PR AND CU MAGNETISM IN (PR1.5CE0.5)SR(2)CU(2)MO(10-DELTA) (M=NB, TA) - CORRELATIONS WITH A SUPPRESSION OF SUPERCONDUCTIVITY

The magnetic properties of nonsuperconducting (Pr1.5Ce0.5)Sr(2)Cu(2)MO (10-delta) with M = Nb, Ta are characterized with de magnetization, sp ecific-heat, and neutron-diffraction experiments. Data for (Pr1.5Ce0.5 )Sr2Cu2NbO10-delta reveal complex Cu magnetism marked by antiferromagn etic order below 200 K, spin structure transitions at 130 and 57 K, bo th collinear and noncollinear antiferromagnetic spin structures, and w eak ferromagnetic behavior below 130 K. The data also indicate an anom alous ordering of the Pr spins near 10 K, a large linear contribution to the low-temperature specific heat, and a Pr 4f crystal-field ground state similar to that found in PrBa2Cu3O7. Furthermore, there is evid ence that the weak ferromagnetic behavior couples to the Pr ordering n ear 10 K. Identical Pr magnetism and similar Cu magnetism are found in (Pr1.5Ce0.5)Sr2Cu2TaO10-delta, deoxygenated (Pr1.5Ce0.5)Sr2Cu2NbO10-d elta, and deoxygenated (Pr1.5Ce0.5)Sr2Cu2TaO10-delta. These results in dicate that superconductivity is suppressed in these compounds in the same phenomenological manner as in PrBa2Cu3O7. We interpret this as ev idence that superconductivity is suppressed by the same mechanism in b oth structures and propose that a general correlation exists between a nomalous Pr magnetism and a lack of superconductivity in these Pr-base d high-T-c cuprates. The significance of these results and analyses to understanding and modeling the suppression of superconductivity by Pr in high-T-c cuprates is discussed.