Large increase in magnetoresistance and cluster-glass behavior in defect Tl2-xMn2O7-y pyrochlores

Moderate-pressure techniques (P = 20 kbar) have been used to prepare defect Tl2-xMn2O7-y pyrochlore-like materials: Neutron powder diffraction (NPD) d ata show that a unique stoichiometry Tl1.94Mn2O6.96 is stabilized under the present synthesis conditions. The defective character of these pyrochlores has a strong influence on the transport and magnetic properties. Both elec trical resistance and magnetoresistance are enhanced with respect to report ed data for stoichiometric Tl2Mn2O7, as a consequence of the spin scatterin g introduced by Tl vacancies. The charge carriers are electrons, and the va lue obtained from the high-field Hall effect is 9.3 x 10(-4) e(-)/unit cell at 5 K. At the same time, a spin-glass behavior develops, subjacent to fer romagnetism, because of the weakening of the ferromagnetic (FM) interaction s between near-neighbor Mn4+ cations. The observation by NPD of a distinct maximum at Q approximate to 0 suggests the existence of FM clusters near ro om temperature (RT), vanishing below T-C to give a long-range-ordered ferro magnetic matrix.