The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened up the path for extensive research activity in the nanoscale world. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. We combined X-ray diffraction, high-resolution transmission electron microscopy, extended X-ray absorption fine structure and magnetic measurements in order to describe the influence of the microstructure and morphology on the magnetic behaviour of NiO nanoparticles (NPs) with sizes ranging from 2.5 to 9 nm. The present findings reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order, typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni²⁺ magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, whereas larger NPs are formed by an uncompensated AFM core with a net magnetic moment surrounded by a SG shell. The coupling at the core–shell interface leads to an exchange bias effect manifested at low temperature as horizontal shifts of the hysteresis loop (∼1 kOe) and a coercivity enhancement (∼0.2 kOe).

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.

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.

Ni(core)@NiO(shell) nanoparticles (NPs) were synthesized through the pyrolysis of an inorganic precursor taking place within the pores of an active carbon matrix at different temperatures between 673 and 1173 K, and a subsequent oxidation in air. For the lowest temperature (673 K), the smallest average size of the NPs (9 nm) and the largest percentage of NiO (82%) are observed. Upon increasing the temperature up to 1173 K, an average diameter of 23 nm is observed while the NiO percentage decreases below 20%. We found that each NP consists of a Ni core surrounded by a structurally disordered NiO shell with a constant thickness of ∼2 nm, regardless of the core size. The spins inside the NiO shell freeze into a spin glass (SG)-like state below T_f ∼ 40 K. The magnetic exchange coupling between the Ni core and the NiO shell spins gives rise to the occurrence of the exchange bias (EB) effect, whose temperature dependence follows a universal exponential trend in all samples. The SG nature of the shell spins yields a vanishing EB above T_f. This is far below the Néel temperature of bulk antiferromagnetic NiO (T_N ∼ 523 K) that usually determines the onset of the EB effect in Ni/NiO interfaces.

The intentional oxidation of metallic Co NPs with different sizes (3–50 nm) gives rise to a wide variety of core/shell morphologies including Co, CoO and Co₃O₄ phases and diverse magnetic behaviours.