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A Size Threshold for Enhanced Magnetoresistance in Colloidally Prepared Cofe2o4 Nanoparticle Solids Benjamin H

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A Size Threshold for Enhanced Magnetoresistance in Colloidally Prepared Cofe2o4 Nanoparticle Solids Benjamin H This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Research Article Cite This: ACS Cent. Sci. 2018, 4, 1222−1227 http://pubs.acs.org/journal/acscii A Size Threshold for Enhanced Magnetoresistance in Colloidally Prepared CoFe2O4 Nanoparticle Solids Benjamin H. Zhou† and Jeffrey D. Rinehart*,†,‡ † ‡ Materials Science and Engineering Program and Department of Chemistry and Biochemistry, University of CaliforniaSan Diego, La Jolla, California 92093, United States *S Supporting Information ABSTRACT: The phenomenon of granular magnetoresist- ance offers the promise of rapid functional materials discovery and high-sensitivity, low-cost sensing technology. Since its discovery over 25 years ago, a major challenge has been the preparation of solids composed of well-characterized, uniform, nanoscale magnetic domains. Rapid advances in colloidal nanochemistry now facilitate the study of more complex and finely controlled materials, enabling the rigorous exploration of the fundamental nature and maximal capabilities of this intriguing class of spintronic materials. We present the first study of size-dependence in granular magnetoresistance using colloidal nanoparticles. These data demonstrate a strongly nonlinear size-dependent magnetoresistance with smaller particles having strong ΔR/R ∼ 18% at 300 K and larger particles ff showing a 3-fold decline. Importantly, this indicates that CoFe2O4 can act as an e ective room temperature granular magnetoresistor and that neither a high superparamagnetic blocking temperature nor a low overall resistance are determining factors in viable magnetoresistance values for sensing applications. These results demonstrate the promise of wider exploration of nontraditional granular structures composed of nanomaterials, molecule-based magnets, and metal-organic frameworks. ontrolling the flow of electrons by switching magnet- With simple device preparation, minimal materials cost, and C ization was one of the most impactful advancements of low equipment investment, such an architecture could rapidly the digital revolution. In particular, the discovery of giant expand the scope and viability of MR sensing devices. magnetoresistance (GMR) led to the first commercial Although early formulations suffered from poor grain applications of spintronic technology nearly 40 years ago.1,2 boundaries and size distributions, advances in colloidal From its initial use in hard drive read-heads, a continuous nanoparticle synthesis over the past few decades now allow stream of advances in giant (GMR), tunnelling (TMR), many types of nanoparticles to be chemically synthesized as anisotropic (AMR), and other forms of MR have led to free-standing particles with tight control over size, morphology, smaller, faster, and more sensitive electrical detection. and surface chemistry. This developing synthetic control has Although reading and writing digital data remains a main led to a re-examining of the viability of granular magneto- driver of MR research, other detection platforms where speed resistance via a bottom-up nanochemistry approach. Downloaded via UNIV OF CALIFORNIA SAN DIEGO on October 19, 2018 at 06:28:21 (UTC). and sensitivity are important have also become prominent. Overwhelmingly, the most studied material to date is the See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. − These technologies include navigation,3 5 biochemical and ferrimagnetic inverse spinel Fe O (magnetite). Ease of − 3 4 chemical detection,6 8 magnetic relaxometry,9,10 and non- synthesis, stability, strong magnetization, and predicted half- − destructive materials testing.11 13 metallicity with full spin polarization have all contributed to One method to improve the sensitivity of an MR-based the prevalence of Fe3O4 nanoparticle composites as research sensor is to increase the number of magnetic layers traversed MR materials. Recent work on granular MR in Fe3O4 has by an electron moving through the device. The magnetization demonstrated its promise by increasing differential magneto- of these layers can be either maximally aligned to enhance the resistance current or antialigned to impede it. In commercial devices, this ΔR RRHH− can be done by deposition of multilayer thin-film devices of =− c × 100% R R increasingly complex architecture. Very early on in the Hc (1) development of MR devices, an alternative geometry was Δ 17 proposed wherein small-grain bulk materials would be pressed from 1.2% ( H = 450 mT) to values exceeding 20% at equivalent ΔH.18 It should be noted that by this definition a together, or bulk mixtures