Neutron and X-Ray Scattering Studies of (FeF2) M (CoF2)n Multilayers

Abstract

ABSTRACTWe have performed elastic neutron scattering measurements of the staggered magnetization in (FeF2) M (CoF2) n superlattices. Thermal expansion Measurements, which are proportional to the magnetic contribution to the specific heat, were also carried out using high resolution x-ray diffraction. One of the two measured samples has thicknesses of M = n = 4.5 and the other M = 26 and n = 28 Monolayers, as determined from high angle x-ray θ — 2θ scans. In the M = n = 4.5 sample, only one transition is observed at T/v = 62.9K. Analysis of the neutron data, including the rounding effects, indicates an effective β ≈ 0.42. This does not compare well with the 3D Ising exponent β = 0.325. The X-ray data also show the existence of only one specific heat anomaly at T = 62.8 K. For the M = 26, n = 28 sample, dips in the staggered magnetization and peaks in the thermal expansion were observed at T ≈ 40 K and 74 K. The higher temperature anomaly, associated primarily with the FeF2 layers, is sharper than the lower one, which is presumably rounded by the staggered ordering field imposed by the long range order in the FeF2 regions on the CoF2 regions.