It is now widely accepted that the magnetic transition in doped manganites that show large magnetoresistance is a type of percolation effect. This paper demonstrates that the transition should be viewed in the context of the Griffiths phase that arises when disorder suppresses a magnetic transition. This approach explains unusual aspects of susceptibility and heat capacity data from a single crystal of La0.7Ca0.3MnO3.The term "colossal magnetoresistance" (CMR) has commonly been used to describe the very large, magneticfield driven changes in electrical resistivity in oxides based on LaMnO 3 near their second-order, ferromagnetic transitions. The largest CMR effects are accompanied by other anomalies in magnetic and thermodynamic properties. Among these are the failure of the magnetic correlation length to increase strongly as the transition temperature T C is approached from above, the persistence of a well defined spin-wave dispersion close to the transition [1], and an unusual shift in the heat capacity peak to higher temperatures in applied magnetic fields.[2] An explanation of this unusual behavior of the heat capacity in the context of a Griffiths singularity [3] is the focus of the present paper.There is now general consensus that the CMR transition is a type of percolation in which, due to the doubleexchange process [4], bonds become metallic as neighboring spins tend to align. The strength of the CMR effect (along with the transition temperature) depends strongly on the ionic size and concentration of the divalent atom that substitutes for La in LaMnO 3 . The effect is nearly absent for La 2/3 Sr 1/3 MnO 3 (T C = 360 K) but quite strong in the present sample La 0.7 Ca 0.3 MnO 3 (T C = 218 K) [5]. In the low temperature metallic phase, the exchange interactions, as measured by the spin-wave stiffness, are the same in Sr-snd Ca-doped crystals [6], demonstrating that the key to the CMR effect is to be found in the non-metallic regime. The ionic size of the divalent substituent exerts its effect on magnetic and electronic properties through local tilting of the oxygen octahedra [7] and the concurrent bending of the active Mn-OMn bonds. This inhibits the formation of metallic bonds and leads to charge localization, polaron formation, and possible charge segregation [8,9]. Evidence in favor of a percolation picture comes from a Monte-Carlo simulation of a random field Ising model that assigns conductivity zero and unity to bonds between neighboring antiparallel and parallel spins respectively [10]. It both produces CMR and emphasizes the importance of randomness. Experimental evidence was provided by Jaime et al.[11] who extracted the field and temperature dependent metallic-bond concentration c(H, T ) from the resistivity via the effective medium approximation, and showed that it also describes the thermoelectric power data. Direct evidence for coexisting polaronic/insulating and metallic components have been reported from neutron [12] Discussions of the CMR effect in percolation terms [16][17][18] have gener...

We explore the possibility that polaronic distortions in the paramagnetic phase of La 0.67 Ca 0.33 MnO 3 manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting fieldand temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the twocomponent system, and find that a limit with low percolation treshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility.

Low-temperature electrical transport and double exchange in La 0.67 (Pb,Ca) AbstractThe resistivity in the ferromagnetic state of flux-grown La 2/3 (Pb,Ca) 1/3 MnO 3 single crystals, measured in magnetic fields up to 7 T, reveals a strong quadratic temperature dependence at and above 50 K. At lower temperatures, this contribution drops precipitously leaving the resistivity essentially temperature independent below 20 K. The Seebeck coefficient also reflects a change of behavior at the same temperature. We attribute this behavior to a cut-off of single magnon scattering processes at long wavelengths due to the polarized bands of a double-exchange ferromagnet.

The heat capacity of single crystal La0.7D0.3MnO3, where D=Ca, Sr, has been measured through the Curie point in fields up to 70 kOe. The magnetic contribution of the Ca sample exhibits a sharp heat capacity peak at TC≃218 K in zero field. The peak broadens and decreases in height with increasing field but, unlike an ordinary ferromagnet, the peak shifts substantially in temperature. As a consequence, the heat capacity data cannot be collapsed into a single scaling function. These features indicate that the transition is not an ordinary second-order ferromagnetic transition. Preliminary heat capacity data from the Sr-doped single crystal, with TC≃360 K, do not exhibit the same shift in peak position with applied field. We attribute the difference in behavior between Ca- and Sr-doped samples to a change in the nature of the phase transition as TC lowers.

This paper reports 139 La NMR measurements of a powder sample of the colossal magnetoresistance compound La 2/3 Ca 1/3 MnO3 (Tc =268K) performed in the paramagnetic state (292K-575K) and in high magnetic fields (2.00-9.40 Tesla). Analysis of the spectrum measured at 575K establishes that the spectrum is a standard powder pattern broadened to a significant degree by a variation in lattice distortions around lanthanum nuclear sites. At lower temperatures, but still above Tc, the spectrum shifts and broadens. Both the shift and broadening exhibit Curie-Weiss behavior, indicating that the shift measures the polarization of the electron spin system, and the broadening reflects a distribution of magnetic susceptibilities. This distribution may result from variations of local susceptibility in the bulk of the sample or from differences in demagnetizing factors among powder grains. Close inspection of the spectrum indicates that the lattice distortions do not change as the temperature lowers. Spectral diffusion measurements suggest that the temperature dependence of the spectrum shape does not result from freezing out of motion of magnetic polarons. Variations in the nuclear spin-lattice relaxation across the spectrum indicate that magnetic fluctuations, not lattice vibrations, dominate nuclear relaxation. Nuclear spin-lattice relaxation therefore measures electron spin dynamics in this system. The magnetic field dependence of the spin-lattice relaxation indicates that the electron spin-spin correlation function adopts simple single exponential behavior with a slow field-independent correlation time of 10 −8 seconds near Tc. The spin-spin correlation function changes form at higher temperatures and can be described by introducing a field dependence to the correlation time and to the magnitude of the fluctuating field. Even at the highest temperatures, the correlation time remains slow, on the order of 10 −9 seconds. The spin-lattice relaxation therefore indicates the presence of extremely slow dynamics above Tc.