A single molecule magnet is an example of a macroscopic quantum system. It possesses a magnetic spin, which can tunnel between different quantum states (Fig. 1), providing a model system for sensitive magnetic measurements. Other examples of macroscopic quantum systems include superconducting devices such as Josephson junctions and Quantum phase-slip junctions.
Sensitive magnetic measurements
One of the challenges of metrology is to make sensitive magnetic measurements at the nanoscale. If we could detect spin flips in a single atom or molecule, we could use the spin to store information. This would enable us to increase the storage capacity of computer hard disks.
SQUID detection of spin flips
There are a number of difficulties with detecting single spin flips. First, we need a device that can respond to extremely small changes in the magnetic field. The most sensitive device available is a Superconducting Quantum Interference Device (SQUID). The main component of a SQUID is a ring of superconducting material (Fig. 2), and the sensitivity of a SQUID to magnetic fields depends on the area of the ring. Therefore, we need to develop a SQUID with the smallest area possible.
Small magnetic particles
The next challenge is to find a suitable atom or molecule, so that we can try to detect its spin. The smallest spin that an atom can carry is equal to the spin of an electron eħ/2me, where e is the charge on an electron, ħ is Planck’s constant over 2π, and me is the mass of an electron. We call this amount of spin the Bohr magneton μB; an electron has a spin of 1 μB. If an atom has many electrons, it can have a spin of a few Bohr magnetons, and a molecule containing many atoms can possess an even larger spin.
A good starting point for trying to detect spin flips is to find a molecule with a spin of several Bohr magnetons. There is a very well studied molecular magnet, Mn12-acetate, which has a spin S = 10 (Figure 3). This molecule is a disc-shaped organic molecule in which twelve Mn ions are embedded. Eight of these form a ring, each having a charge of +3 and a spin S = 2. The other four form a tetrahedron, each having a charge of +4 and a spin S = 3/2. The exchange interactions within the molecule are such that the spins of the ring align themselves in opposition to the spins of the tetrahedron, giving the molecule a total net spin S = 10.
If we could deposit a single molecule of Mn12-acetate close to the centre of a nanoscale SQUID, we could use the SQUID to detect spin flips in the molecule.
However, before we can do that, we need to investigate whether the magnetic behaviour of a single molecule is the same as the magnetic behaviour of a crystal containing many millions of molecules.
Spin avalanches in large assemblies of crystals
Unusual phenomena occur in very large crystals of Mn12-acetate, where individual spin-flips can trigger an avalanche of similar events, causing the magnetisation of the entire crystal to flip at once. We have investigated how spin avalanches are affected by the thermal coupling of the crystal to its surroundings. More information can be found in our article “Influence of thermal coupling on spin avalanches in Mn12-acetate”
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