Low-field magnetoresistance in the pyrochlore Tl2Mn2O7

The discovery of colossal magnetoresistance in perovskite manganites derived from LaMnO3 has renewed interest in these and related materials for possible technological applications(1-7). But the high magnetic fields and narrow temperature windows where the large magnetoresistive responses are observed present daunting limitations. A possible avenue for overcoming these limitations is to improve the low-field response by modifying the manganites to enhance the contribution to magnetoresistance from spin-polarized tunnelling of the conduction electrons; such tunnelling, which can be very sensitive to an applied magnetic field, takes place across grain boundaries (natural or artificial) or between the manganite planes of the layered derivatives of the perovskite compounds(8-13). Here ive show that low-field (H < 1 kOe) grain-boundary magnetoresistance can also be realized in a non-perovskite magnetoresistive oxide, the pyrochlore Tl2Mn2O7. Moreover, this low-field response, which persists for all temperatures below the transition to the ferromagnetic state, does not show the strong temperature-dependent decay characteristic of the perovskite-based systems. We suggest that this improved response is in part due to weaker electron-spin coupling and the lack of strong electron-lattice interactions in the pyrochlore.