Injection Of Spin-polarized Quasiparticles Research Articles

Superconducting spintronics

Traditional studies that combine spintronics and superconductivity have mainly focused on the injection of spin-polarized quasiparticles into superconducting materials. However, a complete synergy between superconducting and magnetic orders turns out to be possible through the creation of spin-triplet Cooper pairs, which are generated at carefully engineered superconductor interfaces with ferromagnetic materials. Currently, there is intense activity focused on identifying materials combinations that merge superconductivity and spintronics to enhance device functionality and performance. The results look promising: it has been shown, for example, that superconducting order can greatly enhance central effects in spintronics such as spin injection and magnetoresistance. Here, we review the experimental and theoretical advances in this field and provide an outlook for upcoming challenges in superconducting spintronics.

Influence of spin injection on the in-plane and out-of-plane transport properties of BSCCO single crystal

We have studied the influence of spin injection on the in-plane and out-of-plane transport properties of $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ . Two different types of samples have been fabricated. One is a mesa composed of $\hbox{Co/Au/Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ for the out-of-plane transport measurement, and another is $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ single crystal bridge for the in-plane transport measurement. For the former, the hysteretic field dependence of $I_{c} $ of intrinsic Josephson junctions (IJJs) in the mesa, due to the spin injection effect, was observed at 77 K. Moreover, it was observed that the hysteresis in the $I_{c}$ – $B$ curves decreased with increasing the number of IJJs in mesas. This suggests that the contribution of the Josephson effect to the $c$ -axis transport property becomes larger than that of the spin injection effect with the increase in the number of junction. On the other hand, for the latter, hysteretic field dependence of the magnetoresistance was observed. These results give us the evidence for the superconductivity suppression of $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{\rm y} $ due to the injection of spin-polarized quasiparticles.

Effect of injected spins with different polarized orientations on the superconductivity of La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 thin films

The influence of spin-polarized quasi-particle injection on the superconductivity of epitaxial La0.7Sr0.3MnO3/La1.85Sr0.15CuO4 composite thin films is systematically investigated. Different electrode configurations on the same specimen are designed to study the role of spin injection. It is found that the upper critical field and flux pinning potential are reduced by the spin injection, especially for the field parallel to ab-plane. The manipulation of superconductivity by the spin polarization orientations of injected quasi-particles possibly originates from the different suppressions on the superconducting pairing strength, which is considered to be related to the local antiferromagnetic spin fluctuations in La1.85Sr0.15CuO4.

Cuprate-Manganite Interfaces-What Controls the Properties of FM/SC Oxide Superlattices?

This is a copy of the slides presented at the meeting but not formally written up for the volume. The physical properties of complex oxides are characterized by the interplay of charge-,spin-,orbital and lattice degrees of freedom. Combining oxides with different spin-,charge- or orbital order opens generate an additional degree of freedom that can give rise to novel physical phenomena. We have studied the interface of the system La0.67.Ca.33MnO3 and YBa2Cu3O7 prepared by conventional PLD as an example representing the combination of ferromagnetic and superconducting oxides by a variety of techniques including neutron diffraction and XMCD analysis. It turns out that at the interface interaction effects are taking place at two different length scales. One is based on the self ? injection of spin-polarized quasiparticles with a length scale of ~ 10nm, the other is a short range exchange interaction with a length scale of ~ 3 nm. The short range interaction gives rise to an antiferromagnetic coupling of the Mn and Cu spins across the interface. The results are discussed in the frame of orbital interactions across the interface. Furthermore, it could be shown that in FM/SC/FM trilayer systems a so far unknown coupling of adjacent magnetic layers occurs when the superlattices are cooled through the superconducting transition temperature. It is well established that two ferromagnetic layers are coupled through an intervening metallic layer by the Ruddermann-Kittel-Kasuya-Yoshida (RKKY) mechanism. The sign of the coupling oscillates from positive to negative with the thickness of the metallic layer. On the other hand, the interaction between ferromagnetic (FM) and superconducting (SC) layers in a FM/SC/FM heterostructure is not well understood. Tentatively, we propose a coupling mechanism mediated by London screening currents to explain our observations.

