Electric-Field-Controlled MRAM: Physics and Applications
151
Fig. 1 a Comparison of various memory technologies in the plot of writing energy per bit versus
writing speed. b Memory-compute hierarchical architecture showing various levels of writing speed
requirements
2 Materials and Techniques
2.1 First-Principles Calculations
Well-motivated by the practicality of electric-field based writing mechanism, there
has been intensive studies in applying this approach on popular MRAM 3d ferromagnet/oxide heterostructures such as CoFeB/MgO [33, 38–40]. In these systems,
the perpendicular magnetic anisotropy (PMA) arises from strong hybridization
between the interfacial (Co)Fe–3d and the O–2p orbitals and also exhibits strong
spin–orbit coupling (SOC) [10, 41, 42]. The introduction of the electric-field in
these systems is argued to be purely an interfacial effect [43], which is why significant modulation of anisotropy by an electric field is observed in thin films even
though it is screened at the ferromagnet/oxide interface. This phenomenon is also
widely known as voltage-controlled magnetic anisotropy (VCMA), and its origin
is generally discussed as the manifestation of relative change in the occupancy of
the 3d orbitals associated with capacitive charging and discharging effect at the
ferromagnet/oxide interface.
Since orbital physics plays a vital role in the physical origin of this effect, firstprinciples calculations can help to understand the electric- field effect as well as accelerate materials discovery to achieve high electric-field efficiency. In particular, by
looking at the K-resolved magnetic anisotropy, determined from first-principles
calculations, one can see a very significant difference in the two-dimensional Brillouin zone with and without electric-field for Ta/CoFe/MgO [44]. There have been
several works on this topic using density functional theory, particularly in heavy
metal/ferromagnet/oxide heterostructure because it is more straightforward to implement this stack structure in MTJs. Studies include materials combined with various
ferromagnets, heavy metals, and even oxides other than MgO. Using different ferromagnets it was shown that the electric-field could tune Co/MgO, Fe/MgO and
151
Fig. 1 a Comparison of various memory technologies in the plot of writing energy per bit versus
writing speed. b Memory-compute hierarchical architecture showing various levels of writing speed
requirements
2 Materials and Techniques
2.1 First-Principles Calculations
Well-motivated by the practicality of electric-field based writing mechanism, there
has been intensive studies in applying this approach on popular MRAM 3d ferromagnet/oxide heterostructures such as CoFeB/MgO [33, 38–40]. In these systems,
the perpendicular magnetic anisotropy (PMA) arises from strong hybridization
between the interfacial (Co)Fe–3d and the O–2p orbitals and also exhibits strong
spin–orbit coupling (SOC) [10, 41, 42]. The introduction of the electric-field in
these systems is argued to be purely an interfacial effect [43], which is why significant modulation of anisotropy by an electric field is observed in thin films even
though it is screened at the ferromagnet/oxide interface. This phenomenon is also
widely known as voltage-controlled magnetic anisotropy (VCMA), and its origin
is generally discussed as the manifestation of relative change in the occupancy of
the 3d orbitals associated with capacitive charging and discharging effect at the
ferromagnet/oxide interface.
Since orbital physics plays a vital role in the physical origin of this effect, firstprinciples calculations can help to understand the electric- field effect as well as accelerate materials discovery to achieve high electric-field efficiency. In particular, by
looking at the K-resolved magnetic anisotropy, determined from first-principles
calculations, one can see a very significant difference in the two-dimensional Brillouin zone with and without electric-field for Ta/CoFe/MgO [44]. There have been
several works on this topic using density functional theory, particularly in heavy
metal/ferromagnet/oxide heterostructure because it is more straightforward to implement this stack structure in MTJs. Studies include materials combined with various
ferromagnets, heavy metals, and even oxides other than MgO. Using different ferromagnets it was shown that the electric-field could tune Co/MgO, Fe/MgO and