Colossal Proximity Effect in a Superconducting Triplet Spin Valve Based on the Half-Metallic Ferromagnet ${\mathrm{CrO}}_2$

Combining superconductors ($S$) and ferromagnets ($F$) offers the opportunity to create a new class of superconducting spintronic devices. In particular, the $S/F$ interface can be specifically engineered to convert singlet Cooper pairs to spin-polarized triplet Cooper pairs. The efficiency of this process can be studied using a so-called triplet spin valve (TSV), which is composed of two $F$ layers and a $S$ layer. When the magnetizations in the two $F$ layers are not collinear, singlet pairs are drained from the $S$ layer, and triplet generation is signaled by a decrease of the critical temperature $T_c$. Here, we build highly efficient TSVs using a 100% spin-polarized half-metallic ferromagnet, ${\mathrm{CrO}}_2$. The application of out-of-plane magnetic fields results in an extremely strong suppression of $T_c$, by well over a Kelvin. The observed effect is an order of magnitude larger than previous studies on TSVs with standard ferromagnets. Furthermore, we clearly demonstrate that this triplet proximity effect is strongly dependent on the transparency and spin activity of the interface. Our results are particularly important in view of the growing interest in generating long-range triplet supercurrents for dissipationless spintronics.

Popular Summary

Hybrid devices of superconductors and ferromagnets hold promise for superconducting electronics since they combine “no dissipation” with switchability. However, the differing properties of superconductors and ferromagnets have been thought to inhibit the creation of hybrid systems: The current-carrying entities in a superconductor (the Cooper pairs) do not possess spin, and the electrons in a magnet do. Cooper pairs are destroyed when they encounter a magnet. This outlook changed with the realization that so-called triplet pairs can be engineered in which the Cooper pairs carry spin and survive in a magnet. Here, we report on the efficiency with which standard pairs can be converted into triplet pairs.

We employ a ${\mathrm{CrO}}_2$ ferromagnet, which is special in the sense that its electrons all have the same spin (as opposed to ferromagnets such as Fe or Co where both spins are present but one type prevails). We show that, in this case, the efficiency of standard pairs turning into triplet pairs can be very high: Singlet-triplet conversion leads to “colossal” changes (about 1 K) in the critical temperature of a specially engineered stack of layers that involves ${\mathrm{CrO}}_2$ and the conventional superconductor MoGe. We describe a pseudospin valve structure consisting of ${\mathrm{CrO}}_2$/$\mathrm{Cu}/\mathrm{Ni}/\mathrm{MoGe}$ in which the application of magnetic fields perpendicular to the layers up to 2 T increasingly generates triplets, as measured by the suppression of the critical temperature of the stack. The observed effects are much larger (almost tenfold) than have been reported for similar experiments involving Co because of the fully spin-polarized nature of ${\mathrm{CrO}}_2$. Furthermore, we note that the quality of the etched ${\mathrm{CrO}}_2$ interface strongly modulates the conversion efficiency.

We expect that our work will pave the way for novel superconducting devices operating in magnetic fields.

Figure 1

Device structure and magnetic characterization. (a) Working principle of a triplet spin valve (TSV) with a half-metallic ferromagnet; the TSV is off (on) when the magnetizations of $F_1$ and $F_2$ are collinear (noncollinear), with a maximum effect when they are orthogonal. (b) Optical micrograph of a typical TSV where a $\mathrm{MoGe}(d_s)/\mathrm{Ni}(1.5\mathrm{nm})/\mathrm{Cu}(5\mathrm{nm})$ trilayer bridge of $10\mu \mathrm{m}$ width is patterned on a 100 nm thin film of ${\mathrm{CrO}}_2$. (c) Magnetization hysteresis loops for ${\mathrm{CrO}}_2(100\mathrm{nm})$ and a multilayer $[\mathrm{Ni}(1.5\mathrm{nm})/\mathrm{Cu}(10\mathrm{nm}){]}_{11}/\mathrm{Ni}(3\mathrm{nm})/\mathrm{Cu}(10\mathrm{nm})$ measured with the magnetic field perpendicular to the sample plane. The magnetization $M$ is normalixed on the saturation magnetization $M_s$, which is $6.8\times {10}^5\mathrm{A}/\mathrm{m}$ for the ${\mathrm{CrO}}_2$ film and $2.2\times {10}^5\mathrm{A}/\mathrm{m}$ for the $\mathrm{Cu}/\mathrm{Ni}$ multilayer.

Figure 2

Colossal triplet spin valve effect. (a) Coordinate system used in angle-dependent magnetotransport measurements, showing the direction of the current $j$, the applied field $H_a$, and the angle $\theta$ between them. (b) Spin valve effect in the two spin valves $\mathrm{MoGe}(d_s)/\mathrm{Ni}(1.5\mathrm{nm})/\mathrm{Cu}(5\mathrm{nm})/{\mathrm{CrO}}_2(100\mathrm{nm})$ with $d_s=5\mathrm{nm}$ (TSV50a, left and middle) and $d_s=25\mathrm{nm}$ (TSV25a, right). Upper panels: Resistive transitions for different $\theta$ as indicated. Lower panels: Variation of $\delta T_{50\%}=T_{50\%}$, $(0°)-T_{50\%}$, $(\theta )$ as a function of $\theta$ at 0.25 T (TSV50a) and 0.5 T (TSV50a, TSV25a), where $T_{50\%}$ is the temperature where the normal state resistance has decreased by 50%. Note that a peak appears in the transition curves for measurements at $\theta =0°$.

Figure 3

Non-triplet-generating layer combinations. (a) Transition curves for $\mathrm{MoGe}(50\mathrm{nm})/\mathrm{Cu}(5\mathrm{nm})/{\mathrm{CrO}}_2(100\mathrm{nm})$ (top), $\mathrm{MoGe}(50\mathrm{nm})/\mathrm{Ni}(1.5\mathrm{nm})/\mathrm{Cu}(5\mathrm{nm})$ (middle), and MoGe(50 nm) (bottom) for $\theta =0°$ and $\theta =90°$ at 0.25 T. The top panel also shows the results of the spin valve device TSV50a as drawn lines. (b) $\delta T_{50\%}$ as a function of $\theta$ for these layered devices and for TSV50a at 0.25 T. (c) Variation of $T_{50\%,\max }=T_{50\%},(0°)-T_{50\%},(90°)$ as a function of applied field for MoGe(50) and TSV50b.

Figure 4

Dependence of TSV effect on interface transparency. Variation of $T_{50\%,\max }$ plotted against the inverse of the normal state resistance $R_N$ (interface transparency), for spin valve devices with different interfaces between the ${\mathrm{CrO}}_2$ layer and the $\mathrm{MoGe}/\mathrm{Ni}/\mathrm{Cu}$ stack. Blue triangles: TSV25 ($d_S=25\mathrm{nm}$), measured at 0.5 T; open symbol: TSV25a. Green circles (red squares): TSV50 ($d_S=50\mathrm{nm}$), measured at 0.5 T (0.25 T). Symbols with $+$ are device TSV50a, the open green circle (red square) is TSV50b.

Figure 5

Normal reflection of triplet Cooper pairs. (a) Resistive transitions of a spin valve device TSV50a, for different values of the in-plane field between 0 and 1 T. (b) Pictorial representation of the effect of an extra ferromagnetic layer $F^{\prime }$ between the mixer layer $F_1$ and the drainage layer $F_2$ on the generation of triplet pairs. The green arrows represent magnetization directions of $F_{1,2}$ (in plane); the blue arrow indicates an interface layer $F^{\prime }$ with magnetization direction out of plane.