Elastic Constants of Fe-Pt Alloys. I. Single Crystalline Elastic Constants of Fe₇₂Pt₂₈

The elastic constants of a disordered Fe 72 Pt 28 single crystal have been measured as a function of temperature. All elastic constants behave highly anomalously below T c due to the Invar properties of this alloy. The magnetic contribution to the shear modes C '=( C 11 - C 12 )/2 and C = C 44 can be described by the relations - C ' m = K 1 ( M / M 0 ) 2 and - C m = K 2 ( M / M 0 ) 2 where K 1 and K 2 are magnetoelastic coupling coefficients. Using these relations the elastic constants at 0K are C '=0.3×10 10 N / m 2 and C =7.5×10 10 N / m 2 giving rise to an elastic anisotropy of A = C / C '=25. The elastic constant C L behaves differently. C L goes through a minimum at 50°C and a magnetically induced reduction is present up to 200° above T c . This is ascribed to a coupling between the longitudinal sound wave and spin fluctuations.