Tunable magnon-phonon cavity via structural phase transition

Abstract

Strong coupling between two quantized excitations in a cavity has the potential to lead to hybridized states that bestow novel quantum phenomena as required for emerging applications. In particular, tunable hybrid magnon-phonon cavities with precise control knobs are in pressing demand for developing quantum functionalities in solid-state platforms. Here, using a combination of synthesis and characterization tools, we present an epitaxial La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) heterostructure that manifests strong couplings between the Kittel magnon and the transverse acoustic phonon. Remarkably, leveraging the magnetoelastic interaction at the epitaxial interface, we demonstrate that when the STO substrate undergoes a cubic-to-tetragonal phase transition at ~105 K, the Kittel magnon of the LSMO thin film splits into three bands due to anisotropic structural strains along the [100], [010], and [001] crystalline axes, hence, resulting in an array of non-degenerate, hybridized magnon-phonon modes. Moreover, we develop an analytical model that can reproduce the interfacial strain-induced magnon splitting and the strength of magnon-phonon coupling. Our work highlights structural phase transitions as a sensitive trigger for generating multistate magnon-phonon hybridization in high-quality magnetoelastic oxide heterostructures - a new route for implementing strain-mediated hybrid magnonics in phononic systems with potential applications in coherent energy and signal transduction.

Figures (4)

• Figure 1: (a) Optical SHG intensity of SrTiO3 substrate. (b) In-plane magnetization as a function of temperature. Dashed lines in (a) & (b) denote the cubic-to-tetragonal phase transition of STO at $T_\mathrm{S}$ = 105 K. (c) Schematic of the magnetoelastic heterostructure composed of LSMO thin films ($d$ = 38 nm) epitaxially grown an STO substrate ($L$ = 0.5 mm). The sample is mounted on a coplanar waveguide. When the STO undergoes the cubic-to-tetragonal phase transition, the elongated axis can be along [100], [010], and [001] crystalline orientations.
• Figure 2: (a) Heatmap of FMR spectral intensity of LSMO/STO heterostructure at 250 K. Dashed curve denotes the magnon mode and the horizontal dashed lines denote the phonon modes. (b) FMR line profiles at selected frequencies and corresponding fits to a superposition of symmetric and antisymmetric Lorentzian functions. FMR line profiles are vertically offset for clarity. Magnified view of anticrossings at (c) $f_1$ = 9.47 GHz, $H_1=1391$ Oe, and (d) $f_2$ = 8.53 GHz, $H_2=1155$ Oe. The white dashed curves show the fits of the anticrossings to the coupled magnon-phonon model. (e) and (f) show the corresponding splitting energy ($\Delta f$) for anticrossings in (c) and (d), respectively. $\Delta f_\mathrm{min}$ represents the anticrossing gap size.
• Figure 3: (a) Heatmap of FMR spectral intensity at 80 K. The strain-driven split magnon bands are locked to the crystalline directions [100] (red), [010] (yellow), and [001] (blue), respectively. (b) Zoom-in of the region enclosed by the dashed box in (a). The white curves are the guide-to-the-eye of the anticrossings. (c) The cooperativity values of the anticrossings from (b) as a function of frequency and magnetic field.
• Figure 4: (a) Simulated FMR of magnon band at 250 K derived from Eq.(3), where STO is in the cubic phase. Simulated FMR of split magnon bands associated with the [100], [010], and [001] crystalline orientations at (b) 80 K and (c) 50 K derived from Eq.(4a)-(4c), where STO is in the tetragonal phase. (d) Magnon-phonon coupling strength $g = \Delta f_{min}/2$ derived from analytical model Eq.(S31) in Sec.10 of Supplemental Materials sm. The measured $g$ values at 250 K (red stars) are overlaid for comparison.