Large low-field magnetoresistance in Fe₃O₄/molecule nanoparticles at room temperature

Abstract

Acetic acid molecule-coated Fe3O4 nanoparticles, 450–650 nm in size, have been synthesized using a chemical solvothermal reduction method. Fourier transform infrared spectroscopy measurements confirm one monolayer acetic acid molecules chemically bond to the Fe3O4 nanoparticles. The low-field magnetoresistance (LFMR) of more than −10% at room temperature and −23% at 140 K is achieved with saturation field of less than 2 kOe. In comparison, the resistivity of cold-pressed bare Fe3O4 nanoparticles is six orders of magnitudes smaller than that of Fe3O4/molecule nanoparticles, and the LFMR ratio is one order of magnitude smaller. Our results indicate that the large LFMR in Fe3O4/molecule nanoparticles is associated with spin-polarized electrons tunnelling through molecules instead of direct nanoparticle contacts. These results suggest that magnetic oxide-molecule hybrid materials are an alternative type of materials to develop spin-based devices by a simple low-cost approach.

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

“…It has been studied extensively and implemented to current information technology with increased data transfer speeds, processing power, memory density and storage capacity. In the efforts of developing more sensitive spintronics devices, metal oxides of CrO 2 , Fe 3 O 4 and La 2/3 Sr 1/3 MnO 3 have attracted much attention due to their unique spin-dependent electronic transport propertiesthey are conductors along one spin channel but become insulators/semiconductors along the other spin channel. − Among these common oxides studied for spintronics, Fe 3 O 4 is especially promising as a practical material owing to its strong ferrimagnetism, good room temperature conductivity and high Curie temperature T c . , To date, various forms of Fe 3 O 4 have been studied, − and granular Fe 3 O 4 are found to be particularly useful to achieve large magnetoresistance (MR). However, when the sizes of the Fe 3 O 4 grains reach sub-20 nm, large fraction of surface atoms of each grain can cause high degree of oxygen (O)-defects, making it difficult to observe high-spin polarization in Fe 3 O 4 .…”

“…However, when the sizes of the Fe 3 O 4 grains reach sub-20 nm, large fraction of surface atoms of each grain can cause high degree of oxygen (O)-defects, making it difficult to observe high-spin polarization in Fe 3 O 4 . To preserve the spin polarization and in turn to enhance the MR ratio, the granular Fe 3 O 4 surface should be compensated with extra “O” from O-rich materials. − For this purpose, capping Fe 3 O 4 nanoparticles (NPs) with O-containing surfactant is particularly advantageous as the coating can not only compensate surface O, but also render the NPs dispersible in a solvent for controlled assembly.…”

Enhancing magnetoresistance in tetrathiafulvalene carboxylate modified iron oxide nanoparticle assemblies

“…When the Fe 3 O 4 NPs are separated more than 1.9 nm but less than 2 nm, a higher COO-binding density around the NP surface can help increase the MR ratios but not the conductivity of the electron transport, which is dominated by the Mott-hopping process. The reported new approach to Fe 3 O 4 NP surface modification and NP stabilization by the TTF-COOsurfactant is advantageous over the conventional method used to build a magnetic NP array/thin film with a common organic surfactant of oleylamine/oleate, 4,6,25,41,42 carboxylate, [7][8][9] sexithienyl, 43 tris(8-hydroxyquinoline), 21,[44][45][46] rubrene, 47 or polymer. 48,49 It should allow one to prepare and assemble even metallic magnetic NPs with much high magnetic moments for spin transport studies.…”

“…In this array, electrons are transported across two adjacent NPs with their conductivity controlled by the length of the surfactant and the cross-surfactant interactions. [3][4][5][6][7][8][9][10][11] Due to its insulating nature, the surfactant does not have the strong spin-orbit coupling and hyperfine interaction with the NP, and as a result, it serves as an electron spin divider between two adjacent NPs, increasing the spin relaxation time of each NP and enhancing the MR effect on the spin-dominated electron transport. [12][13][14] Recent advances in NP syntheses further allow the preparation of magnetic NPs with NP dimensions controlled at the near-atomic-scale precision, [15][16][17] and consequently, the bonding interactions between the organic layer and the ferromagnetic surface can be controlled and the spin-dependent transport may be tuned to optimization.…”

Room-temperature spin-dependent tunneling through molecules

“…We also prepared one reference sample with a significantly larger particle size ϳ550 nm by a chemical solvothermal reduction method. 15 After the alkane molecules coated Fe 3 O 4 nanoparticles were synthesized, they were collected, extensively washed with ethanol to remove physically absorbed molecules, and then dried in a vacuum. The dried nanoparticles naturally coalesced into millimetersized blocks by the van der Waals force, which were sliced into ϳ7 ϫ 7 ϫ 1 mm 3 pieces for transport measurements.…”