New insights into the transport and field-enhancement effects in sol-gel derived colossal magnetoresistive thin films

The transport mechanism underlying the colossal magnetoresistance (CMR) of doped manganites is not yet understood and their technological applications are limited by the high fields (approximately 1 T) required to obtain any significant MR. Following the development of a polymeric chemical synthesis route, we have investigated the O_2p unoccupied density of states in sol-gel derived La_1-xSr_xMnO₃ (0≤x≤0.7) thin films grown epitaxially on LaAlO₃, by electron energy-loss spectroscopy at sub-eV resolution. The spectra show a distinct prepeak in the OK edge at the Fermi level, the intensity of which correlates directly with the conductivity of the film. Similar correlation was also obtained for La_0.7Sr_0.3MnO_3-z annealed in vacuum to obtain well defined oxygen content (z) in the film. This confirms that the charge carriers in these manganese perovskites have significant oxygen 2p hole character and suggests that the `double exchange' mechanism has to be modified. In another set of experiments we have studied room-temperature field amplification effects to enhance the low-field sensitivity of La_0.7Sr_0.3MnO_3-z films sandwiched between two thin rectangular slices of either α-Fe or Mn-Zn ferrite. This field amplification leads to an enhanced low-field MR value as high as 6% at an external field of 500 Oe which is 6 times the value observed without the amplification.