Emergent perpendicular magnetic anisotropy at the interface of an oxide heterostructure

Controlling the magnetic anisotropy in magnetic oxides is critical for the development of oxide spintronics. Here we report on the finding of an emergent interfacial magnetism, with unique perpendicular magnetic anisotropy (PMA), at the interface of a half-doped manganite ${\mathrm{La}}_{1/2}{\mathrm{Sr}}_{1/2}\mathrm{Mn}{\mathrm{O}}_3$ film on $\mathrm{LaAl}{\mathrm{O}}_3$ substrate, through combined electron spin resonance measurements and density functional theory calculations. The interfacial magnetism can be explained by the $3d_{3z^2-r^2}$ orbital-mediated ferromagnetic exchange interaction between Mn ions near the interface, which is enhanced by the compressive strain from the substrate. Our results provide a useful PMA system for possible spintronic applications, as well as atomistic-level insight to the magnetic anisotropy in strained manganite oxide interfaces.

Figure 1

Crystallographic structure and electrical/magnetic characterization. (a) XRD diffraction patterns of epitaxial LSMO films grown on LAO substrate around the (002), (013), and (103) Bragg planes. (b) Left: Fourier-filtered HAADF micrograph of the LSMO film on LAO substrate taken by an aberration-corrected STEM. Right: Intensity line profile along the white arrow. Inset shows the zoom-in of the rectangular area. (c) Temperature-dependent resistance goes through two pronounced metal-insulator transitions. The red curve shows the warming process and black curve shows the cooling process. (d) Temperature-dependent magnetization (M) measured during warming with 500 Oe magnetic field (H) applied along in-plane (orange) and out-of-plane (black) after zero-field-cooling (ZFC).

Figure 2

Characterization of magnetic properties of LSMO films. MFM measurement of the LSMO film at $T=4\mathrm{K}$ (a) and $T=80\mathrm{K}$ with $H=0\mathrm{T}$; the positive (red) and negative (blue) signals are representative of magnetic domain patterns. (c) Magnetization as a function of magnetic field, for fields applied in both in-plane (IP) and out-of-plane (OP) directions. The magnetization is dominated by the bulk and exhibits a typical in-plane magnetic anisotropy. Panels (d) and (e) are coercive field ($H_c$) and saturated magnetization ($M_s$), respectively, extracted from (c).

Figure 3

Spin resonance with H applied perpendicular to the film plane. (a),(b) Differential ESR absorption $dP/dH$ vs H at different temperature measured during warming. H is applied perpendicular to the film plane. Black, purple, and orange arrows are used to label the resonant peaks from paramagnetic bulk, ferromagnetic bulk, and interface magnetism. The starting of resonances from interfacial and bulk are indicated by an orange and a purple solid circle, respectively. Note that the intensity of different curves is normalized for a convenient alignment of different curves, but the curves in middle temperature range have much stronger resonance signals. (c) Enlarged view of the spin excitation of the interfacial magnetism. Note that for a better view, the position of the resonance peaks is marked at the position of the left shoulder in the differential curves instead of at the center of the peak.

Figure 4

Spin excitations with H applied parallel to the film plane. (a) Differential absorption $dP/dH$ vs H at different temperature measured during warming. H is applied parallel to the film plane; the signal from the interfacial and bulk counterparts are marked by red and purple arrows, respectively. (b) Enlarged view of the spin excitation of the interfacial magnetism.

Figure 5

Phase inhomogeneity and distinct magnetic anisotropies. (a) The temperature dependent linewidth $\mathrm{\Delta }{H}_{pp}^{\mathrm{F}{\mathrm{M}}_{\mathrm{bulk}}}$ for H applied perpendicular to the film plane. Inset shows the schematics of the several ground states across the PM/FM phase transition, illustrating the evolution of magnetic domain size with the temperature. The purple arrows represent the locally ordered spins of individual domain. F ferromagnetic state, P paramagnetic state. (b) Temperature-dependent magnetic anisotropy field $H_{\mathrm{anis}}$ of the bulk (black) and interfacial (orange) magnetism.

Figure 6

Mechanism of the interfacial PMA phase. (a) Strain-dependent magnetic anisotropy energy by DFT calculations indicates an orbital-mediated ferromagnetic double-exchange and magnetic anisotropy. (b) The magnetic hysteresis loops are schematically shown for the PMA at the interface and in-plane magnetic anisotropy (IMA) for the bulk film counterparts, with external field applied along in-plane (IP) and out-of-plane (OP), respectively. Strain from the LAO substrate induces $c$-axis lattice tension in the LSMO film near the interface, while the lattice recovers to the initial symmetry when the strain is released.