Revealing magnetic ordering and spin-phonon coupling in Y_1-x Tb _x MnO₃ (0.1  ⩽  x  ⩽  0.3) compounds

The structural and magnetic properties of the Y_1-x Tb _x MnO₃ (0.1  ⩽  x  ⩽  0.3) compounds were investigated. Neutron diffraction patterns for all three samples, recorded at room temperature (RT), were fitted to the nuclear structure confirming the paramagnetic nature of the compounds. At 2.8 K, for the x  =  0.1 sample magnetic moments of the Tb³⁺ ionic as well as Mn³⁺ ionic were ordered. At 5 K for the x  =  0.2 sample only the Mn³⁺ ionic magnetic moments were ordered. There were six sites for Mn atoms. Three on the z  =  0 plane and three on the z  =  0.5 plane (where z corresponds to  +c axis).The Mn³⁺ionic moments were confined to the a-b plane with a net magnitude of 2.78(3) µ _B, and 2.90(3) µ _B for the x  =  0.1 and the x  =  0.2 samples. The Tb³⁺ionic moments had a magnitude of 1.36(4) µ _B at 2.8 K and were aligned antiferromagnetically along the crystallographic c-axis for the x  =  0.1 sample. The low moment in comparison with Mn³⁺ free ions has been attributed to crystalline electric fields similar to that found in the parent compound YMnO₃ and also in another rare earth manganite viz HoMnO₃. The x  =  0.3 sample was found to be a canonical spin glass. To investigate the role of the above spin ordering in Y_1-x Tb _x MnO₃ in governing the phonon dynamics, temperature dependent Raman measurements were carried out. We observed the deviation of the phonon frequency near 685 cm^-1 and its line-width from the expected anharmonic behaviour around magnetic ordering temperature for Tb substituted compounds with x  =  0.1 and 0.2. This was attributed to the spin-phonon coupling in these systems. The anomalous behaviour of this phonon mode in the canonical spin glass compound with x  =  0.3, indicated that the coupling sustained even in the presence of only local magnetic ordering.