PURPOSE: To produce a new barium-containing manganese oligomer having a magnetic property especially suitable for applying to a magnetic thermometer and a magnetic fluid.

CONSTITUTION: A compd. expressed by formula I ((x) is an integer of 70-110 preferably, especially 92; L is a ligand expressed by formula II) is produced. The compd. where 16 manganese ions surround cores of 6 barium ions surrounding carbonate is preferably provided. 16 nuclei manganese aggregate where the cores are surrounded with the 16 manganese ions is provided. The compd. is produced by forming an aq. soln. containing (A) a carbonate ion source, preferably Na2CO3 and/or CO2 and (B) the compd. expressed by formula III (R is H or a hydrocarbon group; (x) is the integer of 0 - about 30) in a molar ratio of A:B = about 1:(1-100), preferably about 1:(2-10) and containing no chloride, and leaving the aq. soln. to stand for a sufficient time of form the compd. expressed by formula I. Further, the (B) can be prepared at a job site.