would be phase-separated into small Δ magnetic domains surrounded by conducting or insulating perfect magnetoresistor will have R/R = 100% instead of material.14,15 Ideally in this geometry, each magnetic grain boundary can be a spin-selecting junction, and the total Received: June 26, 2018 number of junctions is increased by many orders of magnitude. Published: August 22, 2018 © 2018 American Chemical Society 1222 DOI: 10.1021/acscentsci.8b00399 ACS Cent. Sci. 2018, 4, 1222−1227 ACS Central Science Research Article Figure 1. (a) Scheme of the evolution of MR from single-junction thin-film devices to multijunction granular materials to multijunction nanoparticulate materials with exquisite control over grain properties. Transmission electron micrographs and size distribution histograms of (b) − − Fe3O4 and (c g) CoFe2O4 nanoparticles used in this study. The diameter d in parts b f was calculated from the projected area A (dA= 4/π ) while the side length a in part g was calculated as a = A .16 approaching infinity, as in definitions that divide by the lower position of Fe(III) and Co(II) acetylacetonate salts in the value of the resistance (Figure S1). This empirical evidence presence of oleic acid and oleylamine in high-boiling-point suggests that nanostructuring is capable of transforming solvents.32,33 Transmission electron microscopy (TEM) granular MR from a curiosity into a viable technology; statistics were used to verify consistent size and shape for fi however, from a synthetic chemistry perspective, only the ve separate synthetic preparations of CoFe2O4 (d = 5.3, 8.4, barest surface of the materials parameter space has been 12.7, 12.9, 20.7 nm) (Figure 1b−f) as well as an Fe O sample − 3 4 explored.17 28 (d = 8.7 nm) for comparison. Smaller nanoparticles (d =5−9 Magnetic properties such as coercivity, saturation magnet- nm) were roughly spherical in shape, and larger nanoparticles ization, and remanent magnetization are strongly size-depend- exhibited some faceting due to growth along preferential ent in nanomaterials of d =1−20 nm, yet a study on the MR of crystalline faces. The d = 12.7 nm and d = 12.9 nm samples well-defined, colloidally prepared materials is lacking from the showed polyhedral shapes, while convex cubes are observed for literature. In this work we perform the first such study using the d = 20.7 nm sample. Of particular note are d = 12.9 nm nanoparticles of CoFe2O4. The greater anisotropy of CoFe2O4 CoFe2O4 nanoparticles (Figure 1e), which took truncated compared to Fe3O4 has been used to enhance the MR octahedral forms allowing them to self-assemble into semi- 29 ff properties of Fe3O4 through doping and exchange regular lattices. Powder-averaged X-ray di raction (pXRD) 30,31 fi coupling, but the MR of stoichiometric CoFe2O4 alone con rmed the inverse spinel crystal structure of AB2O4 ferrites has not been studied. Intriguingly, we find that single-domain for all samples (Figure S2). ferrimagnetism or even blocked superparamagnetism is As-synthesized nanoparticles form stable colloidal suspen- unnecessary to observe viable MR at 300 K; the most sions in nonpolar solvents due to the presence of long-chain important factor is nanoparticle size. ligands such as oleic acid and oleylamine. Ligand exchange of − Nanoparticles in this work were synthesized according to the native long-chain ligands to the small inorganic BF4 ion literature heat-up processes involving the thermal decom- was performed according to a literature procedure34 in order to 1223 DOI: 10.1021/acscentsci.8b00399 ACS Cent. Sci. 2018, 4, 1222−1227 ACS Central Science Research Article fi fi improve the conductivity of the nal nanoparticle pellets. magnetization (Ms) and coercive eld (Hc) of each sample, Although significant work has been done to improve moment vs field scans were collected from −7 to 7 T at 300 K. conductivity and even control spin-transport through ligand Again, the expected size-dependence is observed, with larger 19,20,28 design, in this work we focus on high-temperature particles displaying stronger Hc and higher Ms. These results properties, and simply decreasing interparticle distance was confirm that all samples are within the superparamagnetic sufficient to achieve viable conductivity. The removal of the regime (Table S1). hydrophobic ligands was evident from the ability to disperse With a structurally and magnetically characterized array of ligand-exchanged nanoparticles in polar solvents such as particle sizes, each material was then tested for magneto- dimethylformamide. TEM also demonstrates a reduced resistive properties. For these measurements, pressed pellets of interparticle spacing in self-assembled layers cast from the each sample were electrically contacted and subjected to a − fi BF4 -exchanged nanoparticles, compared to the TEM of the variable magnetic
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