Half-metallic Ferromagnet La0 Research Articles

Long-range superconducting proximity effect in YBa2Cu3O7/La0.7Ca0.3MnO3 weak-link arrays

The interplay between ferromagnetism and superconductivity has attracted substantial interest due to its potential for exotic quantum phenomena and advanced electronic devices. Although ferromagnetism and superconductivity are antagonistic phenomena, ferromagnets (F) can host spin-triplet superconductivity induced via proximity with superconductors (S). To date, most of the experimental effort has been focused on single S/F/S junctions. Here, we have found the fingerprints of long-range superconducting proximity effect in micrometric weak-link arrays, formed by embedding YBa2Cu3O7 superconducting islands in a half-metallic ferromagnet La0.7Ca0.3MnO3 film. These arrays show magnetoresistance oscillations that appear at temperatures below the critical temperature of YBa2Cu3O7 for currents below a threshold, indicating their superconducting origin. This realization paves the way for device architectures displaying macroscopic quantum interference effects, which are of interest for field sensing applications, among others.

Investigating magnetic and magneto-transport properties of ferromagnetic (FM) structure and antiferromagnetic/ferromagnetic (AFM/FM) heterostructures

Electrical transport properties of functional magnetic oxides on the technologically important silicon substrate have been highly investigated in recent years for the spintronic applications. In this study, we probe the magnetic and magneto-transport properties of a hetero-structure of wide band gap semiconducting antiferromagnetic material (α-Fe2O3) and half metallic ferromagnet La0.7Sr0.3MnO3 (LSMO) and compare it with those of the LSMO single layer (thickness ∼60 nm) deposited on Si (100) substrate using pulsed laser deposition technique. X-ray diffraction θ-2θ scan validates the growth of α-Fe2O3 and LSMO phases, with no other impurity or secondary phase. Magnetic studies reveal that LSMO film shows Curie temperature close to 320 K and the bilayer shows the same at 255 K and another anomaly in the bilayer is at 102 K. LSMO single layer shows fairly high negative magnetoresistance (MR%) value of 41% at 214 K; close to MIT (metal-to-insulator temperature) at 230 K, and at 7 T field, which proves it to be potentially good candidate for spin transport across the heterostructure. α-Fe2O3 film shows no MR. Upon integration of α-Fe2O3 with LSMO, positive MR of about 10% is obtained, which stimulates to carry out further probe in spin excitations either optically or at higher magnetic fields across the heterostructures.

Percolation transitions in d-wave superconductor–half-metallic ferromagnet nanocomposites

Electrical transport properties of random binary networks composed of high-Tc superconductor Bi2Sr2Ca2Cu3O6+x microparticles and half-metallic ferromagnet La0.67Sr0.33MnO3 (LSMO) nanoparticles have been investigated. Two resistive percolation transitions (superconductor–metal–semiconductor) have been observed for the nanocomposites with a volume fraction of the LSMO no more than 30%. The nanocomposites basic attributes (transition critical temperatures, current–voltage characteristics, percolation threshold, etc.), most probably, cannot be quantitatively interpreted within the framework of a conventional percolation model. We have explained the observed behavior by a two-level scale interaction in the system caused by (i) a significant geometric disparity between the constituent components and (ii) proximity-induced superconducting state of the half-metallic manganite.

Low noise all-oxide magnetic tunnel junctions based on a La0.7Sr0.3MnO3/Nb:SrTiO3 interface

All-oxide magnetic tunnel junctions with a semiconducting barrier, formed by the half-metallic ferromagnet La0.7Sr0.3MnO3 and n-type semiconductor SrTi0.8Nb0.2O3, were designed, fabricated, and investigated in terms of their magneto-transport properties as a function of applied bias and temperature. We found that the use of the heavily Nb-doped SrTiO3 as a barrier results in significant improvement in the reproducibility of results, i.e., of large tunnel magnetoresistance (TMR) ratios, and a spectral noise density reduced by three orders of magnitude at low temperature. We attribute this finding to a considerably decreased amount of point defects in SrTi0.8Nb0.2O3, especially oxygen vacancies, compared with the conventional insulating SrTiO3 barrier.

Superconductor-ferromagnet-superconductor nanojunctions from perovskite materials

The lateral superconductor-ferromagnet–superconductor (SFS) nanojunctions based on high critical temperature superconductor YBa2Cu3Ox (YBCO) and half-metallic ferromagnet La0.67Sr0.33MnO3 (LSMO) thin films were prepared to investigate a possible presence of long range triplet component (LRTC) of Cooper pairs in the LSMO. We applied Ga3+ focused ion beam patterning to create YBCO/LSMO/YBCO lateral type nanojunctions with LSMO length as small as 40nm. The resistivity vs. temperature, critical current density vs. temperature and resistance vs. magnetic field dependence were studied to recognize the LRTC of Cooper pairs in the LSMO. A non-monotonic temperature dependence of junction critical current density and a decrease of the SFS nanojunction resistance in increased magnetic field were observed. Only weak manifestations of LRTC and some qualitative agreement with theory were found out in SFS nanojunctions realized from the perovskite materials. The presence of equal-spin triplet component of Cooper pairs in half-metallic LSMO ferromagnet is not such apparent as in SFS junctions prepared from low temperature superconductors NbTiN and half-metallic ferromagnet CrO2.

