Exchange bias in a single ${\mathrm{LaMnO}}_3$ film induced by vertical electronic phase separation

We verify that the exchange bias effect unexpectedly emerges in a single ${\mathrm{LaMnO}}_{3-\delta }$ film, one of the most studied correlated oxides. We combine x-ray absorption spectroscopy results, which serves as a fingerprint of the electronic structure, with microstructure characterizations and magnetization data to explore the origin of the exchange bias behavior. Taken together, these measurements provide compelling evidence that the formation of a Mn${}^{2+}$ component associated with the double exchange between Mn${}^{2+}$-O-Mn${}^{3+}$ produces robust ferromagnetism in the upper part of the film, which is exchanged coupled with the antiferromagnetic bottom part that is dominated by Mn${}^{3+}$. Thus, the Mn ions are found to be distributed unevenly in the depth profile, with vertical electronic phase separation, in contrast to the often accepted view of the lateral phase separation in manganites. The observation of exchange bias in a chemically single film paves the way for interface engineering induced by vertical electronic phase separation towards technological applications.

Figure 1

(a) An expanded view of the (002) peak of the 120 u.c. ${\mathrm{LaMnO}}_{3-\delta }$ films grown with $p_{{\mathrm{O}}_2}$ ranging from 200 to 0.03 mTorr. (b) Mn $L$-edge and (c) O $K$-edge XAS curves of the 120 u.c. ${\mathrm{LaMnO}}_{3-\delta }$ films grown at $p_{{\mathrm{O}}_2}$ = 200–0.03 mTorr. The two broad multiplets of the Mn L-edge are denoted as $L_3$ and $L_2$. (b) The shift of the $L_3$ peak and (c) a continuous decay of the intensity of the peak at 533 eV are highlighted by a vertical dashed line and an arrow, respectively. Curves A, B, C, D, and E correspond to 120 u.c. films grown under 200, 10, 3, 0.3, and 0.03 mTorr oxygen pressure, respectively.

Figure 2

(a) Hysteresis loops of the 120 u.c. ${\mathrm{LaMnO}}_{3-\delta }$ films grown at $p_{{\mathrm{O}}_2}$ in the range of 200–0.03 mTorr. The loops were measured at 4 K after +5 kOe field cooling from 300 K. (b) An expanded view near zero-field, revealing the existence of exchange bias by the asymmetric coercive field, as indicated by the vertical arrows. Here, $H_1$ and $H_2$ are the negative field and the positive field at which the magnetization equals to zero, respectively. Curves A, B, C, D, and E correspond to 120 u.c. films grown under 200, 10, 3, 0.3, and 0.03 mTorr oxygen pressure, respectively. (c) Summarized $H_{\mathrm{EB}}$ (left axis) and $H_{\mathrm{C}}$ (right axis) of the ${\mathrm{LaMnO}}_{3-\delta }$ films. The line is only the guide to the eyes. (d) $M$-$H$ loops of the 120 u.c. ${\mathrm{LaMnO}}_{3-\delta }$ films deposited at $p_{{\mathrm{O}}_2}$ = 200 mTorr measured at 4 K after field cooling from room temperature in +5 kOe (dashed line) and −5 kOe (dash dot dotted line).

Figure 3

(a) $M$-$H$ loops of the 120 u.c. ${\mathrm{LaMnO}}_{3-\delta }$ films deposited at $p_{{\mathrm{O}}_2}$ = 10 mTorr measured at different temperatures (4, 10, 30, and 40 K) after +5 kOe field cooling. The summarized temperature-dependent $H_{\mathrm{C}}$ (left axis) and $H_{\mathrm{EB}}$ (right axis) of the ${\mathrm{LaMnO}}_{3-\delta }$ film is shown in the inset. The line is only a guide to the eyes. (b) $M$-$T$ curves measured under various fields ($H$ = 50, 100, 200, 300, and 500 Oe). The solid and dashed lines are the FC and ZFC data, respectively.

Figure 4

$M$-$H$ loops of the films deposited under $p_{{\mathrm{O}}_2}$ = 10 mTorr with various thicknesses (120, 60, 30, and 15 u.c.), measured at 4 K after + 5 kOe field cooling from room temperature. The summarized $H_{\mathrm{C}}$ (left axis) and $M_{\mathrm{S}}$ (right axis) of the ${\mathrm{LaMnO}}_{3-\delta }$ films is shown in the inset. The $M_{\mathrm{S}}$ was obtained after saturating at 10 kOe. The line is only a guide to the eyes.

Figure 5

(a) Mn $L$-edge XAS spectra of the ${\mathrm{LaMnO}}_{3-\delta }$ films deposited under $p_{{\mathrm{O}}_2}$ = 10 mTorr with various thicknesses (120, 60, 30, and 15 u.c.). The shoulder peak is highlighted by an arrow. (b) Mn $L$-edge spectral differences between films with different thicknesses, which are indicated as follows: between 120 and 60 u.c. (Diff. 1), between 120 and 30 u.c. (Diff. 2), and between 120 and 15 u.c. (Diff. 3). Appropriate weights were chosen for the spectra, namely 60% for the 120 u.c and 40% for the 60, 30, and 15 u.c. spectra, to guarantee that the second peak is almost even for all the difference spectra. The inset of (b) shows the out-of-plane $M$-$H$ loops of [Co(0.5 nm)/Pt(1 nm)]${}_5$/${\mathrm{LaMnO}}_{3-\delta }$ multilayer on an LSAT substrate. The ${\mathrm{LaMnO}}_{3-\delta }$ film is deposited under $p_{{\mathrm{O}}_2}$ = 10 mTorr with a film thickness of 60 u.c. The measurement was performed at 4 K after + 5 kOe field cooling from room temperature.

Figure 6

Sketch of the lattice structure and spin configuration of the ${\mathrm{LaMnO}}_{3-\delta }$ films with exchange bias. Double exchange (DE) and super exchange (SE) effects dominate at the upper and bottom regimes of the sample, respectively.