Magnetic order in disordered dipolar nanostructures

This project is jointly funded by Condensed Matter Physics and the Established Program to Stimulate Competitive Research (EPSCoR). Non-technical abstract

This project aims to identify factors that affect magnetic properties of nanoparticle assemblies, including ferromagnetic liquid droplets and hydrogel sensors, so that they can be used for critical health and technology applications. Ferromagnetic liquid droplets are liquid bodies that can take and sustain virtually any shape while in liquid environment and can transition between a non-magnetic object and a permanent magnet.

These unique properties could find application in drug delivery, microfluidic channels, and actuators. Smart hydrogel sensors are envisioned for continuous health monitoring due to their biocompatibility and sensitivity to, e.g., water acidity, that enlarges or shrinks the hydrogel. Embedded magnetic nanoparticles undergo a phase transition that can be detected without the need for electric wires into or out of the human body.

This project provides mentored research training and career development to graduate and undergraduate students and leverages institutional services to recruit underrepresented students. The principal investigator engages the science-curious public and high school students in the proposed research via existing university services, social media, local conferences, and National Science Foundation-funded outreach programs and integrates his findings into undergraduate and graduate courses.

Technical abstract

Magnetic order by disorder is a complex non-trivial phenomenon that governs the transition between a vanishing (superparamagnetic) and a stable (ferromagnetic) magnetization with long-range, quasi-long-range, or short-range order. This complexity is even larger in liquid systems that undergo a reversible transformation between paramagnetic ferrofluid and ferromagnetic liquid by the assembly and jamming of superparamagnetic nanoparticles on curved liquid-liquid interfaces.

The interfacial assembly is mediated by oppositely charged ligands that anchor the nanoparticles and reduce their electrostatic charge and spacing. The goal of this project is to determine design strategies for disordered ensembles of superparamagnetic nanostructures that combine reversible transformation between superparamagnetism and ferromagnetism with hard-magnetic properties.

The principal investigator accomplishes this by studying the relationship between structural short-range order and magnetic properties in the dried state using nanoparticle assemblies and lithographically patterned nanostructures with different degrees of disorder, symmetry, layer thickness, and spacing and applying these findings to ferromagnetic liquid droplets. Through coordinated experimental and numerical studies using micromagnetic Monte Carlo simulations, magnetometry, ferromagnetic resonance spectroscopy, and advanced x-ray and electron microscopies, the principal investigator corroborates or refutes the following three hypotheses: (1) Materials with increasing disorder favor a transition from ferromagnetism to non-collinear magnetization to superparamagnetism that is delayed in anisotropic arrangements; (2) Structural short-range order governing magnetic order can be inferred from macroscopic properties; and (3) In-field assembly and jamming of nanoparticles at liquid-liquid interfaces enable nanopatterning of the surfactants with an enhanced remanent magnetization and coercive field.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.