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Posted on Thu, Feb 22, 2018

Magnetic properties of nanoparticles

Magnetic properties of nanoparticles are used for drug delivery, therapeutic treatment, contrast agents for MRI imaging, bioseparation, and in-vitro diagnostics.  These nanometer-sized particles are superparamagnetic, a property resulting from their tiny size—only a few nanometers—a fraction of the width of a human hair (nanoparticles are approximately 1/1,000 thinner than human hair). Superparamagnetic nanoparticles are not magnetic when located in a zero magnetic field, but they quickly become magnetized when an external magnetic field is applied. When returned to a zero magnetic field they quickly revert to a non-magnetized state. Superparamagnatism is one of the most important properties of nanoparticles used for biomagnetic separation.

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Magnetization of superparamagnetic properties of nanoparticles

Magnetization is defined as the extent to which a material becomes magnetized when placed within a magnetic field, and is a measure of the net magnetic dipole moment per unit volume. The magnetization is directly related to the applied magnetic field.

Magnetic susceptibility (χ) is a dimensionless quantity that indicates the degree of magnetization of a material in response to a magnetic field. Magnetic susceptibility is equal to the ratio of magnetic dipole moment to magnetic field.

  • M=magnetization (magnetic dipole moment per unit volume)
  • B=applied magnetic field
  • χ=magnetic susceptibility

χ =M/B

Non-magnetic materials, like water, are diamagnetic and experience a small negative response to the magnetic field. In the case of diamagnetic materials, the susceptibility is negative and has a value around -10e-5. Paramagnetic materials, like oxygen molecules, experience a slightly positive response to the magnetic field. In the case of paramagnetic materials, the magnetic susceptibility is around 10e-4 or 10e-5. In both cases, the magnetic dipoles attempt to align with the external magnetic field, and fight against thermal agitation. Diamagnetic and paramagnetic dipoles don’t interact with each other, and their susceptibility at room temperature is very small.

 

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The situation is very different for magnetic materials, which are technically defined as ferro and ferrimagnetic materials. In magnetic materials, each dipole interacts with its nearest neighbors. This inter-dipole interaction creates a high magnetic response throughout the material, with susceptibilities that can reach values higher than 10e3. The drawback is that the materials show magnetic hysteresis: after applying a magnetic field, the material will have ‘memory’, meaning that when the applied field returns to zero the material remains at a certain magnetization. This is a very useful property for developing permanent magnets, but it is a problem for many Life Science applications. Ferromagnetic beads will remain magnetized after the removal of an applied magnetic field, and the beads will form irreversible clumps or aggregates. The ideal material for biological applications would have a high magnetic susceptibility, but no magnetic ‘memory’. They would combine a high response to applied magnetic field with a non-magnetic behavior once the magnetic field is removed. Advances in nanotechnology have provided a way to obtain these superparamagnetic properties of nanoparticles by reducing the size of the ferro- or ferrimagnetic material to few nanometers (below the so-called superparamagnetic diameter). When they are below the superparamagnetic diameter the nanoparticles are able to return quickly to a non-magnetized state after an external magnet is removed. Larger ferro- and ferrimagnetic materials have remnant magnetism after the applied magnetic field returns to zero.

magnetic properties of nanoparticles

 

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