Ultrasonic study of $Gd_{5}(Si_{2}Ge_{2})$ elastic properties

$Gd_{5}(Si_{2}Ge_{2})$ undergoes a magnetic-martensitic transition near room temperature modifying its symmetry from an orthorhombic to a monoclinic structure. A giant magnetocaloric effect ($\Delta T/\Delta B\sim 8K/2T$) and a colossal strain (up to 10000ppm) can be induced both thermally and magnetically. Due to low hysteresis ($<$2K,$<$0.5T), the material has a potential for energy efficient refrigeration and actuation uses. The acoustic phonon properties of the $Gd_{5}(Si_{2}Ge_{2})$ single crystals, grown by tri-arc pulling technique were studied by echo-pulse ultrasonic probing. For the first time we have measured room temperature velocities of longitudinal and transverse sound waves. The measured diagonal elastic constants in the monoclinic phase are: $c_{33}=1.36\times10^{12},c_{44}=5.17\times10^{11},c_{55}=3.39\times10^{11}$dyne/cm$^{2}$. Here x and z are Cartesian axes parallel to crystallographic directions a and c; the later coincides with a two-fold rotation axis of the crystal.Work in Ames is supported by the US DOE.Work in NHMFL is supported by the In-House Research Program, NSF and State of Florida.