'''Superparamagnetism''' is a form of magnetism which occurs only in the presence of an externally applied magnetic field.  Unlike ferromagnetism, superparamagnetic objects do not retain any magnetization in the absence of an externally applied magnetic field.

Specifically, superparamagnetism is a phenomenon in which magnetic materials may exhibit a behavior similar to Curie or the Néel temperature. This is a small length-scale phenomenon, where the energy required to change the direction of the magnetic moment of a particle is comparable to the ambient thermal energy. At this point, the rate at which the particles will randomly reverse direction becomes significant.

Normally, coupling forces in ferromagnetic materials cause the magnetic moments of neighboring atoms to align, resulting in very large internal magnetic fields.  This is what distinguishes ferromagnetic materials from paramagnetic materials. At temperatures above the Curie temperature (or the Neel temperature for antiferromagnetic materials), the thermal energy is sufficient to overcome the coupling forces, causing the atomic magnetic moments to fluctuate randomly.  Because there is no longer any magnetic order, the internal magnetic field no longer exists and the material exhibits paramagnetic behavior. If the material is non-homogeneous, one can observe a mixture of ferromagnetic and paramagnetic clusters of atoms at the same temperature, the superparamagnetic stage. The idea of superparamagnetism is used in [http://ctwc.weizmann.ac.il/spc.html SuperParamagnetic Clustering] algorithm (SPC) as well as in its extension [http://vcclab.org/lab/spc global SPC].

Superparamagnetism occurs when the material is composed of very small crystallites (1–10 nm).  In this case even when the temperature is below the Curie or Neel temperature (and hence the thermal energy is not sufficient to overcome the coupling forces between neighboring atoms), the thermal energy is sufficient to change the direction of magnetization of the entire crystallite.  The resulting fluctuations in the direction of magnetization cause the magnetic field to average to zero.  Thus the material behaves in a manner similar to paramagnetism, except that instead of each individual atom being independently influenced by an external magnetic field, the magnetic moment of the entire crystallite tends to align with the magnetic field.  

The energy required to change the direction of magnetization of a crystallite is called the crystalline anisotropy energy and depends both on the material properties and the crystallite size. As the crystallite size decreases, so does the ''crystalline anisotropy energy'', resulting in a decrease in the temperature at which the material becomes superparamagnetic.

The rate at which particles will lose their direction is governed by the Neel-Arrhenius equation. In particular, it is a function of the exponential of the grain volume. 

==Effect on hard drives==
Superparamagnetism sets a limit on the storage density of hard disk drives due to the minimum size of particles that can be used. This limit is known as the '''superparamagnetic limit'''. Current hard disk technology with longitudinal recording has an estimated limit of 100 to 200 Gbit/in², though this estimate is constantly changing.<ref>Kryder, M.H. (April 2005) "Magnetic recording beyond the superparamagnetic limit". ''Magnetics Conference, 2000. INTERMAG 2000 Digest of Technical Papers. 2000 IEEE International'' pp. 575-575</ref>

One suggested technique to further extend recording densities on hard disks is to use perpendicular recording rather than the conventional longitudinal recording. This changes the geometry of the disk and alters the strength of the superparamagnetic effect. [http://www.hitachigst.com/hdd/hddpdf/tech/chart13.pdf] [http://www.hitachigst.com/hdd/research/recording_head/pr/PerpendicularAnimation.html].Perpendicular recording is predicted to allow information densities of up to around 1 Tbit/in² (1024 Gbit/in²). --reference is on the perpendicular recording page

Another technique in development is the use of HAMR drives, which use materials that are stable at much smaller sizes. But, they require heating before the magnetic orientation of a bit can be changed.

Il '''superparamagnetismo''' è un fenomeno per il quale materiali magnetici possono esibire un comportamento simile al paramagnetismo anche a temperature inferiori alla temperatura di Curie o alla temperatura di Néel. Questo è un fenomeno che avviene su dimensioni di scala ridotta nel caso di materiali per i quali l'energia necessaria per cambiare la direzione del momento magnetico delle particelle è comparabile con l'energia termica del reticolo. In queste condizioni, la percentuale di particelle che invertono casualmente l'orientamento magnetico diviene significativa.

==Analogie con il paramagnetismo==

Si parla di paramagnetismo, la cui teoria fu sviluppata da Paul Langevin, quando si ha un insieme di atomi o molecole ciascuno dei quali possiede un momento di dipolo magnetico permanente. Un aggregato di tali oggetti in generale non detiene una magnetizzazione stabile perché disturbato continuamente dall'agitazione termica.  Applicando un campo magnetico H ad un sistema di particelle del genere, esse si orienteranno seguendo il campo applicato H, ma una volta rimosso il campo esse torneranno ad essere un insieme "magneticamente disordinato". 

Per superparamagnetismo si intende un insieme di oggetti tali per cui il singolo elemento non è un solo atomo ma una regione compatta formata da un aggregato di 1000-100000 atomi. Se un atomo di ferro ha un magnetoni di Bohr, che è l'unità di misura di riferimento nel magnetismo della materia, un oggetto superparamagnetico avrà un momento di dipolo magnetico di 2 000 - 200 000 magnetoni di Bohr. È tuttavia possibile che scendendo a basse temperature - poche decine di ferromagnetico. La temperatura per la quale ciò si verifica prende il nome di temperatura di bloccaggio.