Voltage-controlled magnetic anisotropy effect through a high-k MgO/ZrO2/MgO hybrid tunneling barrier

We investigated the voltage-controlled magnetic anisotropy (VCMA) effect in epitaxial magnetic tunnel junctions (MTJs) with a hybrid MgO/ZrO2/MgO tunnel barrier. A metastable cubic ZrO2(001) thin film was successfully grown on a MgO(001) layer, leading to the high dielectric constant of 26.5. Using the hybrid tunneling barrier, we achieved the large VCMA coefficient of −350 fJ V−1 m−1, which is 70% larger than that observed in the magnetic tunnel junction with the single MgO barrier. Introduction of crystalline high-k dielectric tunneling barrier can open up new pathways to improving the VCMA properties in MTJs for voltage-driven spintronic devices.

T he voltage-controlled magnetic anisotropy (VCMA) effect has been attracting a great deal of attention as a promising approach for energy-efficient control of the magnetization direction, for example, for writing information in voltage-controlled magnetoresistive random-access memories (VC-MRAMs). 1,2)[9][10][11][12] Since the purely electronic VCMA effect does not require chemical reactions or atomic displacements, high-speed response and high cyclic endurance can be realized.][15] In this switching scheme, we need to eliminate the perpendicular magnetic anisotropy (PMA) energy completely during the switching process.On the other hand, as the MTJs element size is reduced to increase the memory capacity, larger PMA energy is required to maintain high thermal stability.To satisfy these two requirements simultaneously, a larger VCMA coefficient is required to ensure the scalability of VC-MRAMs.For example, assuming a 1nm-thick free layer with a PMA energy of 0.3 mJ m −2 and MTJs diameter of 30 nm, a VCMA coefficient of 300 fJ V −1 m −1 is required under a switching electric field of 1 V nm −1 . 2)5][26][27][28] In most previous works, a single MgO barrier was used as the dielectric layer. For the purel electronic VCMA effect, the change in PMA should be proportional to the amount of charge accumulated at the interface.Therefore, introduction of a high-k dielectric material for the tunneling barrier is a promising approach to enable enhancement of the VCMA effect.
Actually, an enhanced VCMA effect has been reported using high-k dielectric layers, such as MgO/HfO 2 , 29,30) MgO/Pb(Zr x Ti 1−x )O 3 /MgO, 31) and SrTiO 3 . 32,33)Recently, we performed a systematic investigation into the relationship between the dielectric constant (ε r ) and the VCMA coefficient in FeB/MgO/HfO 2 and confirmed that there was a positive correlation between them. 34)However, the HfO 2 layer was a thick amorphous layer, which was unsuitable for use as a coherent tunneling barrier in a MTJs.In addition, high-k materials generally have a larger dielectric constant when they are crystallized.Therefore, development of a crystalline high-k tunneling barrier is highly desirable.As a candidate for this, we focused on ZrO 2 , which is known as a high-k gate insulator.First principles calculations predict a higher dielectric constant of ε r = 37 for cubic ZrO 2 than for the amorphous phase (ε r = 22). 35)t has also been reported that ZrO 2 has a low crystallization temperature, 36) which is an advantage when fabricating a crystalline tunneling barrier.
In this study, we investigated the VCMA effect for a MTJs with a hybrid MgO/ZrO 2 /MgO tunneling barrier.We successfully obtained an epitaxial MgO/ZrO 2 /MgO tunnel barrier with metastable cubic ZrO 2 (001).The dielectric constant of the hybrid barrier was 30% larger than that of the single MgO barrier.With this hybrid tunneling barrier the VCMA coefficient reached −350 fJ V −1 m −1 , which is 70% greater than that observed in a MTJs with the MgO barrier.
