Dear Valentina Francia;

Superparamagnetism is a form of magnetism, which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles,with a certain critical size when their size is smaller than 20 nm and having a single magnetic domain can exhibit behaviour similar to paramagnetism despite being below the Curie temperature (for a ferromagnetic substance) or the Néel temperature (for an antiferromagnetic or ferrimagnetic substance magnetization can randomly flip direction under the influence of temperature. The typical time between two flips which is called the Néel relaxation time. In the absence of external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Néel relaxation time, their magnetization appears to be in average zero: they are said to be in the superparamagnetic state. In this state, an external magnetic field is able to magnetize the nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is much larger than the one of paramagnets . for more details can read the following reference,

Superparamagnetic iron oxide nanoparticulate system: synthesis, targeting, drug delivery and therapy in cance

https://pubs.rsc.org/en/content/articlelanding/2019/dt/c9dt00459a#!divAbstract

Dear Valentina Francia;

Superparamagnetism is a form of magnetism, which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles,with a certain critical size when their size is smaller than 20 nm and having a single magnetic domain can exhibit behaviour similar to paramagnetism despite being below the Curie temperature (for a ferromagnetic substance) or the Néel temperature (for an antiferromagnetic or ferrimagnetic substance magnetization can randomly flip direction under the influence of temperature. The typical time between two flips which is called the Néel relaxation time. In the absence of external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Néel relaxation time, their magnetization appears to be in average zero: they are said to be in the superparamagnetic state. In this state, an external magnetic field is able to magnetize the nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is much larger than the one of paramagnets . for more details can read the following reference,

Superparamagnetic iron oxide nanoparticulate system: synthesis, targeting, drug delivery and therapy in cance

https://pubs.rsc.org/en/content/articlelanding/2019/dt/c9dt00459a#!divAbstract

Hi

Superparamagnetism is a type of magnet that appears in small ferromagnetic or ferromagnetic nanoparticles. In nanoparticles small enough, magnetization is affected by temperature changes.

The following articles may be helpful.

Article Superparamagnetic iron oxide nanoparticles: Magnetic nanopla...

Article Magnetic fluid hyperthermia: Focus on superparamagnetic iron...

Article Easy Synthesis and Magnetic Properties of Iron Oxide Nanoparticles

The previous answers have summarized the facts very well. Basically, any ferro- or ferrimagnetic nanomaterial can behave as a superparamagnet, when their size is really small, less than 20 nm or so. At that point, they approach single domain structure. In this following article, a size-dependent study on various properties of Fe3O4 nanoparticles has been made, where magnetic properties are explained in detail using Mossbauer spectra. This may help:

Article Size-modulation of functionalized Fe3O4: Nanoscopic customiz...

Dear all,

you forgot to mention two important parameters, when speaking about the superparamagnetic behaviour, in particular the temperature and the time window of the observation. In order that a system of magnetic particles displays superparamagnetic behaviour it is necessary that the time window of the observation is much larger than the (Neel) relaxation time of the particle which is on the other hand exponentially dependent on the ratio of the anisotropy energy (essentially KV, where K is the effective anisotropy constant and V the characteristic volume of the particle) and the thermal energy kT. In other words, at sufficiently low temperature (more specifically below the "blocking temperature" of the average particle) the system will behave like an ensemble of magnetic moments with possible interparticle interaction. Above the blocking temperature the system is superparamagnetic with the moment of the particle playing the role of the atomic moment in classical paramagnet.