Thermodynamic conditions during growth determine the magnetic anisotropy in epitaxial thin-films of La0.7Sr0.3MnO3

The suitability of a particular material for use in magnetic devices is determined by the process of magnetization reversal/relaxation, which in turn depends on the magnetic anisotropy. Therefore, designing new ways to control magnetic anisotropy in technologically important materials is highly desirable. Here we show that magnetic anisotropy of epitaxial thin-films of half-metallic ferromagnet La0.7Sr0.3MnO3 (LSMO) is determined by the proximity to thermodynamic equilibrium conditions during growth. We performed a series of x-ray diffraction and ferromagnetic resonance (FMR) experiments in two different sets of samples: the first corresponds to LSMO thin-films deposited under tensile strain on (0 0 1) SrTiO3 by pulsed laser deposition (PLD; far from thermodynamic equilibrium); the second were deposited by a slow chemical solution deposition (CSD) method, under quasi-equilibrium conditions. Thin films prepared by PLD show fourfold in-plane magnetic anisotropy, with an overimposed uniaxial term. However, the uniaxial anisotropy is completely suppressed in the CSD films. This change is due to a different rotation pattern of MnO6 octahedra to accommodate epitaxial strain, which depends not only on the amplitude of tensile stress imposed by the STO substrate, but also on the growth conditions. Our results demonstrate that the nature and magnitude of the magnetic anisotropy in LSMO can be tuned by the thermodynamic parameters during thin-film deposition.

Ultrathin junctions based on the LaSrMnO3/Nb:SrTiO3 functional oxide interface

High-quality all-oxide thin film metal–semiconductor junctions, fabricated of a half-metallic ferromagnet La0.7Sr0.3MnO3 and of a niobium doped n-type semiconductor SrTiO3 (SrTi0.8Nb0.2O3) by pulsed laser deposition, were studied in terms of their electronic transport properties in a wide temperature range. A fabrication process for ultrathin film metal–semiconductor junctions with micrometre-sized mesas has been established. The current–voltage characteristics of the junctions were found at variance with the conventional thermionic emission theory often applied to explain the transport properties of Schottky contacts. This discrepancy is tentatively ascribed to the very high electric field in the ultrathin SrTi0.8Nb0.2O3 layer and its effect on carrier depletion and dielectric constant.

Fabrication of YBCO-LSMO-YBCO Lateral Structure with AFM Lithography

We have tried to make the superconductor/half metal/superconductor (SC/HF/SC) Josephson junction to make clear a long range proximity effect. The structure was consisted of high-Tc superconductor YBa2Cu3O7-x and half metallic ferromagnet La0.7Sr0.3MnO3 thin films deposited by pulse laser deposition on SrTiO3(100) single crystal substrates. The SC/HF/SC lateral structure was made by scratching with the atomic force microscope (AFM) probe. We could cut the ditch which has 30nm width and 50nm depth. We have investigated the I-V and R-T measurements of the structure. The structure after the fabrication did not show the superconducting state and we could not find the Josephson current.

Effect of structural and magnetic exchange coupling on the electronic transport of NdNiO3 films intercalated with La0.7Sr0.3MnO3 thin layers

We have fabricated multilayer structure of half-metallic ferromagnet La0.7Sr0.3MnO3 and insulator-metal transition system NdNiO3 on SrTiO3 (100) substrate; thin layers of the former are intercalated in the major matrix of the later. The two structures exhibit pseudomorphic structural coupling with each other. We show that a large increase in the conductivity and an enhanced irreversibility across insulator-metal transition of NdNiO3 may be realized by varying the thickness of La0.7Sr0.3MnO3 and the ratio of thickness of the two layers. Corroborated by the magnetization data, it is shown that the combined effect of structure and magnetism by interfacial coupling is a much better technique than the chemical modification to control the electrical transport of NdNiO3.

Josephson Current in Superconductor-ferromagnet Structure with YBCO-LSMO

Abstract We have studied transport property of superconductor-ferromagnet hybridized structure in nanometer-scale processed by a focused ion beam technique. The structure was consisted of high- T c superconductor YBa2Cu3O 7-x and half metallic ferromagnet La0.7Sr0.3MnO3 thin films deposited by pulse laser deposition on SrTiO3 (100) single crystal substrates. We have measured the I-V characteristics of the sample with 400 nm ferromagnetic layer, but we could not observe the proximity effect.

Orientation dependence of the Schottky barrier height for La0.6Sr0.4MnO3/SrTiO3 heterojunctions

The authors report on the crystallographic orientation dependence of the Schottky properties for heterojunctions between a half-metallic ferromagnet La0.6Sr0.4MnO3 (LSMO) and Nb-doped SrTiO3 semiconductor. The Schottky barrier height determined by in situ photoemission measurements is independent for the substrate orientations (001) and (110), while the magnetic properties of LSMO (110) films are more enhanced than for (001) films. These results suggest that the performance of magnetic devices based on ferromagnetic manganite is improved by using (110)-oriented substrates.

