe3−xO4 core single crystals around which CoO nanocrystals start growing. Conversely, in the in situ route, it is driven by Co2+ cation reactivity with unreacted dissolved Fe3+, conducted to the coating of the preformed Fe3O4 cores by a CoFe2O4-like thin interlayer, itself decorated by outer CoO nanocrystals. These reaction schemes are well confirmed by magnetometry experiments. The low-temperature magnetic properties of the produced composites are consistent with the establishment of an exchange-bias feature into hard-magnet@antiferromagnet and soft-magnet@hard-magnet@antiferromagnet structures, respectively. [ABSTRACT FROM AUTHOR]

The present work focuses on the study of exchange-biased magnetic core/shell nanoparticles at the atomic scale. The nanoparticles (NPs) consist of a perfectly epitaxial crystalline cobalt oxide (CoO) shell on a magnetite (Fe3O4) core. The numerical core/shell is built by taken into account the spinel structure of the core (Fe3O4) and the face-centered cubic structure of the shell (CoO). Two different configurations of core/shell NPs were examined: homogeneous and inhomogeneous core/shell. Our magnetic simulations are based on a 3D classical Heisenberg model. Monte Carlo simulations performing single spin rotation are used to investigate the effect of exchange bias on the spin configurations and hysteresis loops of core/shell nanoparticles. The numerical results reveal, as expected, a significant hysteresis loop shift obtained for a weak interface coupling. In addition, the magnetization reversal is not perfectly uniform in space when the interfacial coupling is different from zero. Finally, the increase in the magnetic interfacial coupling produces an increase in coercive field and a decrease in the exchange bias field. [ABSTRACT FROM AUTHOR]