Structural and magnetic properties of Er thin films and Er/Y superlattices. II. Modification of the commensurate spin states

Continuing the analysis of epitaxial erbium thin films and Er/Y superlattices, we report the effects of basal-plane strain on the modulated spin structure as determined from bulk magnetization and neutron-diffraction measurements detailed in a previous paper [Phys. Rev. B 43, 3123 (1991)]. The phase angle of the c-axis-modulation spin order is larger than that of bulk Er in even the thickest films and is virtually independent of temperature in the superlattices. The sequence and stability of c-axis commensurate states in bulk Er are altered in all samples considered. In the superlattices, the c-axis-modulation net moment state with four spins up followed by three spins down dominates the temperature and field phase diagram. In addition, an additional intermediate spin configuration with a net moment of half the saturation moment develops in the superlattice with the thinnest Er interlayers. A phenomenological calculation of the exchange integral demonstrates that epitaxial strain and lattice clamping can lead to an enhancement of the phase angle. Specifically, additional commensurate phases may arise due to strain-induced variations of the nearest-neighbor and next-nearest-neighbor spin interactions, as shown in the context of the axial-next-nearest-neighbor Ising model