The structural and magnetic properties of Zr/Fe/Zr trilayer structures, composed of ultrathin layers of Fe stacked between two pure Zr layers, have been studied by means of X-ray diffraction (XRD), conversion electron Mossbauer spectroscopy (CEMS) and magnetometry techniques. The XRD results, as well as the room temperature saturation magnetization, show a decrease in the amount of bcc-Fe on decreasing the Fz layer thickness. The thickness dependence of the room temperature coercivity is interpreted as giving evidence for a switch from a Bloch domain wall structure to a Neel interacting cluster behaviour, as the Fe layer thickness is decreased below 16 nm. Below this thickness, the temperature dependence of the coercivity as well as the thermomagnetization behaviour, suggest a magnetic structure composed of crystalline bcc-Fe interacting clusters. This effect can be related to the appearance of an amorphous Fe-Zr alloy phase at the grain boundaries which affects progressively the magnetization process as the magnetic layer thickness is reduced.