Ultrasonic Investigation of a Two-Dimensional Triangular Lattice Ferrimagnet, ErFe₂O_4−δ Single Crystal

Velocities and attenuations of longitudinal sound waves were measured at 100 MHz for a single crystal of non-stoichiometric ErFe₂O_4−δ, along the 〈0, 0, 1〉 and 〈1, 1, 0〉 axes in the hexagonal frame, below 300 k. At room temperature, velocities were 5.22 × 10³ and 5.49 × 10³ m/s along the 〈0, 0, 1〉 and 〈1, 1, 0〉 axes, respectively. These values correspond to the elastic constants of C₁₁ = 1.94 × 10¹² dyne/cm² and C₃₃ = 1.75 × 10¹² dyne/cm². Slight softening was disclosed along the 〈0, 0, 1〉 axis at around 240 k. Correspondingly, a large peak was observed in the attenuation. These anomalies are attributed to the emergence of two-dimensional spin ordering in this crystal at this temperature. On the contrary, no anomaly was detected along the 〈1, 1, 0〉 axis at around 240 k, with rather high attenuation level almost independent of temperature. This absence of anomalies was discussed on the existence of three kinds of magnetic domains. Development of the correlation of Er spins was suggested below 30 k.