Measurement of the critical exponent β by means of optical birefringence on the Heisenberg antiferromagnet RbMnF3

Abstract
The critical exponent beta for the ideal Heisenberg antiferromagnet RbMnF3 was measured by means of optical birefringence. To this end a small part (12%) of Mn2+ ions were substituted by Fe2+ ions. In the critical region 10-4c-T)/Tc-2, beta was determined to be 0.385+or-0.01, in good agreement with the theoretical prediction beta th=0.38+or-0.03. Together with the experimental values for other critical exponents, this value of beta confirms the scaling laws. In a magnetic field up to 1 T, perpendicular to one of the easy axes, no change in the value of beta was observed.