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Neutron scattering has been used to study the nature of the ferromagnetic transition in a single crystal of the perovskite La0. 7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Both the scattering due to the polaron correlations and the anomalous quasielastic component in the magnetic fluctuation spectrum maximize very close to T(C), in a manner remarkably similar to the resistivity, indicating that they have a common origin.

Using the point contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite, La0.7Sr0.3MnO3 (LSMO). Surprisingly, we observed a significant increase in the current spin polarization with the residual resistivity. This counterintuitive trend can be understood as a transition from ballistic to diffusive transport in the contact. Our results strongly suggest that LSMO does have minority spin states at the Fermi level. However, since its current spin polarization is much higher than that of the density of states, this material can mimic the behavior of a true half-metal in transport experiments. Based on our results we call this material a transport half-metal.A half-metallic ferromagnet is a metal that has an energy gap at the Fermi level, E F , in one of the two spin channels. Only the other channel has states available for transport, and thus the electric current is fully spin-polarized. Finding half-metallic or other highly spin-polarized metals would bring about major advances in magnetoelectronics, since device performance improves dramatically as the spin polarization of the metal approaches 100%.1 Although half-metallicity has been predicted in quite a number of materials, the experimental situation is still controversial, especially for the manganese perovskite, La 0.7 Sr 0.3 MnO 3 (LSMO). Theoretical 2 and experimental values 3-6 of the spin polarization of this fascinating material with highly unusual structural, magnetic and electronic properties, obtained by different techniques vary from 35% to 100%. Not surprisingly, when Park et al. concluded from their spin-resolved photoemission spectroscopy measurement that LSMO is completely spin-polarized 3 it attracted immediate attention. This result was important not only from a practical viewpoint, but also as a potential new insight into the microscopic physics of this system, since the values of the spin polarization are extremely sensitive to the band structure of LSMO. Importantly, the measured value of the spin polarization, P n , depends on the experimental technique. It is often possible 10 to define P n in the following form:where N ↑ (E F ), N ↓ (E F ) and v F ↑ , v F ↓ are the majority and minority spin DOS and the Fermi velocities, respectively. This definition allows a direct comparison between different experiments and the theory, since all the quantities in Eq. 1 can be evaluated from the band structure. The spin polarization P 0 (n =0) measured by spinresolved photoemission measurements is determined only by the DOS at the Fermi level.11 Transport experiments measure a different spin polarization, which includes the Fermi velocities (Eq.1). In the ballistic, or Sharvin, limit (mean free path, L, larger than the contact size, d) the DOS is weighted linearly with v F , and P 1 is measured. 12In the diffusive, or Maxwell regime (L < d), as in the classical Bloch-Bolzmann theory of transport in metals, the weighting is quadratic in v F (n=2) and P 2 is measured (a...

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Detailed measurements of the magnetic and transport behavior of the two La(1-x)Ca(x)MnO(3) single crystals exhibiting colossal magnetoresistance are summarized. The x=0.21 sample exhibits unusual exponents (delta = 20+/-1, gamma = 1.71+/-0.1, beta = 0.09+/-0.01, T(C) = 182+/-1 K) and, more importantly, a Griffiths phase characterized by an exponent lambda = 0.70+/-0.2. By contrast, the x=0.20 specimen displays Heisenberg model behavior with no evidence of such a phase. Thus while a Griffiths phase accounts for the behavior of La(1-x)Ca(x)MnO(3) near optimal doping, it does not appear to be a prerequisite for colossal magnetoresistance in this system.

We describe the evolution of the static and dynamic spin correlations of La1−xCaxMnO3, for x=0.1, 0.125 and 0.2, where the system evolves from the canted magnetic state towards the insulating ferromagnetic state, approaching the metallic transition (x=0.22). In the x=0.1 sample, the observation of two spin wave branches typical of two distinct types of magnetic coupling, and of a modulation in the elastic diffuse scattering characteristic of ferromagnetic inhomogeneities, confirms the static and dynamic inhomogeneous features previously observed at x<0.1. The anisotropic q-dependence of the intensity of the low-energy spin wave suggests a bidimensionnal character for the static inhomogeneities. At x=0.125, which corresponds to the occurence of a ferromagnetic and insulating state, the two spin wave branches reduce to a single one, but anisotropic. At this concentration, an anomaly appears at q0=(1.25,1.25,0), that could be related to an underlying periodicity, as arising from (1.5,1.5,0) superstructures. At x=0.2, the spin-wave branch is isotropic. In addition to the anomaly observed at q0, extra magnetic excitations are observed at larger q, forming an optical branch. The two dispersion curves suggest an anti-crossing behavior at some q0' value, which could be explained by a folding due to an underlying perodicity involving four cubic lattice spacings.

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