Photomagnetism (photomagnetic effect) is the effect in which a material acquires (and in some cases loses) its ferromagnetic properties in response to light. The current model for this phenomenon is a light induced

electron transfer Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons. Electrochemical processes ar ...

, accompanied by the reversal of the spin direction of an

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

. This leads to an increase in spin concentration, causing the magnetic transition. Currently the effect is only observed to persist (for any significant time) at very low temperature. But at temperatures such as 5K, the effect may persist for several days.

Mechanism

The magnetisation and demagnetisation (where not demagnetised thermally) occur through intermediate states as shown (right). The magnetising and demagnetising wavelengths provide the energy for the system to reach the intermediate states which then relaxe non-radiatively to one of the two states (the intermediate state for magnetisation and demagnetisation are different and so the photon flux is not wasted by relaxation to the same state from which the system was just excited). A direct transition from the ground state to the magnetic state and, more importantly, vice versa is a

forbidden transition In spectroscopy, a forbidden mechanism (forbidden transition or forbidden line) is a spectral line associated with absorption or emission of photons by atomic nuclei, atoms, or molecules which undergo a transition that is not allowed by a particul ...

, and this leads to the magnetised state being

metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

and persisting for a long period at low temperatures.

Prussian blue analogues

One of the most promising groups of molecular photomagnetic materials are Co-Fe

Prussian blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...

analogues (i.e. compounds with the same structure and similar chemical make up to Prussian blue.) A Prussian blue analogue has a chemical formula M_1-2xCo_1+x e(CN)₆��zH₂O where x and z are variables (z may be zero) and M is an alkali metal. Prussian blue analogues have a face centre cubic structure. It is essential that the structure be

non-stoichiometric In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); m ...

. In this case the iron molecules are randomly replaced by water (6 molecules of water per replaced iron). This non-stoichiometry is essential to the photomagnetism of Prussian blue analogues as regions which contain an iron vacancy are more stable in the non-magnetic state and regions without a vacancy are more stable in the magnetic state. By illumination by the correct frequency one or another of these regions can be locally changed to its more stable state from the bulk state, triggering the phase change of the entire molecule. The reverse phase change can be accomplished by exciting the other type of region by the appropriate frequency.

Mechanism

Prussian blue analogues

See also

References

Further reading