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. Ni 2+ 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). IntroductionThe steadily increasing interest in magnetic nanoparticles (NPs) over the past decade has been motivated by the unusual size-dependent magnetic behaviours and by their numerous applications in modern nano-electronics, magnetic separation or biomedical areas. [1][2][3][4][5][6][7] The growing research activity in this field has been propelled by the development of new chemical routes that allowed the synthesis of NPs with tuneable size distributions and being embedded in different insulating matrices. [8][9][10][11][12][13][14][15][16] It is worth noting that NPs of the same material and similar size but synthetized using different fabrication routes show a strong dependence of the magnetic properties on the morphology, microstructure or the nature of the matrix. 1,8,10,13 Moreover, 3d metal oxide NPs with different core-shell morphologies are also good candidates for a number of applications due to the possibility of tuning the magnetic response and/or the coating with a functional layer. 2,4,[13][14][15] Therefore, a comprehensive study combining advanced structural characterization techniques and meticulous magnetic measurements is needed to elucidate the microstructure-magnetism interplay at the nanoscale.Nickel oxide (NiO) has been under extensive research for decades due to its importance in numerous technological applications (i.e., catalysis, batteries, ceramics, etc.). Nowadays, nanosized NiO particles have generated a renewed interest because the combination of their unique properties (i.e., high surface area, short diffusional paths, exceptional magnetic properties, etc.) opens an avenue for their use in fields as diverse as catalysis, [17][18][19] anodic electrochromism, 20 capacitors, 21 smart windows, 22 fuel and solar cells 23,24 or biosensors. 25 Specially intense research effor...

Both Cu and Fe metals, with face-centered cubic (fcc) crystal structures, are nonmagnetic. However, the substitution of Cu atoms by Fe in the fcc-Cu lattice leads to the formation of a random solid solution and the appearance of ferromagnetic order, with a value of the magnetic moment per Fe atom in a fcc environment even above 2 B . This striking behavior is closely related to magnetovolume effects (Invar), which we have detected by means of lattice thermal expansion and magnetization measurements in FeCu alloys.

The microstructure of ball milled Fe powder as a function of the milling time has been investigated using room temperature x-ray powder diffraction and transmission electron microscopy. The powder microstructure changes when the milling time increases in a twofold way: (i) a reduction of the crystalline grain size to around 20 nm after 80 h of milling time and (ii) a significant amount of microstrain is induced (up to ∼0.75%), together with a slight increase of the crystalline lattice parameter. Moreover, the temperature dependence of the microstructure has been studied by means of in situ neutron powder thermo-diffraction in the range between 300 and 1220 K for the sample milled for 80 h. The heating of the nanostructured powder produces a progressive grain growth starting at around 450 K, and the disappearance of the microstrain above 850 K due to relaxation processes induced by thermally activated atomic diffusion. The kinetics of both processes at two different heating rates of 1 and 10 K min−1 has been compared. A detailed analysis of the diffraction patterns has been performed using the Rietveld method. All this microstructural information can be correlated with the temperature dependence of the magnetization of nanostructured Fe and the differences found with regard to the case of bulk Fe.

No abstract

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 [Formula: see text] 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 ([Formula: see text] K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak ([Formula: see text] K) is a signature of a SG-like freezing of the surface [Formula: see text] 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 [Formula: see text] magnetic moments freeze into a SG-like state below [Formula: see text] K, with no evidence of EB effect.