Preferably the precursor bodies comprise an assembly of plurality of blocks of an RMB alloy, where R comprises at least one rare earth element and M comprises at least one transition metal, for example Fe, Co, or Fe and Co, such as an alloy which most preferably comprises 13-19 atomic percent R, 4-20 atomic percent B and the balance M, where R comprises 50 atomic percent or greater Pr, 0.1-10 atom