Among all antiferromagnetic transition metal monoxides, NiO presents the highest Néel temperature (T_N ∼ 525 K). In this work, the size-dependent reduction of T_N in NiO nanoparticles with average diameters (D) ranging from 4 to 9 nm is investigated by neutron diffraction. The scaling law followed by T_N(D) is in agreement with the Binder theory of critical phenomena in low-dimensional systems. X-ray absorption fine structure measurements link the decrease of T_N to the occurrence of size effects (average undercoordination, bond relaxation and static disorder) in the nearest and next-nearest Ni coordination shells that hold the key for the maintenance of the antiferromagnetic order.

Combining different experimental techniques, investigations in hexagonal $P{6}_3/mmc$ Er${}_2$Fe${}_{17}$ show remarkable magnetovolume anomalies below the Curie temperature, $T_C$. The spontaneous magnetostriction reaches $1.6\times {10}^{-2}$ at 5 K and falls to zero well above $T_C$, owing to short-range magnetic correlations. Moreover, Er${}_2$Fe${}_{17}$ exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy $\Delta S_M$, and adiabatic temperature $\Delta T_{\mathrm{ad}}$ changes [$\Delta S_M\sim -4.7$ J(kgK)${}^{-1}$ and $\Delta T_{\mathrm{ad}}\sim 2.5$ K near the $T_C$, and $\Delta S_M\sim 1.3$ J(kgK)${}^{-1}$ and $\Delta T_{\mathrm{ad}}\sim -0.6$ K at 40 K for $\Delta H=80$ kOe, respectively, determined from magnetization measurements]. The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er and Fe sublattice. The main trends found experimentally for the temperature dependence of $\Delta S_M$ and $\Delta T_{\mathrm{ad}}$ as well as for the atomic magnetic moments are qualitatively well described considering a mean-field Hamiltonian that incorporates both crystalline electric field and exchange interactions. $\Delta S_M\left(T\right)$ and $\Delta T_{\mathrm{ad}}\left(T\right)$ curves are essentially zero at $\sim$150 K, the temperature where the transition from direct to inverse MCE occurs. A possible interplay between the MCE and the magnetovolume anomalies is also discussed.

NiO nanoparticles (NPs) with a nominal size range of 2–10 nm, synthesized via high-temperature pyrolysis of a nickel nitrate, have been extensively investigated using neutron diffraction and magnetic (ac and dc) measurements. The magnetic behavior of the NPs changes noticeably when their diameter decreases below 4 nm. For NPs larger than or equal to this size, Rietveld analysis of the room temperature neutron diffraction patterns reveals that there is a reduction in the expected magnetic moment per ion with respect to bulk NiO, which is linked to the existence of a magnetically disordered shell at the NP surface. The presence of two peaks in the temperature dependence of both the dc magnetization after zero-field-cooling and the real part of the ac magnetic susceptibility is explained in terms of a core (antiferromagnetic, AFM)/shell (spin glass, SG) morphology. The high-temperature peak ( K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak ( K) is a signature of a SG-like freezing of the surface spins. In addition, an exchange bias (EB) effect emerges due to the core/shell magnetic coupling. The cooling field and temperature dependences of the EB effect and the coercive field are discussed in terms of the core size and the effective magnetic anisotropy of the NPs. However, NiO NPs of 2 nm in size no longer show AFM order and the magnetic moments freeze into a SG-like state below K, with no evidence of EB effect.

Iron-carbon nanocomposites have been elaborated by means of a simple chemical procedure based on in situ synthesis of iron nanoparticles within the nanopores of an activated carbon. The Fe nanoparticles present a broad particle-size distribution (5\char21{}40 nm). A combined structural and magnetic study seems to suggest that most of the nanoparticles of mean size $\sim${}15$\hskip0.3em${}nm have exotic ``onionlike'' core-shell morphology of $\gamma${}-Fe nucleus surrounded by a concentric double shell of $\alpha${}-Fe and maghemitelike oxide. The true nature of Fe-oxide was successfully evidenced through room temperature x-ray absorption spectroscopy. The whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to higher blocking temperatures for the largest nanoparticles. M\"ossbauer spectrometry indicates that low temperature para-to-antiferromagnetic transition for the $\gamma${}-Fe phase cannot be discarded. In addition, the external Fe-oxide shell exhibits spin-glass behavior giving rise to the freezing of its magnetic moments at low temperatures. Hence, we propose a competing double magnetic coupling: (i) the oxide shell/$\alpha${}-Fe interaction and (ii) the possible antiferromagnetic coupling between $\gamma${}-Fe nucleus and $\alpha${}-Fe layer; as being both responsible for the observed exchange bias effect at T=5$\hskip0.3em${}K (Hex$\approx${}150$\hskip0.3em${}Oe).