Size Determines Efficacy of Nanoparticle Magnetoresistance.

The discovery of giant magnetoresistance (GMR) occurred in the late 1980s,[1,2] at the dawn of the age of the modern computing. This discovery, and its fortuitous timing, resulted in the rapid commercial deployment of GMR-based devices in the read heads of hard disk drives, accelerating the miniaturization of data storage and driving the digital revolution. The immediate impact of GMR won Albert Fert and Peter Grünberg, the discoverers of this phenomenon, the Nobel Prize in Physics. Today, giant (GMR), tunneling (TMR), and anisotropic (AMR) magnetoresistance are still instrumental in spin valve devices for reading and writing data. Of further interest is that within emerging areas of research there is tremendous promise for application of MR devices in areas requiring sensitive magnetic field sensors, including navigation,[3] biomedical sensing,[4] and more complex applications in spintronics.[5] Creating new magnetoresistive materials is vitally important to developing these new areas. Despite the overwhelming need for new materials, the vast phase space of magnetic materials remains unexplored for MR applications. Reporting in ACS Central Science, Zhou and Rinehart demonstrate a novel approach to the creation of magnetoresistive materials with an intriguing report of tunneling magnetoresistance (TMR) in colloidally prepared CoFe2O4 nanoparticles.[6] Leveraging advances in nanoparticle preparation techniques, Rinehart and co-workers demonstrate how commonly held assumptions about the nature and limitations of nanoparticle-based magnetoresistance need to be questioned. Chief among these assumptions are the defining importance of (1) highly crystalline interfaces and (2) magnetically ordered materials.

TMR arises within conductive ferro- or ferrimagnetic materials at the interface between different magnetic domains. If the magnetization vectors of adjacent magnetic domains are aligned then electrons can move between them relatively unimpeded. If the domains’ magnetization vectors are oppositely aligned, then electron flow between those domains is impeded, resulting in decreased conductivity across the interface (Figure ). This phenomenon underpins spin valve magnetic field sensors used in many different devices. An extension of this phenomenon arises in granular solids which are composed of many independent ferro- or ferrimagnetic single-domain particles which align upon exposure to an external magnetic field.

Figure 1

A simplified schematic of a spin valve in both its (a) low-resistance (on) and (b) high-resistance (off) states. The presence of an external magnetic field to force alignment of the magnetic domains is the trigger to open the spin valve, thus allowing spin valves to serve as potent magnetic field sensors. In spin valve devices, the two layers are separated by a nonmagnetic layer (red), and one magnetic layer is typically pinned in a specific direction by its interaction with an adjacent antiferromagnetic layer (blue). The other layer can change direction upon exposure to an external magnetic field.

For their study, Zhou and Rinehart utilized CoFe2O4, an inverse spinel material structurally similar to Fe3O4—the most commonly studied MR material. CoFe2O4 has been shown to improve the MR behavior of Fe3O4 both as a dopant[7] and in heterostructures,[8] but its bulk MR properties were unknown until now. The authors synthesized CoFe2O4 nanoparticles over a range of sizes (5.3–20.7 nm) and subjected pressed pellets of the nanoparticles to conductivity measurements under a varying magnetic field. With no field applied, they saw high resistance, presumably because of the randomized magnetic domains of the superparamagnetic sample (Figure a). Upon applying a magnetic field and forcing the alignment of the magnetic domains, Zhou and Rinehart saw reduced resistance across the pellet (Figure b). The smallest particles (d = 5.3 nm, 8.4 nm) showed substantial MR (19.2% and 18.4% reduction in resistance at 7 T vs 0 T for 5.3 and 8.4 nm nanoparticles, respectively). These MR values are determined at room temperature, and the majority of the change occurs in a small window at fields less than 0.5 T. These results are all the more impressive given the ease of synthesis and processing, low cost of the material, and the single-component nature of the resulting MR device.

Figure 2

A schematic of a spin valve made of a pressed pellet of nanoparticles. The randomly oriented superparamagnets within the sample restrict electron flow in the absence of an external magnetic field (a). When a field is applied and the domains become aligned, electrons can flow unimpeded through the super-paramagnets (b).

Crucially, Zhou and Rinehart made the surprising discovery that upon increasing the size of the CoFe2O4 nanoparticles to 12.7 nm, differential magnetoresistance of the sample dramatically decreased from 18.4% at d = 8.4 nm to 6.6% at d = 12.7 nm. This counterintuitive decrease occurred despite the higher saturation magnetization values and magnetic blocking temperatures of the larger particles. These results show, for the first time, the powerful nonlinearity of the relationship between the size and magnetoresistive properties of superparamagnetic nanoparticles and demonstrate the presence of a critical size threshold above which MR becomes inoperative.

These results indicate that top-down processing is not a requirement to rationally design and optimize viable magnetoresistive devices. This finding raises many further questions about the material properties that contribute to MR. The observation of a size threshold for granular magnetoresistance in CoFe2O4 begs the question of where and whether this size threshold occurs in other materials and, by extension, what material properties determine this threshold. Exciting prospects in the field include the study of the impact of nanostructure morphology, doping, particle separation distance, and matrix material on MR properties. Crucially, with the realization that simple nanoparticle solids exhibit competitive MR properties, a massive new phase space has opened in the field. This discovery removes the constraint of top-down engineering techniques and delivers a tremendous potential for high throughput discovery. Following this study, new research in magnetoresistance using facilely synthesized nanoparticulate metal–organic frameworks, molecular magnets, and magnetic nanostructures will certainly lead to foundational new insights into this vital phenomenon.