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

Abstract

Pure NiO nanoparticles were subjected to isochronal thermal treatments in open air from 100 to 1000 • C. X-ray diffraction (XRD) patterns indicate that all annealed samples exhibit a single phase of face-centered cubic (FCC) crystalline structure, obvious grain growth occurs only above 400 • C and the average crystallite size increases from 20 to 80 nm. X-ray photoelectron spectroscopy (XPS) shows that only very few amount of Ni 3+ and no impurity element have been found in the annealed samples. Positron annihilation measurements reveal that large number of Ni-vacancy defects exist in the grain surface region. These surface defects begin to recover after annealing above 400 • C, and most of them are removed at 1000 • C. Room temperature ferromagnetism is obviously observed for the samples annealed at 100 and 400 • C. The saturation magnetization gradually decreases with the increase of the annealing temperature, and it almost disappears at 800 and 1000 • C. The disappearance of ferromagnetism shows good coincidence with the recovery of Ni-vacancies. Our results suggest that the anomalous ferromagnetic behavior in NiO nanoparticles might be due to the surface Ni-vacancy defects instead of grain size effects. 2 It is well known that the bulk AF materials are magnetically compensated and have zero net magnetic moment in zero applied field, while fine particles of an AF material should display either weak ferromagnetism (FM) or superparamagnetism (SPM).3 Since Richardson and Milligan first reported anomalous magnetic properties in NiO nanoparticles, 4 ferromagnetic behaviors were found in an increasing number of AF nanomaterials. Consequently, it is important to investigate the magnetic mechanism of AF nanomaterials extensively. Among the various nanostructured AF transition metal monoxides, NiO has been widely investigated, because it is a unique metal-deficient p-type semiconductor presenting a wide band-gap (3.6-4.0eV), and its bulk form possesses a relatively high Néel temperature T N (523 K). As a consequence, it can cause high storage density of magnetic recording media due to allowing magnetic stability at very low volumes far above room temperature. 17 In the recent few decades, great efforts have been devoted to exploring the unexpected magnetic properties in NiO nanomaterials, such as large magnetic moments, enhanced coercivity, spin glass freezing and hysteretic loop shifts. However, up to now the underlying mechanism for the FM is still unclear. It has been reported that the FM might come from the surface and finite size effects, or a net magnetic moment probably arises from uncompensated spins. Several models, such as the two-sublattice model, 3 multi-sublattice model, 10 and core-shell model 9,12-14,18-24 have been used to illustrate the anomalous properties of NiO nanoparticles. However, different groups reported that * Electrochemical Society Fellow.z E-mail: czy2004hust@163.com; chenzq@whu.edu.cn; mascher@mcmaster.ca surface effects and size effects on the ferromagnetic properties of the nanomater...

Impact of Nanostructuration on the Chemical Composition of Nickel Oxide Nanoparticles

“…For this reason, we have measured the magnetic properties of the “NiO” nanoparticles. Many works have indeed reported the appearance of ferromagnetism in NiO nanoparticles. − Figure a displays the RT magnetization ( M ) of the NiO-250 and NiO-800 samples versus magnetic field. Nickel oxide grown at 800 °C with almost no nickel vacancies (largest particle size) exhibits no trace of ferromagnetism, which agrees with the commonly reported antiferromagnetic properties of bulk NiO.…”

“…Our observation clearly suggests that the presence of nickel vacancies in the vicinity of the surface is at the origin of this ferromagnetic behavior. It is therefore directly in line with previous reports that also point out the prominent role of nickel vacancies in the vicinity of the surfaces of nanoparticles in the appearance of this ferromagnetic contribution. − …”

Raman Spectral Probe on Size-Dependent Surface Optical Phonon Modes and Magnon Properties of NiO Nanoparticles

“…Moreover, the hysteresis curves of 5NiO, 7NiO, and 9NiO samples consist of both AFM and FM components and can be split in to two regions as follows: (i) the lower magnetization region, which shows a nonlinear increment of magnetization with the applied fields; and (ii) the higher magnetic field region shows the linear characteristics with the applied field. These kinds of multiphase magnetic behaviors can be explained in terms of the core–shell model. − According to this model, the AFM NiO nanoparticle has an AFM core and FM spin shells. The easily magnetized low-field region in the hysteresis loop is due to the uncompensated AFM spin in the core, and the linear magnetization component in the higher field is due to the surface spins in the shell region.…”

Synthesis of nonstoichiometric NiO nanocatalysts and growth of bamboo‐like carbon nanotubes

“…Conversely, nonstoichiometric Ni‐A displayed ferromagnetism, indicating a smaller particle size and the presence of Ni vacancies near the surface. The Ni vacancies near the surface were reported as the appearance of this ferromagnetic property 26–28 …”