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Superparamagnetism refers to materials which become magnetic in the presence of an external magnet, but revert to a non magnetic state when the external magnet is removed. This is easily seen with a small tube containing iron oxide particles in water; the solution appears brown (rust like); when an external magnet is held to the side of the tube, the FeO particles become magnetic, and are attracted toward the external magnet; the solution becomes clear. When the external magnet is removed, the particles lose their magnetic propertys, and the solution becomes brown again.
Superparamagnetism is a phenomenon by which magnetic materials may exhibit a behavior similar to paramagnetism even when at temperatures below the Curie or the Néel temperature. This is a small length-scale phenomena, 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. Image File history File links Information_icon. ... Magnetic lines of force of a bar magnet shown by iron filings on paper In physics, magnetism is one of the phenomena by which materials exert an attractive or repulsive force on other materials. ... Simple Illustration of a paramagnetic probe made up from miniature magnets. ... The Curie point is a term in physics and materials science, named after Pierre Curie (1859-1906), and refers to a characteristic property of a ferromagnetic material. ... ... A bar magnet. ... 1. ...

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, i.e. superparamagnetic stage. The idea of superparamagnetism is used in SuperParamagnetic Clustering algorithm (SPC) as well as in its extension global SPC. Ferromagnetism is the phenomenon by which materials, such as iron, in an external magnetic field become magnetized and remain magnetized for a period after the material is no longer in the field. ... For other senses of this term, see magnetic field (disambiguation). ... In physics, the Curie point, or Curie temperature, is the temperature above which a ferromagnet loses its ferromagnetic ability to possess a net (spontaneous) magnetization in the absence of an external magnetic field. ... The Neel temperature, TN, is the temperature at which an antiferromagnetic material becomes paramagnetic - that is, the thermal energy becomes large enough to upset the magnetic ordering within the material. ... SPC is an acronym that can stand for: Single Photon Counting, a light measurement technique used in many digital analysis physics-related disciplines Samurai Pizza Cats, a cult classic anime show (known in Japan as Kyattou Ninden Teyandee) Secretariat of the Pacific Community, a Pacific Islands regional intergovernmental organisation Segregated...

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. A crystallite is a domain of solid-state matter that has the same structure as a single crystal. ...

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/sq. inch, though this estimate is constantly changing.^[1] Typical hard drives of the mid-1990s. ...

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. [1] [2].Perpendicular recording is predicted to allow information densities of up to around 1 Tbit/sq. inch (1000 Gbit/sq. inch). --reference is on the perpendicular recording page Perpendicular recording (or Perpendicular Magnetic Recording, PMR) is a recently implemented technology for data recording on hard disk. ...

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. HAMR is an abbrevation for Heat-Assisted Magnetic Recording. ...

Applications of Superparamagnetism

General Applications

• Ferrofluid : Tunable Viscosity
• Sensors : High sensitivity (GMR,BARCIII)
• Self-Assembling

Bio-Medical Applications

• Detection : Magnetic Resonance Imaging (MRI)
• Separation : Cell-, DNA-, protein- separation, RNA fishing
• Treatment : Drug delivery, hyperthermia, magnetofection

References

1. ^ 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

External links

• Perpendicular recording graphically explained
• Perpendicular recording explained through a Flash animation
• D. Weller and A. Moser, “Thermal Effect Limits in Ultrahigh Density Magnetic Recording,” IEEE Trans. Magn. 35, 4423– 4439 (1999).
• L Neel Ann. Geophys. 5 99 (1949)