Strain-Induced Ferromagnetism in Antiferromagnetic ${\mathrm{LuMnO}}_3$ Thin Films

Single phase and strained ${\mathrm{LuMnO}}_3$ thin films are discovered to display coexisting ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism ($\approx 1{\mu }_{\mathrm{B}}$), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth manganite thin films as promising candidate systems for new multifunctional devices.

Figure 1

(a) Sketch of the substrate-induced rhomboidal distortion of the $\mathbf{a}\mathbf{b}$-plane in thin film LMO, as compared with the undistorted lattice of bulk o-LMO. (b) The c-RBS spectrum of the LMO thin film on the YAO substrate, focusing on the contribution due to Lu. Black dots represent the raw data. The red triangles are the simulation. (c) A HRTEM image of a LMO thin film on the YAO substrate at 390 kx magnification. (d) The corresponding diffraction image of panel (c). (e) The reconstructed image of the (110) lattice planes after an inverse FFT. The inset blue window highlights a region of coherently strained film-substrate lattice planes. (f) and (g) show magnified images of the green and red regions in panel (e). Ovals highlight crystal defects.

Figure 2

(a) $M(H)$ loop for the LMO sample ($t\approx 56\mathrm{nm}$) after a zero-field cool (ZFC) to 5 K with ${\mu }_{0}H\parallel (001)$. (b) The same ZFC $M(H)$ loop as in panel (a) (black) and a loop measured after field cooling the sample to 5 K in 4 T (red). (c) Intensities from PNR measurements as a function of $q_z$ at 10 and 150 K. The latter is multiplied by 0.1 for clarity. (d) The normalized spin-up and -down ratio at 10 K. The red (black) lines describe fits to the data using a Gaussian profile for the magnetization that is peaked at the substrate-film interface (surface).

Figure 3

(a) The ($0q_{\mathrm{k}}1$) AFM peak in a ${\mathrm{LuMnO}}_3$ thin film. A Gaussian fit of the main peak gives the peak center at $q_{\mathrm{k}}=0.482(3)$. The solid bar indicates the instrumental resolution of RITA-II. (b) The $T$ dependence of the ($0q_{\mathrm{k}}1$) peak, with lines as guides for the eye. (c) The ${\mu }_{0}H$ dependence of ($0q_{\mathrm{k}}1$) for ${\mu }_{0}H\parallel \mathbf{a}$ up to 10 T. Empty (filled) symbols denote data taken $T=2\mathrm{K}$ ($T=50\mathrm{K}$). Curves are displaced vertically for clarity and peaks are fitted with Gaussian line shapes. (d) Sketch to illustrate the magnetic situation in the LMO film. The FM layer located near the strained film-substrate interface evolves towards a likely cycloidal IC AFM order when moving down the strain gradient towards the film surface.