Bimagnetic CoO Core/CoFe 2 O 4 Shell Nanoparticles: Synthesis and Magnetic Properties Enio Lima, Jr., †,‡ Elin L. Winkler,* ,†,‡ Dina Tobia, †,‡ Horacio E. Troiani, † Roberto D. Zysler, †,‡ Elisabetta Agostinelli, ¶,‡ and Dino Fiorani ¶,‡ † Centro Ató mico Bariloche, CNEA-CONICET, 8400 S.C. de Bariloche, Río Negro, Argentina ‡ Argentine-Italian Joint Laboratory of Nanomagnetism, LIANAM, Laboratorio Resonancias Magnéticas -CNEA / Istituto di Struttura della Materia - CNR ¶ Istituto di Struttura della Materia, CNR, Area della Ricerca di Roma, C.P. 10, I-00016 Monterotondo Staz., Rome, Italy ABSTRACT: In this work we report on the synthesis, the micro- structural characterization, and the magnetic properties of ∼7 nm bimagnetic core/shell nanoparticles prepared by seed-mediated growth high temperature decomposition of organometallic precursor. The nanoparticles are formed by an antiferromagnetic CoO core coated with ferromagnetic CoFe 2 O 4 shell of 2-3 nm of thickness. XRD and electron diffraction patterns show the reflections of the structure of the CoFe 2 O 4 and CoO phases and Dark- and Bright-field TEM images provide evidence of the core-shell morphology of the system. Magnetic measurements show that the system presents a remarkably large coercivity and high squareness (at 5 K, H C = 27.8 kOe and M r /M S = 0.79), compared to CoFe 2 O 4 single phase nanoparticles of comparable size. The enhancement of the effective anisotropy is attributed to the surface and interface exchange coupling effects. KEYWORDS: bimagnetic nanoparticles, CoO/CoFe 2 O 4 nanoparticles, interface exchange coupling, magnetic anisotropy ■ INTRODUCTION Magnetic nanoparticles have been widely investigated for their novel properties as well as for their numerous applications in different technological fields 1 and in biomedicine. 2 Many technological devices, e.g. magnetic storage media and permanent magnets, require high stability of the magnetization. However, further size reduction of single phase nanoparticles has to face the so-called “superparamagnetic limit”, which imposes a lower threshold in the grain size to maintain thermal stability of the magnetization, thus limiting the development of miniaturized devices. 3 Recently, Skumryev et al. 4 have shown that the superparamagnetic threshold can be pushed to a lower grain size in nanostructures with ferromagnetic (FM)/ antiferromagnetic (AFM) interface. This was achieved exploit- ing the interface exchange coupling for increasing the effective magnetic anisotropy of the system. 4-6 The possibility of tuning the magnetic anisotropy through interface exchange coupling between materials with different magnetic structures and different magnetic anisotropies has driven in the last years the research on bimagnetic core/shell nanoparticles. In this frame, new size-controlled synthesis methods were devel- oped. 7-10 Among them the high temperature decomposition of organometallic precursor deserves special attention as this method not only allows the fabrication of monodispersed nanoparticles 7-11 but also makes it possible to prepare high- quality bimagnetic core/shell nanoparticles by seed-mediated growth. 12-17 Within this picture, the motivation of this work is the fabrication of bimagnetic AFM-core/ferrimagnetic (FiM)- shell nanoparticles and the investigation of the magnetic properties with special interest toward the anisotropy enhance- ment with respect to the single phase counterparts. It is well- known that the CoFe 2 O 4 spinel oxide is a hard magnetic material with large coercivity and magnetization, making it an excellent candidate for technological applications. Its coercivity and squareness (defined as the remanence (M r ) normalized to the saturation magnetization (M S ), M r /M S ) is strongly size dependent. 18,19 Chinnasamy and co-workers found that for CoFe 2 O 4 particles the maximun value of the coercivity, H C ∼ 20 kOe, is reached for an average diameter of <ϕ> = 40 nm. Then the coercivity decreases to H C ∼ 10 kOe when the size is reduced down to <ϕ> = 8 nm. We have prepared core/shell systems with the aim of improving the magnetic stability of CoFe 2 O 4 at nanometer scale. In particular, in the present work, we report the synthesis, the microstructural characterization, and an investigation of the magnetic properties of CoO AFM nanoparticles coated with CoFe 2 O 4 FiM shell. The results show an enhancement of the effective anisotropy with respect to the single phase CoFe 2 O 4 nanoparticles. Received: September 27, 2011 Revised: December 21, 2011 Published: December 21, 2011 Article pubs.acs.org/cm © 2011 American Chemical Society 512 dx.doi.org/10.1021/cm2028959 | Chem. Mater. 2012, 24, 512-516