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On the basis of the evolution observed today,
we can estimate that recording densities of 10 million octets/cm2
will be reached by around 2005. At these densities, the size of magnetic
particles that carry information is on the order of 5 nm. The phenomenon
known as superparamagnetism, an impediment to increasing recording densities,
is beginning to be observed. Research carried out by a team of researchers
from the Louis Néel Laboratory (CNRS-Grenoble), the University
of Delaware and the University of Barcelona(1) , provides the keys to
overcoming this phenomenon. These results are published in the June 19,
2003 issue of the journal, Nature.
From magnetic recording (magnetic tape,
computer hard disks) to medicine, a variety of fields of application require
the use of ferromagnetic particles of increasingly small size. These particles
are small permanent magnets: their global magnetic moment, consisting
of the parallel arrangement of all the atomic magnetic moments, is frozen
in a specific direction.
When the size of particles decreases, their moments become increasingly
sensitive to the haphazard effects induced by temperature. The direction
of the moments begins to fluctuate at random. This phenomenon is known
as superparamagnetism. It is assumed that it defines a physical limit
to the increase in recording density since recorded information is lost
at that point.
Scientists have studied the properties of cobalt particles, one of the
simplest ferromagnetic materials, measuring from 3 to 4 nanometers in
diameter. When embedded in a nonmagnetic carbon or alumina matrix(2),
the nanoparticles react normally: they become superparamagnetic as soon
as the temperature is greater than -240°C. Scientists discovered that
the same nanoparticles are no longer superparamagnetic when they are embedded
in an antiferromagnetic matrix . An extra source of energy resulting from
the interaction between the ferromagnetic particle and the antiferromagnetic
matrix therefore exists and forms a very effective barrier against magnetic
moment fluctuations. The use of antiferromagnetic matrices also paves
the way for recording densities greater than the limits reached by current
methods.
1 - D.Givord, V.Skumryev and J.Nogués,
respectively.
2 - Antiferromagnetism defines a large class of magnetic materials, described
for the first time by Louis Néel, in which the spins of atomic
moments alternate, resulting in a global moment equal to zero.
Reference:
Beating the Superparamagnetic Limit with Exchange Bias, Vassil Skumryev,
Stoyan Stoyanov, Yong Zhang, George C. Hadjipanayis, Dominique Givord
and Josep Nogués; Nature, June 19, 2003.
Researcher
contacts:
Dominique Givord
Tel: +33 4 76 88 10 90
E-mail: givord@grenoble.cnrs.fr
Mathematics and Physical Sciences Department
contact:
Frédérique Laubenheimer
Tel: +33 1 44 96 42 63
E-mail: frederique.laubenheimer@cnrs-dir.fr
Press Contact
CNRS:
Muriel Ilous
Tel: +33 1 44 96 43 09
E-mail: muriel.ilous@cnrs-dir.fr
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