Abstract
Systematic transport measurements have been performed on a series of La0.67Ca0.33MnO3 thin films with varying degrees of anisotropic strain. The strain is induced via epitaxial growth on NdGaO3(001) substrates and varied by controlling the thermal annealing time. An antiferromagnetic insulating (AFI) state, possibly associated with charge ordering, emerges upon thermal annealing. The Hall effect in these materials exhibits features that are indicative of a percolative phase transition and correlate closely with the emergence of the AFI state. In the paramagnetic phase, the Hall resistivity takes on two slopes in all samples: a decreasing negative slope with increasing temperature at low fields, which is attributed to the carrier hopping motion, and an almost temperature independent positive slope at high fields due to diffusive transport of holes. Significantly, the crossover fields of the Hall resistivity slope at different temperatures correspond to the same magnetization, which is interpreted as the critical point of a magnetic field-driven percolative phase transition. At lower temperatures near the zero-field metal–insulator transition, pronounced enhancement of the Hall coefficient with the development of the AFI state is observed. The enhancement peaks near the magnetic field-driven percolation; its magnitude correlates with the strength of the AFI state and is suppressed with the melting of the AFI state by an in-plane magnetic field. The observations resemble many features of the enhancement of the Hall coefficient in granular metal films near the composition-driven percolation.
Highlights
electronic phase separation (EPS) has been probed by a wide variety of direct and indirect techniques in the manganites
In the study of the magnetically driven EPS in the semimetallic ferromagnet EuB6, we recently discovered an intriguing manifestation of the EPS and percolative phase transition in the nonlinear Hall effect (HE) [33]
The HE in these materials exhibits two prominent features reflective of a percolative phase transition in an increasingly insulating background. (i) In the paramagnetic phase, the Hall resistivity exhibits a change of slope whose crossover fields at different temperatures correspond to the same magnetization, similar to the observation in EuB6 [33]. (ii) In the vincinity of TC, a pronounced enhancement of the Hall coefficient near the percolation field develops with increasing annealing time
Summary
EPS has been probed by a wide variety of direct (e.g. scanning tunneling microscopy [18, 19, 25] and electron microscopy [20]) and indirect (e.g. noise spectroscopy [26, 27], single domain resistance fluctuation [28, 29] and neutron scattering [30,31,32]) techniques in the manganites. We observed a distinct switch in the slope of the Hall resistivity of EuB6 in its paramagnetic phase, which evolves systematically with temperature. We interpret the critical magnetization as the point of percolation for patches of a more conducting and magnetically ordered phase in a less ordered background This picture appears applicable in the paramagnetic phase of Nd0.7Sr0.3MnO3 [33, 34], overall the HE in the mixed-valence manganites is far more complex [35,36,37,38]. (i) In the paramagnetic phase, the Hall resistivity exhibits a change of slope whose crossover fields at different temperatures correspond to the same magnetization, similar to the observation in EuB6 [33]. The enhancement of the Hall coefficient here is reminiscent of the giant HE (GHE) in granular films of a percolating metallic network in an insulating matrix near the compositiondriven percolation threshold [39], is an additional manifestation of a magnetically driven percolative transition
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