Using a zero-magnetization ferromagnet as the pinning layer in exchange-bias systems

Abstract

Exchange bias properties have been investigated in a $({\text{Sm}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Al}}_{2}/{\text{SmAl}}_{2})$ bilayer (for $x=0.028$) with perpendicular magnetization, an original exchange-coupled system based on the zero magnetization ferromagnet $({\text{Sm}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Al}}_{2})$ as the pinning layer. This unusual magnet exhibits a magnetic compensation temperature where it presents both a zero magnetization and a long-range spin ferromagnetic order. In the $({\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}/{\text{SmAl}}_{2})$ bilayer, a large positive exchange bias has been observed for the ${\text{SmAl}}_{2}$ magnetization reversal, attesting for the exchange coupling between both compounds, even at the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ magnetic compensation temperature; the magnetization of the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ part of the bilayer appears to be pinned but the pinned component is obviously smaller than the one observed in a single uncovered ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$. X-ray Magnetic Circular Dichro\"{\i}sm experiments have been undertaken to investigate the magnetization reversal in both layers independently and more particularly to probe the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ behavior in its zero-magnetization state. They reveal that approximately 20% of the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ layer is driven to reverse by exchange coupling to ${\text{SmAl}}_{2}$. The presence of lateral domains in the ${\text{Sm}}_{1\ensuremath{-}0.028}{\text{Gd}}_{0.028}{\text{Al}}_{2}$ layer likely accounts for the relatively large proportion of rotatable moments and for the observed temperature dependence of exchange-bias field.