  In 1966, Pierre-Gilles de Gennes proposed a non-volatile mechanism for
switching superconductivity on and off in a magnetic device. This involved a
superconductor (S) sandwiched between ferromagnetic (F) insulators in which the
net magnetic exchange field could be controlled through the
magnetisation-orientation of the F layers. Because superconducting switches are
attractive for a range of applications, extensive studies have been carried out
on $F/S/F$ structures. Although these have demonstrated a sensitivity of the
superconducting critical temperature ($T_{c}$) to parallel (P) and antiparallel
(AP) magnetisation-orientations of the F layers, corresponding shifts in $T_c$
(i.e., ${\Delta}T_c = T_{c,AP} - T_{c,P}$) are lower than predicted with
${\Delta}T_c$ only a small fraction of $T_{c,AP}$, precluding the development
of applications. Here, we report $EuS/Au/Nb/EuS$ structures where EuS is an
insulating ferromagnet, Nb is a superconductor and Au is a heavy metal. For P
magnetisations, the superconducting state in this structure is quenched down to
the lowest measured temperature of 20 mK meaning that ${\Delta}T_c/T_{c,AP}$ is
practically 1. The key to this so-called absolute switching effect is a sizable
spin-mixing conductance at the $EuS/Au$ interface which ensures a robust
magnetic proximity effect, unlocking the potential of $F/S/F$ switches for low
power electronics.
