Colossal Magnetoresistive Oxides Research Articles

Dominant Role of a Pseudogap in the Physics of the Colossal Magnetoresistive Oxides

We have performed direct electronic structure measurements of the colosssal magnetoresistive oxides using high energy resolution photoemission spectroscopy. A key result of these measurements is the discovery of a pseudogap which drastically supresses or completely removes the spectral weight (density of states) at the Fermi energy E F . It is shown that the pseudogap plays a dominant role in the "colossal" conductivity changes which occur across the magnetic ordering temperature. We briefly discuss possible origins for the pseudogap.

Finite size effects in microwave loss in colossal magnetoresistance oxides

We present systematic measurements of microwave surface resistance R s at 10 and 26 GHz of colossal magnetoresistance materials with varying thicknesses. It was found that for samples thicker than the electromagnetic skin depth, R s is proportional to ρ 1/2. ρ being the resistivity while for thinner samples, R s varies as ρ −1/2. These observations can be understood in terms of a simple model used to describe R s.

Colossal magnetoresistance in Sol-Gel derived epitaxial thin film of Co-doped La1−xCaxMnOx

Electrical, magnetic, and magnetotransport properties of sol-gel derived epitaxial thin films of Co-doped La1−xCaxMnO3 are reported. The epitaxial thin films, deposited using metal-salt routed sol-gel processing, show excellent quality of epitaxy (FWHM=0.3°). Their surface roughness is about 30A and average grain size is 500A. The thin films exhibit the typical behavior of colossal magnetoresistive oxide, manifesting paramagnetic semiconductor to ferromagnetic metal transition near magnetic transition. The doping of Co reduces electrical conductivity, Curie temperature (Tc) and saturation magnetization (Ms). However, the peak magnetoresistance ratio does not show a monotonous change with increasing Co content. These results are interpreted by spin-disorder scattering, magnetic inhomogeneity, and lattice distortion.

Structure Characterization of Colossal Magnetoresistive Oxides

Magnetoresistance describes a phenomenon in which the electrical resistance of the material depends strongly on an externally applied magnetic field. Colossal magnetoresistance (CMR) was observed in a new class of oxides, (La,A)MnO3 (A = Ca, Sr, or Ba), and the Magnetoresistance ratio of ΔR/R(H)> -100,000% has been reported in epitaxially grown La0.67Ca0.33MnO3 films. The new types of intrinsic magnetoresistive oxides offer exciting possibilities for improved magnetic sensors, magnetoresistive read heads, and magnetoresistive random access memory (MRAM).The CMR magnetic oxides have a perovskite-type crystal structure with ferromagnetic ordering in the a-b planes and antiferromagnetic ordering along the c-axis (Figure 1). The ferromagnetically ordered Mn-O layers of the a-b planes are isolated by a non-magnetic La(A)-O monolayer. This spin-coupling structure is intrinsic. La1-xAxMn03 compounds having the extreme values x = 0, 1 are neither ferromagnetic nor good electrical conductors; they are semiconductors. Only compounds with intermediate values of x are ferromagnetic,

Colossal magnetoresistance oxides: phenomena and questions

Some unusual electronic properties of doped manganites are described. The low temperature resistivity of the metallic ferromagnet has a large $T^2$ term. The residual resistivity can be much larger than the Mott maximum, and depends exponentially on spontaneous magnetization. There is much evidence for strong spatial and temporal fluctuation effects. The available theories are discussed in the light of these phenomena, and it is suggested that a novel localizing mechanism is at work, leading to extreme sensitivity of physical properties to small atomic changes.