Magnetic properties of Tm-Zr multilayers

A 600 A film of thulium and Tm-Zr multilayers in which the Tm layers are separated by 30 A non-magnetic Zr layers were evaporated on superficially oxidized silicon substrates under ultra-vacuum conditions. The thickness of the Tm layers was varied between 8 and 30 A. X-ray diffraction gives evidence for a columnar growth along the c axis of the HCP structure, with in-plane compression of Tm layers thinner than 20 A. The magnetic structure of the film is quite similar to that of bulk Tm. On the contrary, the c-axis modulated antiferromagnetic phase which takes place in the film at TN∼54 K is not observed in the multilayers. This phenomenon is preferentially attributed to an enhancement of the ferromagnetic coupling at the edges of the thulium layers, which favours a structure close to the squared 3-4 antiphase ferromagnetic arrangement of the magnetic moments displayed by the bulk below 30 K. A marked trend to ferromagnetism is observed as the Tm layers become thinner. Contrary to that observed in Dy-Zr and Ho-Zr multilayers, the interface and volume anisotropies do not compensate each other for 8 A Tm layers. The c-axis magnetic anisotropy of Tm is preserved whatever the thickness of the Tm layers. The estimated anisotropies are compared with the results of point-charge crystal-field calculations. (orig.)