Exchange Coupling at the Interfaces of Er–Tb and Er–CoFe Multilayers

Abstract

Strong exchange coupling at the rare-earth–rare-earth­(RE)–RE interfaces could only be observed around the temperature range where both layers were ferromagnetic (FM). In this work, we investigate two highly textured RE–RE (Er–Tb) multilayers: one comprises 5 monolayers (MLs) of Tb, while the other comprises 21 MLs of Tb. For MLTb = 5, the helix can be truncated, while for MLTb = 21, it is expected to be profound. Both samples comprise 21 MLs of Er, which exhibits three temperature-dependent phases of spin configuration. For the MLTb = 5 sample, a significant exchange bias field of up to +170 ± 10 Oe is observed, particularly below 20 K, where Er is known to exist in a conical phase while Tb remains FM. As the Tb thickness is increased from 5 to 21 MLs, the coupling strength diminishes drastically, and a shift in the three magnetic phase transition temperatures in Er is seen. Thus, the state of the helical phase formation in Tb affecting the sinusoidal-to-helical and helical-to-conical phase fractions within Er is emblemized. Additionally, we see a regular FM behavior and no signature of a nanoclustering behavior around the temperature range of the conical-to-helical phase evolution of Er–Tb for the MLTb = 21 sample. As we replace 21 MLs of Tb with 30 MLs of ferromagnet (CoFe), we find usual double hysteresis loops (DHLs) and an increase in the exchange bias field up to −350 ± 18 Oe. The positive exchange bias phenomenon in Er–Tb multilayer below 20 K is attributed to the small spin-imbalance in the number of spins for each sublattice disorder in the conical phase.