Monodomain magnetic nanoparticles, due to their size, demonstrate physical properties not seen in the bulk materials, such as size-dependent magnetization reversal fields. They can be also made into a magnetic suspension or ferrofluid. There is thus growing interest in the application of these materials to ferrofluids, spintronics, directed assembly, as well as for imaging and therapeutic applications. In this article, we provide an overview of these materials, discuss the fundamental physical properties, describe several routes for the “bottom-up” generation of these materials, and identify major challenges for the future of these fields. The articles in this issue describe various aspects of the characterization and application of magnetic nanoparticles.

Using computational modeling, we describe and explain the effects resulting from surfaces and interfaces in core–shell nanoparticles. We outline the basis of the atomistic spin model, which is used to simulate the equilibrium and dynamic magnetic properties of magnetic nanoparticles. The physical origin of magnetic surface anisotropy is described, along with its effect on the magnetic spin configuration and energy landscape. Importantly, it is shown that a cubic anisotropic surface can be induced, which leads to a complex energy landscape with a non-trivial size dependence. Additional microstructural effects in realistic nanoparticle microstructures are investigated, and fundamental magnetic properties can be significantly altered as a result. Finally, an important effect known as exchange bias is also described. Exchange bias causes an enhancement of the thermal stability of magnetic nanoparticles, but due to its atomic origin, it also leads to complicated physical behavior.

This article reviews the principles of magnetic field-directed self-assembly (MFDSA) of magnetic nanoparticles (MNPs), along with recent studies that advance the fundamental understanding and potential capabilities of MNP MFDSA. This technology could eventually find application in manufacturing novel materials and components for biomedicine, energy, optics, functional composites, and microfluidics. In MFDSA, an externally applied field drives the assembly of MNPs. Uniform fields can create complex chains of MNPs, while inhomogeneous fields (such as those created by permanent magnets) apply attractive forces to MNPs that pull them toward the region of strongest field strength. Thus, MNPs can be self-organized as well as directed into user-designed patterns by controlling the external field arrangement. Because of its biocompatibility, nanoscale resolution, and low cost, MFDSA is a highly versatile technique that could enable high volume nanomanufacturing of MNPs into complex, finished materials.

In suspensions of magnetic nanoparticles in appropriate carrier liquids—commonly called ferrofluids—an external magnetic field can control the fluid properties such as viscosity. This article provides an overview of the properties and general makeup of ferrofluids, as well as classical fluid dynamics. Some of the applications of ferrofluids are also described.

Mesoscale (nanometers to microns) magnetic particles are becoming increasingly important in biomedical applications both in vitro for cell and tissue-based research and in vivo for clinical imaging and therapy. These applications generally rely on the fact that, while the body is relatively transparent to magnetic fields, magnetic particles within the body, or in ex vivo biological samples, will couple strongly to applied external fields. By synthesizing bio-functionalized, biocompatible polymer/magnetic particle composites, this remote coupling provides a mechanism for the precisely targeted actuation of cell signaling pathways, delivery of genes, targeted transmission of thermal energy, generation of tissue matrix, and imaging (via magnetic resonance imaging), among others. This article explores a variety of biomedical applications of mesoscale magnetic particles, some of which are routinely used in the clinic and in biomedical laboratories, with others approaching the realm of science fiction.

New instrumentation is being developed to better understand the in vivo properties of magnetic particles suspended in solution or lodged in tissue. We describe three novel methods with the necessary sensitivity to measure the microscopic magnetic properties of individual magnetic particles and complexes quantitatively. The first method is based on proton nuclear magnetic resonance of a magnetic particle suspended in water in a microcapillary probe; the second method uses high-resolution magnetic resonance imaging of water surrounding a magnetic particle; and the third method is based on AC susceptometry with a magnetic cantilever that combines magnetic particle imaging concepts with probe microscopy. We present the physical basis for the measurements, estimate sensitivity limits, and discuss future impacts on the development of magnetic particles for bioimaging and bioassays.

When colloidal nanoparticles made of soft magnetic materials have strong interparticle interactions, new magnetic phases may be formed, such as super-ferromagnets or super-spin glasses, with different local temperature and field-dependent magnetization dynamics. Magneto-transport experiments on nanoscale tunnel junctions formed in magnetic nanoparticle arrays, defined either by scanning probe tips or patterned nanoscale electrodes, can be used as probes of the magnetization dynamics.

Access to affordable and reliable energy has been a cornerstone of the world’s increasing prosperity and economic growth since the beginning of the Industrial Revolution. Our use of energy in the 21st century must also be sustainable. This article provides a techno-economic snapshot of the current energy landscape and identifies several research and development opportunities and challenges, especially where they relate to materials science and engineering, to create the foundation for this new industrial revolution.