ULTRASONIC INVESTIGATION OF A 2-DIMENSIONAL TRIANGULAR LATTICE FERRIMAGNET, ERFE2O4-DELTA SINGLE-CRYSTAL

Velocities and attenuations of longitudinal sound waves were measured at 100 MHz for a single crystal of non-stoichiometric ErFe2O4-delta, a long the [0, 0, 1] and [1, 1, 0] axes in the hexagonal frame, below 30 0 K. At room temperature, velocities were 5.22 x 10(3) and 5.49 x 10(3 ) m/s along the [0, 0, 1] and [1, 1, 0] axes, respectively. These valu es correspond to the elastic constants of C-11 = 1.94 x 10(12) dyne/cm (2) and C-33 = 1.75 x 10(12) dyne/cm(2). Slight softening was disclose d along the [0, 0, 1] axis at around 240 K. Correspondingly, a large p eak was observed in the attenuation. These anomalies are attributed to the emergence of two-dimensional spin ordering in this crystal at thi s 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.