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Pseudo Spin Valve Behavior in Colloidally Prepared Nanoparticle Films Benjamin H Letter Cite This: ACS Appl. Electron. Mater. 2019, 1, 1065−1069 pubs.acs.org/acsaelm Pseudo Spin Valve Behavior in Colloidally Prepared Nanoparticle Films Benjamin H. Zhou† and Jeffrey D. Rinehart*,†,‡ † ‡ Materials Science and Engineering Program and Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States *S Supporting Information 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. KEYWORDS: colloidal nanomaterials, composite materials, spintronics, magnetoresistance, magnetism oon after the revolutionary discovery of giant magneto- Conversely, in a granular or nanocomposite magnetoresistor, S resistance (GMR) in multilayered structures,1,2 an application of an external magnetic field results in individual analogous effect was observed for magnetic grains in particle moments deviating from their original alignment with 3−7 Δ − conducting or insulating matrices. In this formulation, the the local anisotropy axes (maximum R/Rmax; Figure 1a ii,iv) matrix material acts as the spin-selecting junction between toward parallel alignment with the external field axis magnetically oriented grains, and the lack of structural (minimum ΔR/R; Figure1a−i,iii). While this shift in ΔR/ precision is compensated by the sheer number of junctions. R can be large, its sensitivity with respect to applied field is ff max This alternative realization of magnetoresistance o ers the severely limited due to switching between an ordered and tantalizing prospect of tunable, versatile functionality from a disordered micromagnetic structure. single-layer deposition. Advances in colloidal nanosynthesis To achieve spin valve behavior in a nanocomposite and characterization have largely shifted research from milled magnetoresistor, we envisioned a monolithic structure and biphasic solid-state materials to nanoparticle solids. These Downloaded via UNIV OF CALIFORNIA SAN DIEGO on September 3, 2019 at 21:47:45 (UTC). composed of hard and soft magnetic nanoparticles. (Figure See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Δ materials have led to increasing magnetoresistance ratios ( R/ 1b) While there have been reports of multicomponent Rmax) and an emerging understanding of how particle 8−10 11 12 13−17 nanoparticle magnetoresistance (MR) materials, they either composition, size, shape, and surface chemistry 20 do not consist of hard and soft magnetic nanoparticles or modulate the tunneling magnetoresistance (TMR) that arises involve magnetically coupled components.21,22 In this work, we from intrinsic tunnel barriers between individual particles. produce a series of nanocomposite films from high-coercivity Continuing exploration in materials composition and device formulation promise large improvements in the properties of CoFe2O4 and low-coercivity Fe3O4 nanoparticles. By limiting magnetic coupling between the materials, we can generate the these nanocomposite magnetoresistors, yet the fundamental fi structure that enabled magnetoresistance to occupy a central rst example of any form of nanocomposite spin valve role in technology for the past 25 years is still missing: the spin magnetoresistance. Excitingly, these devices display magneto- valve.18,19 A spin valve pins the moment of one magnetic resistive behavior that is both predictable from the magnetism component of the magnetoresistor (the fixed or hard layer) by of the individual nanoparticles and has magnetoresistive using exchange bias or intrinsic anisotropy (the latter sensitivity exceeding either component. mechanism is sometimes termed a pseudo spin valve). Pinning the fixed layer allows very small magnetic fields to flip the Received: March 27, 2019 orientation of the other component (the free or soft layer), Accepted: June 24, 2019 leading to highly sensitive step function responses in resistance. Published: June 24, 2019 © 2019 American Chemical Society 1065 DOI: 10.1021/acsaelm.9b00196 ACS Appl. Electron. Mater. 