Understanding defect-mediated cubic to hexagonal topotactic phase evolution in $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ thin films and associated magnetic and electronic properties

The coupling between the oxygen stoichiometry and strain-induced lattice distortions has huge bearings on the electronic and magnetic properties of $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ (SMO). In this paper we have observed oxygen stoichiometry-mediated epitaxial tetragonal to nonepitaxial-oriented hexagonal topotactic phase transition of $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ thin film on $\mathrm{SrTi}{\mathrm{O}}_3$ (100) substrate. The oxygen stoichiometric hexagonal SMO films are oriented in nature, while the oxygen-deficient epitaxially strained $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$ films are tetragonal in nature. It is observed that oxygen vacancy is obligatory to hold on the epitaxial strain in $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$ thin film, while oxygenated stoichiometric cubic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ thin films are observed to be strain free. The structural phase and stoichiometry are found to govern the nature and mechanism of magnetic interactions in these films. The emergence of unusual weak ferromagnetism in oxygen stoichiometric hexagonal SMO films established a root towards the local structural distortion as confirmed through the Raman spectroscopy and extended x-ray absorption fine structure measurements. This work proves for us the idea that the oxygen deficiency is not the necessary effect of strain; rather, it is the cause of strain for topotactic SMO film.

Figure 1

(a) (200) out-of-plane XRD of tetragonal (T-SMO) and cubic (C-SMO) on STO with (110) reflection of $4H$-SMO on STO (100). Inset figure represents the XRD pattern of bulk $4H$-SMO target. (b1)–(b3) (103) in-plane RSM reflection of tetragonal, cubic, and hexagonal SMO film on STO. (c) XRD scans of hexagonal SMO (STO), SMO (LAO2), SMO (MgO) thin films with x-ray reflectivity with simulated data of SMO (STO) thin film (Inset). (d) GIXRD scans of SMO (Si) and SMO [LAO1] thin films with a schematic of GIXRD measurement geometry.

Figure 2

(a) Mn $L$-edge XAS of tetragonal SMO, cubic SMO, and hexagonal SMO with reference ${\mathrm{Mn}}_2{\mathrm{O}}_3$ and $\mathrm{Mn}{\mathrm{O}}_2$. (b) X-ray absorption Mn $L$-edge spectra of hexagonal SMO (Si) SMO (STO), SMO (LAO1), SMO (LAO2), and SMO (MgO) thin films with bulk SMO and $\mathrm{Mn}{\mathrm{O}}_2$ reference. (c) Simulated ${\mathrm{Mn}}^{4+} L$-edge spectra with SMO (LAO1) experimental data. (d) X-ray absorption O $K$-edge spectra of H-SMO (Si), SMO (STO), SMO (LAO1), SMO (MgO), and SMO (LAO2) thin films with bulk SMO. (e1)–(e5) Fitted O $K$ edge with $t_{2\mathrm{g}}$ and $e_{\mathrm{g}}$ of SMO (STO), SMO (LAO2), SMO (MgO), and SMO (LAO1) thin films with SMO bulk.

Figure 3

(a)–(f) Room-temperature Raman spectra of bulk $4H$-SMO, SMO [Si], SMO [STO], SMO [LAO1], SMO [MgO], and SMO [LAO2] thin films. (g) Zoom view of ${E_{1\mathrm{g}}}^{\left(2\right)}$ and ${E_{2\mathrm{g}}}^{\left(3\right)}$ modes of all the films with bulk SMO. (h) Zoom view of ${A_{1\mathrm{g}}}^{\left(2\right)}$ mode of all the films with bulk SMO. (i) Schematic for the possible vibrational modes of $4H$-SMO.

Figure 4

(a) Variation of crystal field of all oriented films with respect to bulk or polycrystalline SMO films vs change in frequency of inward stretching $A_{1\mathrm{g}}$ mode. (b) Variation of $A_{1\mathrm{g}}$ mode of all hexagonal SMO films with bulk SMO.

Figure 5

(a) Room-temperature EXAFS spectra of SMO [STO] thin film with SMO bulk, $\mathrm{Mn}{\mathrm{O}}_2$, and ${\mathrm{Mn}}_2{\mathrm{O}}_3$. Inset figure represents the zoom view of preedge features of same plot. (b). Represents the variation of real part of χ($R$) with $R$. (c). Experimentally obtained FT-EXAFS [χ($R$) vs $R$] spectra with theoretically simulated model of Mn $K$ edge of both bulk SMO and SMO [STO] thin film. (d) Schematic of local structure of $4H$-SMO (STO) and $4H$-SMO bulk.

Figure 6

(a) Magnetization ($M$) as a function of temperature ($T$) of SMO bulk with SMO (Si) thin film (inset). (b) $M$($T$) (FC) vs $T$ for $T$-SMO and $C$-SMO on STO. (c) $M$($T$) (ZFC and FC) vs $T$ for $4H$-SMO [STO], with STO substrate M-T behavior (inset). (d) Magnetic hysteresis (M-H) loop of SMO [STO], SMO [LAO1], and SMO [MgO], SMO [LAO2] thin films at 100 K zoomed view of M-H curves represents in the inset of figure (d). (e) M-H loops of SMO [STO], SMO [LAO1], and SMO [LAO2] films at 5 K; inset figure represents variation of remanence magnetization and coercive field of SMO [LAO2].