Spin-polarized quasiparticles injection effects in the normal state of YBCO thin films

We report detailed transport studies on ferromagnet-superconductor heterostructures. Epitaxial heterostructures of half-metal colossal magnetoresistive La 2/3Ca 1/3MnO 3 (HM-CMR) and high- T c superconducting YBa 2Cu 3O 7− δ (HTSC) are grown on SrTiO 3 (1 0 0) single crystal substrates using pulsed laser deposition. Using the HM-CMR layer as source for spin-polarized quasiparticles, we show the effect of injection of spin-polarized quasiparticles into the ab-plane and along the c-axis in the normal state of YBCO. The results show a drop in the ab-plane resistance R ab ( T) in the case of injection along the c-axis that is discussed to be related to the opening of a spin-gap in the superconductor.

Dependence of the critical temperature of YBCO thin films on spin-polarized quasiparticle injection

Non-equilibrium effects in cuprate superconductors caused by quasiparticle (QP) injection are investigated by electrical transport measurements. We explore the reduction of Tc in YBa2Cu3O7 thin films by highly spin-polarized QP injection and compare the results with weakly polarized as well as ordinary QP injection. Heterostructures consisting of ferromagnetic La0.67Ca0.33MnO3 and superconducting YBa2Cu3O7 layers were fabricated by pulsed laser deposition techniques in conjunction with shadow mask methods to define an appropriate sample geometry. Standard four probe measurement techniques have been applied for probing the transition to superconductivity of YBCO thin films with and without quasiparticle injection. We observe a 2% decrease of Tc for injection current densities of 5×10−2A/cm2 in the case of unpolarized QPs whereas highly spin-polarized QPs show a much stronger decrease of 14%. This result clearly demonstrates that the spin plays a crucial role in the Tc reduction. Using the theoretical description of QP injection effects of Bhattacharjee and Sardar based on the Owen–Scalapino approach for the description of non-equilibrium effects in superconductors we find a surprisingly good agreement of the experimental data with the model for interlayer tunneling.

Spin-polarized quasiparticle injection effects in YBCO thin films

We report detailed transport studies on ferromagnet-superconductor heterostructures. Epitaxial heterostructures of half-metal colossal magnetoresistive La 2/3Ca 1/3MnO 3 (HM-CMR) and high T c superconducting YBa 2Cu 3O 7 (YBCO) are grown on SrTiO 3 (100) single crystal substrates by pulsed laser deposition. Using the HM–CMR layer as source for spin-polarized quasiparticles, we show the effect of injection of spin-polarized quasiparticles into the ab-plane and along the c-axis of YBCO. The results show a drop in the ab-plane resistance R ab ( T) in the case of injection along the c-axis that is discussed to be related to the opening of a pseudogap.

Spin-polarized quasiparticles injection in La0.7Sr0.3MnO3∕SrTiO3∕Nb heterostructure devices

We report the effect of spin-polarized quasiparticle injection from a ferromagnetic manganite into a conventional Nb superconductor in a La0.7Sr0.3MnO3∕SrTiO3∕Nb heterostructure. A 8-terminal stacked trilayer geometry was used, where the injected current Iinj enters a superconducting microbridge and leaves it symmetrically by tunneling through a SrTiO3 barrier. A high dynamic gain G=−dIc∕dIinj was observed from 1.7 up to 8.5K, where Ic is the microbridge critical current. For a dc injection current, G was measured to be 290 at 1.7K. A SrTiO3 barrier of thickness in the range 3.5–8.5nm was found to provide a suitable spin injection length of the order of the bridge length and a low Joule heating level. In this confined stacked device geometry it was found that Joule dissipation in the manganite and barrier does not contribute significantly to the suppression of Ic, in contrast with previous studies.

Spin injection in High-Tc cuprates: An STM spectroscopy study

To study how the high-Tc order parameter (OP) evolves under the injection of spin-polarized quasiparticles, STM spectroscopy has been performed on superconductor/ferromagnet thin-film heterostructures comprising YBa2Cu3O7−x (YBCO) and La0.7Ca0.3MnO3 (LCMO) at 4.2 K. Quasiparticle-tunneling and Andreev-reflection characteristics measured on the YBCO under spin-injection from the LCMO were analyzed with the d-wave Blonder-Tinkham-Klapwijk theory, to reveal a spectral evolution which provides direct evidence for dynamic magnetic pair-breaking. The spectral analysis also shows the d-wave OP to remain time-reversal invariant as it is suppressed by the spin-injection. These results are discussed in terms of the general search for quantum-critical points in the high-Tc phase diagram.

