Spin-polarized quasiparticle control in a double spin-filter tunnel junction

Spin-polarized quasiparticles can be easily created during spin-filtering through a ferromagnetic insulator (FI) in contact with a superconductor due to pair breaking effects at the interface. A combination FI-N-FI sandwiched between two superconductors can be used to create and analyze such spin-polarized quasiparticles through their nonequilibrium accumulation in the middle metallic (N) layer. We report spin-polarized quasiparticle regulation in a double spin-filter tunnel junction in the configuration NbN-GdN1-Ti-GdN2-NbN. The middle Ti layer provides magnetic decoupling between two ferromagnetic GdN and a place for nonequilibrium quasiparticle accumulation. The two GdN(1,2) layers were deposited under different conditions to introduce coercive contrast. The quasiparticle tunneling spectra has been measured at different temperatures to understand the tunneling mechanism in these double spin-filter junctions. The conductance spectra were found to be comparable to an asymmetric SINI'S-type tunnel junction. A hysteretic R-H loop with higher resistance for the antiparallel configuration compared to parallel state was observed, asserting the spin-polarized nature of quasiparticles. The hysteresis in the R-H loop was found to disappear for subgap bias current. This difference can be understood by considering modification of the interlayer coupling due to nonequilibrium spin-polarized quasiparticle accumulation in the Ti layer.

Figure 1

Temperature dependence of resistance of the NbN(50 nm) – GdN(2 $\mathrm{nm}-8\%$) – Ti(8 nm) – GdN(2 $\text{nm-}4\%$) – NbN(50 nm) double spin-filter tunnel junction. The measurement was done using a current $I$ = 10 $\mu \mathrm{A}$. The upper inset shows $R\left(T\right)$ in the range 0.3 to 1.3 K. Lower inset shows $R\left(T\right)$ close to $T_C$.

Figure 2

(a) Normalized conductance spectra $G\left(V\right)/G\left(0\right)$ of the double spin-filter tunnel junction measured at 40, 30, and 20 K. (b) The $I-V$ and normalized conductance spectra of the same junction measured at 300 mK. (c) Normalized conductance spectra of the junction measured in the temperature range 1.7–11 K. (d) Schematic of the double tunnel junction in the mesa structure with measurement scheme.

Figure 3

(a) $I-V$ and normalized conductance spectra $G\left(V\right)/G_N$ of NbN-GdN(2.3 nm)-NbN tunnel junction. The red solid line represents fitting to the S-I-S tunneling model with fitting parameter $\mathrm{\Delta }$ = 1.415 meV and $\gamma$ = 0.195. (b) $I-V$ and normalized conductance spectra $G\left(V\right)/G_N$ of NbN-Ti(9 nm)-GdN(2.6 nm)-NbN tunnel junction. The red solid line represents fitting to the N-I-S tunneling model with fitting parameter $\mathrm{\Delta }$ = 1.38 meV and $\gamma$ = 0.325. (c) $I-V$ and normalized conductance spectra $G\left(V\right)/G_N$ of NbN-GdN1(2 nm)-Ti(8 nm)-GdN(2 nm)-NbN tunnel junction. The red solid line represents fitting to the S-I-N-I-S tunneling model with asymmetry parameter $a_s$ = 2 (red) and 1.85 (black). (d) Temperature evolution of the fitting parameters $\mathrm{\Delta }$ and $\gamma$ found from fitting Eq. (1) to the conductance spectra shown in Fig. 2. The red ($a_s$ = 2) and blue ($a_s$ = 1.85) solid lines are the fitting to the BCS-type temperature dependence, $\mathrm{\Delta }\left(T\right)=\mathrm{\Delta }\left(0\right)tanh\left[1.74\sqrt{(T_C-T)/T}\right]$ with $T_C=12.11$ K. Black solid line is the fitting to an exponential of the form $\gamma \propto e^{-\zeta /T}$ [51].

Figure 4

R-H loops measured with different bias current: (a) 1 $\mu \mathrm{A}$, (b) 10 $\mu \mathrm{A}$, (c) 50 $\mu \mathrm{A}$, (d) 100 $\mu \mathrm{A}$, and (e) $200\mu \mathrm{A}$. (f) The $I-V$ curve of the junction at the same temperature ($T$ = 300 mK). The arrows indicate different points in the $I-V$ curve where the R-H loop was measured. Gradual disappearance of the hysteresis in the R-H loop can be seen as bias current is reduced to below subgap value.