Fe 3 O 4 Nanocrystals Tune the Magnetic Regime of the Fe/Ni Molecular Magnet: A New Class of Magnetic Superstructures Giorgio Zoppellaro, † Jir ̌ í Tuc ̌ ek, † Radovan Herchel, ‡ Kla ́ ra S ̌ afa ́ r ̌ ova ́ , † and Radek Zbor ̌ il* ,† † Regional Centre of Advanced Technologies and Materials, Departments of Physical Chemistry and Experimental Physics, Faculty of Science, Palacky University, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic ‡ Department of Inorganic Chemistry, Faculty of Science, Palacky University, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic * S Supporting Information ABSTRACT: A new class of organometallic-inorganic magnetic material was engineered by a sonochemically assisted self-assembly process between magnetite nanoparticles (bio- genic Fe 3 O 4 , hard constituent) functionalized with isonicotinic acid and a metamagnetic organometallic complex ([Ni- (en) 2 ] 3 [Fe(CN) 6 ] 2 ·3H 2 O, soft constituent). In such bottom- up methodology, hard and soft counterparts form well- organized microdimensional clusters that showed morpho- logical fingerprints and magnetic behavior clearly distinct from those of the initial building units. In the engineered soft-hard material, the magnetite nanocrystals induced ferromagnetic ordering at room temperature of closer contact layers of [Ni(en) 2 ] 3 [Fe(CN) 6 ] 2 ·3H 2 O, thus demonstrating the ability to sensibly modify the [Ni(en) 2 ] 3 [Fe(CN) 6 ] 2 ·3H 2 O paramagnetic regime. The magnetic ordering of [Ni(en) 2 ] 3 [Fe(CN) 6 ] 2 ·3H 2 O was triggered by the intrinsic local field of the hard magnetic nanocrystals, which resembled, to some extent, the effects promoted by large, external magnetic fields. ■ INTRODUCTION The design and synthesis of core-shell iron oxide nanoparticles (FeNPs), mainly based on naturally occurring magnetite (Fe 3 O 4 ) or maghemite (γ-Fe 2 O 3 ) cores, have been fascinating topics of research for chemists and physicists for almost two decades. 1 Depending on the shell composition, these magnetic systems can become functional units in storage information devices, sensors, probes, contrast agents (magnetic resonance imaging), drug carriers, anticancer agents, and photocataly- sis. 2-6 Functionalization of the nanoparticle’s surface is generally obtained by organic, organometallic, and/or inorganic coating materials. 7-11 FeNPs that merge the ferromagnetic magnetite/maghemite core (FM) with an antiferromagnetic shell (AFM), usually composed of FeO layers, and those containing inverted architectures deliver model materials sensitive to changes in the magnetic ordering at the AFM/ FM interface, a characteristic suitable for exploitation in the field of spintronics. 12-19 FeNPs functionalized with organic molecules containing hydrophilic substituents are envisioned to operate mainly in biological environments. 20 After the organic coating architecture has been selected, the nanoparticle systems can act as sensors, exhibit loading ability of chemotherapeutics (e.g., doxorubicin), or operate as contrast agents in medical diagnostics. 21-28 In particular, the establishment of noncovalent interactions between the organic shell and the drug is an important functional characteristic for the nanoparticle that acts as nanocarrier because it prevents chemical modification of the pharmaceutically active component during the transport process. 20 Such an approach to the synthesis in this category of functional FeNPs takes inspiration directly from many of the processes occurring in nature, where highly organized molecular assemblies and more sophisticated functions (e.g., the genetic information stored in the DNA) can emerge from the combination of distinct building units held together by weak interactions, for example, electrostatic forces, H-bonding, and π-π stacking. While the self-assembly event is well- recognized as an effective path in which evolution acts in nature, the direct application of this strategy on the chemical bench for the synthesis of nanostructured materials is difficult to accomplish, especially when a precise set of chemical/ physical properties is envisioned beforehand. 29 The synthesis of soft-hard magnetic systems based on the noncovalent self- assembly process of diverse units is by far less explored compared to that of soft-hard magnetic hybrids obtained via cold-pressed mixtures of various metal oxides. 30-34 In this work, we report the synthesis and detailed physical properties of a magnetic system composed of soft-hard components which was obtained following the noncovalent approach and the self-assembly pathway in solution. As shown in Figure 1, a hybrid material (micrometer-sized, termed hereafter hybrid) emerged from the combination of magnetite nanoparticles (FeNPs, hard component) surface functionalized with isonicotinic acid and a metamagnetic molecule [Ni- Received: April 8, 2013 Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic4008729 | Inorg. Chem. XXXX, XXX, XXX-XXX