Interplay between microstructure and magnetism in NiO nanoparticles: breakdown of the antiferromagnetic order

Abstract

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...

Size effects on the Néel temperature of antiferromagnetic NiO nanoparticles

“…1 One of the hallmarks of nanomaterials is that their macroscopic physicochemical properties are strongly affected by size reduction, giving rise to a drastic modification of the magnetic behavior compared to their bulk counterparts'. 2,3 As a matter of fact, the magnetic configuration of atomic spins changes progressively as moving from the core, which usually presents a spin arrangement similar to that of the bulk material, to the surface, which exhibits a much higher magnetic disorder, leading to a plethora of glassy magnetic behaviors. The competition between surface and volume magnetic orders has been identified as a cause of high-field irreversibilities, high saturation fields, large coercivities and exchange bias effect.…”

Vacancy-Induced Ferromagnetic Behavior in Antiferromagnetic NiO Nanoparticles: A Positron Annihilation Study

“…[5][6][7][8] However, the mechanism of these different magnetic properties remains debatable. Many mechanistic views have been proposed for the observed FM, such as surface and finite size effects, [9][10][11][12][13][14] chemical valence change, 15,16 superparamagnetic clusters, 16 or vacancy relevant uncompensated surface spins. 7 Consequently, it is important to investigate the magnetic mechanism of AF nanomaterials extensively.…”

Antiferromagnetic structures and electronic energy levels at reconstructed NiO(111) surfaces: ADFT+Ustudy

“…They show that uncompensated ferromagnetic (FM) or a spin-glass like surface shell is exchange coupled to an AFM core. Such a breakdown of the AFM order and the corresponding core-shell magnetic structure have been interpreted and modeled as a finite-size effect of nanoparticles [9][10][11][12][13][14]. At the same time, our atomistic calculations here show that surface reconstruction and hydroxylation/hydrogenation play an equally important role in determining the magnetic properties at the NiO(111) surface.…”

“…Indeed, a break down of the antiferromagnetic order has been suggested near the top layers for some structures, where an uncompensated FM or a spin-glass like surface shell is exchange coupled to an AFM core [15]. In addition, surface morphology was proposed to control magnetic properties such as disappearance of the AFM core and the variation of the EB field and coercivity in small NiO nanoparticles [14]. However, existing studies on NiO nanoparticles mainly focus on size effects on the magnetism of the system.…”

“…In their study, numerical modeling of spin configurations in NiO nanoparticles suggested a new finite-size effect, where the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the particle, by creating multisublattic (eight-, six-, or four-sublattice) spin ordering. Recently, several studies [9][10][11][12][13][14] have shown that NiO nanoparticles would be formed by a spin-glass (SG) like shell strongly coupled to an AFM core. Such a unique magnetic structure leads to the exchange bias (EB) phenomenon [12,15], which can be attributed to the enhanced coercivity and loop shift.…”