Investigating the pairing state of cuprate superconductors via quasiparticle tunneling and spin injection

Scanning tunneling spectroscopic studies of cuprate superconductors revealed that the most eminent phenomena of hole-doped (p-type) cuprates, including the predominant d x 2− y 2 pairing symmetry, spin fluctuations and pseudogap, are absent in the infinite-layer electron-doped (n-type) cuprates Sr 1− x La x CuO 2. The optimally doped Sr 0.9La 0.1CuO 2 exhibits s-wave pairing symmetry, with a superconducting transition temperature T c=43.0 K, a large energy gap Δ=13.0±1.0 meV, and no pseudogap in the normal state. The response of Sr 0.9La 0.1CuO 2 to quantum impurities is also conventional, being less sensitive to non-magnetic impurities than to magnetic impurities, in sharp contrast to the strong effects induced by static non-magnetic impurities in the p-type cuprates. The dynamic injection of spin-polarized quasiparticles into the p-type cuprates is found to incur significant suppression of superconductivity, probably due to the strong influence on the spin fluctuations.

Spin injection and relaxation properties of anisotropic superconductors

We report the spin injection and relaxation properties of the anisotropic YBa 2Cu 3O y(YBCO) superconductor by injection of spin-polarized quasiparticles using a cobalt ferromagnetic injector. The observed current gain depends on the thickness of the Au interlayer and is directly related to the nonequilibrium magnetization due to spin relaxation effects.

Spin polarized carrier injection into high-Tcsuperconductors: A test for the superconductivity mechanism

We point out in this short paper, that be it an s-wave or d-wave superconductor, in a nonequilibrium situation (i.e., in the presence of excess unpolarized and polarized quasiparticles maintained by an injection current) the superconducting gap suppression by the presence of the same amount of excess quasiparticles, can at best differ by a factor of 2, for a conventional BCS superconductor. For the high-${T}_{c}$ superconductors on the other hand, there is a huge difference in gap suppression between unpolarized and polarized quasiparticle injection, as observed in the experiments [V. A. Vasko et al., Phys. Rev. Lett. 78, 1134 (1997); T. A. Venkatesan et al., Appl. Phys. Lett. 71, 1718 (1997)]. We argue that this large anomaly has a natural explanation within the interlayer tunneling mechanism of Wheatley, Hsu, and Anderson [J. M. Wheatley, T. C. Hsu, and P. W. Anderson, Phys. Rev. B 37, 5897 (1988)], and is due to the excess polarized quasiparticles blocking the interlayer pair tunneling process. We also point out that spin polarized quasiparticle injection in a superconductor is a very easy way to distinguish between an s-wave or an anisotropic gap superconductor.

Superconductivity and magnetoresistance in YBa 2Cu 3O 7/SrTiO 3/La 2/3Sr 1/3MnO 3 heterostructures

We report on the design, growth and basic transport characterization of YBa 2Cu 3O 7/SrTiO 3/La 2/3Sr 1/3MnO 3 heterostructures for spin-polarized quasiparticle injection experiments. Trilayers with planar and cross strip geometry have been grown by pulsed-laser ablation, eventually using shadow masks. We have analyzed the microstructure and basic physical properties of the heterostructures.

Spin tunnel injection properties of YBa2Cu3Oy films

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by tunnel injection of spin-polarized quasiparticles using a cobalt ferromagnetic injector. The injection of spin-polarized quasiparticles generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The measurements of transport properties for the directionally oriented spin-polarized quasiparticle injection on the [001] axis and the [110] surfaces of YBCO thin film show anisotropic behaviour according to the junction orientation.

Spin-polarized quasiparticle tunnel injection in a YBa2Cu3Oy/Au/Co junction

We report the strong suppression of YBa2Cu3Oy (YBCO) supercurrent by injection of spin-polarized quasiparticles (QP) using a cobalt ferromagnetic injector. The injection of spin-polarized QP generates a substantially larger nonequilibrium population as compared with that of an unpolarized injection current. The observed current gain depends on the thickness of Au interlayer (dAu) and is directly related to the nonequilibrium magnetization due to spin relaxation effects. For dAu=15 nm, the tunnel characteristic a YBCO/Au/Co junction exhibited a zero bias conductance peak, which may be interpreted by spin scattering processes at a ferromagnetic/d-wave superconductor junction.

