Magnetic properties of pseudomorphic epitaxial films of Pr_{0.7}Ca_{0.3}MnO_3 under different biaxial tensile stresses

In order to analyse the effect of strain on the magnetic properties of narrow-band manganites, the temperature and field dependent susceptibilities of about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001) and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on (001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative of the possible coexistence of antiferromagnetic and ferromagnetic phases. On the contrary, ultrathin films grown on (110) substrates show a robust ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn atom along the easy axis. On the base of high resolution reciprocal space mapping analyses performed by x-ray diffraction, the different behaviours are discussed in terms of the crystallographic constraints imposed by the epitaxy of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the tensile strain on the film c-axis, lying within the substrate plane, favours the ferromagnetic phase, possibly by allowing a mixed occupancy and hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to shed some physics of inhomogeneous states in manganites and on the nature of their ferromagnetic insulating state.

Charge density waves enhance the electronic noise of manganites

The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin films in the temperature range from room temperature to 160 K are reported. It is shown that both the broadband 1/f noise properties and the dependence of resistance on electric field are consistent with the idea of a collective electrical transport, as in the classical model of sliding charge density waves. On the other hand, the observations cannot be reconciled with standard models of charge ordering and charge melting. Methodologically, it is proposed to consider noise-spectra analysis as a unique tool for the identification of the transport mechanism in such highly correlated systems. On the basis of the results, the electrical transport is envisaged as one of the most effective ways to understand the nature of the insulating, charge-modulated ground states in manganites.