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Photomagnetism
The 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, accompanied by the reversal of the spin direction of an electron. This leads to an increase in spin concentration, causing the magnetic transition.〔Pejakovic D.A.,Manson J.L.,Miller, J.S.,Epstein A.J., “Photoinduced Magnetism, Dynamics, and Cluster Glass Behaviour of a molecule-Based Magnet'', Physical Review Letters Vol 85 No 9, 1994-1997 (2000)〕 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.〔Pejakovic D.A.,Manson J.L.,Miller, J.S.,Epstein A.J.,〕
==Mechanism==
The magnetisation and demagnetisation (where not demagnetised thermally) occur through intermediate states 〔Gutlich P., Yann G., Woike T., “Photoswitchable co-ordination compounds”, Coordination Chemistry Reviews pp839-879 (2001)〕 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, and this leads to the magnetised state being metastable and persisting for a long period at low temperatures.
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