Magnetization-induced resistance-switching effects inLa0.67Sr0.33MnO3/YBa2Cu3O7−δbi- and trilayers

We have studied the influence of the magnetization on the superconducting transition temperature (Tc) in bi- and trilayers consisting of the half-metallic ferromagnet La0.67Sr0.33MnO3 and the high-temperature superconductor YBa2Cu3O7â��I´ (YBCO). We have made use of tilted epitaxial growth in order to achieve contacts between the two materials that are partly in the crystallographic ab plane of the YBCO. As a result of uniaxial magnetic anisotropy in the tilted structures, we observe sharp magnetization-switching behavior. At temperatures close to Tc, the magnetization-switching induces resistance jumps in trilayers, resulting in a magnetization dependence of Tc. In bilayers, this switching effect can be observed as well, provided that the interface to the ferromagnetic layer is considerably rough. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor. A simple model describes the observed behavior well. We find no evidence that the switching behavior is caused by a so-called superconducting spin switch, nor by accumulation of spin-polarized electrons. Observation of magnetic coupling of the ferromagnetic layers, through the superconductor, supports the idea of field-induced resistance switching.

Band diagrams of spin tunneling junctions La0.6Sr0.4MnO3∕Nb:SrTiO3 and SrRuO3∕Nb:SrTiO3 determined by in situ photoemission spectroscopy

The authors report on the band diagram for epitaxial Schottky junctions of ferromagnetic metallic oxides [half-metallic ferromagnet La0.6Sr0.4MnO3 (LSMO) and itinerant ferromagnet SrRuO3 (SRO)] on Nb-doped SrTiO3 (Nb:STO) semiconductor substrates using in situ photoemission spectroscopy. The ideal Schottky barrier is formed in SRO/Nb:STO junctions with Schottky barrier height (SBH) of 1.2±0.1eV, while the measured SBH of LSMO/Nb:STO (1.2±0.1eV) is much larger than the prediction from the Schottky-Mott rule (0.7±0.1eV). These results suggest that a certain interface dipole is formed at the LSMO/Nb:STO interface.

Scanning tunneling spectroscopy under pulsed spin injection

An experimental technique combining cryogenic scanning tunneling spectroscopy (STS) and pulsed quasiparticle spin injection has been developed. The spin injection is intended to perturb a superconducting thin film from spin equilibrium, while the STS monitors its steady-state quasiparticle spectrum. A pulsed injection circuit was designed to minimize Joule heating while being both synchronized with and decoupled from the STS circuitry. A detailed description of the technique is presented, along with its application to spin-injection heterostructures comprising the half-metallic ferromagnet La0.7Ca0.3MnO3 and the high-Tc superconductor YBa2Cu3O7−δ.

Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures

Quasiparticle nonequilibrium due to spin-polarized current injection in perovskite superconductor/ferromagnet (S/F) thin-film heterostructures has been studied with the technique of cryogenic scanning tunneling spectroscopy. The spin-injection heterostructures consisted of epitaxial bilayers of the high-Tc superconductor YBa2Cu3O7 − δ and the half-metallic ferromagnet La0.7Ca0.3MnO3, with a spin-polarized quasiparticle current injected into the cuprate layer from the manganite layer. The tunneling conductance measured at 4.2 K on the cuprate layer showed a distinctly nonequilibrium quasiparticle spectrum as a result of the spin injection. Quantitative analysis of the tunneling spectral evolution versus the injection current yielded an estimate of the quasiparticle spin-diffusion depth and the spin-relaxation time in the superconducting cuprate.

Magnetotransport properties of La0.7Ca0.3MnO3 thick films prepared by spray pyrolysis

A spray pyrolysis deposition technique has been used to prepare thick films of half-metallic ferromagnet La0.7Ca0.3MnO3 on LaAlO3 (001) and yttria-stabilized zirconia Y–ZrO2 (100) single-crystal substrates. The films deposited on LaAlO3 (001) exhibited a high degree of crystallinity with both in-plane and out-of-plane texture and a very sharp insulator to metal transition at TIM = 270 K. The intrinsic magnetoresistance (MR) reaches 70% at TIM in 3 T, a similar value as in the bulk material of the same composition. The extrinsic intergrain MR, however, is only 0.5% at 40 K in 3 T, 50 times smaller than for the bulk. By contrast, films grown on YSZ single crystals have a broad insulator to metal transition at a lower TIM of approximately 220 K and poor intrinsic MR. They show however an extrinsic MR as large as in the bulk. The extrinsic MR is interpreted as arising from crystalline and magnetic disorder at the grain boundaries.

Spin-polarized tunneling in the half-metallic ferromagnetsLa0.7−xHoxSr0.3MnO3(x=0and 0.15): Experiment and theory

The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La0.7Sr0.3MnO3, and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.