  Here we show that the B-site cation (Fe/Mo) ordering in the double-perovskite
magnetoresistor, Sr2FeMoO6, is controlled by either kinetics or thermal
equilibrium depending on the temperature range. In order to enhance the
ordering, long synthesis periods at moderate temperatures are thus required.
Employment of a special oxygen-getter-controlled low-O2-pressure encapsulation
technique for the sample synthesis enabled us to use long heating periods
without evident evaporation of Mo, and thereby reach the thermal equilibrium
state and a high degree of B-site cation order. For samples fired at 1150C for
long periods (> 36 hours), record-high Ms values of 3.7 ~ 3.9 muB were
obtained.
