What is the difference between paramagnetic and superparamagnetic

Nanoparticles typically have magnetic anisotropy, meaning they have a preferred direction for magnetization alignment. As a result, the magnetic moment usually has only two stable, antiparallel orientations separated by an energy barrier. The two stable orientations are defined as along the nanoparticle's "easy axis.

In this state, the nanomaterials behave like a normal paramagnet but with a much higher susceptibility. These two states are illustrated in Figure 1 below. It can be calculated by the following equation:. Some typical measurement times for different types of measurements are as follows:. When an external magnetic field is applied to superparamagnetic particles, the particles have a net magnetization because the magnetic moments of the particles will begin to align with the applied field.

Assuming that all the particles are of similar size, there are two possible cases for the net magnetization and the corresponding susceptibility, depending on the temperature T. As mentioned in the introduction, superparamagnetivity occurs due to the small size of the particles. In Figure 2 below, the magnetization of paramagnetic, ferromagnetic and superparamagnetic materials in response to an external magnetic field. As shown in the above figure, the ferromagnetic response has a hysteresis loop.

When ferromagnetic particles increase in size, the magnetic moment increases, which in turn increases the magnetization and allows the magnetization to reach its saturation, or maximum, value quicker. This translates to a thinner hysteresis loop.

Conversely, decreasing the particle size will widen the hysteresis loop until a certain nanoparticle size, called the critical size. Once this size is reached, the hysteresis loop begins to narrow with decreasing size until the superparamagnetic size threshold is reached. When the nanoparticles reach superparamagnetic sizes, response curve retains the sigmoidal shape of a ferromagnetic response but loses the loop. Figure 3 below shows this pattern in a plot of coercivity, or the intensity of the applied magnetic field to yield a zero magnetization, against nanoparticle size.

The superparamagnetic limit refers how superparamagnetism limits the storage capacity of hard drive disks HDDs by setting a minimum size for the magnetic grains used.

In an HDD, data, or bits, is stored on platters of concentric tracks as a sequence of magnetization direction changes. Reading these platters refers to translating the magnetization into a voltage and writing is just the reverse process. Within a single bit cell are many magnetic grains. In order to decrease the transition noise, the size of the magnetic grains must be decreased. Handbook of Porous Silicon pp Cite as. Later version available View entry history.

In this chapter the paramagnetic properties of nanostructured silicon are outlined and furthermore the magnetic properties of a composite material consisting of porous silicon with infiltrated superparamagnetic iron oxide nanoparticles are discussed. The magnetic behavior of the system depends on the nanoparticle size as well as on the magnetic coupling between them.

Both influence the so-called blocking temperature, the transition between superparamagnetic behavior and blocked state. A particle size-related assessment shows that the blocking temperature increases with increasing particle size if the distances between the particles are equal. The blocking temperature can be decreased by weakening the magnetic interaction between the particles. Due to the good biocompatibility of both porous silicon and iron oxide nanoparticles, the composite system is of interest for biomedical applications in the fields of therapy and diagnosis.

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