Spin-polarized quasiparticle injection into YBCO

FET-type devices have been fabricated by using trilayers of Nd/sub 0.7/Sr/sub 0.3/MnO/sub 3/LaNiO/sub 3/ (NSMO) or LaNiO/sub 3/ (LNO) (gate)/LaAlO/sub 3/ (LAO) (barrier)/YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) (channel) in order to investigate effect of quasiparticle injection into YBCO. Here, NSMO and LNO were used as gate electrodes for injection of spin-polarized and spin-unpolarized quasiparticles into the superconducting channel, respectively. When injecting along the c-axis of YBCO, the critical current was suppressed with spin-polarized quasiparticles 30 times more efficiently than with spin-unpolarized quasiparticles. Differential current gain, defined as a differential change of the critical current to injection current change, has been achieved up to /spl sim/16 for c-axis YBCO. If the response time is limited by quasiparticle relaxation time of /spl sim/10 ps, the device may be useful for fast electronics. Preliminary high-speed measurements indicate that part of the critical current suppression may be caused by quasiparticle injection, not all by heating. When injecting along a-axis, no significant dependence on quasiparticle polarization was observed. Other superconductors such as Pr/sub 1.85/Ce/sub 0.15/CuO/sub 4/ (PCCO) and PbIn have been tested in similar devices for comparison.

Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures

Scanning tunneling spectroscopy was performed at 4.2 K on epitaxial thin-film heterostructures comprising YBa2Cu3O7−δ and La0.7Ca0.3MnO3, to study the microscopic effects of spin-polarized quasiparticle injection from the half-metallic ferromagnetic manganite on the high Tc cuprate superconductor. The quasiparticle tunneling characteristics observed were consistent with d-wave pairing symmetry, with a gap-maximum Δ0≈22 meV, up to at least 35 mA (7×103 A/cm2) injection. Spectral smearing observed at higher injections could be fitted to elevated effective quasiparticle temperatures, even though negligible sample heating was detected by in situ thermometry. The overall spectral evolution with the injection current also appears to be nonthermal in character, showing a nonmonotonic change in both the zero-bias tunneling conductance and the area under the conductance spectrum. We discuss general implications of these results for the scenario of dynamic pair breaking by a nonequilibrium distribution of spin-polarized quasiparticles.

Novel high-Tc transistors with manganite oxides

One viable approach for transistor-like high-Tc three terminal devices is the quasiparticle injection device (QPID). We have fabricated Au/YBa2Cu3O7/LaAlO3/Nd0.7Sr0.3MnO3 (NSMO) and Au/YBa2Cu3O7/LaAlO3/LaNiO3 (LNO) heterostructures on (100) LaAlO3 substrates by pulsed laser deposition for studies of quasiparticle injection effects in high-Tc superconducting thin films. The effect of the injection of spin-polarized quasiparticles from a ferromagnetic NSMO gate was compared to that of unpolarized quasiparticles from a nonmagnetic metallic LNO gate. A current gain greater than nine has been attained for spin-polarized QPIDs, which is an order of magnitude larger than the gain of spin unpolarized QPIDs. Such large effects could be useful in a variety of active high-Tc/colossal magnetoresistance heterostructure devices.

Spin-polarized quasiparticle injection devices using Au/YBa2Cu3O7/LaAlO3/Nd0.7Sr0.3MnO3 heterostructures

Oxide heterostructures were used for studies of quasiparticle injection effects in high-Tc superconducting thin films. The effect of injection of spin polarized quasiparticles from a ferromagnetic gate layer was compared to that of unpolarized quasiparticles from a nonmagnetic metallic gate. Transport measurements of the superconducting layer showed strong suppression in the supercurrent by the injection of spin-polarized quasiparticles, and a current gain of as large as five was attained. This is 10 to 30 times larger than the gain of unpolarized injection devices. Such large effects could be useful in a variety of active high-Tc superconductor/colossal magnetoresistance heterostructure based devices.