Abstract
Understanding the cohabitation arrangements of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface remains an open challenge. Probing this coexistence with sub-Kelvin magnetotransport experiments, we demonstrate that a hysteretic in-plane magnetoresistance develops below the superconducting transition for 0.15 T, independently of the carrier density or oxygen annealing. This hysteresis is argued to arise from vortex depinning within a thin ( nm) superconducting layer, mediated by discrete ferromagnetic dipoles located solely above the layer. The pinning strength may be modified by varying the superconducting channel thickness via electric field-effect doping. No evidence is found for bulk magnetism or finite-momentum pairing, and we conclude that ferromagnetism is strictly confined to the interface, where it competes with superconductivity. Our work indicates that oxide interfaces are ideal candidate materials for the growth and analysis of nanoscale superconductor/ferromagnet hybrids.
Highlights
The groundwork for the study of coexistent superconductivity and ferromagnetism in oxide heterostructures was laid ten years ago, when a nanoscale metallic channel was discovered at epitaxial interfaces between the band insulators LaAlO3 and SrTiO3 [1]
Our data clarify much of the confusion that has surrounded the origin of ferromagnetism in LaAlO3/SrTiO3
The inhomogeneity in the ferromagnetism which we deduce from our data and its independence from n2D suggest that it originates from dxy electrons localized by static defects, in agreement with recent models [18,19,20]
Summary
The groundwork for the study of coexistent superconductivity and ferromagnetism in oxide heterostructures was laid ten years ago, when a nanoscale metallic channel was discovered at epitaxial interfaces between the band insulators LaAlO3 and SrTiO3 [1]. Measurements of the electrical transport in LaAlO3/SrTiO3 subsequently revealed a Kondo effect and hysteretic magnetoresistance (MR) [2], which are indicative of scattering off local magnetic moments and ferromagnetic ordering, respectively. This apparent emergence of ferromagnetism at the interface was a surprise, since in bulk form neither LaAlO3 nor SrTiO3 is magnetic, even when doped with carriers. All the known coexistence mechanisms for ferromagnetism and spin-singlet superconductivity relevant to LaAlO3/SrTiO3 require a real-space modulation of the superconducting order parameter1 This may be achieved either by spontaneous vortex formation [14], finitemomentum electron pairing, i.e. the formation of a helical Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state due to strong Rashba spin–orbit coupling [15], or macroscopic spatial dispersion of ferromagnetism and superconductivity [9]. Our data show that the interplay between vortices and dipoles in LaAlO3/SrTiO3 constitutes a uniquely tunable superconductor/ ferromagnet hybrid behaviour
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