We report a modulation of the in-plane magnetotransport in artificial manganite superlattice [(NdMnO3)n/(SrMnO3)n/(LaMnO3)n]m by varying the layer thickness n while keeping the total thickness of the structure constant. Charge transport in these heterostructures is confined to the interfaces and occurs via variable range hopping. Upon increasing n, the interfacial separation rises, leading to a suppression of the electrostatic screening between carriers of neighboring interfaces and the opening of a Coulomb gap at the Fermi level (EF). The high-field magnetoresistance (MR) is universally negative due to progressive spin alignment. However, at a critical thickness of n = 5 unit cells (u.c.), an exchange field coupling between ferromagnetically ordered interfaces results in positive MR at low magnetic field (H). Our results demonstrate the ability to geometrically tune the electrical transport between regimes dominated by either charge or spin correlations.
References
We chose this data-set to attempt a polaron hopping fit since its low VRH exponent and the narrow interfacial spacing suggest that it is our closest-lying SL to the bulk 3D samples in which polaronic transport has previously been observed.14
A crossover from 3-D VRH to ES-VRH has previously been observed for (SrMnO3)n/(LaMnO3)2n bilayers upon increasing the layer thickness.21 However, the metallic behaviour typically exhibited by the thinnest SLs (Refs. 18, 19, and 21) indicates that 3D-VRH transport is a result of bulk conduction. In contrast, our observation of a 2D to ES VRH crossover upon increasing n confirms that the electrical transport is purely interfacial in our SLs.