According to the group theory approach, linear magnetoelectric effect (ME) can not be obtained for the spatial group of YMn _2O _5, which was known to be mmm. Regard to the magnetic structure of these type of materials, we propose a magnetic group structure for the YMn _2O _5 by considering spin orientation of the Mn ~(3+) and Mn ~(4+) ions. According to the landau theory of phase transition it can be shown, how symmetrical rules result in relationship between quantities such as magnetic order, polarization, and etc. This relation shows a weak ferromagnetic state, associated with spontaneous polarization, arisen by Dzyaloshinskii-Moriya type interaction and a field induced change in magnetoelectrical susceptibility.
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