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

Abstract

Uniform hexagonal single phase Ni1–x Fe x O (x = 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 Ni1–x Fe x 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 Fe3+ 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 Ni2+ vacancies surrounding an Fe3+ 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.

Room Temperature Magnetic Memory Effect in Cluster-Glassy Fe-Doped NiO Nanoparticles

“…Such a complex structure could result in interesting magnetic properties. According to previous reports, bare and the Fe-doped NiO NPs both exhibit low-temperature magnetic memory effect [ 16 , 28 , 29 ]. Such a type of nanoscale system can be used as a “thermal assistant memory cell” in digital information storage [ 30 , 31 , 32 ].…”

“…Moreover, the obtained conventional and spontaneous EB field from NiO NPs is consistent with previous findings and it can be attributed to the formed uncompensated core and disordered shell-type structure [ 3 , 4 ]. The observed reduction in the EB field from Fe-doped NiO NPs and its further reduction with the increase of Fe-concentration could be understood by assuming the presence of a 4:1 defect cluster in the core of the NPs (inset of Figure 3 b) [ 16 ]. The 4:1 defect cluster consists of tetravalent interstitial iron Fe i 4+ and four V Ni , in total, being four times negatively charged.…”

“…The observed decrease in the EB field with the increase of Fe-doping concentration and the absence of memory effect in strongly exchange-coupled NiO NPs (having size distribution, i.e., anisotropy distribution) suggested RT memory effect and EB phenomenon are independent. Hence, it appears that, along with particles uniaxial anisotropy [ 17 ], the intraparticle interactions due to formed 4:1 defect clusters [ 16 , 23 ] may have paved the way for creating an additional anisotropy energy barrier, resulting in an appearance of RT memory effect from Fe-doped NiO NPs. In conclusion, Ni 1-x Fe x O NPs with x(%) varying from 0 to 1% and having a similar structure as that of NiO without any impurity phase was successfully synthesized while using the co-precipitation method.…”

“…According to recent findings, RT ferromagnetism in NiO NPs can be achieved through transition metal (TM)-doping, for example, Fe (either due to substitution or the formed defect clusters), which opens up their potential applications in the future advanced spintronic devices [ 13 , 14 , 15 ]. The properties of such a system can be tailored by controlling both the particle size and Fe-dopant concentration, which results in a complex magnetic property [ 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. For instance, the doping of Fe 3+ ions in NiO either could replace the Ni ions or occupy an interstitial site.…”

“…The substituted Fe 3+ ions can alter the Ni 2+ ─O 2− ─Fe 3+ superexchange interaction and so the AF properties. Whereas, Fe ions at the interstitial site could form 4:1 defect cluster consisting of tetravalent interstitial and four V Ni , in total being four times negatively charged [ 16 , 23 , 24 , 25 , 26 , 27 ]. Such a complex structure could result in interesting magnetic properties.…”