Nonvolatile Superconductive Memory Using Exchange Field

Efficient cryogenic memory is increasingly important with growing power demands in data centers and efforts toward quantum computing [1]. One way to create nonvolatile memory is to sandwich a superconductor (SC) between two ferromagnetic (FM) layers in an SC spin valve. The relative magnetization (M) states of the two FM layers control the SC state, driven by the exchange field acting on the SC from the FM layers: when the two M are (anti-) parallel, SC is (superconducting) resistive [2]. We achieved nonvolatile memory states with MBE-grown EuS/V/EuS ultrathin film hybrid structures, where V is a SC and EuS is a FM insulator. To switch M with no applied field, spin-orbit torque (SOT) from a Pt layer inserted between an SC and FM layer may be used. By patterning the films into μm-scale devices functioning as bits, we show that the magnetic orientations of individual bits can be controlled electrically. However, thermal effects from the SOT switching current may dominate the switching mechanism, especially at high current densities. A tiny ambient field (<5 Oe) can assist in switching individual bits [3], showing that FM/SC/FM hybrids are strong contenders for the next generation of electrically-tunable cryogenic memories.