Raman and Infrared studies of the multiferroics TbMn2O5 and YMn2O5

Orthorhombic manganites of the $RMnO_{3}$ and $RMn_{2}O_{5}$ families develop an electric polarization induced, flipped and flopped by application of a magnetic field. The $Tb(Y)Mn_{2}O_{5}$ Anti-ferromagnetic order is incommensurate between $T_{N}=42K$ ($44K$) and $T_{C}=38K$ and becomes commensurate between $T_{C}=38K$ and $T=24K$ ($18K$); at this temperature ferroelectricicity appears and remains incommensurate below $24K$ ($18K$)[1,2]. We have studied the $TbMn_{2}O_{5}$ and $YMn_{2}O_{5}$ Raman active phonons as a function of temperature and compared their behavior to similar multiferroic compounds $Bi(Dy)Mn_{2}O_{5}$ phonons where the $A_{g}$ high frequencies modes soften between $T^{*}\sim 70K$ and $T_{N}$ and harden below $T_{N}\sim 42K$ [3]. The over-hardening of the phonon frequencies above anharmonicity at low temperatures confirm that the spin-lattice interaction plays an important role in the magnetoelectric properties. The $TbMn_{2}O_{5}$ infrared active phonon frequencies are also studied as a function of temperature and under applied magnetic field (up to 10 Tesla). As predicted theoretically the $TbMn_{2}O_{5}$ infrared-active phonon frequencies soften below $T_{N}$. Evolutions of the phonon frequencies under the applied magnetic field are reported and analyzed.