Structural and Magnetic X-RAY Scattering Measurements of Epitaxial Dy thin Films

We report structural and magnetic X-ray scattering measurements of a 2000Å thick Dy film, sandwiched by LaF3 films on a GaAs(111) substrate. The structure was grow by molecular beam epitaxy with the c-axis of the Dy parallel to the LaF3 c-axis and GaAs[111] axis. For the magnetic X-ray scattering, we used a polarization analyzer and an X-ray energy near the Dy L111 absorption edge. ∆t this energy there is a resonant enhancement in the magnetic scattering intensity, and we obtained ≃-50 counts per second in the magnetic diffraction peaks.Between 85 and 179K, bulk Dy forms a helical antiferromagnetic structure, and below 85K, it transforms into a ferromagnet. We find that the c-axis lattice constant of the Dy film has a similar temperature dependence to bulk Dy from room temperature to about 110K, but below this, the film behaves differently from bulk and is strained. The temperature dependence of the turn-angle in the helical antiferromagnetic state is similar to bulk, although a weaker dependence is observed below ∼1 10K due to magnetoelastic effects. Surprisingly, the magnetic coherence length (≃310Å) is significantly shorter than the structural coherence length (≃730Å). The transition to ferromagnetic ordering at 86K exhibits temperature hysteresis as is evident in the structural and magnetic X-ray data and in SQUID magnetometry data. We believe this hysteresis arises from a combination of the strain-energy barrier accompanying the transition and magnetic inhomogeneities in the film.