Multilayered structures consisting of a MgO seed layer (5 nm)/Cr buffer layer (50 nm)/Fe (0.5 nm)/Ir (0.06 nm)/Co (0.1 nm)/MgO (1.0 nm)/ZrO 2 (0.5 nm)/MgO (0.5 nm)/Fe (10 nm)/Ta (5 nm)/Ru (7 nm) were deposited on single crystal MgO(001) substrates using a combination of MBE and magnetron sputtering [Fig.1(a)].The 5 nm thick MgO seed layer and the 50 nm thick Cr buffer layer were grown at 200 °C by electron-beam evaporation in the MBE chamber.After deposition of the Cr buffer layer, the films were annealed at 800 °C to obtain an atomically flat surface.Ultrathin Fe/Ir/Co layers were deposited at 150 °C followed by annealing at 250 °C.This trilayer structure works as the perpendicularly-magnetized free layer, whose PMA can be controlled by the VCMA effect.After cooling the substrate to RT, a 1 nm thick MgO layer was deposited and annealed at 250 °C.Then, the sample was moved to a sputtering deposition chamber through an ultrahigh vacuum transfer chamber.A 0.5 nm thick ZrO 2 layer was deposited at RT by direct rf sputtering from a ZrO 2 sintered target with a low input power of 0.5 W cm −2 .The sample was moved to the MBE chamber again, and then, a 0.5 nm thick MgO layer and 10 nm thick Fe top layer were deposited at RT and 150 °C, respectively.Finally, Ta/Ru capping layers were deposited by sputtering.The top Fe layer is used as the in-plane magnetized reference layer to evaluate the PMA and VCMA properties from TMR measurements as discussed later.As a reference sample, we also prepared a MTJs without inserting the ZrO 2 layer, i.e. a MTJs with a 2 nm thick MgO tunneling barrier.To evaluate the dielectric constant and VCMA properties, the thin films were patterned into MTJs devices by employing conventional optical lithography, ionmilling, and lift-off processes.The resistance-area products of the MgO/ZrO 2 /MgO and MgO MTJs were 81 kΩμm 2 and 11 kΩμm 2 , respectively.It should be noted that the hybrid MgO/ZrO 2 /MgO barrier exhibits smaller breakdown voltage of about 1.7 V compared to that of the single MgO barrier (2.1 V), probably due to the low breakdown voltage of high-k dielectric material. 37)igure 1 We also performed cross-sectional scanning transmission electron microscopy (STEM) analyses of the MTJs structure with the MgO/ZrO 2 /MgO trilayer dielectric.The bright field (BF)-STEM image, shown in Fig. 2(a), confirms that the interface between the Fe/Ir/Co free layer and the bottom MgO (001) tunneling barrier is both epitaxial and flat.Although it is difficult to distinguish interface between the MgO and ZrO 2 layers from the BF-STEM image, it does confirm continuous epitaxial growth of the MgO(001)/ZrO 2 (001) layers.We can also see the degradation in crystallinity of the top MgO layer.This tendency is consistent with the results of the RHEED observation.Figure 2(b) shows a cross-sectional high-angle annular dark field (HAADF)-STEM image for the same sample.Since the HAADF-STEM signal is proportional to the square of the atomic number of the element, the contrast between the Mg and Zr atoms becomes clear.We can clearly see the bright layer around the center part of the tunneling barrier, suggesting the presence of a ZrO 2 middle layer as designed.For further confirmation, energy dispersive X-ray spectrometry (EDS) elemental line profiles (O, Mg, Cr, Fe, Co, Zr, Ir) were also taken normal to the film plane as shown in Fig. 2(c).A clear Zr peak can be seen between the two MgO layers.These results reveal that there is no clear diffusion of Zr atoms from the tunneling barrier.
