Spin hopping in ultrathin La/sub 0.7/Ca/sub 0.3/MnO/sub 3/ films

Summary form only given. Granular ferromagnets show a magnetoresistance due to spin-polarized tunnelling between individual grains. Within a simple model Inoue et al. (1996) calculated the magnetoresistance ratio and obtained [R/sub 0/-R(H)]/R(H) = P/sup 2/. Here R(H) denotes the resistance in an applied field H and R/sub 0/ = R(H = 0); since the spin-polarization P is limited, P /spl les/ 1, the magnetoresistance cannot exceed unity. La/sub 0.7/Ca/sub 0.3/MnO/sub 3/ films (LCMO) films with various thicknesses were prepared by pulsed laser deposition onto LaAlO/sub 3/ substrates. Films with a nominal thickness larger than 4 nm were continuous and showed a metal-insulator transition above 200 K. Films with a smaller nominal thickness were discontinuous as evidenced by atomic force microscopy. These films are superparamagnetic and show hopping conduction. The resistance follows a exp[(To/T)/sup 3/5/] law for temperatures T < 200 K; this might be related to a thermally assisted tunnelling process in a two dimensional granular structure formed by surface diffusion. The magnetoresistance [Ro-R(H)]/R(H) of the 2.2 nm film exceeds the theoretical limit. The data can be convincingly fitted by an exponential dependence, exp[J/sub m/H/sup 2//kBT]-1, where J/sub m/H/sup 2/ is an activation energy. This indicates that the intergrain coupling energy plays a major role in the tunnelling process. We discuss this data and present a more detailed model for tunnelling in granular materials.