  We use a recently developed optical fiber Bragg grating technique, in
continuous and pulsed magnetic fields in excess of 90T, to study magnetoelastic
correlations in magnetic materials at cryogenic temperatures. Both insulating
UO2 and metallic CeRhIn5 present antiferromagnetic ground states, at T_N =
30.3K and T_N = 3.85K respectively. A strong coupling of the magnetism to the
crystal lattice degrees of freedom in UO2 is found, revealing piezomagnetism as
well as the dynamics of antiferromagnetic domain switching between spin
arrangements connected by time reversal. The AFM domains become harder to
switch as the temperature is reduced, reaching a record value H_PZ(T = 4K) =
18T. The effect of strong magnetic fields is also studied in CeRhIn5, where an
anomaly in the sample crystallographic c-axis of magnitude Delta_c/c = 2 ppm is
found associated to a recently proposed electronic nematic state at H_en = 30T
applied 11o off the c-axis. Here we show that while this anomaly is absent when
the magnetic field is applied 18o off the a-axis, strong magnetoelastic quantum
oscillations attest to the high quality of the single crystal samples.
