Structural and Magnetic Properties of Epitaxial GADOLINIUM(0001) Films on TUNGSTEN(110)

Epitaxial Gd films on W(110) have been investigated in a ultra-high vacuum (UHV) system equipped with different surface sensitive techniques. The UHV system provided the facility for surface preparation and characterization, surface analysis and data acquisition. Spin-polarized secondary electrons studies showed that the surface magnetization vector is canted out of the Gd film's surface plane and that the surface magnetic ordering temperature is enhanced up to 60^ circC higher than the bulk Curie temperature. The in-plane component of surface magnetization aligns ferromagnetically to the bulk. The enhanced surface magnetic ordering temperature is sensitive to the surface roughness. It ranges from 22^circK to 60^ circK above the bulk Curie temperature. The surface relaxation of the epitaxial Gd(450A) films on W(110) was studied at two different growth conditions by video-LEED. From the comparisons between dynamical LEED calculations and our experimental results we find a first interlayer spacing contraction of 2.4% and a second interlayer spacing expansion of 1%. This is in good agreement with a similar LEED analysis of the (0001) surface of bulk Gd crystals. Using the FWHM of the intensity profile of LEED spots, we demonstrate that films grown at elevated temperature show evidence of larger surface roughness compared to films deposited at room temperature and then annealed. The magnetization of thick epitaxial Gd(0001) films was investigated as a function of annealing temperature and film thickness by magneto-optical Kerr effect (MOKE) measurements. After annealing to a sufficiently high temperature, hysteresis loops of thick films (>=400A) show evidence for a canted magnetization and a strong anomaly of the coercive field just below the bulk Curie temperature T_{rm c}. In in-plane susceptibility measurements by MOKE, a second peak below the ferromagnetic-paramagnetic transition peak at the Curie temperature appears for thicker films (>=350A). This peak shifts to lower temperature with increasing film thickness. The origin of this second susceptibility peak has been identified as a reorientation transition of the magnetization. Results from micromagnetic calculations using bulk Gd parameters are fully consistent with the observed behavior. The out -of plane AC susceptibility measurement also confirmed the same behavior.