Strain-Driven Orbital and Magnetic Orders and Phase Separation in Epitaxial Half-Doped Manganite Films for Tunneling Devices

Mixed-valence manganites ${\mathrm{La}}_{1-x}{A}_x{\mathrm{MnO}}_3$ ($A=\mathrm{Sr}$, Ca) with $x\approx 0.5$ can be driven from a ferromagnetic-metallic to an antiferromagnetic-insulating state by a small modification ($\mathrm{\Delta }x$) of the carrier density ($\mathrm{\Delta }x/x<1$). For this reason, these oxides have received renewed attention due to their potentially advantageous integration in ferroelectric tunnel junctions of adjustable tunnel barrier width. Interestingly, in thin films, epitaxial strain can modify the electronic and magnetic ground state strongly affecting their magnetotransport properties. Here we exploit the extreme sensitivity of linearly and circularly polarized x-ray absorption to orbital anisotropy and magnetic ordering to explore the role of structural distortions and electronic bandwidth on the orbital occupancy and spin ordering of $\mathrm{Mn}\hspace{0.17em}3d$ states in ${\mathrm{La}}_{0.5}{A}_{0.5}{\mathrm{MnO}}_3$ films under various strain states. ${}^{55}\mathrm{Mn}$ NMR experiments are used to get information about the electronic and magnetic phase separation and orbital ordering occurring in these films. These results combined with the corresponding structural, magnetic, and electrical characterization allow us to map the strain-dependent orbital and magnetic phase diagrams of half-doped manganites and its dependence on the electronic bandwidth.

Figure 1

Normalized x-ray absorption spectra obtained with in-plane ($I_{ab}$, solid line) and out-of-plane ($I_c$, dashed line) x-ray polarization of LSMO (a) and LCMO (b) samples measured at 300 K. Bottom of (a): difference XAS spectra between LSAT and DSO samples and simulated ${\mathrm{Mn}}^{2+}$ spectrum. Resultant XLD spectra ($I_{ab}-I_c$) of LSMO (c) and LCMO (d) samples (shaded areas indicate region of integration as indicated in the text).

Figure 2

Integrated area under XLD as a function of the $c/a$ ratio for LSMO (solid squares) and LCMO (empty circles) films grown on DSO (black symbols), STO (blue), LSAT (green), YAO (red), and LAO (orange) substrates. Lines correspond to linear fittings of $I_{\mathrm{XLD}}$ ($c/a$) for all LSMO samples (solid line) and for all LCMO samples except film on STO (dashed line).

Figure 3

XMCD spectra of LSMO (a) and LCMO (d) samples measured at 5 and 50 K, respectively (shaded areas indicate region of integration after subtraction of ${\mathrm{Mn}}^{2+}$ contribution as indicated in the text). XMLD [XLD (5 or 50 K)–XLD (300 K)] spectra of LSMO (b) and LCMO (e) samples. Normalized integrated area under XMCD at the $L_2$ edge (left axis, solid squares, solid line) and intensity difference between the first and second peak of XMLD at the $L_2$ edge (right axis, empty circles, dashed line) for LSMO (c) and LCMO (f) samples as a function of the $c/a$ ratio. All measurements taken with applied field $B=2\mathrm{T}$.

Figure 4

Spin-echo intensity in LSMO (36-nm) thin films at 376 MHz (films on LSAT, YAO) and 330 MHz (film on STO) as a function of rf field strength expressed in the units of restoring field ($\beta h_1$). Dashed curves are fittings to the log-normal function. Vertical arrows indicate $h_{1\mathrm{opt}}$ corresponding to $H_{\mathrm{rest}}$, horizontal arrows indicate maximum signal intensity $I_{\mathrm{opt}}$ corresponding to $I_0$.

Figure 5

${}^{55}\mathrm{Mn}$ NMR spectra recorded at 4.2 K from 20- and 36-nm LSMO films grown on STO (a), LSAT (b), and YAO (c). ${}^{55}\mathrm{Mn}$ NMR spectrum recorded from bulk LSMO is also shown in the middle panel (b). Arrow lines indicate the frequencies $f_0$ and $f_1$. All ${}^{55}\mathrm{NMR}$ spectra are corrected for enhancement factor following the Panissod procedure [23], and the spectrum intensity at each frequency point is proportional—with a common proportionality factor for all spectra taking into account samples mass—to the number of nuclei (Mn atoms) characterized by this resonance frequency.

Figure 6

${}^{55}\mathrm{Mn}$ NMR spectrum intensity registered at 4.2 K as a function of magnetization in LSMO 20-nm (solid symbols) and 36-nm (empty symbols) thin films for LSAT, YAO, and STO substrates. NMR intensity is obtained by integrating spectra in the 300–420 MHz range ($f_0+f_1$ lines) (black symbols), 300–350 MHz range ($f_1$ line, yellow symbols), 350–420 MHz range ($f_0$ line, purple symbols), and normalized to layer thickness for comparison of 20- and 36-nm-thick layers.

Figure 7

(a) Sketch of electronic-phase-separated ${\mathrm{Mn}}^{3+/4+}$ and ${\mathrm{Mn}}^{4+}$ ferromagnetic regions in optimally doped ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_3$ as inferred from ${}^{55}\mathrm{Mn}$ NMR spectroscopy [16, 18, 19, 50, 51, 52]. (b),(c) sketches of the ($x^2-y^2$) and ($3z^2-r^2$) orbitals and associated $A$-type and $C$-type AFM spin ordering and the corresponding dipolar hyperfine field ($H_{\mathrm{dip}}$) (blue and red arrows) and hyperfine Fermi contact field ($H_{\mathrm{con}\mathrm{t}}$) (black arrows) acting on the ${}^{55}\mathrm{Mn}$ nuclei [54].

Figure 8

Strain phase diagram of half-doped manganite films indicating the orbital and magnetic phase derived from experiments and theoretical calculations. Shaded areas indicate regions where samples show metallic character.