Unexpected observation of spatially separated Kondo scattering and ferromagnetism in Ta alloyed anatase TiO2 thin films

We report the observation of spatially separated Kondo scattering and ferromagnetism in anatase Ta0.06Ti0.94O2 thin films as a function of thickness (10–200 nm). The Kondo behavior observed in thicker films is suppressed on decreasing thickness and vanishes below ~25 nm. In 200 nm film, transport data could be fitted to a renormalization group theory for Kondo scattering though the carrier density in this system is lower by two orders of magnitude, the magnetic entity concentration is larger by a similar magnitude and there is strong electronic correlation compared to a conventional system such as Cu with magnetic impurities. However, ferromagnetism is observed at all thicknesses with magnetic moment per unit thickness decreasing beyond 10 nm film thickness. The simultaneous presence of Kondo and ferromagnetism is explained by the spatial variation of defects from the interface to surface which results in a dominantly ferromagnetic region closer to substrate-film interface while the Kondo scattering is dominant near the surface and decreasing towards the interface. This material system enables us to study the effect of neighboring presence of two competing magnetic phenomena and the possibility for tuning them.


Temperature dependent thermopower behavior
An independent validation for Kondo scattering is the observation of a maximum in the Seebeck coefficient close to the Kondo temperature. Figure S2 shows the Seebeck coefficient S as a function of temperature for 200 nm sample. The comparison between resistivity and Seebeck coefficient versus temperature indicates that as long as the resistivity is metallic (positive temperature coefficient), the Seebeck coefficient is linear in T, as predicted by Mott law: which yields for a degenerate semiconductor in 3D: where α describes the functional dependence of the scattering time on the energy τ ∼ E α and its value depends on the dominant scattering mechanism. In many cases α ≈ 0 is assumed for simplicity and for lack of a solid theoretical back up, α ≈ -0.5 for scattering with acoustic phonons and for most other scattering mechanisms, α is between 0 and -1. As the carrier concentration measured by Hall effect is virtually constant as a function of temperature in our films, eq. (2) well describes the linear temperature dependence of the Seebeck curves above ~50 K, assuming effective masses around 2m 0 , where m 0 is the bare electron mass.
At lower temperatures there is an onset of resistivity upturn and correspondingly also the Seebeck coefficient changes its temperature derivative sign, consistently with other literature data 1 .
At lower temperatures there is an onset of resistivity upturn and correspondingly also the Seebeck coefficient changes its temperature derivative sign. In the hypothesis of Kondo regime, we can assume that below T K a Kondo resonance yields an extra term in the Seebeck coefficient, derived from the Mott eqn. (1) 2,3 . This term can be phenomenologically expressed as : The coefficient a should be related to the energy position of the Kondo resonance (T K ) and to the strength of the magnetic scattering processes, while the coefficient b should be related to the energy width of the Kondo resonance. We can fit the experimental data in Fig. S2 as a sum of Mott linear Seebeck term (eq. (2)) plus a Kondo Seebeck term (eq. (3)). Associating fitting coefficients a (0.00015 ) and b (5) to meaningful physical parameters is not easy as the spin, charge and orbitals are coupled strongly in oxide systems.

Figure S2:
The thermo power as a function of temperature along with the theoretical fit. Figure S3: The temperature dependence of susceptibility (χ) for 200 nm film is shown in the temperature range from 2 to 200 K under an external magnetic field of 6 Tesla. Kondo behavior is identified from the saturation of magnetic susceptibility. However, the high temperature susceptibility could be described by Curie-Weiss χ = C/T-ϴ p where C is the Curie consatant and ϴ p is the paramagnetic Curie temperature. Figure S3 shows the susceptibility below ~6 K tends to saturate which deviates from the Curie-Weiss law. This deviation is similar to that reported by other groups 4 which support our Kondo model . Figure S4: The normalised PL spectra obtained at 20 K for Ti 1-x Ta x O 2 thin films for different Ta concentrations (0 to 8%) at the low energy range (1.8 eV to 3.0 eV). We observed the Stoke-shifted self-trapped exciton peak centered at 2.35 eV for the undoped TiO 2 film. This is in accordance with the previous PL studies done on TiO 2 . The origin of this broad peak has been assigned to oxygen vacancies in the TiO 2 crystal which form defect levels deep inside the bandgap. The broad PL peak for undoped TiO 2 thin films was deconvoluted into four Gaussian peaks centered at around 2.05, 2.2, 2.3 and 2.5 eV. Next, the PL spectra from a bare LAO substrate (treated under the same conditions under which the films were grown) were obtained. It was found out that LAO itself has a broad PL peak, which in turn is deconvoluted into two Gaussian components. These two Gaussians matched pretty closely with the 2.05 and 2.2 eV peaks for the undoped TiO 2 ( Figure S4). Hence, we can safely assign these two lower energy peaks to the oxygen vacancies in the LAO substrates. As a result, only the other two peaks at 2.3 and 2.5 eV can then be argued to be coming from TiO 2 . We further observed that the Gaussians at 2.3 and 2.5 eV (due to TiO 2 ) were sharply quenched on the slightest Ta incorporation for the Ti 1-x Ta x O 2 thin films. It may be concluded here that the signal obtained for the Ti 1-x Ta x O 2 thin films were actually due to the LAO substrate. Ta incorporation in TiO 2 enhances free donor electrons in the system.

Photoluminescence spectra for different concentrations of Ta in TiO 2
Consequently, the self compensating nature of the crystal acts to enhance the formation energy for any electron donor defects in the system. This explains the quenching of the PL peaks (arising due to oxygen vacancies) in Ti 1-x Ta x O 2 thin films. A new peak is found to increase in intensity with Ta concentration at 2.27 eV which may be related to cationic defects which tend to increase with Ta concentration.