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 δM plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 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 magnetizations states in nanoparticles.Additional information on the δM and the in-field Mössbauer techniques, table with the complete results of the Mössbauer spectra fits, details of the Monte Carlo simulations, FC and ZFC magnetization curves of the VST series (Fig. S1a), Langevin scaling of M(H;T) data measured in VST45 (Fig. S1b), details on the estimate of the "magnetic size" from Langevin fits, δM plots of all the VST series and graphical analysis of the intraparticle and interparticle contributions to the dip (Fig. S2), example of hysteresis loops measured after ZFC and FC (for sample VST17, Fig. S3); X-ray diffraction patterns and lattice parameter across of the maghemite cores of different size (Fig. S4); complete results from Monte Carlo simulations showing the dependence of δM on core anisotropy (Fig. S5), surface anisotropy ( Fig. S6), exchange coupling constant ( Fig. S7) and disordered surface thickness (Fig. S8).

The fundamental question as to the relative importance of interparticle superexchange versus dipolar interaction between oxide magnetic particles in direct physical contact is addressed by examining the magnetic properties of a series of compacted samples comprising identical maghemite particles (8 nm in diameter) coated by nonmagnetic shells (oleic acid or silica) of varying thickness that control the distance between the magnetic cores and hence the packing density (particle volume fraction). A remarkably narrow maghemite particle size distribution is established by electron microscopy and small-angle X-ray scattering. The series includes a sample made up of bare particles in a random-close-packed configuration (therefore in direct contact) that exhibits ideal superspin-glass behavior with a relatively high freezing transition temperature. It is shown that interparticle superexchange interactions between the nanoparticles in this sample play a minor role compared to classical dipolar interactions in establishing the collective, superspin-glass state. This follows from the freezing temperature of the most concentrated samples in the series (those with 0 ≤ shell thickness < 3 nm), which are found to vary in direct proportionality with the volume fraction of the maghemite cores and therefore with the strength of dipolar interactions.

A simple single-phase material, a random close-packed (volume fraction 67%) ensemble of highly monodisperse bare maghemite (γ-Fe2O3) nanoparticles, is shown to exhibit ideal superspin-glass behavior (mimicking that of model spin-glasses), namely, an unprecedentedly sharp onset of the absorption component of the ac susceptibility, narrow memory dips in the zero-field-cooled magnetization and a spin-glass characteristic field-dependence of the magnetic susceptibility. This ideal behavior is attributed to the remarkably narrow dispersion in particle size and to the highly dense and spatially homogeneous configuration ensured by the random close-packed arrangement. This material is argued to constitute the closest nanoparticle analogue to a conventional (atomic) magnetic state found to date.

Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications in ultra-high density recording among other applications. Here we demonstrate that low blocking temperature ferromagnetic (FM) Co nanoparticles (T B <70 K) become magnetically stable above 400 K when embedded in a high Néel temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable T B enhancement is due to a magnetic proximity effect between a thin CoO shell (with low Néel temperature, T N ; and high anisotropy, K AFM ) surrounding the Co nanoparticles and the NiO matrix (with high T N but low K AFM ). This proximity effect yields an effective AFM with an apparent T N beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed T B increase. Mean-field calculations provide a semi-quantitative understanding of this magnetic-proximity stabilization mechanism. 2The current miniaturization trend in magnetic applications has led to a quest to suppress spontaneous thermal fluctuations (superparamagnetism) in ever-smaller nanostructures [1][2][3][4][5], which is a clear example of the fundamental efforts of condensed matter physics to meet technological challenges [6] (e.g., the continued growth of recording density [7]). Despite the foreseeable change of recording paradigm from continuous to patterned media, where each bit is recorded in an individual nanostructure [7], the key for sustained storage density increase will remain the introduction of progressively more anisotropic (high K) materials [8], which allow for magnetic stability at very small volumes, V (i.e., blocking temperature, T B ∝ KV, above room temperature, RT). Two main strategies are largely investigated to achieve high K (both of them with implications in other active technologies beyond information storage, such as permanent magnets, magnetic hyperthermia or even sensors [5,[9][10][11]): (i) the use of compounds with intrinsically high magnetocrystalline anisotropy (such as FePt [3,8]) and (ii) the design of exchange-coupled nanocomposites [4,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].Unfortunately, most high-K materials require high-temperature annealing processes to obtain the desired phase, which could hamper their implementation in certain structures. Thus, FM-AFM exchange coupling alternatives may be an appealing option. In fact, it has been demonstrated [4] that ferromagnetic-antiferromagnetic (FM-AFM) interfacial exchangecoupling is an effective method, later patented by Seagate [12], to increase the effective K of FM nanoparticles. However, a T B enhancement beyond RT using this approach has been rarely reported [22][23][24][25][26] (where often broad particle size distribution can partly account for the "apparent" T B increase [22][23][24][25]). The reason for this scarcity is that high Néel temperature (T N ) AFMs tend to h...

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