Influence of spin injection on the critical current density in La_0.7Sr_0.3MnO₃/La_1.85Sr_0.15CuO₄ heterostructure

Since the discovery of high temperature superconductors (HTSs), the vortex dynamics and critical current density in the mixed state have garnered tremendous fundamental and technological interest,^1–3 due to their significance in major applications of superconductors. Recently, much attention has been paid to the pinning behaviors of a special HTS heterostructure system consisting of colossal magnetoresistance (CMR) materials and high temperature superconductors. In CMR/HTS bilayers, the spin-polarized quasi-particle injection from the ferromagnetic (F) layer can result in the suppression of superconductivity due to the breaking of time-reversal symmetry of the Cooper pairs,⁴ which can provide not only insights on spin dependent pinning mechanisms of HTSs⁵ but also methods to design spintronic devices.⁶ Thus, it is interesting and important to understand the effect of F layers on the vortex dynamics in HTSs. Samal et al. reported that the injection of spin-polarized carriers from a ferromagnet can obviously suppress the critical current in a superconductor.⁷ Several studies revealed that the pinning potentials in YBa₂Cu₃O_7-δ (YBCO)/La_0.7Ca_0.3MnO₃ and YBCO/La_0.7Sr_0.3MnO₃ heterostructures are much smaller than the values obtained for HTS/nonmagnetic heterostructures,^8–10 and the polarized spin orientation of the F layer may play a crucial role for the suppression of pinning potential.¹¹ On the contrary, it was found in YBa₂Cu₃O_7-δ/La_0.67Sr_0.33MnO_3-δ (YBCO/LSMO) bilayer that the flux pinning in YBCO film can be improved by the magnetic inhomogeneity of the underlying LSMO, indicating that the magnetic microstructures and correlated defects in cuprate/manganite bilayers may increase the pinning force.¹² Thus, it is still controversial how the ferromagnetic layer influences the flux pinning in CMR/HTS heterostructures. On the other hand, in type-II superconductors, based on the collective pinning model for the randomly distributed defects, there exist two pinning mechanisms associated with disorders in the transition temperature T_c (δT_c pinning) or from spatial variations in the charge carrier mean free path l near lattice defects (δl pinning).¹³ For CMR/HTS heterostructure, the influence of spin injection on the pinning behaviors is still unknown.

Another important issue in HTS is the angular dependent critical current density J_c,^14,15 which is also crucial for the practical application of HTS.¹⁶ It has been reported that the angular dependence of J_c in YBCO can usually be described by the anisotropic Ginzberg-Landau (G-L) model, indicating the pinning is due to uncorrelated defects randomly distributed over angular space.¹⁷ However, for a magnetic field parallel to the ab-plane of YBCO or Bi₂Sr₂CaCu₂O₈ (BSCCO) films, the intrinsic interlayer pinning plays a dominant role and the critical current can be scaled by Tachiki-Takahashi model¹⁸ or Lawrence-Doniach model¹⁹ in this case. Since the spin injection is important for applications, it is necessary to investigate influence of spin injection on the angular dependent pinning behaviors of cuprate.

In this paper, we report the spin injection effect on the angular resolved critical current density J_c in a La_0.7Sr_0.3MnO₃/La_1.85Sr_0.15CuO₄ (LSMO/LSCO) bilayer with different electrode configurations to realize the control of spin injection. It is found that both δT_c and δl pinning mechanisms coexist in the LSMO/LSCO bilayer and injected spin-polarized quasiparticles can increase the contribution of δT_c pinning by breaking the time-reversal symmetry of the Cooper pairs. From the angular dependent critical current J_c(θ) measurements, we find that the intrinsic pinning has a dominant contribution to the total pinning potential in the vicinity of H//ab. Besides, the relative changes of J_c affected by spin injection are also discussed.

