⁸⁹Y NMR Studies of MBE Grown DyFe₂/YFe₂ Multilayer Films

Abstract

⁸⁹Y (spin I=1/2) NMR studies of DyFe₂/YFe₂ multilayer films at 4.2 K are reported and discussed. Results are presented for a 1000 Å thick film of YFe₂, and a 24,000 Å thick superlattice film [300 Å DyFe₂/300 Å YFe₂]×40. For comparative purposes, measurements were also made on bulk powdered YFe₂ samples. In practice, there are significant differences between the ⁸⁹Y NMR in thin film and powder form. In the MBE films the NMR frequency (45.85±0.2 MHz) is shifted down in frequency by ∼0.1 MHz, with respect to bulk (powdered) YFe₂ (45.94±0.09 MHz). The origin of this shift is attributed to the expanded nature of the MBE films (∼0.8%), relative to bulk YFe₂. In addition, the NMR line width is broader in the MBE films by about a factor of two. This indicates either the presence of strain within the MBE films, and/or differing dipolar fields associated with the DyFe₂/YFe₂ interfaces. This feature is also confirmed by the spin–spin lattice relaxation time T ₂, which is almost ten times longer in the superlattice film (T ₂=5.1 ms), than in bulk YFe₂ (T ₂=0.6 ms). It is argued that this is due to the increased inhomogeneous line-width, which inhibits energy-conserving mutual spin–spin-flops.