Interplay between magnetism and interface-induced effects in ultrathin manganites

Optimally doped manganite ${[\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO)] is a ferromagnetic material with perspective use in applications due to colossal magnetoresistance and $T_c$ around room temperature. However, utilizing LSMO in thin-film form and complex heterostructures is limited due to the appearance of a magnetic dead layer, but interfacing LSMO with other transition metal oxides (TMOs) can reduce the magnetic dead layer. For instance, the LSMO layer in proximity to ${\mathrm{SrRuO}}_3$ (SRO) remains magnetic down to 1–2 u.c. To illuminate this effect, we study the magnetic properties and orbital anisotropy of ultrathin LSMO deposited on ${\mathrm{SrTiO}}_3$ (STO) compared with and without an SRO intermediate layer by resonant x-ray spectroscopy. We found that two events occur at the LSMO-SRO interface: Orbital rearrangement and charge transfer. Both effects cooperate, enhancing in-plane double exchange in ultrathin LSMO. Based on quantitative analysis and theoretical simulation of the x-ray spectra, magnetic stability mechanisms in LSMO/SRO are discussed in detail.

Figure 1

Ligand field multiplet simulations of x-ray linear dichroism (XLD) spectra for ${\mathrm{Mn}}^{3+}$. Parameters are given in Table 1, where a single $e_g$ electron occupies (a) the $x^2\text{−}{y}^2$ orbital or (b) the $3z^2\text{−}{r}^2$ orbital.

Figure 2

(a) X-ray diffraction (XRD) ω-2θ scans for $\frac{2}{20}$ and $\frac{15}{20}$. (b) XRD reciprocal space map along ${\mathrm{SrTiO}}_3$ (STO) (103) of $\frac{15}{20}$. (c) X-ray reflectivity (XRR) data and simulated curve for $\frac{2}{20}$.

Figure 3

(a) Difference between Mn x-ray magnetic circular dichroism (XMCD) intensity at $L_3$ and $L_2$ edges plotted as a function of temperature. (b) Mn $L_{2,3}$ XMCD comparison at two temperatures: 80 and 130 K. Data in (a) and (b) were obtained in grazing incidence geometry at 50 mT applied after saturation for ultrathin ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO; 2 and 4 u.c. thick) for three different interface layers (0, 3, and 20 u.c. thick).

Figure 4

(a) Comparison of Mn $L_{2,3}$ x-ray magnetic circular dichroism (XMCD) data measured at 25 K in grazing in 50 mT after 6 T between ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO)/20 ${\mathrm{SrRuO}}_3$ (SRO) bilayers with different LSMO thickness followed by (b) magnetic hysteresis at the Mn $L_3$ edge measured in grazing incidence geometry. Arrows indicate direction of measurements.

Figure 5

Total magnetic moment $M_{\mathrm{tot}}$ per Mn atom calculated as a sum of magnetic spin and orbital moment extracted from corresponding x-ray magnetic circular dichroism (XMCD) data at 25 K in 6 T and 50 mT using Eq. (1).

Figure 6

(a) X-ray linear dichroism (XLD) data collected at Mn $L_{2,3}$ edges in grazing at room temperature. (b) Corresponding linear combination of two simulations labeled as weight of the $3z^2\text{−}{r}^2$ orbital. (c) Calculated hole ratio $X$ as a function of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO) thickness calculated from experimental XLD data and from density functional theory (DFT) calculation from Ref. [34].

Figure 7

Comparison of x-ray absorption spectroscopy (XAS) at Mn $L_{2,3}$ edges measured at 300 K between 4 u.c. ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO) interfaced with different ${\mathrm{SrRuO}}_3$ (SRO) layers (0, 3, and 20 u.c. thick) and reference data (${\mathrm{SrMnO}}_3$ and $\frac{11}{0}$).