Studying Single Molecule Superparamagnetism

Moving smoothly on from yesterday’s post about ferromagnetism, comes today’s on paramagnetism. Paramagnetism is where a material placed in a magnetic field, develops tiny induced magnetic fields within it lining up with the exterior magnetic field. This results in the material being attracted to the source of the magnetic field along the field lines. This is, in fact, the nature of ferromagnetic materials when they are above their Curie temperature. More specifically, this recent paper is on the topic of superparamagnetism. This is where a ferromagnetic nanoparticle can randomly switch its magnetisation due to thermal fluctuations in the vicinity. Overall, when a long observation period is taken, the net magnetisation is in fact zero. The similarity with regular paramagnetism is that when an external field is applied, the nanoparticles tend to align along the applied field, ending with a net magnetisation, normally of greater strength than regular paramagnetic materials.

Now because it’s temperature that causes the random switching of the magnetic field, cooling the particles reduces the rate at which they switch. If the time of measurement is much longer than the average time taken between the magnetic field switching direction, then, as already stated, the net field is zero. If the time of measurement is very short then the particle will not have time to switch direction and so it will act like a permanent magnet during this time. Normally the time period of measurement is kept constant and the temperature is varied. The temperature above which the magnetic field is measured as zero is called the blocking temperature. The aim of creating single molecule magnets that exist at higher and higher temperature is very important.

It has been previously shown that one of the best ways of increasing the blocking temperature is to first use lanthanides (one of those fifteen metal ions taken from the bottom of the periodic table).

Paper links: Single molecule magnet with an unpaired electron trapped between two lanthanide ions inside a fullerene


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