  Voltage controlled magnetic anisotropy (VCMA) is an efficient way to
manipulate the magnetization states in nanomagnets, promising for low-power
spintronic applications. The underlying physical mechanism for VCMA is known to
involve a change in the d-orbital occupation on the transition metal interface
atoms with an applied electric field. However, a simple qualitative picture of
how this occupation controls the magnetocrystalline anisotropy (MCA) and even
why in certain cases the MCA has opposite sign still remains elusive. In this
paper, we exploit a simple model of orbital populations to elucidate a number
of features typical for the interface MCA and the effect of electric field on
it, for 3d transition metal thin films used in magnetic tunnel junctions. We
find that in all considered cases including the Fe (001) surface, clean
Fe1-xCox(001)/MgO interface and oxidized Fe(001)/MgO interface, the effects of
alloying and electric field enhance the MCA energy with electron depletion
which is largely explained by the occupancy of the minority-spin dxz,yz
orbitals. On the other hand, the hole doped Fe(001) exhibits an inverse VCMA,
where the MCA enhancement is achieved when electrons are accumulated at the Fe
(001)/MgO interface with applied electric field. In this regime we predict a
significantly enhanced VCMA which exceeds 1pJ/Vm. Realizing this regime
experimentally may be favorable for a practical purpose of voltage driven
magnetization reversal.
