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Magneto-transport and strain driven meta-stable phases in ultra thin manganite films


Ultra thin manganite films exhibits many functional behaviours including very large anisotropic magneto-resistance (AMR) and tunnelling magneto-resistance (TMR).  In the first part of the talk we will show our results on the AMR in ultrathin La-based manganite films grown on various substrates. It was found that depending on the strain state, the AMR in some of these systems exceeds 100%. These changes are very dramatic when compared to the few percent change in AMR in conventional ferromagnets. In this part we will also show some results on manganite based magnetic tunnel junctions with particular emphasis on the tunnelling anisotropic magneto-resistance (TAMR).

In the second part of the talk we will concentrate on the phase separation tendencies in ultra thin manganite films. It is known that the epitaxial strain associated with a lattice mismatched substrate in ultra thin manganite films can cause new electronic behaviour, not found in bulk materials or thicker films of the same chemical composition.  In this case resistive (insulator to metal) and magnetic transition broadens due to the large distribution of the transition temperatures. The temperature dependence of the fraction of the metallic phase could be obtained by assuming a Gaussian distribution of random transition temperatures, whose width increases with an increasing lattice strain. The resistivity $\rho$(T) calculated from a simple percolative model reproduces well the experimental ones for La-based manganite crystals and strained films. In this part of the talk we will also show that the resistivity of the films grown on LaAlO 3 substrates with thicknesses less than 15 nm exhibit strong relaxation effects; the resistivity of the films change dramatically  when measured as a function of time in a constant magnetic field.  The resistive relaxation effects are less pronounced in the films grown on SrTiO 3 substrates indicating that the observed changes are strain driven.  Moreover, the ultrathin films grown on LaAlO 3 substrates age dramatically faster than the thicker counterparts or the films of the same thicknesses grown on SrTiO 3 substrates. Aging cause an upturn in the temperature dependence of the resistivity at lower temperatures where material is supposed to be ferromagnetic metal.