Evolution of the magnetoresistance lineshape with temperature and electric field across Nb-doped SrTiO3 interface

Abstract

We report on the temperature and electric field driven evolution of the magnetoresistance lineshape at an interface between Ni/AlOx and Nb-doped SrTiO3. This is manifested as a superposition of the Lorentzian lineshape due to spin accumulation and a parabolic background related to tunneling anisotropic magnetoresistance (TAMR). The characteristic Lorentzian line shape of the spin voltage is retrieved only at low temperatures and large positive applied bias. This is caused by the reduction of electric field at large positive applied bias which results in a simultaneous reduction of the background TAMR and a sharp enhancement in spin injection. Such mechanisms to tune magnetoresistance are uncommon in conventional semiconductors.Spin voltage measured at different semiconducting interfaces have been widely studied using different combination of materials as spin contacts and employing different measurement techniques. Such studies are commonly performed using the popular three terminal (3T) and four terminal non-local (NL) geometries 1-6 . In spite of the fact that both these electrical transport schemes fail to resolve outstanding issues related to the precise understanding, origin and magnitude of spin accumulation across semiconducting interfaces, these are more accentuated using the 3T geometry 7-11 . This is rooted in the inability of the 3T scheme to clearly ascertain the origin and magnitude of the spin voltage, possible considerations being spin accumulation in the semiconductor or localized states either in the tunneling barrier or at the semiconducting surface 12 . Earlier studies involving amorphous tunnel barriers showed that the tunneling conductance and spin polarization can be strongly influenced by the presence, concentration and type of impurities in the tunneling barrier via the formation of impurity mini bands and highly conducting multiresonant channels [13][14][15] . Attempts to mitigate such impurities by designing epitaxial barriers has also proved non-trivial in this context 4,11 . Additionally, the nature and type of impurities offer further challenges to validate proposed theories that seek to explain experimental observations using either spin injection (ferromagnet/tunnel barrier) or non-magnetic (metal/tunnel barrier) contacts 8,9 . Increasing the parameter space by using new transport schemes and/or choosing different materials will be an useful approach to understand the experimental findings related to the origin of spin accumulation across semiconducting interfaces. One such material interface that enables tunability of electronic properties, relevant for spin transport is that of complex oxides 16 . Although such material interfaces are commonly replete with oxygen vacancies and surface charge 17-19 , the tunability of several functional properties with temperature, electrical field, stress and strain has led to the unexpected emergence of new phenomena not encountered in other material systems.In this context SrTiO 3 (STO) is a relevant material. STO single crystals exhibit a large...