Optical birefringence in Dy3Al5O12(DAG) in fields along [111]

In fields along [111] DAG approximates a very special Ising antiferromagnet--one in which applied fields couple to the antiferromagnetic order parameter (H/sub a/--M/sub s/ coupling). The linear magnetic birefringence of DAG in H parallel [111] with light traveling along [1 anti 1 0] (perpendicular to the field) was measured. The H/sub a/--M/sub s/ coupling allows us to produce antiferromagnetic DAG in either of the two time reversed states; A+ which is absolutely stable in positive fields, and A- which is only metastable. For these two states Δn(H/sub a/) is remarkably different (at 1.370K and 3 kOe Δn(A+) - Δn(A-) = 10-5). The metamagnetic transition, A+ - P shows as a change in Δn of about .75 x 10-5. These are the first measurements of linear magnetic birefringence across the A+ - A- and A - P transitions. Both contrasts are substantial and make possible microscopy of the mixed phase