Single Abrikosov vortices as quantized information bits

Abstract

Superconducting digital devices can be advantageously used in future supercomputers because they can greatly reduce the dissipation power and increase the speed of operation. Non-volatile quantized states are ideal for the realization of classical Boolean logics. A quantized Abrikosov vortex represents the most compact magnetic object in superconductors, which can be utilized for creation of high-density digital cryoelectronics. In this work we provide a proof of concept for Abrikosov-vortex-based random access memory cell, in which a single vortex is used as an information bit. We demonstrate high-endurance write operation and two different ways of read-out using a spin valve or a Josephson junction. These memory cells are characterized by an infinite magnetoresistance between 0 and 1 states, a short access time, a scalability to nm sizes and an extremely low write energy. Non-volatility and perfect reproducibility are inherent for such a device due to the quantized nature of the vortex.

Figure 1. Operation of a memory cell with a Josephson spin-valve read-out.

(a) Sketch of a Josephson spin-valve device. (b) Magnetic field dependence of Josephson critical current through the JSV#1 at T=2.4 K. A jump, marked by a circle, is due to antivortex exit. (c) Magnetic field dependence of a.c. resistance for JSV#1 at I_a.c.=50 (bottom plot) and 100 (top plot) μA. It is seen that the vortex exit field depends on the current. (d) High-bias spin-valve MR for a JSV#2 at T=1.8 K. Multiple branches at high fields are caused by entrance of vortices. The number of vortices (0–3) is marked at each branch. (e) Scanning electron microscopy image of JSV#3 AVRAM cell with a vortex trap. Scale bar, 500 nm. (f) Demonstration of write and erase operations for the same cell at T=1.8 K and H=2.4 kOe. Top plot shows applied current pulses, bottom plot the a.c. resistance.

Figure 2. Operation of a memory cell with planar Josephson junctions.

(a) Scanning electron microscopy (SEM) image and sketch of a planar AVRAM cell with a vortex trap and two read-out planar Josephson junctions. The scale bar in SEM image is 1 μm. (b) Magnetic field modulation of the critical current for the first read-out junction without (black) and with a vortex (red). (c) Magnetic field dependence of a.c. resistance without a vortex (black), with a vortex (red) and with an antivortex (blue line). (d) Demonstration of write and erase operations by current pulses of different amplitudes. (e) Demonstration of controllable 0–1 switching in a broad field range. (f) Evolution of the device state on applying a pulse train with growing amplitude. Note excellent half-selection stability. (g) Dependence of the final state on the pulse amplitude. (h) Demonstration of high-endurance 0–1 switching at zero applied field.