Optical Studies of Phase Transition in Ferroelectric-ferromagnetic Materials

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The optical measurements have been undertaken to clarify the mechanism of the structural phase transitions in solid state materials where the ferroelectricity and the ferromagnetism or antiferromagnetism coexist. BaMnF_4, which is a typical material with both the ferroelectricity and antiferromagnetism, has a incommensurate structural phase transition at 250 K. Unlike most materials which exhibit the incommensurate phase transition, however, BaMnF_4 crystal never recovers to a commensurate phase at a finite temperature by cooling from 250 K. The optical measurements such as the photoluminescence, luminescence decay, time-resolved luminescence spectra, linear dichroism and circular dichroism spectra have been performed to find the reason of such an unusual structural phase transition in BaMnF_4, in addition to the heat-capacity measurement using an AC calorimeter. Various magnetic materials and ferroelectric materials (including K_2ZnCl_4) have been also investigated to compare with the results of BaMnF_4.An anomaly was observed at 250 K in BaMnF_4 in the temperature dependences of luminescence lifetime, circular dichroism and linear dichroism. From the analysis it was confirmed that the phase transition is of the first order one and the crystal structure is changed into a monoclinic one. From the luminescence spectra and lifetime was found an energy transfer from Mn ions into uninstitutionally doped impurities. It was suggested that the behavior of Mn ions gives an influence to the structural phase transition.Various materials such as three-dimensional (3D) antiferromagnet MnF_2, Rb_2MnCl_4 which is the same 2D antiferromagnet as BaMnF_4, 2D ferromagnet FeCl_2 and 1D antiferromagnet [(CH_3)_3NH]MnCl_3・2H_2O have been investigated spectroscopically to examine the energy transfer in magnetic materials in detail and to make clear the difference with the results on BaMnF_4. It was confirmed that a magneto-electric effect is involved in the phase transition in BaMnF_4.