  The diode effect is fundamental to electronic devices and is widely used in
rectifiers and AC-DC converters. At low temperatures, however, conventional
semiconductor diodes possess a high resistivity, which yields energy loss and
heating during operation. The superconducting diode effect (SDE), which relies
on broken inversion symmetry in a superconductor may mitigate this obstacle: in
one direction a zero-resistance supercurrent can flow through the diode, but
for the opposite direction of current flow, the device enters the normal state
with ohmic resistance. The application of a magnetic field can induce SDE in
Nb/V/Ta superlattices with a polar structure, in superconducting devices with
asymmetric patterning of pinning centres, or in superconductor/ferromagnet
hybrid devices with induced vortices. The need for an external magnetic field
limits their practical application. Recently, a field-free SDE was observed in
a NbSe2/Nb3Br8/NbSe2 junction, and it originates from asymmetric Josephson
tunneling that is induced by the Nb3Br8 barrier and the associated NbSe2/Nb3Br8
interfaces. Here, we present another implementation of zero-field SDE using
noncentrosymmetric [Nb/V/Co/V/Ta]20 multilayers. The magnetic layers provide
the necessary symmetry breaking and we can tune the SDE by adjusting the
structural parameters, such as the constituent elements, film thickness,
stacking order, and number of repetitions. We control the polarity of the SDE
through the magnetization direction of the ferromagnetic layers. Artificially
stacked structures, as the one used in this work, are of particular interest as
they are compatible with microfabrication techniques and can be integrated with
devices such as Josephson junctions. Energy-loss-free SDEs as presented in this
work may therefore enable novel non-volatile memories and logic circuits with
ultralow power consumption.
