Memory-functionality superconductor/ferromagnet/superconductor junctions based on the high-$T_c$ cuprate superconductors ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and the colossal magnetoresistive manganite ferromagnets ${\mathrm{La}}_{2/3}{X}_{1/3}{\mathrm{MnO}}_{3+\delta }(X=\mathrm{Ca},\mathrm{Sr})$

Complex oxides exhibit a variety of unusual physical properties, which can be used for designing novel electronic devices. Here we fabricate and study experimentally nanoscale superconductor/ferromagnet/ superconductor junctions with the high-$T_c$ cuprate superconductors ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and the colossal magnetoresistive (CMR) manganite ferromagnets ${\mathrm{La}}_{2/3}{X}_{1/3}{\mathrm{MnO}}_{3+\delta }(X=\mathrm{Ca}\text{or}\mathrm{Sr})$. We demonstrate that in a broad temperature range the magnetization of a manganite nanoparticle, forming the junction interface, switches abruptly in a monodomain manner. The CMR phenomenon translates the magnetization loop into a hysteretic magnetoresistance loop. The latter facilitates a memory functionality of such a junction with just a single CMR ferromagnetic layer. The orientation of the magnetization (stored information) can be read out by simply measuring the junction resistance in a finite magnetic field. The CMR facilitates a large readout signal in a small applied field. We argue that such a simple single-layer CMR junction can operate as a memory cell both in the superconducting state at cryogenic temperatures and in the normal state up to room temperature.

Figure 1

(a) SEM image of a YBCO/LCMO/YBCO junction on the LC_10 sample. (b) SEM image of a YBCO/LSMO/YBCO junction on LS_11. (c) Sketches of a bridge (top) and a junction (bottom) nanosculptured by focused Ion Beam from SFS trilayers (not in scale). Red arrows indicate the current flow path through the structures. Black arrows indicate field orientations. (d) The dc magnetization versus temperature obtained from the unpatterned trilayers while cooling in 1000 Oe applied in plane. (e) $M\left(H\right)$ hysteresis loops for the same films at 100 K (field in plane). (f) $R\left(T\right)$ showing the superconducting transition of the YBCO electrodes, obtained from patterned bridges on LC_10 and LS_11, respectively. The resistances are measured by applying a small ac current of 0.3 $\mu \mathrm{A}$ for LC_10 and 1 $\mu \mathrm{A}$ for LS_11.

Figure 2

(a)–(c) $R$ vs $T$ curves of the SFS junctions, obtained while cooling in different magnetic fields with applied ac currents of (a) 0.1 $\mu \mathrm{A}$ and (b) 50 $\mu \mathrm{A}$ on a junction on LC_10 and (c) 1 $\mu \mathrm{A}$ on a junction on LS_11. The dotted vertical lines indicate $T_c$ for YBCO in zero field. (d) and (e) Field dependence of the $I-V$ curves at $T=7\mathrm{K}$ on the same LC_10 junction with the field oriented (d) in plane and (e) out of plane. (f) In-plane field dependence of the $I-V$ curves for LS_11 at 7 K. Inset: close-up of the low-bias parts of $I-V$. $I-V$ are measured in a field ranging from 0 to 16 T with an increment of 1 T.

Figure 3

Magnetoresistance (MR) curves of a junction on the LC_10 sample. (a)–(c) High-field MR curves measured with a low ac current of 0.1 $\mu \mathrm{A}$ at (a) $T>T_{\mathrm{Curie}}$, (b) $T_{\mathrm{Curie}}>T>T_c$, and (c) $T<T_c$. (d)–(i) MR curves in a small field range at different temperatures, measured with a higher ac current of 100 $\mu \mathrm{A}$. Arrows indicate the direction of the field sweep. All fields are in plane oriented. The appearance of hysteretic MR loops is clearly seen, with an abrupt switching due to the remagnetization of the manganite interlayer.

Figure 4

MR of a junction on the YBCO/LSMO/YBCO sample. (a) and (b) High-field MR curves, measured with 1-$\mu \mathrm{A}$ ac bias at (a) $T_{\mathrm{Curie}}>T>T_c$ and (b) $T<T_c$. (c)–(e) MR curves in a small field range at different temperatures, measured with 14-$\mu \mathrm{A}$ ac current. Arrows indicate the direction of the field sweeping. Fields are in plane oriented. The abrupt $R\left(H\right)$ switching indicates a monodomain magnetic configuration of the manganite nanoparticle.

Figure 5

Examples of magnetization loops reconstructed from the MR curves for the junction on YBCO/LCMO/YBCO (a) at 15 K and (b) at 100 K and for the junction on YBCO/LSMO/YBCO at (c) 10 K and (d) at 100 K. (e) and (f) Temperature dependence of $H_{\mathrm{Coer}}\left(T\right)$ obtained from the hysteresis loops.

Figure 6

Sketch of the operation principle of a bistable memory cell based on the CMR junctions.