⁵⁵Mn NMR observation of colossal magnetoresistance effect in Sm_0.55Sr_0.45MnO₃

Temperature dependent ⁵⁵Mn NMR study of Sm_0.55Sr_0.45MnO₃ is reported. Previous bulk magnetization measurements have shown that below T _C ~ 125 K the sample is ferromagnetic metallic (FMM) and above T_C it is charge ordered and insulating. In present report, we show that from zero-field NMR a single line double-exchange (DE) signal is observed at temperatures up to 139 K, which is due to a presence of FMM clusters also above T _C. The intensity of the DE line follows the temperature dependence of the magnetization measured at 0.01 T. When a magnetic field up to 2 T is applied at 139 K (i.e. 14 K above T _C), a strong increase in NMR intensity of the DE line is observed indicating that content of FMM regions increases. This reveals that metallicity is induced in the material by the applied magnetic field and explains the observed colossal magnetoresistance (CMR) effect at the microscopic level. The observation agrees with previous results, which confirm that the percolation of the FMM clusters is responsible for the CMR effect. The shift of the resonant frequency in the applied field is three times smaller compared to decrease expected from gyromagnetic ratio, which indicates an antiferromagnetic coupling between the FMM clusters.