Study of Magnetic Properties of A_2B^'NbO_6 (A=Ba,Sr, (BaSr): and B^'=Fe   and Mn) double perovskites
  We have studied the magnetic properties of Ba_2FeNbO_6 and Ba_2MnNbO_6. it is seen that Ba_2FeNbO_6 is an antiferromagnet with a weak ferromagnetic behaviour at 5K while Ba_2MnNbO_6 shows two magnetic transitions one at 45 K and the other at 12K. Electron spin resonance (ESR) measurements at room temperature show that the Mn compound does not show any Jahn-Teller distortion. It is also seen that the Neel temperature of the A_2FeNbO_6 (A=Ba,Sr, BaSr) compounds do not vary significantly. However variations in the average A-site ionic radius influence the formation of short range correlations that persist above T_N. 
N. Rama, J.B. Philipp, M. Opel, K. Chandrasekaran, V.   Sankaranarayanan, R. Gross, and M.S. Ramachandra Rao