We report a study of the effect of red light (lambda = 650 nm) on the magnetic properties of GdBaCo2O5+delta. The ferromagnetic transition temperature decreases by 4 K and the intermediate to high spin state transition temperature increases by 3 K. We propose that electron capture in the oxygen vacancy along Co-O chain and competing interaction between the orbital ordering and ferromagnetic phase are responsible for the above observed results. A 'butterfly-wing' shaped hysteresis loop along with a field induced transition has been observed in the temperature window 190 less than or equal to T less than or equal to 215 K. (C) 2003 Elsevier Ltd. All rights reserved. References: 20
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