Environmental Engineering Reference
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20-30 nm in size contain a single magnetic domain with a single magnetic
moment and exhibit superparamagnetism [64]. A material in a paramag-
netic phase is characterized by randomly oriented (or uncoupled) mag-
netic dipoles, which can be aligned only in the presence of an external
magnetic i eld and along its direction. h is type of material has no coer-
civity nor remanence, which means that when the external magnetic i eld
is switched of , the internal magnetic dipoles randomize again; no extra
energy is required to demagnetize the material and hence the initial zero
net magnetic moment is spontaneously recovered. A nanoparticle with
such magnetic behavior is superparamagnetic (SPM).
h e size reduction of magnetic materials shows interesting advantages
that make them more suitable for therapeutic, diagnostic and environmen-
tal techniques compared to their bulk counterparts. Magnetic parameters
such as the coercivity of the nanoparticles can be i nely tuned by decreas-
ing their size. Moreover, a further reduction of the size below a certain
value of the radius, the so-called superparamagnetic radius (rSP), induces
a magnetic transition in particles where both ferro- and ferrimagnetic
nanoparticles (FM) become superparamagnetic and, as previously stated,
high magnetic moments are observed under the ef ect of a magnetic
i eld, but no remanent magnetic moment will be present when the exter-
nal magnetic i eld is removed. Superparamagnetism is a property strictly
associated with nanostructured magnetic materials and arises when the
thermal energy is sui ciently high to overcome the magnetic stabiliza-
tion energy of the particle. With increasing size, the particles will become
blocked as thermal energy becomes insui cient to allow the free rotation
of spins. Superparamagnetic particles are usually ordered below a blocking
temperature.
11.5
Synthesis of Magnetic Nanoparticles
During the past few years, a large number of the published articles about
nanoparticles have described ei cient routes to attain shape-controlled,
highly stable, low-cost, ecofriendly, and narrow size distribution mag-
netic nanoparticles. Currently, several popular methods including co-
precipitation, microemulsion, thermal decomposition, sonochemical,
microwave assisted, chemical vapor deposition, combustion synthesis,
carbon arc, laser pyrolysis synthesis, hydrothermal synthesis and sol-gel
synthesis have been reported for synthesis of magnetic nanoparticles.
h e synthesis of superparamagnetic nanoparticles is a complex process
because of their colloidal nature. Over the last decades, much research has
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