Expanding the operational temperature window of a superconducting spin valve

To increase the efficiency of the superconducting spin valve (SSV), special attention should be paid to the choice of ferromagnetic materials for the F1/F2/S SSV multilayer. Here, we report the preparation and superconducting properties of the SSV heterostructures where Pb is used as the superconducting S layer. In the magnetic part of the structure, we use the same starting material, the Heusler alloy ${\mathrm{Co}}_2{\mathrm{Cr}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Al}}_y$, for both F1 and F2 layers. We utilize the tunability of the magnetic properties of this alloy, which, depending on the deposition conditions, forms either an almost fully spin-polarized half-metallic F1 layer or a weakly ferromagnetic F2 layer. We demonstrate that the combination of the distinct properties of these two layers boosts the generation of the long-range triplet component of the superconducting condensate in the fabricated SSV structures and yields superior values of the triplet spin-valve effect of more than 1 K and of the operational temperature window of the SSV up to 0.6 K.

Figure 1

Design of the prepared SSV heterostructures (see the text for details).

Figure 2

Magnetic hysteresis loops for the ${\mathrm{HA}}^{\mathrm{hot}}$ ($d_{{\mathrm{HA}}^{\mathrm{hot}}}=20\mathrm{nm}$) and ${\mathrm{HA}}^{\mathrm{RT}}$ ($d_{{\mathrm{HA}}^{\mathrm{RT}}}=3\mathrm{nm}$) single layer samples measured at $T=30\mathrm{K}$. The magnetization curve of the ${\mathrm{HA}}^{\mathrm{RT}}$ sample is scaled to unity whereas the respective curve of the ${\mathrm{HA}}^{\mathrm{hot}}$ sample is scaled to the ratio of the absolute saturation magnetizations of the two layers $M_s^{\mathrm{hot}}/M_s^{\mathrm{RT}}$.

Figure 3

Dependence of $T_c$ on the angle $\alpha$ between the direction of the cooling field used to fix the direction of the magnetization of the ${\mathrm{HA}}^{\mathrm{RT}}$ layer and the applied magnetic field that rotates the magnetization of the ${\mathrm{HA}}^{\mathrm{hot}}$ layer: (a) for the ${\mathrm{HA}}^{\mathrm{hot}}$(20 nm)/Al(4 nm)/${\mathrm{HA}}^{\mathrm{RT}}$(2 nm)/Al(1.2 nm)/Pb(60 nm) sample at the applied magnetic field $H_0=1\mathrm{kOe}$; (b, c) for the ${\mathrm{HA}}^{\mathrm{hot}}$(20 nm)/Al(4 nm)/${\mathrm{HA}}^{\mathrm{RT}}$(5 nm)/Al(1.2 nm)/Pb(60 nm) sample at the applied magnetic fields $H_0=2$ and $4\mathrm{kOe}$, respectively. Solid line, theoretical curve with the parameters presented in Sec. 4; dashed lines, reference curves (see Sec. 4 for details).

Figure 4

Superconducting transition curves for the P ($\alpha =0^{\circ },{360}^{\circ }$), AP ($\alpha ={180}^{\circ }$), and PP ($\alpha ={90}^{\circ }$) configuration of the cooling field used to fix the direction of the magnetization of the ${\mathrm{HA}}^{\mathrm{RT}}$ layer and the applied magnetic field $H_0=4\mathrm{kOe}$ that rotates the magnetization of the ${\mathrm{HA}}^{\mathrm{hot}}$ layer for the sample ${\mathrm{HA}}^{\mathrm{hot}}$(20 nm)/Al(4 nm)/${\mathrm{HA}}^{\mathrm{RT}}$(5 nm)/Al(1.2 nm)/Pb(60 nm). The shaded area marks the operational temperature window of the SSV. The three transition curves produce the three points $T_c\left(0^{\circ }\right)$, $T_c\left({90}^{\circ }\right)$, and $T_c\left({180}^{\circ }\right)$ in Fig. 3.