  We report magnetization, resistivity and thermopower in the charge-orbital
ordered antiferromagnet Nd0.75Na0.25MnO3. Magnetic-field induced collapse of
antiferromagnetism is found to be accompanied by a giant negative
magnetothermopower (= 80-100% for a field change of 5T) over a wide temperature
(T = 60-225K) and giant magnetoresistance. While the field-induced metamagnetic
transition in magnetization is reversible upon field-cycling at T > 40 K, it is
irreversible at lower temperatures and this has impact on magnetoresistance,
magnetothermopower as well as change in the temperature of the sample. Our
results indicate high sensitivity of thermopower to changes in the magnetic
state of the sample.
