Magnetocapacitance and exponential magnetoresistance in manganite–titanate heterojunctions

Abstract

We present a rectifying manganite-titanate heterojunction exhibiting a magnetic field tunable depletion layer. This creates a large positive magnetocapacitance, a direct measure of the fieldinduced reduction of the effective depletion width across the junction. Furthermore, the reduction of the junction barrier shifts the forward bias characteristics, giving exponentially-enhanced differential magnetoresistance, occurring despite the absence of a spin filter. These results provide a unique probe of a Mott insulator/band insulator interface, and further suggest new electronic devices incorporating the magnetic field sensitivity of these strongly correlated electron materials. PACS numbers: 75.70.Cn, 75.47.Lx, 73.40.Ei 1 There is a continual search for new methods and materials to utilize magnetic response in electronic devices, ranging from dilute magnetic semiconductors, 1 metal superlattices, 2 magnetic oxides, 3 as well as hybrid structures among them. Perovskite manganites exhibit strong electronspin coupling, manifesting dramatic magnetoresistance near the simultaneous magnetic and metal-insulator transition, 3 as well as a fully spin-polarized ground state. 4 The later feature has been explored in spin-polarized transport at grain boundaries 5 and thin film tunnel junctions. 6 Electron-doped SrTiO 3 has long been used as a semiconducting element in rectifying junctions. 7,8 Oxygen vacancies, as well as substitutional doping such as Nb on the Ti-site in SrTiO 3 , generate conduction electrons. 9 Recently, diode characteristics have been demonstrated in manganite-titanate junctions, 10-13 where the manganite hole concentration was initially minimized to resemble semiconductor p-i-n or p-n junctions, corresponding to bulk insulating concentrations. Ferromagnetic metallic manganites are formed by doping holes in the antiferromagnetic Mott insulator LaMnO 3 , with the metal-insulator transition occurring for ~ 17 % hole doping in La 1-x Sr x MnO 3 . 3 We have found that rectification can be observed even for much higher hole concentrations, and focus here on junctions using La 0.7 Sr 0.3 MnO 3-δ to vary across the ferromagnetic state. For La 0.7 Sr 0.3 MnO 3-δ /Nb:SrTiO 3 junctions, we find a significant increase in the junction capacitance in an applied magnetic field. Corresponding to this decrease in the depletion width, the reduction of the current barrier gives rise to exponential differential magnetoresistance. The modification of the electronic structure at the heterointerface by magnetic field is enabled by the unusual features arising from strong charge-spin coupling, giving a direct probe of the correlated electron equivalent of semiconductor band bending.

Magnetocapacitance without magnetoelectric coupling

“…Qualitatively, the frequency dependence is very similar to that reported for superlattices, 11 although the sign of the magnetocapacitance is different. MR has been assumed here to reside in the manganite layers, but spin-dependent tunneling across the ferroelectric is also possible, as is magnetic tuning of size of the depletion layers at the manganite-titanate junctions; 30 in either case the sign of the magnetocapacitance would be the opposite of that calculated here. The frequency dependence can be interpreted as follows.…”

A heteroepitaxial perovskite metal-base transistor

“…In summary, the present work demonstrates a new three-terminal device platform for perovskite oxide heterostructures, which should provide useful for hot-electron spectroscopy of heterostructures incorporating strong electron interactions and quantum wells 25 , as well as magnetically active junctions 26 . In the hot-electron regime, the low current gains are far from commercial relevance as basic transistors, and they are comparable to those of metal-base spin-valve transistors 27 .…”

Electric field penetration in Au/Nb:SrTiO3 Schottky junctions probed by bias-dependent internal photoemission

“…With the current movement towards advanced functionality in complex oxide devices, various studies have examined the high sensitivity of perovskite Schottky junctions to external perturbations such as magnetic field 1,2 , light irradiation 3 and electric field 4 . Furthermore, by applying atomic scale growth techniques in oxide thin films, oxide Schottky barrier heights (SBH) can be modulated by tuning interface dipoles 5 and introducing resonant artificial interface states 6 .…”