Magnetic Nonuniformity and Thermal Hysteresis of Magnetism in a Manganite Thin Film

We measured the chemical and magnetic depth profiles of a single crystalline $({\mathrm{La}}_{1-x}{\mathrm{Pr}}_x{)}_{1-y}{\mathrm{Ca}}_y{\mathrm{MnO}}_{3-\delta }$ ($x=0.52\pm 0.05$, $y=0.23\pm 0.04$, $\delta =0.14\pm 0.10$) film grown on a ${\mathrm{NdGaO}}_3$ substrate using x-ray reflectometry, electron microscopy, electron energy-loss spectroscopy, and polarized neutron reflectometry. Our data indicate that the film exhibits coexistence of different magnetic phases as a function of depth. The magnetic depth profile is correlated with a variation of chemical composition with depth. The thermal hysteresis of ferromagnetic order in the film suggests a first-order ferromagnetic transition at low temperatures.

Figure 1

(a)–(c) Depth sensitive EELS measurements: $\mathrm{O}:\mathrm{Mn}$ ratio (a), atomic percentage (b) of different elements (La, Pr, and Ca), and (c) Mn oxidation state calculated using both the chemical concentrations (●) and the Mn $L_{2,3}$ intensity ratio (▴) along the depth of the film (see text). (d) $M(H)$ hysteresis curve of the sample measured at $T=20\mathrm{K}$ along two in-plane directions ($1\overline{1}0$) and (001) of ${\mathrm{NdGaO}}_3$ substrate. (e) transport measurements of the film with and without magnetic field.

Figure 2

(a) PNR Data from the sample at $T=200\mathrm{K}$ and 20 K. For clarity the PNR data at 200 K has been offset by a factor of 5. Inset of (a) show the XRR data from the film. $\mathrm{asym}=(R^{+}-R^{-})/(R^{+}+R^{-})$, measurements at $T=200\mathrm{K}$ (b) and 20 K (c). The open and closed triangles on transport data (d) represents the temperature during cooling and warming where the PNR data were simultaneously acquired with the transport data. Fig. (e) shows the electron SLD (ESLD) depth profile which yields the solid curve in the inset of (a). Nuclear SLD (f) and magnetization (g) depth profile, which yields the solid curves in (a) at $T=200$ and 20 K.

Figure 3

Spin asymmetry (asym) during field cooling [(a)–(h)] and warming [(i)–(p)] across MIT. (q)–(r) show the magnetization ($M$) depth profile corresponding to (a)–(p).

Figure 4

(a)–(c) $M(T)$ curves for the different regions during cooling (blue) and warming (red). $M(T)$ normalized to $M(20\mathrm{K})$ for the different regions during cooling (d) and warming (e).