  Quantum-well (QW) devices have been extensively investigated in semiconductor
structures. More recently, spin-polarized QWs were integrated into magnetic
tunnel junctions (MTJs). In this work, we demonstrate the spin-based control of
the quantized states in iron $3d$-band QWs, as observed in experiments and
theoretical calculations. We find that the magnetization rotation in the Fe QWs
significantly shifts the QW quantization levels, which modulate the
resonant-tunneling current in MTJs, resulting in a tunneling anisotropic
magnetoresistance (TAMR) effect of QWs. This QW-TAMR effect is sizable compared
to other types of TAMR effect, and it is present above the room-temperature. In
a QW MTJ of Cr/Fe/MgAl$_2$O$_4$/top electrode, where the QW is formed by a
mismatch between Cr and Fe in the $d$ band with $\Delta_1$ symmetry, a QW-TAMR
ratio of up to 5.4 % was observed at 5 K, which persisted to 1.2 % even at
380K. The magnetic control of QW transport can open new applications for
spin-coupled optoelectronic devices, ultra-thin sensors, and memories.
