Synthesis and magneto-transport properties of FeO@Fe3O4@MgO@Fe3O4 onion-like magnetic nanoparticles

J. NUÑEZ; G.F. GOYA; E.L. WINKLER; F. RIVADULLA; M. AGUIRRE; V. LEVORAN; R.D. ZYSLER

S.C. de Bariloche

Congreso; International workshop on Spintronics 2022 (SPIN 2022); 2022

The core/shell (CS) architecture makes it possible to combine in the same nanoparticle (NP) different materials, increasing the degrees of freedom to design and manufacture new nanostructures [1]. Recent studies in CS NPs show that, by systematically varying the shell composition, it is possible to fine-tune the magnetic and electrical transport properties of these systems [2-3]. In addition, new properties are observed in CS bimagnetic materials due to the exchange interaction at the interface [4], as exchange-bias [5] or exchange-spring [6].Since the switching field in tunneling magnetoresistance (TMR) devices is determined by the anisoptropy of the magnetic materials [7], the design and fabrication of more complex and higher quality NPs is a key factor to develop new multifunctional nanoparticles for advanced applications. In this work we show that high quality onion-like NPs can be growing by adapting the seed-mediated growth method proposed by Sun et al [8].FeO@Fe3O4@MgO@Fe3O4 monodispersed NPs were synthesized by thermal decomposition in a three-step process. Their structure and morphology were characterized by different techniques of transmission electron microscopy (TEM) and powder X-ray diffraction. By analyzing TEM images, we obtain a monodisperse size distribution with mean size of (24±4) nm. CSS structure and composition was confirmed by high angular annular dark field scanning transmission electron microscopy (HAADF STEM) images and from elemental mapping performed by electron energy loss spectroscopy (EELS). The magnetic properties were studied from magnetization measurements as a function of the applied field (MvsH) and temperature (MvsT), in a range of ±2.5T and 5K-380K. In field cooling MvsH curves the presence of a bias field was observed below 125K. Magneto transport mesurements were performed on self-assembled NPs films in a range of ±1T and 90K-300K. The results are analyzed in terms of the magnetic coupling between the soft Fe3O4 and hard FeO@Fe3O4 magnetic phases, and the role of the nonmagnetic MgO separator is discussed.