SPIN DYNAMICS IN THE LA1.85SR0.15CU1-XFEXO4 SYSTEM PROBED BY ESR

We study the magnetic properties of the La1.85Sr0.15Cu1-xFexO4 system (x = 0-0.1) in the vicinity of the metal-insulator (MI) transition usi ng static susceptibility measurements and the electron-spin resonance (ESR) of the Fe ions. Spin-glass (SG) freezing is present for all nons uperconducting specimens. The iron ESR line broadens on approaching th e freezing temperature, similarly to the effect observed in canonical spin glasses. This broadening can be attributed to the influence of th e slowing down of spin fluctuations on the spin-spin relaxation rate. It depends differently on x on the two different sides of the MI trans ition suggesting the existence of two different SG phases: insulating (ISG) where the Fe spins couple to the Cu-spin array by superexchange interactions, and metallic (MSG), where there exists, in addition, Rud erman-Kittel-Kasuya-Yosida-like coupling mediated by the free carriers . The proximity of the MI transition suppresses this coupling, giving rise to the observed dependence of the paramagnet-SG phase boundary on x. In the MSG phase, at high temperatures, the spin-lattice relaxatio n mechanism is also mediated by the free carriers. It differs from the analogous process observed in conventional metals in that the ESR lin ewidth increases with increasing T faster than linearly. We explain th is behavior by assuming that the effective magnetic field felt by the Fe moments originates mainly from the spins of holes located on the ne arest-neighbor oxygen ions. The linewidth divided by T which probes th e dynamical susceptibility at the Fe site varies as a + bT. Here a goe s to zero as the MI transition is approached and so plays the role of the Pauli susceptibility of the free carriers, whereas b is independen t of x in the vicinity of the MI transition but decreases for small Fe dopings away from the MI transition and may be identified as originat ing from the antiferromagnetic spin fluctuations in the system.