Spin-orbit interaction and Kondo scattering at the PrAlO₃/SrTiO₃ interface : effects of oxygen content

Mozaffari, Shirin
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The broken translational symmetry at an interface between complex metal oxides gives rise to a number of remarkable phenomena with rich physics often due to charge, spin, orbital, or atomic reconstruction, such as superconductivity, metal-insulator transitions, and giant magnetoresistance. These phenomena can be entirely different from those innate to the constituent materials. A recent example of such interface phenomena is the two-dimensional electron gas (2DEG) at the interface of LaAlO₃ (LAO) and TiO₂-terminated SrTiO₃ (STO). Despite that LAO and STO are both band insulators and non-magnetic, their interface is found to be conductive, superconductive, ferromagnetic, and exhibits an electric-field tunable metal-insulator transition, which all are the result of electronic reconstruction at the interface. It is generally agreed that the electron gas is formed by transfer of electrons from the polar layers of LAO to the top TiO₂ layer of STO. In addition to the electronic reconstruction at the interface, doping the interface by interdiffusion of La/Sr atoms and by the presence of oxygen vacancies that are created in the STO substrate during thin film deposition contribute to the conductivity. We examined, in detail, the effects of oxygen content on the electrical and magnetic properties of PrAlO₃ (PAO) on STO. Among the aluminates, PAO has the closest lattice constant to LAO (due to the multivalency of Ce, it is hard to make stable thin films of CeAlO₃), and thus this helps to lessen the effect of strain as much as possible. Also, the effective magnetic moment of the Pr³⁺ f electron states is 3.6 Bohr magnetons, due to often hybridized, while closed shell La³⁺ is diamagnetic, thus this rare-earth substitution can lead to magnetic properties in an aluminate structure. PAO is known to be paramagnetic in bulk and has mostly been studied for its structural transitions below room temperature which are absent in LAO. We report the effects of oxygen pressure during growth (P [subscript O₂]) on the electronic and magnetic properties of PrAlO₃ films grown on TiO₂-terminated SrTiO₃ substrates. Resistivity measurements show an increase in the sheet resistance as P [subscript O₂] is increased. The saturation of the sheet resistance down to 0.3 K is consistent with Kondo theory for P [subscript O₂] ≥ 10⁻⁵ torr. Resistivity data fits indicate Kondo temperatures of 16--18 K. For the 10⁻⁴ sample, we measured a moderate positive magnetoresistance (MR) due to a strong spin-orbit (SO) interaction at low magnetic fields that evolves into a larger negative MR at high fields due to the Kondo effect. Analysis of the MR data permitted the extraction of the SO interaction critical field for the P [subscript O₂] = 10⁻⁵ torr interface (H [subscript so] = 1.25 T). We observed high positive MR for the least oxygenated sample, where a fraction of the n-type carriers are derived from oxygen vacancies and possible cation interdiffusion; for this 6 x 10⁻⁶ torr sample, Hall effect data indicate a thick conducting layer. Its extremely high MR (~400%) is attributed to classical behavior due to a distribution of mobilities.