Enhancing low-field magnetoresistance in magnetite nanoparticles via zinc substitution.

We report a strategy to enhance the room temperature low-field magnetoresistance (LFMR) behavior of Fe3O4 nanoparticle (NP) assemblies by controlled Zn-substitution. The Zn-substituted 7 nm ZnxFe3-xO4, (x = 0 to 0.4) NPs are prepared by thermal decomposition of metal acetylacetonates (M(acac)n, M = Fe2+, Fe3+, and Zn2+). The substitution increases NP magnetic susceptibility (χ) and makes the magnetic moment more sensitive to low magnetic fields. As a result, the Zn0.3Fe2.7O4 NP assembly with NPs separated by tridecanoate exhibits a large magnetoresistance (MR) ratio of -14.8% at 300 K under a 4.5 kOe magnetic field. The demonstrated approach to control NP substitution to enhance low-field magnetoresistance of the NP assemblies provides an attractive new strategy to fabricate Fe3O4-based magnetic NP assemblies with desirable transport properties for sensitive spintronic applications.

Enhancing magnetoresistance in tetrathiafulvalene carboxylate modified iron oxide nanoparticle assemblies.

We report a facile approach to stabilize Fe3O4 nanoparticles (NPs) by using tetrathiafulvalene carboxylate (TTF-COO(-)) and to control electron transport with an enhanced magnetoresistance (MR) effect in TTF-COO-Fe3O4 NP assemblies. This TTF-COO-coating is advantageous over other conventional organic coatings, making it possible to develop stable Fe3O4 NP arrays for sensitive spintronics applications.

Tuning Electron-Conduction and Spin Transport in Magnetic Iron Oxide Nanoparticle Assemblies via Tetrathiafulvalene-Fused Ligands.

We report a strategy to coat Fe3O4 nanoparticles (NPs) with tetrathiafulvalene-fused carboxylic ligands (TTF-COO-) and to control electron conduction and magnetoresistance (MR) within the NP assemblies. The TTF-COO-Fe3O4 NPs were prepared by replacing oleylamine (OA) from OA-coated 5.7 nm Fe3O4 NPs. In the TTF-COO-Fe3O4 NPs, the ligand binding density was controlled by the ligand size, and spin polarization on the Fe3O4 NPs was greatly improved. As a result, the interparticle spacing within the TTF-COO-Fe3O4 NP assemblies are readily controlled by the geometric length of TTF-based ligand. The shorter the distance and the better the conjugation between the TTF's HOMO and LUMO, the higher the conductivity and MR of the assembly. The TTF-coating further stabilized the Fe3O4 NPs against deep oxidation and allowed I2-doping to increase electron conduction, making it possible to measure MR of the NP assembly at low temperature (<100 K). The TTF-COO-coating provides a viable way for producing stable magnetic Fe3O4 NP assemblies with controlled electron transport and MR for spintronics applications.