Superparamagnetism Scientific Article By Dr.Nasma Adnan Date: 10/11/2022 | Views: 170

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Superparamagnetism is a form of magnetism exhibited by small ferromagnetic or ferrimagnetic nanoparticles. At sizes of less than a hundred nanometers, the nanoparticles are single-domain particles, allowing the magnetization of the nanoparticles to be approximated as one giant magnetic moment by summing the individual magnetic moments of each constituent atom. This approximation is called the “macro-spin approximation.” When the nanoparticles are small enough, the energy barriers for magnetization reversal, which are proportional to grain volume, are relatively low compared to thermal energy.
When the nanoparticles are small enough, the energy barriers for magnetization reversal, which are proportional to grain volume, are relatively low compared to thermal energy. With enough thermal energy, their magnetization can flip direction randomly over short periods of time and the time between two flips in direction is called the Neel relaxation time. The superparamagnetic state refers to how the average magnetization of the nanoparticles averages to zero when no external magnetic field is applied and the measurement time for the magnetization of the nanoparticles is greater than the Neel relaxation time.
With an external magnetic field applied, the nanoparticles are magnetized like paramagnets, but with much greater susceptibility. While any ferromagnetic or ferromagnetic material can exhibit paramagnetic behavior, the difference is that this usually occurs above the Curie temperature whereas in superparamagnets, this occurs below the Curie temperature.