Anomalous Elastic Properties of Fe₂TiO₄

Abstract

The temperature dependence of elastic constants c₁₁−c₁₂ and c₄₄ of Fe_2.05Ti_0.95O_4.00 is measured by the ultrasonic pulse echo method, using a small (110) platelet with approximate dimension of 5×7×1.364 mm, for the temperature range of 77°∼300°K. c₁₁−c₁₂ decreases anomalously with decreasing temperature in the paramagnetic phase, whereas c₄₄ increases. An extraordinarily large attenuation is observed around the Curie temperature in the 1\bar10 shear mode which couples to c₁₁−c₁₂. The elastic constants obtained, c₁₁−c₁₂=0.265×10¹² dyne/cm² and c₄₄=0.396×10¹² dyne/cm² at room temperature, are almost an order of magnitude smaller than those of magnetite and spinel, and provide a reasonable explanation for the giant magnetostriction observed in the crystal. The soft mode in the crystal is interpreted to be due to the Jahn-Teller relaxation effect of Fe²⁺ ions in the tetrahedral site.