The intensity of the sample is depends on miller indices (h k l) of a sample. The miller indices depends on the Crystallinity of the sample. The crystal structure of the sample changed during the phase transformation when calcination occured. As a result,when the phase transformation of Fe occurs, the change in crystal structure will be changed. For this reason,you got higher intensity for alpha-Fe. 

There are a lot of reasons why this could happen, but you really need to explain the type of instrument you used and how the measurements were taken before anyone could begin to offer and helpful ideas. I can tell you one thing for sure, though. Nothing Sujoy wrote has anything to do with either what you measured or physical reality.