Electric-field-dependent spin polarization in GdN spin filter tunnel junctions

Vertical transport and tunnelling in rare-earth nitride heterostructures

We report an investigation of the ferromagnetic semiconductor rare earth nitrides (RENs) for their potential for cryogenic-temperature electronics and spintronics applications. We have identified ohmic contacts suitable for the device structures that demand electron transport through interface layers, and grown REN/insulator/REN heterostructures that display tunnelling characteristics, an enormous 400% tunneling magnetoresistance and a hysteresis promising their exploitation in non-volatile magnetic random access memory.

Low temperature characterization of high efficiency spin-filter Josephson junctions

The interplay between superconducting and ferromagnetic order pa¬rameters in S/F interfaces gives rise to a wide range of peculiar properties with applications in high-efficiency computation and in the emerging field of super¬conducting spintronics. In NbN/GdN/NbN Josephson junctions, GdN barriers give unique properties due to the double insulting and ferromagnetic nature of the material, as demonstrated in previous works. Here we focus on tunneling spectroscopy of these junctions down to 0.3 K when changing the barrier thick¬ness, which contributes to complete a consistent picture on the physics of these junctions and supports the previous indications of equal-spin Cooper pairs con¬tributing to the total supercurrent of the devices.

Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport

The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-π transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3 K, and the first evidence of an incomplete 0-π transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations.

All-spinel oxide Josephson junctions for high-efficiency spin filtering

Obtaining high efficiency spin filtering at room temperature using spinel ferromagnetic tunnel barriers has been hampered by the formation of antiphase boundaries due to their difference in lattice parameters between barrier and electrodes. In this work we demonstrate the use of LiTi₂O₄ thin films as electrodes in an all-spinel oxide CoFe₂O₄-based spin filter devices. These structures show nearly perfect epitaxy maintained throughout the structure and so minimise the potential for APBs formation. The LiTi₂O₄ in these devices is superconducting and so measurements at low temperature have been used to explore details of the tunnelling and Josephson junction behaviour.

Spectroscopic evidence of odd frequency superconducting order

Spin filter superconducting S/I/N tunnel junctions (NbN/GdN/TiN) show a robust and pronounced Zero Bias Conductance Peak (ZBCP) at low temperatures, the magnitude of which is several times the normal state conductance of the junction. Such a conductance anomaly is representative of unconventional superconductivity and is interpreted as a direct signature of an odd frequency superconducting order.

Macroscopic quantum tunnelling in spin filter ferromagnetic Josephson junctions

The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F), is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunnelling in Josephson junctions with GdN ferromagnetic insulator barriers. We show a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions, which present clear signatures of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits.

Strongly bias-dependent tunnel magnetoresistance in manganite spin filter tunnel junctions

A highly unconventional bias-dependent tunnel magnetoresistance (TMR) response is observed in Sm0.75 Sr0.25 MnO3 -based nanopillar spin filter tunnel junctions (SFTJs) with two different behaviors in two different thickness regimes of the barrier layer. Thinner barrier devices exhibit conventional SFTJ behaviors; however, for larger barrier thicknesses, the TMR-bias dependence is more complex and reverses sign at higher bias.

Spin manipulation with magnetic semiconductor barriers

Magnetic semiconductors with unique spin-filtering property and the ability to create excessive internal magnetic fields can open myriads of new phenomena.

The interface between superconductivity and magnetism: understanding and device prospects

Ferromagnetism and conventional singlet superconductivity can be regarded as competing ordering phenomena. A considerable body of theoretical work over the past twenty years has predicted that at interfaces between the two systems competition or coupling between superconducting and magnetic phenomena are possible. Despite the very short lengthscales over which some of the phenomena exist, many of these predictions have been experimentally realized. The aim of this topical review is to provide an overview of the experimental position and to discuss the potential developments and applications of existing results.

Nanopillar spin filter tunnel junctions with manganite barriers

The potential of a manganite ferromagnetic insulator in the field of spin-filtering has been demonstrated. For this, an ultrathin film of Sm0.75Sr0.25MnO3 is integrated as a barrier in an epitaxial oxide nanopillar tunnel junction and a high spin polarization of up to 75% at 5 K has been achieved. A large zero-bias anomaly observed in the dynamic conductance at low temperatures is explained in terms of the Kondo scattering model. In addition, a decrease in spin polarization at low bias and hysteretic magneto-resistance at low temperatures are reported. The results open up new possibilities for spin-electronics and suggest exploration of other manganites-based materials for the room temperature spin-filter applications.