Tunability of exchange bias in Ni@NiO core-shell nanoparticles obtained by sequential layer deposition

Abstract

Films of magnetic Ni@NiO core-shell nanoparticles (NPs, core diameter d ≅ 12 nm, nominal shell thickness variable between 0 and 6.5 nm) obtained with sequential layer deposition were investigated, to gain insight into the relationships between shell thickness/morphology, core-shell interface, and magnetic properties. Different values of NiO shell thickness t(s) could be obtained while keeping the Ni core size fixed, at variance with conventional oxidation procedures where the oxide shell is grown at the expense of the core. Chemical composition, morphology of the as-produced samples and structural features of the Ni/NiO interface were investigated with x-ray photoelectron spectroscopy and microscopy (scanning electron microscopy, transmission electron microscopy) techniques, and related with results from magnetic measurements obtained with a superconducting quantum interference device. The effect of the shell thickness on the magnetic properties could be studied. The exchange bias (EB) field H(bias) is small and almost constant for ts up to 1.6 nm; then it rapidly grows, with no sign of saturation. This behavior is clearly related to the morphology of the top NiO layer, and is mostly due to the thickness dependence of the NiO anisotropy constant. The ability to tune the EB effect by varying the thickness of the last NiO layer represents a step towards the rational design and synthesis of core-shell NPs with desired magnetic properties.

Growth of semi-coherent Ni and NiO dual-phase nanoparticles using hollow cathode sputtering

“…For example, oxidizing pure Ni nanoparticles (Koga and Hirasawa 2013), or using chemical reactive processes to grow Ni (Gang Wu et al 2010) or NiO (El-Kemary et al 2013). A common issue is the difficulty in precisely controlling the level of oxidation of the nanoparticles (i.e., the relative composition of Ni and NiO) or to have a structure other than a metallic core and oxide shell in a single-step approach (Spadaro et al 2015).…”

Investigation of Ni@CoO core-shell nanoparticle films synthesized by sequential layer deposition

“…In this way, the EB can pin the spins in the FM material and stabilize their alignment along a magnetically pre-definite direction, increasing the magnetization stability [2,6]. This effect was exploited in core-shell NPs with a FM metal core and an AFM oxide shell, like Co@CoO and Ni@NiO systems [2,4,7,8]. In the case of Co@CoO NP films, it was found that T B , coercivity, and the EB field increase with the NP areal density, due to a modification of the magnetic properties caused by the neighboring oxide shells coming into contact, and providing a more efficient exchange interaction with the FM cores [3,4].…”

“…Alternatively, in the so-called sequential layer deposition mode, the NP can be embedded within two layers evaporated by MBE. The shells can be made of either metals or oxides, if the codeposition is performed in oxygen atmosphere [7,[20][21][22]. By this method it was possible to perform a systematic and accurate investigation of the morphology, structure and magnetic properties of Ni@NiO core-shell NPs by a number of different techniques [7].…”

Large exchange bias and enhanced coercivity in strongly-coupled Ni/NiO binary nanoparticles

“…[5][6][7][8] In particular particle systems consisting of 3d transition metals and their oxides have shown the exchange bias (EB) effect and related phenomena with interesting applications, which can improve the performance of permanent magnetic materials, i.e. an enhancement of the coercivitytypically a shi of hysteresis loop, 9,10 and combating the superparamagnetic limit in magnetic recording media. 11,12 The Co/ CoO core-shell structure is one of the most thoroughly studied systems whose EB behavior was observed in the coreshell structure at the ferromagnetic/antiferromagnetic (FM/ AFM) interface for the rst time.…”

“…These factors are critical especially in nanosized crystals and are strongly affected by the synthetic route. Remarkable progress has been made in recent years regarding the synthesis of the Ni/NiO binary manoparticles., 10,12,[17][18][19][20] Highly crystalline and monodisperse Ni/NiO nanoparticles have been prepared by the physical techniques including laserinduced reductive sintering, 3 sequential layer deposition, 10 CVD method, 21 nanosecond laser irradiation, 22 cluster-beam deposition, 23 and pulsed laser deposition. 24 However, universal agreement of the origin of the EB effect and other related anomalous magnetic properties is still under debate.…”