Ultrasonic investigation of critical behavior in the random-field Ising system Dy(AsxV1−x)O4

Abstract

Elastic-constant measurements, made with use of a high-precision ultrasonic interferometer, have allowed determination of the susceptibility critical exponent γ in the random-field Ising system Dy(${\mathrm{As}}_{\mathit{x}}$ ${\mathrm{V}}_{1\mathrm{-}\mathit{x}}$)${\mathrm{O}}_4$. For pure ${\mathrm{DyVO}}_4$, a value γ=1.15±0.05 was obtained, which is between the values γ=1 for a mean-field system and γ=1.25 for a three-dimensional Ising system with nearest-neighbor interactions, and consistent with conclusions reached by previous authors that there is a strong short-range component to the exchange interactions. Random-field samples yielded a significantly higher value, specifically γ=1.79±0.07, consistent with the two-dimensional nearest-neighbor Ising value γ=7/4 and with results of simulations by other authors, but in disagreement with certain scaling-theory predictions. This is the most reliable value obtained thus far for γ in a random-field Ising system in three dimensions. Results of optical-birefringence measurements on the ultrasonics samples, along with numerical simulations, show that the measurement of γ was not significantly affected by arsenic-concentration inhomogeneities.