2019, 1, 1065−1069 ACS Applied Electronic Materials Letter of Fe(III) or Fe(III)/Co(II) oleate were decomposed in the presence of oleic acid in refluxing octadecene. The resulting nanoparticles were washed through precipitation with ethanol and centrifugation. Transmission electron microscopy (TEM) revealed monodisperse spherical nanoparticles of CoFe2O4 (d ± ± = 13.4 1.4 nm) and Fe3O4 (d = 7.8 0.8 nm) (Figure 1c,d). Powder-averaged X-ray diffraction (pXRD) data are consistent with the inverse spinel crystal structure of AB2O4 ferrites (Figure S1). These synthetic components were chosen as our initial formulation to balance three factors: (1) the components possessed size, shape, and size distribution homogeneity to maximize the chances of a random mixture upon deposition; (2) nanoparticle sizes were small enough to fi Δ expect signi cant R/Rmax according to our previously observed size dependence;11 and (3) the coercivity difference between CoFe2O4 and Fe3O4 was large enough to impose a definitive spin valve effect. In layered devices, the pseudo spin valve effect is achieved by deposition of three different material layers (hard, spacer, and soft), a process complicated by lattice mismatch and interdiffusion between layers. In this granular system, the interfaces between hard and soft materials are created simply Figure 1. Relationship between the micromagnetic state and by the intimate mixing of Fe3O4 and CoFe2O4 nanoparticles in magnetoresistance of (a) single component and (b) multicomponent colloidal suspensions. Hexane suspensions of Fe3O4 and nanogranular materials. Magnetization vectors of nanoparticles within CoFe2O4 of equal concentration were mixed in the desired the granular material are indicated by arrowheads in blue (high proportionbyvolume(1:0,3:1,1:1,1:3,and0:1 coercivity material) and orange (low coercivity material) circles. In CoFe2O4:Fe3O4). These mixed suspensions were subsequently (b) the orange and blue hysteresis curves depict the independent stripped of their native oleic acid ligands by treatment with magnetic switching behavior of high and low coercivity components 26 NOBF4 in dichloromethane to improve the conductivity of of the material, respectively, leading to a pseudo spin valve the final nanoparticle films. Devices for MR measurements magnetoresistance behavior. Diagrams indicate magnetic saturation fi (low resistance; i, iii) and net-zero magnetization points (maximum were formed by dropcasting lms from the ligand-stripped resistance; ii, iv). Below are transmission electron micrographs nanoparticles in dimethylformamide suspensions onto litho- (TEM) and size histograms of (c) CoFe2O4 and (d) Fe3O4 graphically patterned gold electrodes on silicon substrates nanoparticles. (Figure S2). Energy-dispersive X-ray spectroscopy shows an even distribution of both cobalt and iron throughout the films CoFe2O4 and Fe3O4 nanoparticles were synthesized (Figure S3), consistent with a lack of segregation of CoFe2O4 fi 23−25 according to modi ed literature procedures. Complexes and Fe3O4 nanoparticles upon deposition. Figure 2. (a−e) Reduced magnetization (magnetization normalized to the saturation value, circles) at 200 K vs magnetic field for a series of fi fi nanoparticle lms with composition varying from pure CoFe2O4 to pure Fe3O4. Plots for the mixed composition lms show agreement with a mass- − Δ fi weighted linear combination (colored lines) of the magnetizations of the component nanoparticles. (f j) R/Rmax (%) (circles) for lms at 200 K with fits to eq 1 (lines). The reverse scans (+7 to −7 T) in each plot are faded to enhance the clarity of the plots. 1066 DOI: 10.1021/acsaelm.9b00196 ACS Appl. Electron. Mater. 2019, 1, 1065−1069 ACS Applied Electronic Materials Letter A pseudo spin valve requires that hard and soft materials field was increased to H =7T(Figure 2f−j). Each film reaches have individual coercive fields as different as possible and that its minimal resistance at large positive and negative fields, the composite material lacks exchange coupling between the reflecting their maximally aligned magnetic state (m ≈ ±1). As materials. Thus, magnetic measurements were performed to the magnetization of the films approaches zero at small fields characterize the individual CoFe2O4 and Fe3O4 nanoparticle (the minimally aligned magnetic state, m =0;H = HC), their magnetic characteristics. Ideally, CoFe2O4 (our hard material) resistance reaches its maximum. While the MR of the fi fi should have a maximal coercive eld and remanent moment, magnetically hard CoFe2O4 lm shows a gradual change in fi while Fe3O4 (our soft material) should have a low coercive resistance across the entire eld range and a separation fi fi eld, high susceptibility,
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