  A major issue in the development of spintronic memory devices is the
reduction of the power consumption for the magnetization reversal. For this
purpose, the artificial control of the magnetic anisotropy of ferromagnetic
materials is of great importance. Here, we demonstrate the control of the
carrier-energy dependence of the magnetic anisotropy of the density of states
(DOS) using the quantum size effect in a single-crystal ferromagnetic material,
GaMnAs. We show that the mainly two-fold symmetry of the magnetic anisotropy of
DOS, which is attributed to the impurity band, is changed to a four-fold
symmetry by enhancing the quantum size effect in the valence band of the GaMnAs
quantum wells. By combination with the gate-electric field control technique,
our concept of the usage of the quantum size effect for the control of the
magnetism will pave the way for the ultra-low-power manipulation of
magnetization in future spintronic devices.
