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J. Lourembam and J. Huang
CoFe/MgO structures with a Co-rich interface having higher efficiency [45, 46].
First-principles calculations predicted that the electric-field efficiency in heterostructures Fe/Co/MgO and FeCo/MgO values were larger by a factor of 10.9 and 6.75,
respectively, as compared to that of Fe/MgO, which was reported to be +130 fJV
−1
m
−1 . In this case, the direction of the electric-field points towards the MgO at the
interface. In this convention, the perpendicular magnetic anisotropy energy (MAE)
increases as the electron density decreases (by applying a positive voltage) at the
free layer/MgO interface. For the sake of simplicity, we will adopt this convention
for the rest of the chapter unless otherwise stated.
Interestingly, while the electric-field tuning of magnetic anisotropy is thought
to be limited to the ferromagnet/oxide interface, the underlayer or the cap layer
was also reported to influence the electric-field efficiency [47, 48]. Depending on the
heavy metal used as an underlayer, the sign of the efficiency may change from positive
to negative. Interestingly, Pt as the underlayer was found to show substantially more
efficiency compared to Ta or Au [47, 48]. An exhaustive first-principles investigation
of different heavy materials to achieve high VCMA is still lacking. The choices of
heavy metal could be expanded to Ir, Nb, Hf, Zr etc. Hf underlayer on CoFe/MgO
structures could see electric field efficiency as high as − 387 fJV
−1 m
−1 [44].
First-principles calculations can also guide us in the invention of new stack structures such as the insertion of an oxide or a metal layer between the ferromagnet and
the barrier oxide [49]. Insertion of an oxide monolayer can significantly affect the
redistribution of d-electrons near the Fermi level. By studying monolayer insertions
with oxides such as FeO, CoO, NiO, PdO and ZnO, Minggang et al. found that ZnO
insertion shows the highest efficiency amongst these systems reaching E-field efficiency of + 166 fJV
−1 m
−1 [50]. VCMA effect under select monolayer insertions
are shown in Fig. 2, and Magnetocrystalline anisotropy (MCA) shows quasi-linear
dependence with electric-field. Here, the direction of the positive electric-field points
away from the MgO. Such oxide insertion schemes can be easily realised practically by introducing an oxidation step in the MTJ stack deposition process. In the
next section, experimental results and methods on electric-field efficiency will be
described in detail.
2.2 Experiments—Single Ferromagnet Structures
One of the popular methods of determining E-field efficiency is by isolating the
free-layer from the full MTJ stack and studying it independently, e.g. gated-Hall
bar structure. Even though VCMA can be determined directly from MTJs, the fabrication of these devices can be costly and time-consuming especially for the discovery
of new materials. At the same time, the electric-field can also induce secondary effects
which can complicate the understanding of materials dependent E-field efficiency.
Let us first discuss the gated-Hall bar approach, the schematic of which is given
Fig. 3a and the VCMA calculation method is described in the following paragraphs.
J. Lourembam and J. Huang
CoFe/MgO structures with a Co-rich interface having higher efficiency [45, 46].
First-principles calculations predicted that the electric-field efficiency in heterostructures Fe/Co/MgO and FeCo/MgO values were larger by a factor of 10.9 and 6.75,
respectively, as compared to that of Fe/MgO, which was reported to be +130 fJV
−1
m
−1 . In this case, the direction of the electric-field points towards the MgO at the
interface. In this convention, the perpendicular magnetic anisotropy energy (MAE)
increases as the electron density decreases (by applying a positive voltage) at the
free layer/MgO interface. For the sake of simplicity, we will adopt this convention
for the rest of the chapter unless otherwise stated.
Interestingly, while the electric-field tuning of magnetic anisotropy is thought
to be limited to the ferromagnet/oxide interface, the underlayer or the cap layer
was also reported to influence the electric-field efficiency [47, 48]. Depending on the
heavy metal used as an underlayer, the sign of the efficiency may change from positive
to negative. Interestingly, Pt as the underlayer was found to show substantially more
efficiency compared to Ta or Au [47, 48]. An exhaustive first-principles investigation
of different heavy materials to achieve high VCMA is still lacking. The choices of
heavy metal could be expanded to Ir, Nb, Hf, Zr etc. Hf underlayer on CoFe/MgO
structures could see electric field efficiency as high as − 387 fJV
−1 m
−1 [44].
First-principles calculations can also guide us in the invention of new stack structures such as the insertion of an oxide or a metal layer between the ferromagnet and
the barrier oxide [49]. Insertion of an oxide monolayer can significantly affect the
redistribution of d-electrons near the Fermi level. By studying monolayer insertions
with oxides such as FeO, CoO, NiO, PdO and ZnO, Minggang et al. found that ZnO
insertion shows the highest efficiency amongst these systems reaching E-field efficiency of + 166 fJV
−1 m
−1 [50]. VCMA effect under select monolayer insertions
are shown in Fig. 2, and Magnetocrystalline anisotropy (MCA) shows quasi-linear
dependence with electric-field. Here, the direction of the positive electric-field points
away from the MgO. Such oxide insertion schemes can be easily realised practically by introducing an oxidation step in the MTJ stack deposition process. In the
next section, experimental results and methods on electric-field efficiency will be
described in detail.
2.2 Experiments—Single Ferromagnet Structures
One of the popular methods of determining E-field efficiency is by isolating the
free-layer from the full MTJ stack and studying it independently, e.g. gated-Hall
bar structure. Even though VCMA can be determined directly from MTJs, the fabrication of these devices can be costly and time-consuming especially for the discovery
of new materials. At the same time, the electric-field can also induce secondary effects
which can complicate the understanding of materials dependent E-field efficiency.
Let us first discuss the gated-Hall bar approach, the schematic of which is given
Fig. 3a and the VCMA calculation method is described in the following paragraphs.