Ferromagnetic transition in EuS-PbS multilayers

The magnetic properties of multilayers of ferromagnetic EuS intercalated wi th diamagnetic PbS were studied as a function of the EuS layer thickness (v arying from 2 to 200 ML). The critical temperature T-C of the paramagnet-fe rromagnet phase transition was determined from magnetization vs temperature measurements and was found to depend on the underlying substrate [KCl (100 ) vs BaF2 (111)] as well as on the thickness of the EuS layer. For thick la yers (d(EuS) approximate to 200 ML), which mimic semibulk EuS, the T-C valu es were found shifted with respect to the bulk EuS (about 1 K up for layers grown on KCI and about 3 K down for layers grown on BaF2). This effect is attributed to stress resulting mainly from the difference of thermal expans ion coefficients between the substrate and the structure. For thin layers ( d(EuS) < 10 ML), a systematic reduction of T-C With decreasing EuS layer th ickness was observed. This behavior is discussed from two points of view: ( a) the reduction of the average number of magnetic neighbors because of the increasing role of the interface for the thin layers, and (b) the three-di mensional/two-dimensional (3D/2D) crossover from a 3D Heisenberg-type ferro magnet to a 2D XY or Ising-like system. The dependence of the magnetic anis otropy on the EuS layer thickness was studied by ferromagnetic resonance me asurements. The energy of magnetic anisotropy can be well described as a su m of a thickness-independent (volume) part and a 1/d(EuS) (surface) term. W e found that EuS layers with d(EuS) > 2 Angstrom magnetize in the plane of the structure. [S0163-1829(99)03045-3].