Observation of intermediate spin states in Er/Y superlattices

Abstract

Low temperature hysteresis measurements, performed on a series of Er/Y superlattices with Er interlayer thickness ranging from 37 to 90 Å, suggest the existence of intermediate spin states that are stabilized in a field. While bulk Er becomes ferromagnetic at 20 K, even at 10 K these samples transform to a state carrying (1)/(7) the saturation moment. This state, which is observed only at high temperatures in bulk Er, is stable at 10 K to 15 kOe, beyond which the magnetization saturates. The widths of hysteresis loops associated with the low and high field transitions, respectively, are 0.5 and 2.5 kG, many times that of Er. Just below the critical field the spin structure passes through a narrow field range where the magnetization is approximately Ms/2. No such state is observed in bulk Er in zero field. This intermediate state is more apparent on reducing the field after saturation, and becomes more stable as the Er interlayer thickness is decreased. The magnetization is not found to decay appreciably when the field is held for 10 h on the Ms/2 plateau. Preliminary data on thin films also show the existence of the Ms/7 state at low temperatures, but with the range of stability decreasing with increasing Er film thickness. We discuss these phenomena in terms of modifications of the exchange interaction due to epitaxial strains.