What characterizes superparamagnetic nanoparticles?

Superparamagnetic nanoparticles are characterized by their unique behavior in the presence of an external magnetic field. Unlike ferromagnetic materials, which retain magnetization even after the external magnetic field is removed, superparamagnetic nanoparticles do not exhibit permanent magnetization. Instead, when subjected to an external magnetic field, these nanoparticles can quickly align their magnetic moments, leading to a strong response to the magnetic field.

This property makes superparamagnetic nanoparticles particularly useful in a variety of applications, such as targeted drug delivery and magnetic resonance imaging (MRI), where a temporary and reversible magnetic response is advantageous. The ability to switch their magnetic properties on and off in response to an external field allows for precise control and manipulation in biomedical applications.

In contrast, the other options describe characteristics that do not pertain to superparamagnetic nanoparticles. These materials do not retain magnetization indefinitely (which distinguishes them from ferromagnetic materials), they are very responsive to magnetic fields rather than being unresponsive, and while stability under extreme temperature changes is significant in many contexts, it is not a defining characteristic of superparamagnetism.