Why is manganese not ferromagnetic?

Pure manganese is not ferromagnetic because the distance between manganese atoms is too small. At this distance, the nearest neighbor moments have a lower energy when they point in opposite directions. read more

The austenitic form of Iron is paramagnetic not ferromagnetic, so just changing the crystal structure slightly can switch between ferromagnetism and paramagnetism. In Manganese the balance of the interactions prevents ferromagnetism, though note Manganese alloys like Heusler alloy can be ferromagnetic. read more

The Curie temperature in a ferromagnetic metal corresponds to the temperature where spin up/down bands spontaneously split, and the condition for this happening is set by the Stoner criterion (tl;dr: ferromagnets not only stick to your fridge, but they also get you high AF). read more