Magnetic homogeneity of colossal-magnetoresistance thin films determined by alternating current magnetic susceptibility

We report measurements of the alternating current (ac) magnetic susceptibility, ${\chi }_{\mathrm{ac}}{\mathrm{=\chi }}^{\prime }{\mathrm{+i\chi }}^{″},$ performed on colossal-magnetoresistance (CMR) materials. We have studied thin film samples of ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{Ca}}_{\mathrm{0.33}}{\mathrm{MnO}}_{\mathrm{3}}$ and ${\mathrm{Nd}}_{\mathrm{0.7}}{\mathrm{Sr}}_{\mathrm{0.3}}{\mathrm{MnO}}_{\mathrm{3}}.$ For homogeneous samples, the temperature of the peak observed in ${\chi }^{″}\mathrm{(T)}$ is in agreement with the temperature of peak resistivity ${\mathrm{(T}}_P)$ obtained from transport measurements. This agreement is not found for inhomogeneous samples, where ${\chi }^{″}\mathrm{(T)}$ shows multiple peaks. The analysis of ${\chi }^{″}\mathrm{(T)}$ enables one to determine the quality of the CMR materials. The results obtained in thin films of ${\mathrm{La}}_{\mathrm{0.67}}{\mathrm{Ca}}_{\mathrm{0.33}}{\mathrm{MnO}}_{\mathrm{3}}$ and ${\mathrm{Nd}}_{\mathrm{0.7}}{\mathrm{Sr}}_{\mathrm{0.3}}{\mathrm{MnO}}_{\mathrm{3}},$ are consistent with those obtained from an homogeneous single crystal of ${\mathrm{La}}_{\mathrm{0.80}}{\mathrm{Sr}}_{\mathrm{0.20}}{\mathrm{MnO}}_{\mathrm{3}}.$ We show that the contactless ac magnetic susceptibility technique is a quick method to reveal inhomogeneities which are not directly evident in direct current transport measurements.