Electrostatic Shaping of Magnetic Transition Regions in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$

We report a magnetic transition region in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ with gradually changing magnitude of magnetization, but no rotation, stable at all temperatures below $T_C$. Spatially resolved magnetization, composition and Mn valence data reveal that the magnetic transition region is induced by a subtle Mn composition change, leading to charge transfer at the interface due to carrier diffusion and drift. The electrostatic shaping of the magnetic transition region is mediated by the Mn valence, which affects both magnetization by ${\mathrm{Mn}}^{3+}\text{−}{\mathrm{Mn}}^{4+}$ double exchange interaction and free carrier concentration.

Figure 1

Off-axis EH results. (a) Representative off-axis electron hologram of the LSMO film on a Nb-doped STO substrate covered by a protective carbon layer with the analysis region marked. (b) Mean inner and electrostatic potential contribution to the phase reconstructed from (a). (c) Reconstructed magnetic phase shift at 290 K with the mean inner and electrostatic potential contribution removed. (d)–(f) Magnetic induction maps with a phase contour spacing of $\pi$ radians at 290, 240, and 94 K, respectively. The denser are the phase contours in the specimen, the stronger is the magnetization. In the upper left corner of the LSMO layer, focused ion beam damage is present and suppresses ferromagnetism.

Figure 1

Off-axis EH results. (a) Representative off-axis electron hologram of the LSMO film on a Nb-doped STO substrate covered by a protective carbon layer with the analysis region marked. (b) Mean inner and electrostatic potential contribution to the phase reconstructed from (a). (c) Reconstructed magnetic phase shift at 290 K with the mean inner and electrostatic potential contribution removed. (d)–(f) Magnetic induction maps with a phase contour spacing of $\pi$ radians at 290, 240, and 94 K, respectively. The denser are the phase contours in the specimen, the stronger is the magnetization. In the upper left corner of the LSMO layer, focused ion beam damage is present and suppresses ferromagnetism.

Figure 2

(a) Line profile of the magnetic phase shift $\phi$ (black) and its first derivative $d\phi /dx$ (blue) at 240 K vs distance $x$ in [001] direction through the LSMO film. The orange dashed lines are linear fits. The red line represents a hyperbolic tangent fit using $M(x)=\mathrm{\Delta }M\times \tanh (1.76x/w)$ [27], with the amplitude difference $\mathrm{\Delta }M$ and the full width at half maximum (FWHM) $w$. The fit is used to smooth the noise in the further analysis and to determine the transition region’s width. The gray vertical dashed lines indicate the approximate width of the transition region. (b) Map of projected in-plane magnetization $\mathit{M}$ (green arrows) reconstructed from the magnetic phase image at 240 K. The lengths and directions of the arrows indicate the magnitudes and directions of the magnetization.

Figure 2

(a) Line profile of the magnetic phase shift $\phi$ (black) and its first derivative $d\phi /dx$ (blue) at 240 K vs distance $x$ in [001] direction through the LSMO film. The orange dashed lines are linear fits. The red line represents a hyperbolic tangent fit using $M(x)=\mathrm{\Delta }M\times \tanh (1.76x/w)$ [27], with the amplitude difference $\mathrm{\Delta }M$ and the full width at half maximum (FWHM) $w$. The fit is used to smooth the noise in the further analysis and to determine the transition region’s width. The gray vertical dashed lines indicate the approximate width of the transition region. (b) Map of projected in-plane magnetization $\mathit{M}$ (green arrows) reconstructed from the magnetic phase image at 240 K. The lengths and directions of the arrows indicate the magnitudes and directions of the magnetization.

Figure 3

Remanent magnetization magnitude $M$ and Curie temperature $T_C$ within the LSMO layer. (a) $M$ vs temperature and position in [001] direction. (b) $T_C$ (black squares, left axis) and $M$ at 0 K (blue dots, right axis) vs distance, derived from fits of the temperature dependent magnetization data $M(T)$ [$M(T)$ examples at positions $-50$ and $+75\mathrm{nm}$ with fits are shown in the inset]. The dashed lines in (b) represent fits to $M(0\mathrm{K})$ and $T_C$ using the hyperbolic tangent expression of Fig. 2.

Figure 4

Chemical composition and electronic properties. (a) Relative La, Sr, O, and Mn composition profiles measured by EDX (La, Sr, Mn) and EELS (O) vs distance. (b) Ratio of integrated $\mathrm{Mn}\text{−}{L}_3$ and $L_2$ edges measured by EELS vs distance, revealing a change of the Mn valence. The red solid lines in (a) and (b) represent hyperbolic tangent fits. (c) Charge distribution vs distance derived from the measured Mn valence, under consideration of the Mn composition profile in (a). The red solid line represents the theoretical charge distribution for two LSMO layers with different free carrier concentrations calculated using the continuity and Poisson equations. (d) Electrostatic phase shift profiles measured by EH compared to a simulation (red solid line), based on the above electrostatic and the mean inner potential change induced by the Mn composition change. The agreement corroborates the presence of charge redistribution. (e) Temperature dependence of the transition region widths of the magnetization (black circles) compared to that of the Mn valence (red diamond), Mn composition (orange triangle), and $T_C$ (green star).