The LSMO/LSCO bilayer was epitaxially grown on a commercial (001)-oriented LaSrAlO₄ (LSAO) single crystal substrate by a dc magnetron sputtering. LSCO layer of thickness t = 45 nm was first deposited on LSAO substrate and then a 50 nm thick LSMO film was deposited on the top of the LSCO layer through a mask to yield a bilayer stripe 3.5 mm × 0.7 mm. Au electrodes were prepared on both LSMO and LSCO surfaces, and the distance between two voltage measurement electrodes is about 1 mm (Fig. 1(a)). The in-plane electrical resistivity broadening and critical current were measured in magnetic fields up to 9 T by a standard four-probe method with current I⊥H configuration using Physical Property Measurement System (PPMS-9). The angle θ = 0° is defined as the applied magnetic field parallel to the c-axis. The J_c data were collected using I-V curves with a criterion of E_c = 10 μV cm⁻¹. The critical temperature T_c is defined as the zero resistance temperature.

Figure 1(a) displays the two different electrode configurations for our LSMO/LSCO sample. Because of the much smaller resistivity of LSCO (about 0.35 mΩ cm at 100 K) than that of LSMO (about 1∼10 mΩ cm at 100 K),²⁰ almost all current will flow through LSCO especially below T_c. Thus, for the current electrodes on LSMO layer, almost all the current will be spin polarized by LSMO, and this electrode configuration is defined as a strong spin injection (S-INJ). While for current electrodes on LSCO layer, it is regarded as a weak spin injection (W-INJ).¹¹ The superconducting transition temperatures T_c are 26.0 K and 22.6 K for W-INJ and S-INJ, respectively, suggesting the Cooper pair breaking by spin injection. The critical currents for these two electrode configurations were obtained through the I-V measurements, and two representative curves at 10 K are shown in Fig. 1(b). The critical current J_c is obviously reduced by the strong spin injection due to pair-breaking effect and the relative change of J_c is about 45% at 10 K and 0 T. To decrease the influence of Joule heating, the critical current measurements are performed by means of a pulsed current technique. The pulse width and period are 30 ms and 2 s, and in this condition the relative change of J_c caused by Joule heating observed for the current scanning from zero to high or from high to zero is only about 0.4%, which is much less than the influence of spin injection, see supplementary information, indicating the influence of Joule heating is relatively small.²¹ Figures 1(c) and 1(d) show the temperature dependent critical current densities at different magnetic fields parallel to ab-plane and c-axis, respectively. On the other hand, J_c decreases with increasing magnetic fields, especially for H//c, indicating an anisotropic characteristic.

Figures 2(a) and 2(b) display the temperature dependencies of the normalized critical current densities J_c(T)/J_c(4K) for LSMO/LSCO bilayer at zero magnetic field. Generally, the vortices interact with a pinning site either through the spatial variations in the T_c throughout the material (δT_c pinning) or by scattering of charge carriers with reduced mean free path l near lattice defects (δl pinning).¹³ As we know, the temperature dependence of J_c(T) in different magnetic fields related to the randomly distributed pinning defects can be described as

Here J_dp (0) is the depinning current density at T = 0 K, J(t) and g(t) have different temperature dependencies for δT_c and δl pinning mechanisms, μ is the glassy exponent and C is a fitting constant.²² If both the δT_c and δl pinnings coexist, J_c(T) data can be analyzed within the following expression:

where $J c l ( T )$ and $J c T c ( T )$ are the critical current expressions for the δl and δT_c pinnings, respectively, P₁ and P₂ are the fitting parameters. It can be seen in Figs. 2(a) and 2(b) that the experimentally obtained normalized critical current density values for W-INJ and S-INJ deviate from either the δl or the δT_c pinning and can be well described by Eq.(2), indicating that δl and δT_c pinning mechanisms coexist in our sample. The fitting parameters P₁ and P₂ in zero field are 0.46 and 0.54 for W-INJ, 0.14 and 0.86 for S-INJ, respectively, and one can see that the fitting curve for S-INJ is very close to the δT_c pinning. It implies that the breaking down of Cooper pairs due to the spin-polarized quasi-particles makes the spatial variations in the transition temperature T_c play a more important role. The magnetic field dependences of P₁ and P₂ are shown in Figs. 2(c) and 2(d). Both δl and δT_c pinnings coexist in magnetic fields lower than 7 T. The δT_c pinning is dominant in lower magnetic fields and its contribution decreases with increasing field. While the δl pinning shows an opposite trend, because the quasiparticle mean free path l decreases with increasing scattering from vortices at higher field²³ in a disordered vortex array. Besides, as compared with the weak injection condition, for spin injection the δT_c pinning mechanism always plays a more important role.

