Critical slowing down of longitudinal spin relaxation in La1-xCaxMnO3

Abstract

Using original modulation technique, the longitudinal electron spin-relaxation time T1 has been measured directly in the paramagnetic state of three La1-xCaxMnO3 samples (x = 0.2, 0.25, and 0.33). Well above the phase-transition temperature Tc, the longitudinal relaxation times are found to be equal to the transversal ones (T2) as determined from the electron-paramagnetic-resonance (EPR) linewidth, whereas a steep slowing down of T1 with a critical exponent α≃0.5 was observed as Tc was approached in all the materials. Various models are discussed, including extremely slow internal dynamics, formation of magnetic clusters, and inhomogeneous EPR broadening near Tc. © 2001 The American Physical Society.