Surface-spin magnetism of antiferromagnetic NiO in nanoparticle and bulk morphology

Abstract

The surface-spin magnetism of the antiferromagnetic (AFM) material NiO in nanoparticle and bulk morphology was investigated by magnetic measurements (temperature-dependent zero-field-cooled (zfc) and field-cooled (fc) dc susceptibility, ac susceptibility and zfc and fc hysteresis loops). We addressed the question of whether the multisublattice ordering of the uncompensated surface spins and the exchange bias (EB) effect are only present in the nanoparticles, originating from their high surface-to-volume ratio or if these surface phenomena are generally present in the AFM materials regardless of their bulky or nanoparticle morphology, but the effect is just too small to be detected experimentally in the bulk due to a very small surface magnetization. Performing experiments on the NiO nanoparticles of different sizes and bulk NiO grains, we show that coercivity enhancement and hysteresis loop shift in the fc experiments, considered to be the key experimental manifestations of multisublattice ordering and the EB effect, are true nanoscale phenomena only present in the nanoparticles and absent in the bulk.

Doping of Ga in antiferromagnetic semiconductor α-Cr2O3 and its effects on magnetic and electronic properties

“…Recently, antiferromagnetic (AFM) systems (NiO, CoO, Fe 2 O 3 , Cr 2 O 3 ) in nanoparticle (NP) form have received a significant amount of research interest for fundamental understanding of new magnetic phenomena, such as surface superparamagnetism, size-induced ferromagnetism, quantum confinement effect [1][2][3][4][5][6][7]. The enhanced ferromagnetism and magnetic exchange bias effect has provided the experimental evidence of modified spin order (core-shell structure) and magnetic anisotropy in AFMNP [2][3][4].…”

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.…”

“…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.…”

“…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. 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].…”

SIZE-DEPENDENT MAGNETIZATION FLUCTUATIONS IN NiO NANOPARTICLES

“…Thus, an enhancement of surface and interface effects make the antiferromagnetic nanoparticles an interesting area of research. 2,3,4,5,6,7,8,9,10,11,12,13 Nickel Oxide (NiO) has been considered as a prototype for antiferromagnetism, as it is one of the first few materials in which antiferromagnetism was studied. 14 One of the first serious concerns with NiO nanoparticle was, evidenced from the experimental study of Richardson and Milligan,15 that these nanoparticles show a large magnetic moment as the size becomes smaller than 100nm, apart from anomalous behavior of the magnetic susceptibility.…”