Employing a broadband microwave reflection configuration, we have measured the complex surface impedance, $Z_S(\omega ,T),$ of single-crystal ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{MnO}}_3,$ as a function of frequency $(0.045\mathrm{}–45\hspace{0.5em}\mathrm{GHz})$ and temperature $(250\mathrm{}–325\hspace{0.5em}\mathrm{K}).\mathrm{}$ Through the dependence of the microwave surface impedance on the magnetic permeability, $\hat{\mu }(\omega ,T),$ we have studied the local magnetic behavior of this material, and have extracted the spontaneous magnetization, $M_0\mathrm{}\left(T\right),$ in zero applied field. The broadband nature of these measurements and the fact that no external field is applied to the material provide a unique opportunity to analyze the critical behavior of the spontaneous magnetization at temperatures very close to the ferromagnetic phase transition. We find a Curie temperature $T_C=305.5\mathrm{}\pm 0.5\hspace{0.5em}\mathrm{K}$ and scaling exponent $\beta =0.45\mathrm{}\pm 0.05,$ in agreement with the prediction of mean-field theory. We also discuss other recent determinations of the magnetization critical exponent in this and similar materials and show why our results are more definitive.

• Received 4 June 1999

DOI:https://doi.org/10.1103/PhysRevB.61.R870