Nickel Colloidal Superparticles: Microemulsion-Based Self-Assembly Preparation and Their Transition from Room-Temperature Superparamagnetism to Ferromagnetism

Abstract

The construction of magnetic colloidal superparticles can utilize the properties from both individual assembly units and their collective interactions; thus, it can meet wider demands in multiple fields. Although many studies have focused on the magnetic oxide superparticles, very limited study has been conducted on magnetic metal superparticles. Herein, we report on the microemulsion-based preparation of Ni colloidal superparticles with tunable sizes and magnetic properties. The prepared Ni colloidal superparticles have a spherical morphology with an average size that can vary from 130 to 700 nm. The key influential factors in the formation of superparticles are being investigated. Magnetic measurements reveal room-temperature superparamagnetic characteristics for all superparticle samples. The calculated interaction energy values indicate that strong magnetic interactions are present in Ni superparticles. Although Ni superparticles with a size of about 700 nm are still room-temperature superparamagnetic, annealed Ni superparticles with a much smaller size of about 390 nm instead exhibit room-temperature ferromagnetic characteristics. This behavior is quite different from that of magnetic nanoparticles. Our results demonstrate that the construction of magnetic colloidal superparticles through adjustments to the parameters of the assembling units may provide them with rich magnetic properties, thus making them useful in a wider range of applications.