Ferromagnetism and increased ionicity in epitaxially grown ${\text{TbMnO}}_3$ films

Thin films of ${\text{TbMnO}}_3$ have been grown on ${\text{SrTiO}}_3$ substrates. The films grow under compressive strain and are only partially clamped to the substrate. This produces remarkable changes in the magnetic properties and, unlike the bulk material, the films display ferromagnetic interactions below the ordering temperature of $\sim 40\text{K}$. X-ray photoemission measurements in the films show that the $\text{Mn}3s$ splitting is 0.3 eV larger than that of the bulk. Ab initio embedded-cluster calculations yield $\text{Mn}3s$ splittings that are in agreement with the experiment and reveal that the larger observed values are due to a larger ionicity of the films.

Figure 1

(Color online) (a) Sketch of the ${\text{TbMnO}}_3$ orthorhombic unit cell, the pseudocubic ${\text{TbMnO}}_3$ lattice (dashed lines), and the cubic substrate (shaded). (b) Sketch of the four types of orthorhombic domains present in the films.

Figure 2

(Color online) Orthorhombic (a) and pseudocubic (b) lattice parameters of a series of ${\text{TbMnO}}_3$ films with different thicknesses grown at ${\text{PO}}_2=0.9\text{mbar}$. Encircled are the bulklike values corresponding to the relaxed part of the 80 nm film.

Figure 3

(Color online) (a) Magnetization $\left(M\right)$ as a function of temperature $\left(T\right)$ for a 40-nm-TMO film grown at 0.9 mbar. Data were recorded upon warming, with a 500 Oe field applied parallel to the film’s surface; although qualitatively similar results were obtained when the field was applied in the perpendicular direction. The diamagnetic contribution arising from the STO substrate was measured in a separate control experiment and subtracted from the raw magnetization. (b) Inverse susceptibility $\left({\chi }^{-1}\right)$ as a function of $T$. (c) $M$ versus magnetic field $\left(H\right)$ at different temperatures. Both the diamagnetism of the substrate and the paramagnetism of Tb ions have been deducted from the raw data. (d) Saturation magnetization $\left(M_S\right)$ as a function of $T$.

Figure 4

(Color online) XPS spectra around the (a) $\text{Mn}3s$ and (b) $\text{Mn}2p$ edges for two films grown at two very different oxygen pressures. The observed spectra are independent of oxygen pressure during growth and film thickness. The small peak appearing at 93 eV is likely to be related to an unidentified surface contamination.