LOW-FIELD MAGNETORESISTANCE IN THE PYROCHLORE TL2MN2O7

The discovery of colossal magnetoresistance in perovskite manganites d erived from LaMnO3 has renewed interest in these and related materials for possible technological applications(1-7). But the high magnetic f ields 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 magnetores istance from spin-polarized tunnelling of the conduction electrons; su ch tunnelling, which can be very sensitive to an applied magnetic fiel d, takes place across grain boundaries (natural or artificial) or betw een the manganite planes of the layered derivatives of the perovskite compounds(8-13). Here ive show that low-field (H < 1 kOe) grain-bounda ry magnetoresistance can also be realized in a non-perovskite magnetor esistive oxide, the pyrochlore Tl2Mn2O7. Moreover, this low-field resp onse, which persists for all temperatures below the transition to the ferromagnetic state, does not show the strong temperature-dependent de cay characteristic of the perovskite-based systems. We suggest that th is improved response is in part due to weaker electron-spin coupling a nd the lack of strong electron-lattice interactions in the pyrochlore.