Temperature
4.8 Analyses
other hand, the deviation of the resistivity data from the polaron model increases with the increasing substrate-induced lattice distortion at low temperatures, suggesting that the residual resistivity and magnetoresistance of those samples are largely determined by the extrinsic lattice distortions such as domain walls and grain boundaries.
A noteworthy correlation of the functions G(T) and G0(H) with the normalized magnetization m ≡M/Ms, whereM is the magnetization and Ms is the saturation mag- netization is illustrated in Fig. 4.7(a) and (b). Here, the unnormalized experimental magnetization data M(T) and M(H) are taken using a SQUID magnetometer and are plotted in the insets in Fig. 4.7(a) and (b). This correlation suggests the relevance of mag- netic ordering to the electrical conduction, particularly for temperatures nearTCurie. The derived polaron binding energy (Eb0 ≈0.35 eV) compares favorably with the Jahn-Teller energy of∼0.5 eV for undoped LaMnO3[88, 96], and is much larger than that for the mag- netic polarons due to the electron-spin interaction [109]. Since the equations given in the polaron model are only approximate and limited to the high-temperature region, the exact forms of G(T) and G0(H) should not be taken literally. However, the analyses do provide a consistent picture of two types of contribution to the resistivity and magnetoresistance, with strong evidence for polaron hopping conduction at high temperatures, and a different scattering mechanism associated with the lattice distortion at low temperatures.
The low-temperature scattering mechanism may be understood by comparing M(T) data for all LCMO films and that for the bulk in Fig. 4.7(a). The slower rise of magnetization belowTCurie for samples of larger lattice distortion, either intrinsic (strain) or extrinsic (relaxation), appears to be correlated with larger resistivity and magnetoresis-
Figure 4.7: a) TheG(T)−T curves for LCMO/LAO, LCMO/STO, and LCMO/YAOfilms (solid lines) at H = 0 andH = 60 kOe. The inset shows the temperature dependence of the magnetic momentsM(T) for LCMO/LAO, LCMO/STO, and LCMO/YAO films, and bulk LCMO, taken at H = 6 kOe. b) The representative G0(H)−H isotherms for the LCMO/LAOfilm. The corresponding M−H data are shown in the inset [58].
tance, suggesting increasing electron scattering induced by larger lattice distortion. One possible consequence of larger lattice distortion is a larger number of magnetic domains.
Although all domains may undergo a ferromagnetic phase transition at the sameTCurie, the incomplete alignment of the moments of the magnetic domains resulting from inhomogene- ity or pinning by local defects below TCurie gives rise to slower rising magnetization and larger scattering of conduction electrons. Therefore, an applied magnetic field has more significant effect on reducing the resistivity in samples with larger lattice distortion through the aligning of the magnetic domains [53].
The picture of magnetic domain wall scattering is consistent with the larger re- sistivity and magnetoresistance at low temperatures where the polaron contribution be- comes insignificant. The distinct change of slope in low-temperature ρ−H isotherms of LCMO/STO and LCMO/YAO, which are samples with larger lattice distortion, also sug- gests better alignment of magnetic domains in sufficiently highfields. In contrast, theρ−H isotherms of the least distorted LCMO/LAO can be described by the polaron conduction equation over a large magneticfield range, consistent with less significant magnetic domain wall scattering [53].
In the La0.5Ca0.5CoO3 epitaxial films, despite comparable lattice relaxation and lattice strain to the manganites in LCCO/YAO, the magnitude and temperature dependence of the resistivity in the LCMO and LCCO systems exhibit sharp contrasts. Since the higher electron mobility in the cobaltites tends to inhibit the formation of lattice polarons, the resistivity of the cobaltites may be understood in terms of the combination of conventional impurity, phonon and disorder-spin scattering, with the disorder-spin scattering being the
only magnetic field-dependent term. For both LCCO/LAO and LCCO/YAO samples, a faster decrease in the zero-field resistivity occurs below the Curie temperatureTCurie≈180 K, indicating that the magnetic ordering belowTCurie reduces the resistivity and that the small negative magnetoresistance nearTCurieis due to thefield-induced suppression of spin
fluctuations and of the corresponding scattering near TCurie. It appears that the physical
origin of the negative magnetoresistance in the cobaltites is fundamentally different from that in the manganites, and the formation of lattice polarons seems to be essential for the occurrence of CMR effects in the manganites. However, recent STM studies [110] and numerical simulations [111, 112] have shown that for certain range of doping levels, afirst- order phase transition can take place either atTCurieor under a large applied magneticfield, and that nanoscale inhomogeneities associated with competing phases of ferromagnetism and antiferromagnetism can occur. This scenario complements the lattice polaron model that restricts to high temperatures and provides feasible account for the occurrence of first-order phase transitions in certain manganites and for the conductivity over a wide
temperature range.
Finally, it should be remarked that the cobaltites La1−xMxCoO3(M =Ca, Sr) also exhibit very interesting and unique physical properties. In particular, giant ferromagnetic Hall effect with interesting doping and temperature dependence has been discovered in studies from this group [113, 114]. A record ferromagnetic Hall coefficient among all single-phase ferromagnets is found in La0.8Ca0.2CoO3, which has the doping composition near the magnetic percolation threshold [113]. However, those results are not directly related to the spin-injection studies, so will not be discussed further in this thesis.