Understanding the Magnetic Memory Effect in Fe-Doped NiO Nanoparticles for the Development of Spintronic Devices

Uniform hexagonal single phase Ni<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O (<i>x</i> = 0, 0.01, 0.05, and 0.1) nanoparticles synthesized by a standard hydrothermal method are characterized with an enhanced lattice expansion along with a decrease in the microstrain, crystal size, and Ni occupancy as a function of the Fe concentration. The observed anomalous temperature and field dependent magnetic properties as a function of the Fe content were explained using a core–shell type structure of Ni<sub>1–<i>x</i></sub>Fe<sub><i>x</i></sub>O nanoparticle such that the effect of Fe-doping has led to a decrease of disordered surface spins and an increase of uncompensated-core spins. Perfect incorporation of Fe<sup>3+</sup> ions at the octahedral site of NiO was observed from the low Fe concentration; however, at a higher Fe content, 4:1 defect clusters (four octahedral Ni<sup>2+</sup> vacancies surrounding an Fe<sup>3+</sup> tetrahedral interstitial) are formed in the core of the nanoparticles, resulting in the transition of spin-glassy to the cluster-glassy system. An enhanced thermal magnetic memory effect is noted from the cluster-glassy system possibly because of increased intraparticle interactions. The outcome of this study is important for the future development of diluted magnetic semiconductor spintronic devices and the understanding of their fundamental physics.