Pseudo Spin Valve Behavior in Colloidally Prepared Nanoparticle Films

Abstract

Magnetoresistance provides a high-precision, versatile sensing mechanism that has continuously advanced in tandem with methods for top-down engineering of high-precision multilayer devices. Materials composed of solution-prepared nanocrystals can replicate some of the behavior of traditional magnetoresistive stacks but are limited by their intrinsic response mechanism: a gradual shift from low to high resistance with field. To fundamentally alter this behavior, we demonstrate the first instance of any form of spin valve behavior arising from a nanoparticle composite magnetoresistor. Single-layer films of varying CoFe2O4 to Fe3O4 nanoparticle ratios are used to show how the low-field sensitivity can be tuned to values exceeding those of either single-component material. Additionally, the sensitivity is shown to be a simple function of the magnetization of the individual component materials.

Ultrathin, High-Aspect Ratio, and Free-Standing Magnetic Nanowires by Exfoliation of Ferromagnetic Quasi-One-Dimensional van der Waals Lattices

“…Nanoscale low-dimensional magnetic devices have enabled studies of a vast range of exotic physical phenomena − such as spin-Peierls transitions in spin chains and superconductivity in spin-ladder systems and have given rise to technologies spanning magnetic refrigeration, , implantable bioelectronics, , and data storage devices. − Many of these technologies benefit from the use of one-dimensional (1D) magnetic nanostructures, which have smaller footprints, greater sensitivity toward external stimuli, and enhanced coercivity enabled by their inherent shape anisotropy . The advantages conferred by nanowires have led to numerous advances in bottom-up synthetic routes, such as vapor- and solution-phase growth, − electrodeposition, − and lithographic methods, , which can yield freestanding magnetic nanowires with well-defined diameters, lengths, compositions, and phase purity.…”

Size-Tunable Magnetite Nanoparticles from Well-Defined Iron Oleate Precursors

“…40,41 Among the many demonstrated precursor materials, iron oleate is arguably the most popular, as it is nontoxic, can be made on a large scale, 2 and has been shown to produce particles of a variety of sizes (d = 1−40 nm), 13,14 with considerable shape control. 15,16 Many research groups, 10,12,14,42 including our own, 23 have reported structural and magnetic data for magnetite nanoparticles using iron oleate syntheses, yet somewhat counterintuitively, these syntheses continue to have challenges. The formulations of the iron oleate used as a synthetic precursor to magnetic nanoparticles have been shown, through careful characterization, to be highly sensitive to minor variations in its synthesis due to its propensity to retain water, oleic acid, and other reaction byproducts.…”

“…Even slight variations in phase, morphology, homogeneity, and heterostructure can limit or even negate their functional magnetic capabilities . Especially with iron oxide, these capabilities are vital for optimizing responses in applications such as magnetic hyperthermia, , nanocomposite magnetoresistance, − smart fluids, magnetic particle imaging, − magnetic particle spectroscopy, , and thermometry. , …”

“…Many research groups, ,,, including our own, have reported structural and magnetic data for magnetite nanoparticles using iron oleate syntheses, yet somewhat counterintuitively, these syntheses continue to have challenges. The formulations of the iron oleate used as a synthetic precursor to magnetic nanoparticles have been shown, through careful characterization, to be highly sensitive to minor variations in its synthesis due to its propensity to retain water, oleic acid, and other reaction byproducts. ,, …”

Shell-mediated control of surface chemistry of highly stoichiometric magnetite nanoparticles

“…Magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ) have been approved, for example, as agents for medical imaging, magnetic fluid hyperthermia and drug delivery systems [1][2][3][4][5] . At the same time, self-assembled Fe 3 O 4 nanoparticles (NPs) and thin films have attracted attention in the field of spin-dependent electronic transport for the design of novel magnetoresistive devices [6][7][8][9][10][11][12][13] , in which the half-metallic nature and the spinpolarization of Fe 3 O 4 play a central role. However, most nanomaterials contain variable amounts of Fe 3 O 4 and γ-Fe 2 O 3 due to the relatively easy oxidation of Fe 2+ to Fe 3+ under environmental conditions [14][15][16][17] .…”