Magnetic tunnel junction (MTJ) devices with a heterogeneous free layer structure particularly suited for efficient spin-torque-transfer (STT) magnetic random access memory (MRAM) (STT MRAM) are disclosed. In one aspect, a MTJ structure with a reduced thickness first pinned layer section provided below a first tunnel magneto-resistance (TMR) barrier layer is provided. The first pinned layer section includes one pinned layer magnetized in one magnetic orientation. In another aspect, a second pinned layer section and a second TMR barrier layer are provided above a free layer section and above the first TMR barrier layer in the MTJ. The second pinned layer is magnetized in a magnetic orientation that is anti-parallel (AP) to that of the first pinned layer section. In yet another aspect, the free layer comprises first and second heterogeneous layers separated by an anti-ferromagnetic coupling spacer, the first and second heterogeneous layers differing in their magnetic anisotropy.