Exchange-spring-driven spin flop in an ErFe2/YFe2 multilayer studied by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism at the Er ${M}_{4,5}$ edge is used to study the switching behavior of the hard ErFe${}_{2}$ layers in an epitaxial [ErFe${}_{2}$(70 \AA{})/YFe${}_{2}$(150 \AA{})] \ifmmode\times\else\texttimes\fi{} 25 exchange-spring superlattice. Magnetic hysteresis loops for the Er magnetization, at temperatures $T$ 200 K, reveal a single irreversible switch between a vertical exchange spring and its reversed state. Experiments at $T$ g 200 K reveal a crossover to a regime with two irreversible switching processes. Computational modeling for this system gives good agreement with the experiment, revealing that the observed high-temperature switching behavior is due to an exchange-spring-driven spin-flop-like transition. In contrast to the conventional spin-flop transition in an antiferromagnet, the increase in anisotropy energy of the hard magnetic ErFe${}_{2}$ layers and Fe-Fe exchange energy is overcome by a decrease in overall Zeeman energy. Computational studies also reveal two types of transitions between vertical exchange-spring and spin-flop states with first-order and second-order character.