  Pr0.9Sr0.1CoO2.99 sample exhibits magnetoresistivity (MR) of up to 40 % at 5
K with a strong hysteresis/memory effect. Magnetisation measurements on
Pr0.9Sr0.1CoO2.99 in an applied field of 100 Oe show that, as temperature
decreases, the zero-field-cooled (ZFC) and field-cooled (FC) magnetisation
curves branch clearly at 50 K, and a cusp appears in the ZFC branch at Tcusp 20
K. Magnetisation measurements in various fields between 100 and 10,000 Oe show
that both the ZFC-FC branching temperature and, Tcusp, decrease with increasing
field. The magnetization-field isotherms at 5 and 10 K show hysteresis loops
typical of ferromagnets. No appreciable MR is seen in this compound at 50 K,
i.e. at a temperature close to ZFC-FC branching temperature. At 20 K, negative
MR of above 16% is observed without any hysteresis effect. We believe that the
appearance of a ferromagnetic component at 5 K and 10 K (i.e. at temperatures
below Tcusp) within the spin glass state of Co spins is responsible for both
large MR and the prominent hysteresis/memory effect in MR.
