Y-89 NMR studies of MBE grown DyFe2/YFe2 multilayer films

Y-89 (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 Angstrom thick film of YFe2, and a 24,000 Angstrom thick superlattice film [300 Angstrom DyFe2/300 Angstrom YFe2] x 40. For comparative purposes, measurements were also made on bulk powdered YFe2 samples. In practice, there are significant differences between the Y-89 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 similar to0.1 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 (similar to0.8%), 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.