Bridging exchange bias effect in NiO and Ni(core)@NiO(shell) nanoparticles

Rinaldi-Montes, Natalia; Gorría, P.; Martínez-Blanco, David; Fuertes, Antonio B.; Barquı́n, L. Fernández; Puente‐Orench, Inés; Blanco, J.A.

doi:10.1016/j.jmmm.2015.07.060

Cited by 19 publications

(11 citation statements)

References 25 publications

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“…This shift can be caused by the exchange bias effect due to the magnetic exchange interaction between ferromagnetic and antiferromagnetic phases. It was reported that Ni(core)@NiO(shell) nanoparticles indicated the exchange bias effect because Ni is ferromagnetic and NiO is antiferromagnetic and it became more pronounced as the particle size decreased [32,33]. The present result also indicates that the horizontal shift becomes more pronounced as the particle size decreases with increase in the TOP/Ni ratio.…”

Section: Resultssupporting

confidence: 68%

“…The present result also indicates that the horizontal shift becomes more pronounced as the particle size decreases with increase in the TOP/Ni ratio. Although the XRD spectra of the obtained Ni nanoparticles do not exhibit peaks corresponding to NiO, thin NiO layers can be formed naturally on Ni surfaces in an air atmosphere and may have incomplete crystal structures or disorder which are not easy to be identified by XRD [32,33]. In the Discussion section, the surface analysis of the Ni nanoparticles is presented.…”

Section: Resultsmentioning

confidence: 99%

“…The supposed model of the Ni nanoparticles capped by TOPO is shown in Figure 10. Additionally, since the interior Ni atoms are ferromagnetic and surface-oxidized Ni atoms are antiferromagnetic, the exchange bias effect occurs in the magnetization curves [32,33]. …”

Section: Discussionmentioning

confidence: 99%

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Effect of Particle Size on the Magnetic Properties of Ni Nanoparticles Synthesized with Trioctylphosphine as the Capping Agent

2016

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Magnetic cores of passive components are required to have low hysteresis loss, which is dependent on the coercive force. Since it is well known that the coercive force becomes zero at the superparamagnetic regime below a certain critical size, we attempted to synthesize Ni nanoparticles in a size-controlled fashion and investigated the effect of particle size on the magnetic properties. Ni nanoparticles were synthesized by the reduction of Ni acetylacetonate in oleylamine at 220 °C with trioctylphosphine (TOP) as the capping agent. An increase in the TOP/Ni ratio resulted in the size decrease. We succeeded in synthesizing superparamagnetic Ni nanoparticles with almost zero coercive force at particle size below 20 nm by the TOP/Ni ratio of 0.8. However, the saturation magnetization values became smaller with decrease in the size. The saturation magnetizations of the Ni nanoparticles without capping layers were calculated based on the assumption that the interior atoms of the nanoparticles were magnetic, whereas the surface-oxidized atoms were non-magnetic. The measured and calculated saturation magnetization values decreased in approximately the same fashion as the TOP/Ni ratio increased, indicating that the decrease could be mainly attributed to increases in the amounts of capping layer and oxidized surface atoms.

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Section: Resultssupporting

confidence: 68%

Section: Resultsmentioning

confidence: 99%

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Effect of Particle Size on the Magnetic Properties of Ni Nanoparticles Synthesized with Trioctylphosphine as the Capping Agent

2016

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“…26 As for NiO, Rinaldi-Montes et al [12][13][14] reported on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. In our study, however, the RT FM of NiO nanoparticles decreases with an increase of grain size, possibly due to the decrease of vacancy defect concentrations at the surfaces of nanoparticles, which is supported by our positron annihilation measurements.…”

Section: Resultsmentioning

confidence: 99%

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

Chen¹,

Chen²,

Zhang³

et al. 2017

ECS J. Solid State Sci. Technol.

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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...

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“…The impact of size reduction on the magnetic properties of solid MNPs, caused by the large contribution of a disordered surface spin layer, has been widely studied for different materials, such as CoFe 2 O 4 MNPs of 4.5 nm (H EB = 735 Oe) [11] or NiO MNPs of 4 nm (H EB = 900 Oe) [12], and, more intensely, in γ-Fe 2 O 3 MNPs [13][14][15][16]. EB observed in solid MNPs is attributed to the exchange coupling between disordered surface spins that may become frozen in a spin-glass-like state and inner-ordered spins [5].…”

Section: Introductionmentioning

confidence: 99%

Significant Surface Spin Effects and Exchange Bias in Iron Oxide-Based Hollow Magnetic Nanoparticles

et al. 2022

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Exchange bias (EB) properties have become especially important in hollow magnetic nanoparticles (MNPs) due to the versatility and reduced size of these materials. In this work, we present the synthesis and study of the EB properties of iron-oxide-based hollow MNPs and their precursors Fe/iron oxide MNPs with core/void/shell structure. The two mechanisms involved in EB generation were investigated: the frozen spins present in the nanograins that form the nanoparticles and the surface spins. The effect of external parameters on the coercivity (HC), remanence (MR), exchange bias field (HEB) and frozen spins, such as cooling field (HFC) and temperature, was investigated. Both HC and HEB present a maximum threshold above which their values begin to decrease with HFC, showing a new trend of HEB with HFC and allowing modulation on demand. The existence of surface spins, present on the outer and inner surfaces, was demonstrated, and an intrinsic EB phenomenon (HEB = 444 Oe for hollow iron oxide-based MNPs of 13.1 nm) with significant magnetization (MS~50 emu/g) was obtained. Finally, core/void/shell MNPs of 11.9 nm prior to the formation of the hollow MNPs showed a similar behavior, with non-negligible HEB, highlighting the importance of surface spins in EB generation.

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Bridging exchange bias effect in NiO and Ni(core)@NiO(shell) nanoparticles

Cited by 19 publications

References 25 publications

Effect of Particle Size on the Magnetic Properties of Ni Nanoparticles Synthesized with Trioctylphosphine as the Capping Agent

Effect of Particle Size on the Magnetic Properties of Ni Nanoparticles Synthesized with Trioctylphosphine as the Capping Agent

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

Significant Surface Spin Effects and Exchange Bias in Iron Oxide-Based Hollow Magnetic Nanoparticles

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