Strain-induced magnetic damping anomaly in La₁₋ₓSrₓMnO₃ ($x=0.3$-$0.5$) thin films

Magnetic properties of La₁₋ₓSrₓMnO₃ (LSMO) are highly sensitive to various factors such as the Sr doping level x, lattice strain, and oxygen stoichiometry due to the strongly correlated nature of $3d$ electrons. For the development of energy-efficient spintronic devices with ultra-low magnetic damping of LSMO, a thorough understanding of its complex magnetization dynamics is of great importance. In this work, we have measured ferromagnetic resonance of LSMO thin films on Nb-doped SrTiO₃ (Nb-STO) substrates over a wide temperature and frequency range and observed an anomalous increase in the Gilbert damping constant and a decrease in the effective saturation magnetization at temperatures below 100 K. The anomalies become more pronounced as the LSMO thickness decreases while they are not observed for LSMO on (LaAlO₃)0.3(Sr₂TaAlO₆)0.7 substrates with relatively small epitaxial strain. The results suggest that the epitaxial strain-induced magnetically dead layer at the LSMO/Nb-STO interface acts as a spin sink and leads to the anomalies in the magnetization dynamics.