Subsequently, we assessed ε r of the MgO/ZrO 2 /MgO hybrid tunneling barrier using an impedance analyzer (Keysight, E4990A with a 42941A impedance probe).Evaluating the intrinsic capacitance component of small MTJs devices proved challenging due to non-negligible parasitic capacitances, including those from interlayer SiO 2 films [see Fig. 3(a)].To address this problem, we investigated the device-size dependence of capacitance. 34)Figure 3(b) shows MTJs device area (S) dependence of the capacitance multiplied by the dielectric layer thickness (t barrier ): C p t barrier .Here, parasitic capacitance components, which can be evaluated from the intercept of S dependence of measured C p t barrier , are already subtracted.C p t barrier varies linearly with S and its slope correlates with the dielectric constant ε r (= C p t barrier /ε 0 S).We observe a larger slope for the MgO/ZrO 2 /MgO hybrid tunneling barrier compared to that of the MgO single barrier, suggesting that a higher ε r is obtained for the MgO/ZrO 2 /MgO hybrid barrier.The ε r values evaluated from the slopes are 26.5 ± 0.3 and 21 ± 2 for the MgO/ZrO 2 /MgO and MgO barriers, respectively.By inserting the ZrO 2 layer, the dielectric constant has been increased by about 30%.It should be noted that the dielectric constant of the single MgO dielectric layer is larger than that of the bulk value (ε r = 9.8).This increase in ε r may be related to distortion of the MgO layer, which we will discuss elsewhere.
Finally, we examined the VCMA effect for a MTJs with the MgO/ZrO 2 /MgO hybrid tunneling barrier.The bias voltage dependences of normalized TMR curves measured under in-plane magnetic fields are shown in Fig. 4 for MTJs with (a) the MgO barrier and (b) the MgO/ZrO 2 /MgO hybrid barrier.In-plane magnetic fields tilt the magnetization of the perpendicularly-magnetized free layer in the film plane while that of the reference layer stays in the film plane.Therefore, the saturation properties of the tunneling resistance reflect the magnetization process of the free layer in the magnetic hard axis direction.Typical TMR ratios of orthogonally-magnetized MTJs with the MgO single barrier and the MgO/ZrO 2 /MgO hybrid barrier were 18% and 7%, respectively.Lower TMR ratio in the MTJs with the hybrid barrier is probably attributed to the poorer crystallinity of ZrO 2 and top MgO as discussed above.Clear shifts in the saturation fields are observed in both cases due to the VCMA effect.Here, the sign of the bias electric field is defined with respect to the top electrode, indicating that positive (negative) bias induces electron accumulation (depletion) at the interface between the Fe/Ir/Co trilayer and the dielectric layer.Employing the feature that the tunneling conductance depends on the relative angle between the free and reference layers, the ratio of the in-plane magnetization component of 023001-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd the free layer (M in-plane ) to its saturation magnetization (M S ) can be calculated from the TMR curves (normalized M in-plane − H curve).The PMA energies K PMA can be evaluated from the M in-plane (H) area by combining it with the M S value obtained from independent vibrating sample magnetometer measurements.The values of saturation magnetization of Fe/Ir/Co trilayer was evaluated to be about 2.09 ± 0.03 T for both structures with single MgO and hybrid MgO/ZrO 2 /MgO barriers, however we observed a slightly thicker magnetic dead layer of about 0.10 nm for the hybrid MgO/ZrO 2 /MgO barrier compared to that of the single MgO (0.09 nm).More details about the evaluation process can be found in Ref. 28.The bias electric field dependences of K PMA t free are summarized in Fig. 4 for the MTJs with (c) the MgO barrier and (d) the MgO/ZrO 2 /MgO hybrid barrier.