In order to thoroughly investigate the pinning mechanisms of LSMO/LSCO bilayer, the angular dependence of J_c is measured in different magnetic fields with different angles θ for the sample rotated from H//c to H//ab. We analyzed the angular dependence of J_c at 4.2 K by using the G-L anisotropic scaling approach.^24,25 If the pinning is due to the randomly distributed pinning centers such as uncorrelated point defects, then J_c(H, θ)  =  J_c[Hε(θ)], where ε(θ) = [cos²(θ)+γ⁻²sin²(θ)]^1/2 and γ² is the electronic mass anisotropy parameter. Figs. 3(a) and 3(b) show the scaling results for the bilayer at different magnetic fields. It is noteworthy that the measured J_c(H, θ) at 4.2 K can be reasonably fitted from 0° up to about 45° except in the vicinity of H//ab. This means that random defects are dominant in this region (0°-45°), while the failure of scaling close to H//ab is owing to the intrinsic pinning associated with the modulation of the superconducting order parameter along the c-axis, which is similar to that observed in (Y, Gd)Ba₂Cu₃O_x²⁶ and Ba(Fe_1-xCo_x)₂As₂ thin films.²⁷ 

The results mentioned above indicate the pinning mechanisms for LSMO/LSCO bilayer are complex. In order to find out the influence of spin injection with different orientations on pinning properties, we analyze the J_c data as a function of applied magnetic field at 4.2 K for H//ab and H//c, respectively, as shown in Fig. 4(a). Compared with H//ab, the J_c for H//c shows much stronger decrease with increasing magnetic fields. Defining the suppression difference of J_c due to spin injection as:

where $J c W−INJ$ and $J c S−INJ$ are the critical current densities with weak and strong spin injections, respectively. It is found that with increasing fields δJ_c/J_c increases for H//c while it decreases for H//ab, as shown in Fig. 4(b). Two effects may be responsible for these results. For H//c, the pinning due to randomly distributed defects is dominant and spin injection suppresses this kind of pinning more obviously with increasing fields due to the increasing Cooper pair breaking effect. While for H//ab at lower temperature such as 4.2 K where ξ_c ≈ 0.24 nm ≈ 0.19 c (ξ_c at 4.2 K is estimated from ξ_c(0) and c is the c-axis lattice constant),²⁸ the vortices parallel to the ab-plane are small enough to go in between superconducting layers, so the intrinsic pinning from the layered structure prevents the critical current from dropping too quickly for H//ab compared with H//c. On the other hand, with increasing fields the vortex core size r₀ in type-II superconductors decreases due to the increased strength of the vortex-vortex interactions,²⁹ making the intrinsic pinning play a more important role. Thus the coexistence of intrinsic pinning and spin injection effect makes a different variation trend of δJ_c/J_c with increasing field for H//ab.

In summary, the influence of spin-polarized quasi-particle injection on the critical current density J_c in LSMO/LSCO bilayer with different electrode configurations indicates that spin injection suppresses J_c but enhances the contribution of δT_c pinning. With increasing magnetic fields, the reduction in the quasiparticle mean free path l due to the increased scattering from vortices causes the δl pinning more dominant. One of the important results is that because of the intrinsic pinning effect the change of field dependent J_c for H//ab with spin injection is different from that for H//c, which reflects the different interactions between injected spins and intrinsic pinning or random pinning in La_1.85Sr_0.15CuO₄. Our results indicate that the spin injection effect may benefit for designing superconducting spintronic devices.⁶