Unravelling the onset of the exchange bias effect in Ni(core)@NiO(shell) nanoparticles embedded in a mesoporous carbon matrix

Abstract

The onset of the exchange bias effect in air-oxidized Ni@NiO nanoparticles is mainly driven by the properties of the magnetically disordered NiO shell, and less dependent on the size of the metallic Ni core.

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

“…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.…”

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

“…In both systems, EB arises as a consequence of the exchange coupling at the interface between two magnetic phases, each one of them having a However, it cannot be discerned because the magnetization arising from the uncompensated spins at the surface of the Ni@NiO NPs is much smaller than the magnetization from the Ni core spins. The above features suggest that the drastic fall of the onset temperature for the exchange bias effect in Ni@NiO NPs is caused by the NiO shell spins being in a SG-like rather than in an AFM state, and having a freezing temperature one order of magnitude below the Néel temperature of bulk NiO [10].…”

“…The percentage of NiO increases drastically when reducing the NP size, due to the higher surface to volume ratio. Considering D TEM as the diameter of the entire NP, the thickness of the NiO shell can be calculated by geometric arguments [10], providing d NiO $ 2 nm, in good agreement with XRD and HRTEM results. From the above structural characterization, the Ni(core)@NiO(shell) morphology seems to be confirmed for samples Ni-9, Ni-11 and Ni-23 (Fig.…”

“…One sample of NiO NPs (NiO-4) and three samples of Ni@NiO NPs (Ni-9, Ni-11, Ni-23) were prepared via high-temperature pyrolysis of a Ni-nitrate taking place within the restricted volume formed by the porosity of an activated porous carbon matrix, followed by oxidation in air at room temperature [10,14]. The composition of the NPs (NiO or Ni@NiO) was controlled by means of the surrounding gas during the synthesis process (air or N 2 , respectively).…”

“…In particular, among all bi-magnetic NPs consisting of a transition metal core enveloped by a native transition metal oxide shell, Ni@NiO ones exhibit an onset temperature for the EB effect that is reduced up to one order of magnitude with respect to the Néel temperature of bulk AFM NiO (T N $523 K) [9][10][11][12][13]. In addition, EB effect has also been observed in nominally AFM NiO NPs and explained according to a model where uncompensated magnetic Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm moments located on the NP surface freeze in a spin glass (SG)-like state and get exchange coupled to the AFM core [14][15][16][17].…”