Here, t free is the total thickness of the Fe/Ir/Co trilayer and the electric-field is given by V bias /t barrier .K PMA t free changes linearly with the applied electric field.The PMA decreases (increases) under a positive (negative) electric field.This sign is the same as that observed for Fe-based alloy/MgO structures. 28)The VCMA coefficients evaluated from the slopes of the linear fittings are −210 fJ V −1 m −1 and −350 fJ V −1 m −1 for the MTJs with a MgO barrier and a MgO/ZrO 2 /MgO hybrid barrier, respectively.Inserting the ZrO 2 layer increased the VCMA coefficient by about 70%.Interestingly, the VCMA coefficient increases more than the dielectric constant.This implies that the observed enhancement of the VCMA effect originates not only from the increase in dielectric constant but also from some other mechanism.One possible cause is the influence of weak oxidation at the interface between the free layer and the MgO layer.Nakamura et al. predicted an enhancement in the VCMA effect due to the formation of a weakly oxidized FeO layer at the Fe/MgO interface, based on first-principles calculations. 38)Recently, we systematically investigated the influence of the surface oxidation state on the VCMA properties of Fe/Ir/Co/ultrathin MgAlO x /MgO/Fe MTJs by controlling the oxidation conditions for the ultrathin MgAlO    023001-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd layer.Although the PMA and TMR properties were maximized when the oxidation conditions were optimized, we found an increase in the VCMA coefficient if the surface was weakly-oxidized, where the PMA was slightly reduced. 17)In this work, we also observed a decrease in K PMA t free for the MTJs with the MgO/ZrO 2 /MgO (0.25 mJ m −2 ) layer compared to the one with the single MgO (0.51 mJ m −2 ) layer.This tendency suggests the weak oxidation at the bottom interface between the Fe/Ir/Co layers, which can occur during growth of the ZrO 2 layer on the MgO layer, leading to the increased VCMA coefficient.In summary, we investigated the influence of inserting a thin ZrO 2 layer into a MgO tunnel barrier on the VCMA properties.We successfully obtained an epitaxial cubic ZrO 2 (001) thin film by growing it on a MgO(001) tunneling barrier.The dielectric constant of the MgO/ZrO 2 /MgO hybrid barrier was 30% greater than that of a single MgO barrier.We observed a large VCMA coefficient of −350 fJ V −1 m −1 for a MTJs with the hybrid barrier.This represents an increase by 70% compared with the VCMA coefficient for the MTJs with a single MgO barrier, which is a larger increase than the increase in dielectric constant.This larger increase in the VCMA coefficient may be related to a difference in the interfacial oxidation state.The introduction of a high-k tunneling barrier and precise control of the interfacial oxidation state can provide us with a valuable approach to improving the performance of MTJs for VC-MRAMs.023001-4 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd (b) shows reflection high energy electron diffraction (RHEED) images for the bottom MgO, ZrO 2 and top MgO layers.The incident electron beam is parallel to the [100] direction of the MgO layer.These images confirm the growth of a cubic ZrO 2 (001) layer on the MgO(001), although the streak patterns are a little obscure.The effect of the ZrO 2 layer on the crystalline quality of the top MgO layer is apparent, being of poorer quality compared to that of the bottom MgO layer.

Fig. 1 .
Fig. 1.(a) Schematic illustration of the sample structure.(b) RHEED patterns taken along the MgO[100] direction of the top MgO (upper), ZrO 2 (middle), and bottom MgO (lower) layers, corresponding to the region outlined in red in (a). x

Fig. 2 .
Fig. 2. Nanostructural analysis results of a MTJs with the MgO/ZrO 2 /MgO hybrid tunneling barrier: (a) BF-STEM image, (b) high-angle annular dark-field scanning TEM (HAADF-STEM) image, and (c) energy dispersive EDS elemental line profiles taken along the film-plane.Distance = 0 corresponds to the peak of Mg signal obtained from the bottom MgO layer.Peak positions of Mg signals for two MgO layers are indicated by red arrows.

Fig. 3 .
Fig. 3. (a) Schematic illustration of the microfabricated MTJs device.(b) MTJs device area (S) dependence of the capacitance of the tunneling barriers.

Fig. 4 .
Fig. 4. Bias voltage dependence of normalized TMR curves for MTJs with (a) a MgO single barrier and (b) a MgO/ZrO 2 /MgO hybrid barrier and the bias electric field dependence of K PMA t free for MTJs with (c) a MgO single barrier and (d) a MgO/ZrO 2 /MgO hybrid barrier.